In the diode pumped solid state laser technique several resonator configurations and pumping schemes are utilized. One of interesting setups with unique properties is bounce geometry resonator configuration (Alcock, 1997) when a slab active material is used and the laser beam experiences total internal reflection and amplification on the pumped crystal side. This arrangement has several advantages. The slab crystal shape allows good heat dissipation, because it is relatively thin and can be cooled from one or both sides. As a pump source, high power laser diode single bar or bar stacks can be used without bulky additional optics or radiation delivery fibers. In addition, laser resonator mode can be optimized to match the pumped region profile. Efficient operation of the bounce geometry laser pumped by the laser diode bar requires active medium with a high absorption coefficient for the pump radiation in order to obtain thin region with high gain near the pumping side. Therefore, materials as Nd:GdVO4 and Nd:YVO4 has challenged Nd:YAG with standard doping concentration of around 1 at.%. When recently Nd:YAG laser crystals with higher doping level (more than 2 at.%) became available (Urata, 2001), efficient operation of these crystals under bounce geometry has been demonstrated operating at 1.06 and 1.3 *μ*m (Sauder, 2006; 2007). Nd:YAG crystals have better thermal and mechanical properties, and better energy storage capacity due to the longer upper state life-time in comparison with vanadates giving potential to generate more energetic pulses in the Q-switched and mode-locked regime.

Highly-energetic pulses in the nanosecond range can be obtained from the oscillator using active or passive Q-switching, which is well-known and widespread technique, mostly used under continuous pumping. In order to obtain more energetic pulses, quasi-continuous (QCW) pumping can be employed. Using the 2 at.% Nd:YAG in bounce geometry under QCW pumping, passively Q-switched operation at 1.06 *μ*m has been reported with maximum

**2. Nd:YAG crystal with doping concentration of 2.4 at.%**

face 30 × 2 mm was antireflection coated at the 808 nm pump wavelength.

wavelength tuning around 808 nm to match the Nd:YAG absorption peak.

Fig. 1. Fluorescence lifetime measurement of 2.4 at.% Nd:YAG slab crystal.

The laser free-running operation was investigated at the output wavelengths of 1.06, 1.3, and 1.4 *μ*m and is described in details in Kubeˇcek (2009b). Schematic of the laser oscillator is

Generation at the wavelength of 1.06 *μ*m was studied at first. The 17 cm long laser resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC. Various OC reflectivities of 82, 50 and 30 % at 1.06 *μ*m were tested.

**3. Free-running laser operation at 1.06, 1.3 and 1.4** *μ***m**

**2.1 Nd**3+ **lifetime measurement**

shown in Fig. 2.

High neodymium concentration results in shorter upper-state (fluorescence) lifetime caused by concentration quenching. At the typical Nd concentration of 1 at.% the fluorescence lifetime is about 230 *μ*s (Koechner, 2006). At the concentration of 2 at.% the lifetime shortening to 145 - 180 *μ*s and further at 3 at.% to 120 - 130 *μ*s (Dong, 2005; Urata, 2001) was reported.

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 185

In our experiments we have used a Nd:YAG slab crystal with dimensions of 30 × 5 × 2 mm grown by Czochralski method with Nd concentration of 2.4 at.%. Two 5 × 2 mm end faces were angled at 68 degrees. This configuration allows the laser cavity beam to be incident at Brewster's angle to these faces and to experience total internal reflection from the diode pumped face which results in laser operation in the horizontally polarized state. The pumping

As a pump source the linear single bar quasi-continuous diode array LD with the fast axis collimation, nominal output power of 180 W and repetition rate up to 100 Hz was used. This diode was mounted on the copper plate with Peltier cooling enabling fine generated radiation

The integral fluorescence decay from our crystal for excitation at the wavelength of 808 nm was measured using a Si photodiode in the spectral region above 1000 nm and is shown in Fig. 1. Calculated lifetime of 135 *μ*s is in the good agreement with other published results.

output energy of 2.3 mJ in a 12 ns pulse (Sauder, 2006). At 1.3 *μ*m, actively Q-switched operation with maximum pulse energy of 1.8 mJ in 50 ns pulse was produced (Sauder, 2007).

In order to generate pulses in the picosecond and sub-picosecond range, the laser has to be operated in the mode-locking (ML) regime. In the solid state lasers, mostly passive ML techniques are used. These techniques can be divided into several groups. One large group is based on saturable absorbers based on different materials and media. The next group of techniques involves usage of nonlinear effects of different orders. The second order nonlinearity is involved in nonlinear mirror ML (Stankov, 1988; Thomas, 2010) and the third order in Kerr-lens ML (Agnesi, 1994; Spence, 1991). In the case of saturable absorbers, the dye saturable absorber (Schafer, 1976) was widely used in the previous decades, but because of the dye long-term instability, toxicity, and difficult manipulation, it was in the last decade replaced by solid state saturable absorbers. With the semiconductor technique development, the possibility of fabrication of the semiconductor saturable absorber with requested parameters appeared. Such absorber is based on one or more quantum wells grown on the semiconductor substrate. At the wavelength of about 1 *μ*m, the quantum well is usually formed by the InGaAs layer with the thickness of several nanometers surrounded by the GaAs layers. As the InGaAs energy band gap is lower than GaAs band gap, it serves as a quantum well. According to the number of quantum wells and other growing parameters, the resulting characteristics, such as modulation depth, relaxation time constant, and saturation fluence can be adjusted according to the required application and laser operation conditions. Such saturable absorber is placed into the resonator in the transmission mode, usually under the Brewster's angle to avoid refections from the GaAs with high-refractive index. Alternatively, the saturable absorber structure can be grown on the Bragg mirror forming so called semiconductor saturable absorber mirror (SAM), which is placed as a rear mirror in the laser resonator and which has been widely used (Keller, 1996). Use of SAM as the ML technique has also some disadvantages. At first the laser output power is limited by the SAM structure damage threshold. The output pulse duration is also limited by the SAM relaxation time constant. The another approach to the solid-state saturable absorber development utilizes carbon nanotubes or graphene. Mode-locking operation of Nd-doped solid state lasers using carbon nanotubes SA (Yim, 2008) or graphene SA (Lee, 2010; Tan, 2010) was recently demonstrated.

Using semiconductor SAM, the output train of low energy pulses usually under continuous pumping is produced. In order to obtain pulses with higher energy directly from the oscillator, the semiconductor saturable absorber containing multiple quantum wells with higher modulation depth can be used in the laser resonator under QCW pumping. Bounce geometry can be considered as a proper resonator configuration for efficient mode-locking operation in this pumping regime. Efficient operation of quasi-continuously pumped 1 at.% doped Nd:YAG (Kubeˇcek, 2008) and also Nd:YVO4 and Nd:GdVO4 (Kubeˇcek, 2009a; 2010) using the saturable absorber in the transmission mode was already reported.

For applications requiring more energetic pulses or higher average power, the laser oscillator output can be further amplified using a laser amplifier in various configurations involving regenerative amplifier (Luhrmann, 2009), zigzag slab geometry (Cui, 2009; Fu, 2010), multiple amplification media (Fu, 2009), or hybrid fiber and solid-state system (Wushouer, 2010). These configurations are usually based on Nd:YAG or Nd:YVO4 active materials with concentrations below 1 at.%. Much simpler amplifier configuration providing high gain involve grazing incidence geometry based on 1 at.% Nd:YVO4 slab crystal (Agnesi, 2008; 2010).

#### **2. Nd:YAG crystal with doping concentration of 2.4 at.%**

High neodymium concentration results in shorter upper-state (fluorescence) lifetime caused by concentration quenching. At the typical Nd concentration of 1 at.% the fluorescence lifetime is about 230 *μ*s (Koechner, 2006). At the concentration of 2 at.% the lifetime shortening to 145 - 180 *μ*s and further at 3 at.% to 120 - 130 *μ*s (Dong, 2005; Urata, 2001) was reported.

In our experiments we have used a Nd:YAG slab crystal with dimensions of 30 × 5 × 2 mm grown by Czochralski method with Nd concentration of 2.4 at.%. Two 5 × 2 mm end faces were angled at 68 degrees. This configuration allows the laser cavity beam to be incident at Brewster's angle to these faces and to experience total internal reflection from the diode pumped face which results in laser operation in the horizontally polarized state. The pumping face 30 × 2 mm was antireflection coated at the 808 nm pump wavelength.

As a pump source the linear single bar quasi-continuous diode array LD with the fast axis collimation, nominal output power of 180 W and repetition rate up to 100 Hz was used. This diode was mounted on the copper plate with Peltier cooling enabling fine generated radiation wavelength tuning around 808 nm to match the Nd:YAG absorption peak.

#### **2.1 Nd**3+ **lifetime measurement**

2 Will-be-set-by-IN-TECH

output energy of 2.3 mJ in a 12 ns pulse (Sauder, 2006). At 1.3 *μ*m, actively Q-switched operation with maximum pulse energy of 1.8 mJ in 50 ns pulse was produced (Sauder, 2007). In order to generate pulses in the picosecond and sub-picosecond range, the laser has to be operated in the mode-locking (ML) regime. In the solid state lasers, mostly passive ML techniques are used. These techniques can be divided into several groups. One large group is based on saturable absorbers based on different materials and media. The next group of techniques involves usage of nonlinear effects of different orders. The second order nonlinearity is involved in nonlinear mirror ML (Stankov, 1988; Thomas, 2010) and the third order in Kerr-lens ML (Agnesi, 1994; Spence, 1991). In the case of saturable absorbers, the dye saturable absorber (Schafer, 1976) was widely used in the previous decades, but because of the dye long-term instability, toxicity, and difficult manipulation, it was in the last decade replaced by solid state saturable absorbers. With the semiconductor technique development, the possibility of fabrication of the semiconductor saturable absorber with requested parameters appeared. Such absorber is based on one or more quantum wells grown on the semiconductor substrate. At the wavelength of about 1 *μ*m, the quantum well is usually formed by the InGaAs layer with the thickness of several nanometers surrounded by the GaAs layers. As the InGaAs energy band gap is lower than GaAs band gap, it serves as a quantum well. According to the number of quantum wells and other growing parameters, the resulting characteristics, such as modulation depth, relaxation time constant, and saturation fluence can be adjusted according to the required application and laser operation conditions. Such saturable absorber is placed into the resonator in the transmission mode, usually under the Brewster's angle to avoid refections from the GaAs with high-refractive index. Alternatively, the saturable absorber structure can be grown on the Bragg mirror forming so called semiconductor saturable absorber mirror (SAM), which is placed as a rear mirror in the laser resonator and which has been widely used (Keller, 1996). Use of SAM as the ML technique has also some disadvantages. At first the laser output power is limited by the SAM structure damage threshold. The output pulse duration is also limited by the SAM relaxation time constant. The another approach to the solid-state saturable absorber development utilizes carbon nanotubes or graphene. Mode-locking operation of Nd-doped solid state lasers using carbon nanotubes

SA (Yim, 2008) or graphene SA (Lee, 2010; Tan, 2010) was recently demonstrated.

using the saturable absorber in the transmission mode was already reported.

incidence geometry based on 1 at.% Nd:YVO4 slab crystal (Agnesi, 2008; 2010).

Using semiconductor SAM, the output train of low energy pulses usually under continuous pumping is produced. In order to obtain pulses with higher energy directly from the oscillator, the semiconductor saturable absorber containing multiple quantum wells with higher modulation depth can be used in the laser resonator under QCW pumping. Bounce geometry can be considered as a proper resonator configuration for efficient mode-locking operation in this pumping regime. Efficient operation of quasi-continuously pumped 1 at.% doped Nd:YAG (Kubeˇcek, 2008) and also Nd:YVO4 and Nd:GdVO4 (Kubeˇcek, 2009a; 2010)

For applications requiring more energetic pulses or higher average power, the laser oscillator output can be further amplified using a laser amplifier in various configurations involving regenerative amplifier (Luhrmann, 2009), zigzag slab geometry (Cui, 2009; Fu, 2010), multiple amplification media (Fu, 2009), or hybrid fiber and solid-state system (Wushouer, 2010). These configurations are usually based on Nd:YAG or Nd:YVO4 active materials with concentrations below 1 at.%. Much simpler amplifier configuration providing high gain involve grazing The integral fluorescence decay from our crystal for excitation at the wavelength of 808 nm was measured using a Si photodiode in the spectral region above 1000 nm and is shown in Fig. 1. Calculated lifetime of 135 *μ*s is in the good agreement with other published results.

Fig. 1. Fluorescence lifetime measurement of 2.4 at.% Nd:YAG slab crystal.

#### **3. Free-running laser operation at 1.06, 1.3 and 1.4** *μ***m**

The laser free-running operation was investigated at the output wavelengths of 1.06, 1.3, and 1.4 *μ*m and is described in details in Kubeˇcek (2009b). Schematic of the laser oscillator is shown in Fig. 2.

Generation at the wavelength of 1.06 *μ*m was studied at first. The 17 cm long laser resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC. Various OC reflectivities of 82, 50 and 30 % at 1.06 *μ*m were tested.

Fig. 4. Comparison of the laser operation at different wavelengths: 1.06, 1.3, and 1.4 *μ*m.

**4. Passively Q-switched laser operation at 1.06 and 1.3** *μ***m**

of 4.1 % and slope efficiency of 8.4 %.

is described in details in Jelínek (2011a).

is shown in the inset of the figure.

The lowest curve in Fig. 4 shows the output pulse energy at 1.4 *μ*m versus pump pulse energy. The maximum output energy of 1.5 mJ for the pump energy of 38 mJ was obtained using output coupler with reflectivity of 96 %. This corresponds to the optical to optical efficiency

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 187

Q-switched laser operation was investigated at the output wavelengths of 1.06 and 1.3 *μ*m and

The laser resonator schematic was similar to that used in the free-running regime (shown in Fig. 2). At 1.06 *μ*m the 80 mm long optical resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 50 %. In order to obtain Q-switched regime, two 5 mm long anti-reflection (AR) coated Cr:YAG crystals with low signal transmission of 58 % (corresponding to the total low power transmission of 34 %) were introduced into the resonator close to the output coupler OC. Stable Q-switched operation was obtained for the pump energy of 17 mJ. The single 5 ns output pulse with energy of 1.3 mJ was generated directly from the oscillator and its oscillogram is shown in Fig. 5. The output beam spatial profile was Gaussian in both axes and

At 1.3 *μ*m the 100 mm long optical resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 70 % at 1.3 *μ*m. Both dichroic mirrors have low reflectivity at wavelength of 1064 nm. As a saturable absorber V:YAG crystal with low signal transmission of 60 % and AR coating was used. Because of possible AR coating damage the saturable absorber was placed close to the rear high reflective mirror HR, where the fundamental laser mode radius is larger. Stable Q-switched operation at 1.3 *μ*m was obtained for the pump energy of 30 mJ, which is nearly two times higher than the pump energy for the operation at 1.06 *μ*m, caused by lower gain at 1.3 *μ*m transition. Generation at the single wavelength of 1338 nm was confirmed by the spectrometer (Ocean Optics NIR512). The 13 ns output pulse with energy of 600 *μ*J was generated directly from the oscillator and its oscillogram is shown in Fig. 6. The output beam spatial profile shown in the inset of the figure was investigated by a standard silicon CCD

Fig. 2. Schematic of the Nd:YAG laser oscillator in bounce geometry operated in the free-running regime.

The Nd:YAG crystal was inserted approximately in the middle of the resonator. The output pulse energy with respect to the pump pulse energy is shown in Fig. 3. The pump pulse duration was set to 200 *μ*s. For the output coupler with reflectivity of 82 % the slope efficiency of 50 % was obtained. The maximum output energy of 17 mJ for the pump energy of 38 mJ corresponds to the optical to the optical efficiency of 44.6 %. The output beam spatial profile was elliptical in the horizontal axis and is shown in the inset of Fig. 3 (for the output coupler reflectivity of 50 %).

Fig. 3. The laser output characteristics in the free-running regime at 1.06 *μ*m with various output couplers.

The slab shape with uncoated faces allowing the laser beam to be incident on the Nd:YAG at the Brewster's angle has the advantage in possibility of using the same crystal for generation at different wavelengths. We have successfully investigated efficient operation at 1.3 and 1.4 *μ*m and the results were compared with values obtained for the generation at 1.06 *μ*m. For the generation at 1.3 *μ*m the HR mirror (radius of curvature of 150 mm) was highly reflective in this spectral region and had low reflectivity at 1.06 *μ*m. The laser resonator was 95 mm long. For the generation at 1.4 *μ*m the curvature of this mirror was the same, but the mirror was highly reflective at 1.4 *μ*m. The output pulse energy with respect to the pump pulse energy is shown in Fig. 4. The maximum output energy at 1.3 *μ*m was 8.5 mJ for the pump energy of 38 mJ and was obtained using the output coupler with reflectivity of 94 %. This corresponds to the optical to optical efficiency of 22.5 % and slope efficiency of 26 %.

4 Will-be-set-by-IN-TECH

Fig. 2. Schematic of the Nd:YAG laser oscillator in bounce geometry operated in the

The Nd:YAG crystal was inserted approximately in the middle of the resonator. The output pulse energy with respect to the pump pulse energy is shown in Fig. 3. The pump pulse duration was set to 200 *μ*s. For the output coupler with reflectivity of 82 % the slope efficiency of 50 % was obtained. The maximum output energy of 17 mJ for the pump energy of 38 mJ corresponds to the optical to the optical efficiency of 44.6 %. The output beam spatial profile was elliptical in the horizontal axis and is shown in the inset of Fig. 3 (for the output coupler

Fig. 3. The laser output characteristics in the free-running regime at 1.06 *μ*m with various

the optical to optical efficiency of 22.5 % and slope efficiency of 26 %.

The slab shape with uncoated faces allowing the laser beam to be incident on the Nd:YAG at the Brewster's angle has the advantage in possibility of using the same crystal for generation at different wavelengths. We have successfully investigated efficient operation at 1.3 and 1.4 *μ*m and the results were compared with values obtained for the generation at 1.06 *μ*m. For the generation at 1.3 *μ*m the HR mirror (radius of curvature of 150 mm) was highly reflective in this spectral region and had low reflectivity at 1.06 *μ*m. The laser resonator was 95 mm long. For the generation at 1.4 *μ*m the curvature of this mirror was the same, but the mirror was highly reflective at 1.4 *μ*m. The output pulse energy with respect to the pump pulse energy is shown in Fig. 4. The maximum output energy at 1.3 *μ*m was 8.5 mJ for the pump energy of 38 mJ and was obtained using the output coupler with reflectivity of 94 %. This corresponds to

free-running regime.

reflectivity of 50 %).

output couplers.

Fig. 4. Comparison of the laser operation at different wavelengths: 1.06, 1.3, and 1.4 *μ*m.

The lowest curve in Fig. 4 shows the output pulse energy at 1.4 *μ*m versus pump pulse energy. The maximum output energy of 1.5 mJ for the pump energy of 38 mJ was obtained using output coupler with reflectivity of 96 %. This corresponds to the optical to optical efficiency of 4.1 % and slope efficiency of 8.4 %.

#### **4. Passively Q-switched laser operation at 1.06 and 1.3** *μ***m**

Q-switched laser operation was investigated at the output wavelengths of 1.06 and 1.3 *μ*m and is described in details in Jelínek (2011a).

The laser resonator schematic was similar to that used in the free-running regime (shown in Fig. 2). At 1.06 *μ*m the 80 mm long optical resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 50 %. In order to obtain Q-switched regime, two 5 mm long anti-reflection (AR) coated Cr:YAG crystals with low signal transmission of 58 % (corresponding to the total low power transmission of 34 %) were introduced into the resonator close to the output coupler OC. Stable Q-switched operation was obtained for the pump energy of 17 mJ. The single 5 ns output pulse with energy of 1.3 mJ was generated directly from the oscillator and its oscillogram is shown in Fig. 5. The output beam spatial profile was Gaussian in both axes and is shown in the inset of the figure.

At 1.3 *μ*m the 100 mm long optical resonator was formed by a spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 70 % at 1.3 *μ*m. Both dichroic mirrors have low reflectivity at wavelength of 1064 nm. As a saturable absorber V:YAG crystal with low signal transmission of 60 % and AR coating was used. Because of possible AR coating damage the saturable absorber was placed close to the rear high reflective mirror HR, where the fundamental laser mode radius is larger. Stable Q-switched operation at 1.3 *μ*m was obtained for the pump energy of 30 mJ, which is nearly two times higher than the pump energy for the operation at 1.06 *μ*m, caused by lower gain at 1.3 *μ*m transition. Generation at the single wavelength of 1338 nm was confirmed by the spectrometer (Ocean Optics NIR512). The 13 ns output pulse with energy of 600 *μ*J was generated directly from the oscillator and its oscillogram is shown in Fig. 6. The output beam spatial profile shown in the inset of the figure was investigated by a standard silicon CCD

with 100 QW measured to be ∼25 % and ∼50 %, respectively. In the case of SA with 33 QW the modulation depth is lower. The GaAs substrate can be used also as a nonlinear element for passive negative feedback by beam defocusing (Corno, 1990; Diels, 2006). The laser resonators differ in length between 25 and 116 cm and also in the ratio of the beam radius inside the Nd:YAG crystal and on the SA. The experimental details are described in Jelínek (2010; 2011b).

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 189

A schematic diagram of the mode-locked laser oscillator in the configuration (A) is shown in Fig. 7a. The 250 mm long laser resonator was formed by the spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 30 %. The Nd:YAG crystal was inserted at a distance of 130 mm from the OC. The fundamental laser beam profile inside the resonator was calculated by software Psst (University of St Andrews, United Kingdom). According to this calculation the beam diameter in the crystal was approximately 600 *μ*m and its diameter on the SA was 400 *μ*m. In order to obtain better spatial overlap between the pump and smaller laser beam, a cylindrical focusing lens L1

Fig. 7. Schematic of the laser in the mode-locked regime: oscillator (A) 250 mm long and

nonlinear element for passive negative feedback by beam defocusing.

The laser was passively mode-locked by a multiple quantum well semiconductor saturable absorbers (SA) inserted into the resonator close to the OC under the Brewster's angle in the transmission mode. Two different samples of saturable absorbers, consisted of 33 and 100 quantum wells (QW), were investigated. In addition its GaAs substrate served also as

Using the SA consisting 33 QW, the mode-locking threshold was achieved for pump energy of 17.5 mJ in 150 *μ*s pulse. Pulse duration evolution along the train was measured by a fast oscilloscope LeCroy SDA 9000 and a PIN photodiode EOT ET-3500 and a typical output pulse train oscillogram together with details of the pulses at the beginning and the end of the train

The measured pulse width was consequently recalculated using the experimentally determined oscilloscope-photodiode instrumental limit and *sum of square* method, which is in details described in Jelínek (2011c); Kubeˇcek (2009d). Although the absolute calculated pulse duration may not be precise, pulse shortening from initial 120 ps to final 35 ps can be clearly observed and in summary is shown in Fig. 9. The whole train consisted of 100 pulses and its

are shown in Fig. 8. The oscilloscope-photodiode system response time was 75 ps.

**5.1.1 Laser operation with short resonator**

(f = 50 mm) was used to focus the pumping radiation.

oscillator (B) 1160 mm long.

Fig. 5. Oscillogram of the Q-switched output pulse at 1.06 *μ*m.

Fig. 6. Oscillogram of the Q-switched output pulse at 1.34 *μ*m.

camera. Because of its low sensitivity in this region the measured signal was quite low but it can be deduced that the beam profile was approximately Gaussian in both axes.

#### **5. Passively mode-locked laser operation at 1.06** *μ***m**

In order to obtain mode-locked laser operation, two types of semiconductor saturable absorbers (SA) were tested. In the first case the SA in the transmission mode was used and the output characteristics were compared with the characteristics of the laser mode-locked using the semiconductor saturable absorber mirror (SAM).

#### **5.1 Laser mode-locking by saturable absorber in transmission mode**

In the transmission mode two saturable absorbers were investigated in two resonator configurations. The saturable absorbers were grown by molecular beam epitaxy at the Center for High Technology Materials of the University of New Mexico (Diels, 2006; Kubeˇcek, 2009c). The structures were grown on the 400 *μ*m thick GaAs substrates and consisted of 33 or 100 quantum wells (QW) formed by the periods of 9 nm thick In0.275Ga0.725As and 11 nm thick GaAs layers grown at low temperature (around 350◦ C) in order to decrease the carrier lifetime to the value less than 50 ps (Gupta, 1992). Number of the QWs determines the saturable absorber modulation depth and low-power transmission which were in the case of the SA with 100 QW measured to be ∼25 % and ∼50 %, respectively. In the case of SA with 33 QW the modulation depth is lower. The GaAs substrate can be used also as a nonlinear element for passive negative feedback by beam defocusing (Corno, 1990; Diels, 2006). The laser resonators differ in length between 25 and 116 cm and also in the ratio of the beam radius inside the Nd:YAG crystal and on the SA. The experimental details are described in Jelínek (2010; 2011b).

#### **5.1.1 Laser operation with short resonator**

6 Will-be-set-by-IN-TECH

Fig. 5. Oscillogram of the Q-switched output pulse at 1.06 *μ*m.

Fig. 6. Oscillogram of the Q-switched output pulse at 1.34 *μ*m.

**5. Passively mode-locked laser operation at 1.06** *μ***m**

**5.1 Laser mode-locking by saturable absorber in transmission mode**

the semiconductor saturable absorber mirror (SAM).

camera. Because of its low sensitivity in this region the measured signal was quite low but it

In order to obtain mode-locked laser operation, two types of semiconductor saturable absorbers (SA) were tested. In the first case the SA in the transmission mode was used and the output characteristics were compared with the characteristics of the laser mode-locked using

In the transmission mode two saturable absorbers were investigated in two resonator configurations. The saturable absorbers were grown by molecular beam epitaxy at the Center for High Technology Materials of the University of New Mexico (Diels, 2006; Kubeˇcek, 2009c). The structures were grown on the 400 *μ*m thick GaAs substrates and consisted of 33 or 100 quantum wells (QW) formed by the periods of 9 nm thick In0.275Ga0.725As and 11 nm thick GaAs layers grown at low temperature (around 350◦ C) in order to decrease the carrier lifetime to the value less than 50 ps (Gupta, 1992). Number of the QWs determines the saturable absorber modulation depth and low-power transmission which were in the case of the SA

can be deduced that the beam profile was approximately Gaussian in both axes.

A schematic diagram of the mode-locked laser oscillator in the configuration (A) is shown in Fig. 7a. The 250 mm long laser resonator was formed by the spherical high reflective mirror HR (radius of curvature of 300 mm) and a flat output coupler OC with reflectivity of 30 %. The Nd:YAG crystal was inserted at a distance of 130 mm from the OC. The fundamental laser beam profile inside the resonator was calculated by software Psst (University of St Andrews, United Kingdom). According to this calculation the beam diameter in the crystal was approximately 600 *μ*m and its diameter on the SA was 400 *μ*m. In order to obtain better spatial overlap between the pump and smaller laser beam, a cylindrical focusing lens L1 (f = 50 mm) was used to focus the pumping radiation.

Fig. 7. Schematic of the laser in the mode-locked regime: oscillator (A) 250 mm long and oscillator (B) 1160 mm long.

The laser was passively mode-locked by a multiple quantum well semiconductor saturable absorbers (SA) inserted into the resonator close to the OC under the Brewster's angle in the transmission mode. Two different samples of saturable absorbers, consisted of 33 and 100 quantum wells (QW), were investigated. In addition its GaAs substrate served also as nonlinear element for passive negative feedback by beam defocusing.

Using the SA consisting 33 QW, the mode-locking threshold was achieved for pump energy of 17.5 mJ in 150 *μ*s pulse. Pulse duration evolution along the train was measured by a fast oscilloscope LeCroy SDA 9000 and a PIN photodiode EOT ET-3500 and a typical output pulse train oscillogram together with details of the pulses at the beginning and the end of the train are shown in Fig. 8. The oscilloscope-photodiode system response time was 75 ps.

The measured pulse width was consequently recalculated using the experimentally determined oscilloscope-photodiode instrumental limit and *sum of square* method, which is in details described in Jelínek (2011c); Kubeˇcek (2009d). Although the absolute calculated pulse duration may not be precise, pulse shortening from initial 120 ps to final 35 ps can be clearly observed and in summary is shown in Fig. 9. The whole train consisted of 100 pulses and its

Fig. 10. Oscillogram of the output pulse train using the SA consisted of 100 QWs from the laser resonator A (250 mm long) - upper trace and details of the pulse shapes in the middle of

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 191

In order to obtain shorter pulse trains with longer interval between pulses, which is necessary for cavity dumping, a longer resonator configuration was used. The experimental setup is in

A schematic diagram of this investigated laser oscillator in configuration B is shown in Fig. 7b. The 116 cm long optical resonator was formed by a flat output coupler OC with the reflectivity of 30% at 1.06 *μ*m, a high reflective folding mirror M1 with the radius of curvature of 1 m, a flat mirror M2, and a flat rear mirror M3. The laser was passively mode-locked by a multiple quantum well saturable absorber SA consisted of 100 quantum wells, inserted into the resonator under the Brewster's angle in transmission mode. The fundamental laser beam profile inside the crystal was approximately 1080 *μ*m and its diameter on the SA was 640 *μ*m. The mode-locking operation threshold was obtained for the pump energy of 39 mJ in 270 *μ*s long pulse. The output pulse train energy reached 900 *μ*J and its spatial profile was approximately Gaussian in both axes but slightly elliptical in the horizontal axis and is shown in the inset of Fig. 11. The train consisted of 7 pulses (at half-maximum) and the recorded oscillogram is shown in Fig. 11. Lower graphs show pulse duration histograms of the highest pulse (pulse no. 1) and the pulse no. 4. The Gaussian approximations of the histograms show only small decrease in the pulse duration from initial 76 to 69 ps. Increase in pulse duration stability is also observable, the Gaussian fit FWHM of the histogram decreased from 24 to

In order to obtain single mode-locked pulse, the cavity dumping technique was used. Into the resonator a KD\*P Pockels cell PC and a Glan laser polarizer P were inserted. The radiation transmitted through the output coupler OC was used to trigger a high voltage generator supplying quarter wave step 4.8 kV signal (rise time below 7 ns) for the Pockels cell. The single pulse with the maximum amplitude from the train (pulse no. 1) was extracted with the energy of 30 *μ*J. In order to obtain maximum energy in the single pulse directly from the oscillator, the 50% output coupler was used. The pump energy of mode-locking operation

the train - lower traces.

19 ps.

**5.1.2 Laser operation with long resonator**

**5.1.3 Single pulse extraction using cavity dumping**

details described in Jelínek (2011b).

Fig. 8. Oscillogram of the output pulse train using the SA consisted of 33 QWs from the laser resonator (A) (250 mm long) - upper trace, and details of the pulse shapes from the beginning and the end of the train - lower traces.

energy was of 170 *μ*J. The output beam spatial profile was slightly elliptical in the horizontal axis but it was approximately Gaussian in both axes.

Fig. 9. Pulse duration evolution along the train for the laser resonator (A) mode-locked by the SA consisted of 33 and 100 quantum wells (QW).

Usage of the saturable absorber with 100 quantum wells resulted in higher mode-locking threshold (pump energy of 25 mJ in 180 *μ*s pulse) and also to lengthening of the pulse train. The typical output pulse train oscillogram together with detail of the pulses in the middle of the train is shown in Fig. 10. In this case significant pulse shortening was not observed, the pulse duration of 30 ps remained at approximately constant value from the beginning along the whole train, as can be seen in Fig. 9. The whole train consisted of more than 300 pulses and its energy reached 500 *μ*J.

Using both saturable absorbers the extended pulse trains typical for passive negative feedback regime of operation were obtained. The corresponding pulse shortening along the train was observed only for pulses with initial pulse duration of 120 ps. With higher depth of modulation the shortening of initially 30 ps long pulses was not observed probably due to the limit of resolution of our detection system.

Fig. 10. Oscillogram of the output pulse train using the SA consisted of 100 QWs from the laser resonator A (250 mm long) - upper trace and details of the pulse shapes in the middle of the train - lower traces.

#### **5.1.2 Laser operation with long resonator**

8 Will-be-set-by-IN-TECH

Fig. 8. Oscillogram of the output pulse train using the SA consisted of 33 QWs from the laser resonator (A) (250 mm long) - upper trace, and details of the pulse shapes from the beginning

energy was of 170 *μ*J. The output beam spatial profile was slightly elliptical in the horizontal

Fig. 9. Pulse duration evolution along the train for the laser resonator (A) mode-locked by

Usage of the saturable absorber with 100 quantum wells resulted in higher mode-locking threshold (pump energy of 25 mJ in 180 *μ*s pulse) and also to lengthening of the pulse train. The typical output pulse train oscillogram together with detail of the pulses in the middle of the train is shown in Fig. 10. In this case significant pulse shortening was not observed, the pulse duration of 30 ps remained at approximately constant value from the beginning along the whole train, as can be seen in Fig. 9. The whole train consisted of more than 300 pulses

Using both saturable absorbers the extended pulse trains typical for passive negative feedback regime of operation were obtained. The corresponding pulse shortening along the train was observed only for pulses with initial pulse duration of 120 ps. With higher depth of modulation the shortening of initially 30 ps long pulses was not observed probably due to the

and the end of the train - lower traces.

axis but it was approximately Gaussian in both axes.

the SA consisted of 33 and 100 quantum wells (QW).

and its energy reached 500 *μ*J.

limit of resolution of our detection system.

In order to obtain shorter pulse trains with longer interval between pulses, which is necessary for cavity dumping, a longer resonator configuration was used. The experimental setup is in details described in Jelínek (2011b).

A schematic diagram of this investigated laser oscillator in configuration B is shown in Fig. 7b. The 116 cm long optical resonator was formed by a flat output coupler OC with the reflectivity of 30% at 1.06 *μ*m, a high reflective folding mirror M1 with the radius of curvature of 1 m, a flat mirror M2, and a flat rear mirror M3. The laser was passively mode-locked by a multiple quantum well saturable absorber SA consisted of 100 quantum wells, inserted into the resonator under the Brewster's angle in transmission mode. The fundamental laser beam profile inside the crystal was approximately 1080 *μ*m and its diameter on the SA was 640 *μ*m.

The mode-locking operation threshold was obtained for the pump energy of 39 mJ in 270 *μ*s long pulse. The output pulse train energy reached 900 *μ*J and its spatial profile was approximately Gaussian in both axes but slightly elliptical in the horizontal axis and is shown in the inset of Fig. 11. The train consisted of 7 pulses (at half-maximum) and the recorded oscillogram is shown in Fig. 11. Lower graphs show pulse duration histograms of the highest pulse (pulse no. 1) and the pulse no. 4. The Gaussian approximations of the histograms show only small decrease in the pulse duration from initial 76 to 69 ps. Increase in pulse duration stability is also observable, the Gaussian fit FWHM of the histogram decreased from 24 to 19 ps.

#### **5.1.3 Single pulse extraction using cavity dumping**

In order to obtain single mode-locked pulse, the cavity dumping technique was used. Into the resonator a KD\*P Pockels cell PC and a Glan laser polarizer P were inserted. The radiation transmitted through the output coupler OC was used to trigger a high voltage generator supplying quarter wave step 4.8 kV signal (rise time below 7 ns) for the Pockels cell. The single pulse with the maximum amplitude from the train (pulse no. 1) was extracted with the energy of 30 *μ*J. In order to obtain maximum energy in the single pulse directly from the oscillator, the 50% output coupler was used. The pump energy of mode-locking operation

Fig. 12. Extracted pulse duration histogram.

Fig. 14.

in Jelínek (2011d).

absorber mirror (SAM).

**5.2.1 Single pulse extraction using cavity dumping**

time constant of 5 ps was used. The fundamental laser beam diameter was calculated to be

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 193

Stable mode-locked laser operation was obtained at the pump energy of 25 mJ in 180 *μ*s long pulse. The 500 *μ*J output pulse train consisted of ∼ 15 pulses (at half-maximum) is shown in

The output beam spatial profile was close to Gaussian in both axes and is shown in the inset of Fig. 14. Duration of the pulse with the maximum amplitude in the train was measured by the streak camera resulting in the value of 15 ps. The pulse duration stability measured from 10 successive laser shots was better than 2 ps and pulse train stretching was not observed. The duration of the pulse with maximum amplitude was also confirmed by the SHG autocorrelator. Assuming the Gaussian pulse shape the real pulse duration (measured from 1000 laser shots) was calculated to be 16 ps which is in the good agreement with the streak camera measurement. The streak camera and the autocorrelator records can be found

Fig. 13. Schematic of the Nd:YAG laser oscillator in the mode-locked regime using saturable

A single pulse was extracted from the output pulse train by the cavity dumping technique, as it was described in section 5.1.3. In order to obtain high energy single pulse directly from

approximately 1 mm in the Nd:YAG crystal and 700 *μ*m on the SAM.

Fig. 11. Upper trace: oscillogram of the oscillator output pulse train. Lower traces: pulse duration histograms of the highest pulse (pulse no. 1) and the pulse no. 4.

threshold decreased to 26 mJ in 180 *μ*s long pulse. The whole output train energy with this output coupler was 500 *μ*J (measured behind M1 without cavity dumping). The extracted single pulse energy increased to 75±7 *μ*J and the contrast was better than 10−3. The pulse duration histogram was also measured and is shown in Fig. 12. Its Gaussian approximation gives the mean pulse duration of 113 ps. It can be seen that using the 50% output coupler the pulse duration is longer than using 30 % output coupler which corresponds to the lower saturated gain coefficient in the Nd:YAG.

#### **5.2 Laser mode-locking by saturable absorber mirror**

Simpler mode-locked laser configuration was obtained using a saturable absorber mirror (SAM) with high modulation depth. The experimental details and results listed below are in details described in Jelínek (2011d).

The laser oscillator schematic is shown in Fig. 13. The 112 cm long optical resonator was formed by a flat output coupler OC with reflectivity of 30 % at 1.06 *μ*m, a highly reflective folding mirror M1 with the radius of curvature of 1 m, and a flat mirror M2. For passive mode-locking regime a commercially-available semiconductor anti-resonant saturable absorber mirror (SAM) with absorbance of 50 %, modulation depth of 25 %, and relaxation

Fig. 12. Extracted pulse duration histogram.

10 Will-be-set-by-IN-TECH

Fig. 11. Upper trace: oscillogram of the oscillator output pulse train. Lower traces: pulse

threshold decreased to 26 mJ in 180 *μ*s long pulse. The whole output train energy with this output coupler was 500 *μ*J (measured behind M1 without cavity dumping). The extracted single pulse energy increased to 75±7 *μ*J and the contrast was better than 10−3. The pulse duration histogram was also measured and is shown in Fig. 12. Its Gaussian approximation gives the mean pulse duration of 113 ps. It can be seen that using the 50% output coupler the pulse duration is longer than using 30 % output coupler which corresponds to the lower

Simpler mode-locked laser configuration was obtained using a saturable absorber mirror (SAM) with high modulation depth. The experimental details and results listed below are

The laser oscillator schematic is shown in Fig. 13. The 112 cm long optical resonator was formed by a flat output coupler OC with reflectivity of 30 % at 1.06 *μ*m, a highly reflective folding mirror M1 with the radius of curvature of 1 m, and a flat mirror M2. For passive mode-locking regime a commercially-available semiconductor anti-resonant saturable absorber mirror (SAM) with absorbance of 50 %, modulation depth of 25 %, and relaxation

duration histograms of the highest pulse (pulse no. 1) and the pulse no. 4.

saturated gain coefficient in the Nd:YAG.

in details described in Jelínek (2011d).

**5.2 Laser mode-locking by saturable absorber mirror**

time constant of 5 ps was used. The fundamental laser beam diameter was calculated to be approximately 1 mm in the Nd:YAG crystal and 700 *μ*m on the SAM.

Stable mode-locked laser operation was obtained at the pump energy of 25 mJ in 180 *μ*s long pulse. The 500 *μ*J output pulse train consisted of ∼ 15 pulses (at half-maximum) is shown in Fig. 14.

The output beam spatial profile was close to Gaussian in both axes and is shown in the inset of Fig. 14. Duration of the pulse with the maximum amplitude in the train was measured by the streak camera resulting in the value of 15 ps. The pulse duration stability measured from 10 successive laser shots was better than 2 ps and pulse train stretching was not observed. The duration of the pulse with maximum amplitude was also confirmed by the SHG autocorrelator. Assuming the Gaussian pulse shape the real pulse duration (measured from 1000 laser shots) was calculated to be 16 ps which is in the good agreement with the streak camera measurement. The streak camera and the autocorrelator records can be found in Jelínek (2011d).

Fig. 13. Schematic of the Nd:YAG laser oscillator in the mode-locked regime using saturable absorber mirror (SAM).

#### **5.2.1 Single pulse extraction using cavity dumping**

A single pulse was extracted from the output pulse train by the cavity dumping technique, as it was described in section 5.1.3. In order to obtain high energy single pulse directly from

Fig. 16. Streak camera record of the ML Nd:YAG laser extracted pulse temporal profile (dots)

Highly Doped Nd:YAG Laser in Bounce Geometry Under Quasi-Continuous Diode Pumping 195

from 200 to 300 *μ*s with the energy from 29 to 42 mJ was used. Resulting amplification factor

Fig. 17. Nd:YAG laser crystal in bounce geometry amplification under input fluence from

corresponding to the strong signal amplification about 3 times.

It can be seen that in the weak signal regime (∼ 1 *μ*J/cm2), amplification up to 93 was obtained. In the strong signal regime (3 J/cm2), the 1.3 mJ seed pulse was amplified to 3.5 mJ

Operation of the highly 2.4 at.% doped Nd:YAG laser in bounce geometry under quasi-continuous pumping was investigated. The laser was operated in free-running, Q-switched and mode-locked regime. In the free-running regime, efficient operation at the wavelengths of 1.06, 1.3 and 1.4 *μ*m was demonstrated with slope efficiencies of 50, 26, and 8 %, respectively. In the passively Q-switched regime at 1.06 *μ*m, the 5 ns output pulses with the energy of 1.3 mJ were generated. At 1.34 *μ*m, the 13 ns output pulses with energy of 600 *μ*J

In order to obtain passively mode-locked regime, two types of saturable absorbers inserted into the resonator either in the transmission or reflection mode were tested. Using the

and its Gaussian fit (red line).

is shown in Fig. 17.

1 *μ*J/cm2 to 3 J/cm2.

**7. Conclusion**

were generated.

Fig. 14. Output pulse train without cavity dumping.

the oscillator, the output coupler with reflectivity of 70 % was used. Stable mode-locked laser operation was obtained at the pump energy of 19 mJ. Oscillogram of the output pulse train (measured behind the OC) and the extracted single pulse with energy of 25 *μ*J are shown in Fig. 15. The beam spatial profile was nearly Gaussian in both axes and is shown in the inset of the figure. Duration of the pulse was 17 ps with the stability better than 2 ps. The temporal profile measured by the streak camera is shown in Fig. 16. Inset shows the streak camera record of the pulse and its replica reflected from the 5 mm BK7 plate corresponding to the delay of 50 ps between pulses. In comparison with our previous results described in section 5.1.3, significant (about 5 times) pulse duration shortening and stability improvement was achieved.

Fig. 15. Oscillogram of the ML Nd:YAG laser output pulse train (blue upper trace) and the extracted pulse (red lower trace).

#### **6. Diode pumped Nd:YAG amplifier in bounce geometry**

Performance of the Nd:YAG laser crystal in bounce geometry was also investigated in the single pass amplifier configuration and is in details described in Jelínek (2011a;b). The input pulse diameter was adjusted by a spherical lens to obtain good overlap with the pumped region. Several laser sources generating pulses in the range from 100 ps to 5 ns were used to provide the amplifier input fluence in the range from ∼ 1 *μ*J/cm2 to 3 J/cm2. According to the laser source performance and certain amplifier setup the pump pulse duration ranging

Fig. 16. Streak camera record of the ML Nd:YAG laser extracted pulse temporal profile (dots) and its Gaussian fit (red line).

from 200 to 300 *μ*s with the energy from 29 to 42 mJ was used. Resulting amplification factor is shown in Fig. 17.

Fig. 17. Nd:YAG laser crystal in bounce geometry amplification under input fluence from 1 *μ*J/cm2 to 3 J/cm2.

It can be seen that in the weak signal regime (∼ 1 *μ*J/cm2), amplification up to 93 was obtained. In the strong signal regime (3 J/cm2), the 1.3 mJ seed pulse was amplified to 3.5 mJ corresponding to the strong signal amplification about 3 times.

#### **7. Conclusion**

12 Will-be-set-by-IN-TECH

the oscillator, the output coupler with reflectivity of 70 % was used. Stable mode-locked laser operation was obtained at the pump energy of 19 mJ. Oscillogram of the output pulse train (measured behind the OC) and the extracted single pulse with energy of 25 *μ*J are shown in Fig. 15. The beam spatial profile was nearly Gaussian in both axes and is shown in the inset of the figure. Duration of the pulse was 17 ps with the stability better than 2 ps. The temporal profile measured by the streak camera is shown in Fig. 16. Inset shows the streak camera record of the pulse and its replica reflected from the 5 mm BK7 plate corresponding to the delay of 50 ps between pulses. In comparison with our previous results described in section 5.1.3, significant (about 5 times) pulse duration shortening and stability improvement

Fig. 15. Oscillogram of the ML Nd:YAG laser output pulse train (blue upper trace) and the

Performance of the Nd:YAG laser crystal in bounce geometry was also investigated in the single pass amplifier configuration and is in details described in Jelínek (2011a;b). The input pulse diameter was adjusted by a spherical lens to obtain good overlap with the pumped region. Several laser sources generating pulses in the range from 100 ps to 5 ns were used to provide the amplifier input fluence in the range from ∼ 1 *μ*J/cm2 to 3 J/cm2. According to the laser source performance and certain amplifier setup the pump pulse duration ranging

**6. Diode pumped Nd:YAG amplifier in bounce geometry**

Fig. 14. Output pulse train without cavity dumping.

was achieved.

extracted pulse (red lower trace).

Operation of the highly 2.4 at.% doped Nd:YAG laser in bounce geometry under quasi-continuous pumping was investigated. The laser was operated in free-running, Q-switched and mode-locked regime. In the free-running regime, efficient operation at the wavelengths of 1.06, 1.3 and 1.4 *μ*m was demonstrated with slope efficiencies of 50, 26, and 8 %, respectively. In the passively Q-switched regime at 1.06 *μ*m, the 5 ns output pulses with the energy of 1.3 mJ were generated. At 1.34 *μ*m, the 13 ns output pulses with energy of 600 *μ*J were generated.

In order to obtain passively mode-locked regime, two types of saturable absorbers inserted into the resonator either in the transmission or reflection mode were tested. Using the

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saturable absorber with lower modulation depth (33 quantum wells) in the transmission mode, pulse duration shortening along the extended output pulse train from initial 120 to 35 ps was observed. Whole pulse train energy was 170 *μ*J. Using the saturable absorber with higher modulation depth (100 quantum wells), the 500 *μ*J train of 300 pulses with duration of 30 ps was generated without significant shortening effect. In order to be able to extract single pulse from the train, longer laser resonator was constructed. Using the cavity dumping technique, the single 75 *μ*J pulse with the duration of 113 ± 22 ps was extracted from the resonator.

Simpler mode-locked laser configuration was obtained using a saturable absorber mirror (SAM) with modulation depth of 25 %. The 500 *μ*J output pulse train consisted of 15 pulses with duration of 15 ps. By the cavity dumping technique, the single 25 *μ*J pulse with stable duration of 17 ps and Gaussian spatial profile was extracted from the resonator.

Single pass amplification using the Nd:YAG crystal in bounce geometry was also investigated in the weak and also strong signal regime. In the weak signal regime (∼ 1 *μ*J/cm2), amplification up to 93 was obtained. In the strong signal regime (3 J/cm2), the 1.3 mJ seed pulse was amplified to 3.5 mJ corresponding to the strong signal amplification about 3 times.

#### **8. Acknowledgements**

This research has been supported by the Czech Science Foundation under grant No. 102/09/1741, the research projects of the Czech Ministry of Education MSM<PHONE_NUMBER> "Laser Systems, radiation and modern optical applications" and ME 10131 "Picosecond solid state lasers and parametric oscillators for sensors of rotation and other physical quantities."

#### **9. References**

14 Will-be-set-by-IN-TECH

saturable absorber with lower modulation depth (33 quantum wells) in the transmission mode, pulse duration shortening along the extended output pulse train from initial 120 to 35 ps was observed. Whole pulse train energy was 170 *μ*J. Using the saturable absorber with higher modulation depth (100 quantum wells), the 500 *μ*J train of 300 pulses with duration of 30 ps was generated without significant shortening effect. In order to be able to extract single pulse from the train, longer laser resonator was constructed. Using the cavity dumping technique, the single 75 *μ*J pulse with the duration of 113 ± 22 ps was extracted from the

Simpler mode-locked laser configuration was obtained using a saturable absorber mirror (SAM) with modulation depth of 25 %. The 500 *μ*J output pulse train consisted of 15 pulses with duration of 15 ps. By the cavity dumping technique, the single 25 *μ*J pulse with stable

Single pass amplification using the Nd:YAG crystal in bounce geometry was also investigated in the weak and also strong signal regime. In the weak signal regime (∼ 1 *μ*J/cm2), amplification up to 93 was obtained. In the strong signal regime (3 J/cm2), the 1.3 mJ seed pulse was amplified to 3.5 mJ corresponding to the strong signal amplification about 3 times.

This research has been supported by the Czech Science Foundation under grant No. 102/09/1741, the research projects of the Czech Ministry of Education MSM<PHONE_NUMBER> "Laser Systems, radiation and modern optical applications" and ME 10131 "Picosecond solid state lasers and parametric oscillators for sensors of rotation and other physical quantities."

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**8. Acknowledgements**

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**9. References**

**11**

*Latvia*

*Riga Technical University*

**Application of Nd:YAG Laser in**

**Semiconductors' Nanotechnology**

Artur Medvid', Aleksandr Mycko, Pavels Onufrijevs and Edvins Dauksta

The main tendency in development of modern electronics and optoelectronics is the use of functional objects of small size. Unique properties of the nanoobjects are mainly determined by their atomic and electronic processes occurring in the structure, which has a quantum character. Surface nanostructures and their unique properties play a significant role in such objects as highly dispersed systems - adsorbents and catalysts, fillers, composite materials, film and membrane systems (Crommie et al., 1993; Beton et al., 1995; Junno et al., 1995). Formation on the surfaces of ordered and disordered ensembles of nanoparticles allow creating materials with new unique physical properties. Nanostructuring of the surface leads to improvement of optical, electrical, thermal, mechanical and field electron emission properties of materials, for example, reducing of the work function of electron from silicon (Evtukh et al., 2010), enhancing biocompatibility with implants in living tissue and prosthetic devices used in orthopaedics and dentistry. Such materials find application in selective nanocatalyse, microelectronics, nanophotonics, photovoltaic, spectroscopy, and optics. On their base devices are created for recording and storing information with ultra-high density, as well as

Study of different mechanisms of nanostructured materials' formation is required due to the need to develop new and effective methods of forming low-dimensional nanostructures when at least one dimension of crystal is less or equal Bohr' radius of electron, hole or exciton. At this condition the quantum confinement effect manifests in the system, and it

One promising direction is to use laser radiation to form nanostructures on solid surfaces. Lasers have been used for more than 50 years in diverse fields of application starting from simple laser micromachining processes for micro-electro-mechanical systems (MEMS), such as, cutting, drilling, to more smarter ones, e.g.: restoration of art works (laser stone cleaning), pulse laser deposition of coatings and films; local defect annealing after ion implantation; formation the precipitation areas of impurities in Si in medicine, medical

Among solid state lasers, Nd:YAG laser has an important role due to its high efficiency, possibility to tune it in different wavelengths from infrared (λ=1064 nm) till ultraviolet

(λ=213 nm) and change pulse duration from milliseconds down to picoseconds.

**1. Introduction**

light-emitting devices (Vu et al., 2010).

diagnosis, treatment, or therapy etc.

leads to the essential change of material physical properties.

## **Application of Nd:YAG Laser in Semiconductors' Nanotechnology**

Artur Medvid', Aleksandr Mycko, Pavels Onufrijevs and Edvins Dauksta *Riga Technical University Latvia*

#### **1. Introduction**

16 Will-be-set-by-IN-TECH

198 Nd YAG Laser

W. D. Tan, et. al. (2010). Mode locking of ceramic Nd:yttrium aluminum garnet with graphene

G. M. Thomas, et al. (2010). Nonlinear mirror modelocking of a bounce geometry laser, *Optics*

Y. Urata, et al. (2001). Laser performance of highly neodymium-doped yttrium aluminum

X. Wushouer, et al. (2010). High peak power picosecond hybrid fiber and solid-state amplifier

J. H. Yim, et al. (2008). Fabrication and characterization of ultrafast carbon nanotube saturable

absorbers for solid-state laser mode locking near 1 *μ*m, *Applied Physics Letters*, 93:

as a saturable absorber, *Applied Physics Letters* 96: 031106.

*Express* 18: 12663-12668.

161106.

garnet crystals, *Opt. Letters*, 26: 801–803.

system, *Laser Phys. Letters* 7: 644-649.

The main tendency in development of modern electronics and optoelectronics is the use of functional objects of small size. Unique properties of the nanoobjects are mainly determined by their atomic and electronic processes occurring in the structure, which has a quantum character.

Surface nanostructures and their unique properties play a significant role in such objects as highly dispersed systems - adsorbents and catalysts, fillers, composite materials, film and membrane systems (Crommie et al., 1993; Beton et al., 1995; Junno et al., 1995). Formation on the surfaces of ordered and disordered ensembles of nanoparticles allow creating materials with new unique physical properties. Nanostructuring of the surface leads to improvement of optical, electrical, thermal, mechanical and field electron emission properties of materials, for example, reducing of the work function of electron from silicon (Evtukh et al., 2010), enhancing biocompatibility with implants in living tissue and prosthetic devices used in orthopaedics and dentistry. Such materials find application in selective nanocatalyse, microelectronics, nanophotonics, photovoltaic, spectroscopy, and optics. On their base devices are created for recording and storing information with ultra-high density, as well as light-emitting devices (Vu et al., 2010).

Study of different mechanisms of nanostructured materials' formation is required due to the need to develop new and effective methods of forming low-dimensional nanostructures when at least one dimension of crystal is less or equal Bohr' radius of electron, hole or exciton. At this condition the quantum confinement effect manifests in the system, and it leads to the essential change of material physical properties.

One promising direction is to use laser radiation to form nanostructures on solid surfaces.

Lasers have been used for more than 50 years in diverse fields of application starting from simple laser micromachining processes for micro-electro-mechanical systems (MEMS), such as, cutting, drilling, to more smarter ones, e.g.: restoration of art works (laser stone cleaning), pulse laser deposition of coatings and films; local defect annealing after ion implantation; formation the precipitation areas of impurities in Si in medicine, medical diagnosis, treatment, or therapy etc.

Among solid state lasers, Nd:YAG laser has an important role due to its high efficiency, possibility to tune it in different wavelengths from infrared (λ=1064 nm) till ultraviolet (λ=213 nm) and change pulse duration from milliseconds down to picoseconds.

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 201

(Benda, Gowar , et al. 1999) and molecular beam epitaxy (Tomas, Jennings, et al., 2007) are only a few methods to form a p-n junction. The main drawback for these methods is a high

A possibility of p-n junction formation by laser radiation was shown in several p- and ntype semiconductors: p-Si (Mada & Ione, et at., 1986; Blums & Medvid', et al., 1995), p-CdTe (Medvid' , Litovchenko et al., 2001), p-InSb (Fujisawa, 1980; Medvid', Fedorenko, et al., 1998), p-InAs (Kurbatov & Stojanova, 1983), p-PbSe (Tovstyuk, Placko, et al., 1984), p-Ge (Kiyak & Savitsky, 1984) and n-HgCdTe (Dumanski, Bester, et al., 2006) due to inversion of conductivity type. Different mechanisms have been proposed to explain the nature of inversion of conductivity type, for example, impurities' segregation, defects' generation (Tovstyuk, Placko, et al., 1984), amorphization (Ljamichev, Litvak, et al. 1976) and oxygen related donor generation (Mada & Ione, et at., 1986). However, n-type impurities in Si irradiated by laser cannot be oxygen atoms, according to paper (Blums & Medvid', et al., 1995). Several authors have tried to explain p-n junction formation in n-type HgCdTe by defects generation, based on a model of defects formation related to interstitial mercury diffusion (Dumanski, Bester, et al., 2006). On the other hand, the authors of those papers did not take into account the effect of temperature gradient on the diffusion of atoms in solid solution. For example, diffusion of Hg interstitials toward the bulk of the semiconductor is impeded due to gradient of temperature directed to the irradiated surface of the semiconductor. Moreover, it is theoretically shown, that the p-n junction can be formed by redistribution of impurities in co-doped Si in gradient temperature field (Medvid' & Kaupuzs, 1994). In this case the atoms with bigger effective radii than Si atom radius drift toward the irradiated surface of the Si crystal, but those with radii smaller than Si atom

Characteristics of p-n junction, formed by laser radiation in semiconductors, are comparable to the commercial ones, that is why laser technology has a promising future. The main advantages of this technology are low cost and the possibility to locate p-n junction fast and precisely.

Unfortunately, the mechanism of p-n junction formation by laser radiation has not been clear until now. In this paper a new mechanism of p-n junction formation in intrinsic

According to the model, p-n junction is formed in a semiconductor by strongly absorbed laser radiation due to generation and redistribution of intrinsic point defects (interstitial atoms and vacancies) in temperature gradient field, so called Thermo gradient effect

For this purpose i-Ge crystal is irradiated by Nd:YAG laser with different energy of quantum. I-type Ge crystal was used in the experiments as a model material because the concentration of impurities in this material is lower than the concentration of intrinsic point

In the experiments i-Ge single crystals with *Na* = 7.4 × 1011 cm-3, *Nd* = 6.8 × 1011 cm-3 ,where *Na* and *Nd* are acceptors' and donors' concentration, and slab dimensions of 16.0 × 3.5 × 2.0 mm3 were used. Samples were mechanically polished with diamond paste and chemically treated with H2O2 and CP-4 (HF:HNO3:CH3COOH in volume ratio 3:5:3), therefore all surfaces of samples are characterised by minimum surface recombination velocity of Smin = 100 cm/s

cost per p-n junction, since the equipment for these methods is expensive.

radius – to the opposite direction.

(Medvid', 2002).

(Pozela, 1980).

defects at room temperature.

semiconductor by laser radiation is proposed.

Nowadays, lasers are used for formation of the following semiconductors' structures: p-n junctions, heterostructures Ge/SiGe, CdTe/CdZnTe, and graded band gap (Medvid' & Fedorenko 1999; Medvid', Mychko et al., 2007; Medvid', I.Dmytruk et al., 2007; Medvid', P.Onufrijevs et al., 2008; Medvid', Mychko et al., 2008; Medvid', Onufrijevs et al., 2010; Medvid', Mychko et al., 2010).

Moreover, the most actual application of lasers could be in nanotechnology. We have elaborated a new laser method for nanocones' formation on a surface of semiconductors by laser radiation. The proposed model is characterized by two stages – Laser Redistribution of Atoms (LRA) and Selective Laser Annealing (SLA).

In the case of Si0.7Ge0.3 solid solutions the first stage, LRA, is characterized by formation of hetero-structures, such as: Ge/Si due to drift of Ge atoms toward the irradiated surface of the sample in the gradient of temperature, the so-called Thermogradient effect (Medvid', 2002). This process is characterized by positive feedback: after every laser pulse the gradient of temperature increases due to increase of Ge atoms' concentration at the irradiated surface. New Ge phase is formed at the end of the process. Ge atoms are localized at the surface of Si like a thin film. As a result, LRA stage gradually transits to SLA stage.

The second stage, SLA, is characterized by formation of nanocones on the irradiated surface of a semiconductor by selective laser heating of the top layer with a following mechanical plastic deformation of the layer as a result of relaxation of the mechanical compressive stress arising between these layers due to mismatch of their crystal lattices and selective laser heating. SLA occurs due to higher absorption of the laser radiation by the top layer than the buried layer.

In the case of the elementary semiconductors, at the first stage of the process a thin top layer with mechanical compressive stress due to separation and redistribution of interstitials and vacancies in gradient temperature field (Medvid', 2002) on the irradiated surface of the semiconductors is formed. As a result, interstitials are concentrated at the irradiated surface of the semiconductor, forming the top layer. Vacancies are concentrated under the top layer forming a buried layer with mechanical tension due to absence of atoms. Sometimes vacancies form nanocavities (Medvid, 2009). At the second stage of the process nanocones are formed on the irradiated surface of the semiconductors due to plastic deformation of the top layer in the same way as in the previous case with semiconductor solid solutions.

A study of the nanocones' optical, mechanical and electrical properties which are used for construction of third generation solar cells, Si white light emitting diode, photondetector with selective or "bolometer" type spectral sensitivity, Si tip for field electron emitting with low work function of electron is conducted in the chapter.

#### **2. Applications of laser redistribution of atoms**

#### **2.1 P-n junction formation in i-Ge crystal by laser radiation**

P-n junction is the main component of many semiconductor devices such as diodes, transistors, microchips etc (Makino, Kato, et al. 2008). Thermodiffusion, ion implantation

Nowadays, lasers are used for formation of the following semiconductors' structures: p-n junctions, heterostructures Ge/SiGe, CdTe/CdZnTe, and graded band gap (Medvid' & Fedorenko 1999; Medvid', Mychko et al., 2007; Medvid', I.Dmytruk et al., 2007; Medvid', P.Onufrijevs et al., 2008; Medvid', Mychko et al., 2008; Medvid', Onufrijevs et al., 2010;

Moreover, the most actual application of lasers could be in nanotechnology. We have elaborated a new laser method for nanocones' formation on a surface of semiconductors by laser radiation. The proposed model is characterized by two stages – Laser Redistribution of

In the case of Si0.7Ge0.3 solid solutions the first stage, LRA, is characterized by formation of hetero-structures, such as: Ge/Si due to drift of Ge atoms toward the irradiated surface of the sample in the gradient of temperature, the so-called Thermogradient effect (Medvid', 2002). This process is characterized by positive feedback: after every laser pulse the gradient of temperature increases due to increase of Ge atoms' concentration at the irradiated surface. New Ge phase is formed at the end of the process. Ge atoms are localized at the surface of Si like a thin film. As a result, LRA stage gradually transits to

The second stage, SLA, is characterized by formation of nanocones on the irradiated surface of a semiconductor by selective laser heating of the top layer with a following mechanical plastic deformation of the layer as a result of relaxation of the mechanical compressive stress arising between these layers due to mismatch of their crystal lattices and selective laser heating. SLA occurs due to higher absorption of the laser radiation by the top layer

In the case of the elementary semiconductors, at the first stage of the process a thin top layer with mechanical compressive stress due to separation and redistribution of interstitials and vacancies in gradient temperature field (Medvid', 2002) on the irradiated surface of the semiconductors is formed. As a result, interstitials are concentrated at the irradiated surface of the semiconductor, forming the top layer. Vacancies are concentrated under the top layer forming a buried layer with mechanical tension due to absence of atoms. Sometimes vacancies form nanocavities (Medvid, 2009). At the second stage of the process nanocones are formed on the irradiated surface of the semiconductors due to plastic deformation of the top layer in the same way as in the previous case with

A study of the nanocones' optical, mechanical and electrical properties which are used for construction of third generation solar cells, Si white light emitting diode, photondetector with selective or "bolometer" type spectral sensitivity, Si tip for field electron emitting with

P-n junction is the main component of many semiconductor devices such as diodes, transistors, microchips etc (Makino, Kato, et al. 2008). Thermodiffusion, ion implantation

Medvid', Mychko et al., 2010).

SLA stage.

than the buried layer.

semiconductor solid solutions.

low work function of electron is conducted in the chapter.

**2. Applications of laser redistribution of atoms**

**2.1 P-n junction formation in i-Ge crystal by laser radiation**

Atoms (LRA) and Selective Laser Annealing (SLA).

(Benda, Gowar , et al. 1999) and molecular beam epitaxy (Tomas, Jennings, et al., 2007) are only a few methods to form a p-n junction. The main drawback for these methods is a high cost per p-n junction, since the equipment for these methods is expensive.

A possibility of p-n junction formation by laser radiation was shown in several p- and ntype semiconductors: p-Si (Mada & Ione, et at., 1986; Blums & Medvid', et al., 1995), p-CdTe (Medvid' , Litovchenko et al., 2001), p-InSb (Fujisawa, 1980; Medvid', Fedorenko, et al., 1998), p-InAs (Kurbatov & Stojanova, 1983), p-PbSe (Tovstyuk, Placko, et al., 1984), p-Ge (Kiyak & Savitsky, 1984) and n-HgCdTe (Dumanski, Bester, et al., 2006) due to inversion of conductivity type. Different mechanisms have been proposed to explain the nature of inversion of conductivity type, for example, impurities' segregation, defects' generation (Tovstyuk, Placko, et al., 1984), amorphization (Ljamichev, Litvak, et al. 1976) and oxygen related donor generation (Mada & Ione, et at., 1986). However, n-type impurities in Si irradiated by laser cannot be oxygen atoms, according to paper (Blums & Medvid', et al., 1995). Several authors have tried to explain p-n junction formation in n-type HgCdTe by defects generation, based on a model of defects formation related to interstitial mercury diffusion (Dumanski, Bester, et al., 2006). On the other hand, the authors of those papers did not take into account the effect of temperature gradient on the diffusion of atoms in solid solution. For example, diffusion of Hg interstitials toward the bulk of the semiconductor is impeded due to gradient of temperature directed to the irradiated surface of the semiconductor. Moreover, it is theoretically shown, that the p-n junction can be formed by redistribution of impurities in co-doped Si in gradient temperature field (Medvid' & Kaupuzs, 1994). In this case the atoms with bigger effective radii than Si atom radius drift toward the irradiated surface of the Si crystal, but those with radii smaller than Si atom radius – to the opposite direction.

Characteristics of p-n junction, formed by laser radiation in semiconductors, are comparable to the commercial ones, that is why laser technology has a promising future. The main advantages of this technology are low cost and the possibility to locate p-n junction fast and precisely.

Unfortunately, the mechanism of p-n junction formation by laser radiation has not been clear until now. In this paper a new mechanism of p-n junction formation in intrinsic semiconductor by laser radiation is proposed.

According to the model, p-n junction is formed in a semiconductor by strongly absorbed laser radiation due to generation and redistribution of intrinsic point defects (interstitial atoms and vacancies) in temperature gradient field, so called Thermo gradient effect (Medvid', 2002).

For this purpose i-Ge crystal is irradiated by Nd:YAG laser with different energy of quantum. I-type Ge crystal was used in the experiments as a model material because the concentration of impurities in this material is lower than the concentration of intrinsic point defects at room temperature.

In the experiments i-Ge single crystals with *Na* = 7.4 × 1011 cm-3, *Nd* = 6.8 × 1011 cm-3 ,where *Na* and *Nd* are acceptors' and donors' concentration, and slab dimensions of 16.0 × 3.5 × 2.0 mm3 were used. Samples were mechanically polished with diamond paste and chemically treated with H2O2 and CP-4 (HF:HNO3:CH3COOH in volume ratio 3:5:3), therefore all surfaces of samples are characterised by minimum surface recombination velocity of Smin = 100 cm/s (Pozela, 1980).

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 203

012345678

Intensity, MW/cm2

Fig. 2. Rectification ratio as a function of irradiation intensity of Nd:YAG laser for different

The rectification ratio of 9.3 at 350 laser pulses was observed after irradiation with the highest quantum energy hν4=4.66 eV, but the highest rectification ratio of 400 and potential barrier height 0,472 eV were observed at 650 laser pulses for the second harmonics hν2=2.32

Fig. 3. Barrier height and rectification ratio as a function of number of laser pulses with

Current-voltage characteristics of samples irradiated with energies of laser radiation quanta hν1=1.16 eV and hν2=2.32 eV are not shown here because they are similar to current-voltage characteristics in Fig. 1. Decrease of the threshold intensity with increase of energy of laser

1 I

1 h=1.16 eV 2 h=2.32 eV 3 h=4.66 eV

th1

1

2

3

4

Rectification ratio

eV as can be seen in Fig. 3.

2

energies of the laser radiation quantum and 350 laser pulses.

energy of laser radiation quantum hν2=2.32 eV.

Ith2

I 3 th3

5

6

7

8

Different intensities and energies of laser radiation quanta: hν1=1.16 eV (λ1 = 1064 nm), hν2=2.32 eV (λ2 = 532 nm), and hν4=4.66 eV (λ4 = 266 nm), where ν1- fundamental frequency, ν2- second harmonics, ν4- fourth harmonics and λ- wavelength, of nanosecond Nd:YAG laser with pulse duration τ1 = 6.0 ns, τ2 =4.0 ns and τ4 =3.0 ns, were used to irradiate the samples. Current-voltage characteristics were measured for the non-irradiated and the irradiated samples. Measurements of current-voltage characteristics were done by soldering tin electrical contacts directly on the irradiated surface of i-Ge and on the opposite side. Also, current-voltage characteristics were measured at different number of laser pulses in order to determine the mechanism of the effect. Maximum electric field applied to the samples was 100 V/cm. Measurements of current-voltage characteristics were done at room temperature (T) and atmospheric pressure. Rectification ratio was calculated at constant 30 V voltages.

Current-voltage characteristics of i-Ge samples before and after irradiation by Nd:YAG laser with energy of laser radiation quantum hν4=4.66 eV and different intensities, are shown in Fig. 1. The current-voltage characteristic of the non-irradiated sample is linear. It means, current-voltage characteristics obey Ohm's law and therefore there are no potential barriers between the electric contacts and the sample. But after irradiation by the laser currentvoltage characteristics becomes diode-like. Moreover, this process takes place in threshold manner - resistance of the sample increases by 10 times and rectification effect appears at a certain intensity of the laser radiation.

Fig. 1. Current-voltage characteristics of a non-irradiated and an irradiated i-Ge sample by Nd:YAG laser with different intensities of quantum energy hν4=4.66 eV and 350 laser pulses.

Threshold intensity (Ith) decreases with increase of energy of laser radiation quantum, as seen in Fig. 2. The rectification effect of current-voltage characteristics, quantitively determined by rectification ratio, is increased with intensity and energy of laser radiation quantum, as shown in Fig. 1. and 2. Threshold intensities are observed at the fundamental frequency Ith1= 5.5 MW/cm2, the second harmonic Ith2= 1.5 MW/cm2 and the fourth harmonic Ith4= 1.15 MW/cm2.

Different intensities and energies of laser radiation quanta: hν1=1.16 eV (λ1 = 1064 nm), hν2=2.32 eV (λ2 = 532 nm), and hν4=4.66 eV (λ4 = 266 nm), where ν1- fundamental frequency, ν2- second harmonics, ν4- fourth harmonics and λ- wavelength, of nanosecond Nd:YAG laser with pulse duration τ1 = 6.0 ns, τ2 =4.0 ns and τ4 =3.0 ns, were used to irradiate the samples. Current-voltage characteristics were measured for the non-irradiated and the irradiated samples. Measurements of current-voltage characteristics were done by soldering tin electrical contacts directly on the irradiated surface of i-Ge and on the opposite side. Also, current-voltage characteristics were measured at different number of laser pulses in order to determine the mechanism of the effect. Maximum electric field applied to the samples was 100 V/cm. Measurements of current-voltage characteristics were done at room temperature (T) and atmospheric pressure. Rectification ratio was calculated at constant 30

Current-voltage characteristics of i-Ge samples before and after irradiation by Nd:YAG laser with energy of laser radiation quantum hν4=4.66 eV and different intensities, are shown in Fig. 1. The current-voltage characteristic of the non-irradiated sample is linear. It means, current-voltage characteristics obey Ohm's law and therefore there are no potential barriers between the electric contacts and the sample. But after irradiation by the laser currentvoltage characteristics becomes diode-like. Moreover, this process takes place in threshold manner - resistance of the sample increases by 10 times and rectification effect appears at a

Threshold intensity (Ith) decreases with increase of energy of laser radiation quantum, as seen in Fig. 2. The rectification effect of current-voltage characteristics, quantitively determined by rectification ratio, is increased with intensity and energy of laser radiation quantum, as shown in Fig. 1. and 2. Threshold intensities are observed at the fundamental frequency Ith1= 5.5 MW/cm2, the second harmonic Ith2= 1.5 MW/cm2 and the fourth

Voltage (V)

0

Fig. 1. Current-voltage characteristics of a non-irradiated and an irradiated i-Ge sample by Nd:YAG laser with different intensities of quantum energy hν4=4.66 eV and 350 laser pulses.

200

400

600

800

1000

Current (mA)

1200

V voltages.

certain intensity of the laser radiation.

I

I

I

harmonic Ith4= 1.15 MW/cm2.

Non-irradiated

min=0.6 MW/cm2

med=1.15 MW/cm2

max=2.7 MW/cm2

Fig. 2. Rectification ratio as a function of irradiation intensity of Nd:YAG laser for different energies of the laser radiation quantum and 350 laser pulses.

The rectification ratio of 9.3 at 350 laser pulses was observed after irradiation with the highest quantum energy hν4=4.66 eV, but the highest rectification ratio of 400 and potential barrier height 0,472 eV were observed at 650 laser pulses for the second harmonics hν2=2.32 eV as can be seen in Fig. 3.

Fig. 3. Barrier height and rectification ratio as a function of number of laser pulses with energy of laser radiation quantum hν2=2.32 eV.

Current-voltage characteristics of samples irradiated with energies of laser radiation quanta hν1=1.16 eV and hν2=2.32 eV are not shown here because they are similar to current-voltage characteristics in Fig. 1. Decrease of the threshold intensity with increase of energy of laser

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 205

Nonmonotonous dependence of rectification ratio as function of the laser intensity and decrease of rectification ratio at maximal intensities of laser irradiation are explained by formation of nanocones on the irradiated surfaces of the samples (Medvid', Onufrijevs, et al., 2011) and therefore a partial destruction of the n-type layer on the irradiated surface of the samples. The current-voltage characteristics measured at a different number of the laser pulses (N) from 350 to 880 at irradiation of the samples by the second laser harmonic and laser intensity 1.5 MW/cm2 are shown in Fig. 3. We can see that rectification ratio of the current-voltage characteristics is a non-monotonous function of N with maximum at 650 pulses, as can be seen in Fig.3. At the initial stage of the function rectification ratio increases with N, but at 650 it sharply decreases. Increase of rectification ratio with N (Fig. 3.) we explain by accumulation effect: after every laser pulse concentration of interstitials at the irradiated surface and concentration of vacancies in the buried layer of the sample increases

and, of course, n-p junction barrier height increases, too, as shown in Fig. 3, curve 1.

1500

U (V)

, (1)

is the Richardson constant, which is

*b* - the

0

500

1000

I (mA)

 Non-irradiated 350 pulses 550 pulses 650 pulses 750 pulses 880 pulses

where A is the irradiated area (0.3 × 0.3 cm2) of i-Ge, A

laser pulses.

40.8 A cm-

barrier height.

Yakuphanoglu, et al., 2011):

² K-

Fig. 5. Current-voltage characteristics of i-Ge samples before and after Nd:YAG laser irradiation at intensity 1.5 MW/cm2 and quantum energy hν2=2.32 for different number of

To calculate barrier height equation 1 was rearranged to the express barrier height (Gupta,

2 b sI AA T exp kT

A sharp decrease of rectification ratio at N more than 650 pulses is explained by nanostructure formation on the irradiated surface of the sample due to relaxation of

² for i-Ge (Chua, Nikolai, et al., 2003), k is the Boltzmann constant,

radiation quantum and appearance of current-voltage characteristics rectification effect we explain in the following way: irradiation of the sample by strongly absorbed laser radiation leads to transformation of the light energy to the thermal one. Heating up the sample by laser radiation increases additional generation of intrinsic defects in crystal - interstitials and vacancies which are quenched in crystal lattice after the end of the laser pulse. The formation of a potential barrier takes place due to separation of vacancies and interstitials in temperature gradient field (Medvid, 2002). Calculations of Tmax on the surface of the sample at threshold intensity of laser radiation using the heat balance equation in approach of adiabatic process show that Tmax1 in the case of fundamental frequency is lower than Tmax2 for the second and Tmax4 for the fourth harmonics.

Table 1. i-Ge and p-n junction parameters

It means that gradient of temperature has the main role in formation of the potential barrier, because gradient of T is proportional to the light absorption coefficient in i-Ge crystal which increases with energy of laser radiation quantum, as seen in Table 1.

Decrease of the threshold intensity of laser radiation with increase of laser radiation quantum, as can be seen in Fig. 2, is an evidence of this suggestion. Concentration of interstitials at the irradiated surface and vacancies in the buried layer of the sample leads to formation of n-p junction because interstitials are donors and vacancies are acceptors in Ge (Shaw, 1973). Schematic illustration of n-p-i structure formed by the laser in i-Ge is shown in Fig. 4.

Fig. 4. Schematic illustration of n-p-i structure after irradiation of i-Ge sample. Interstitial atoms are known to be of n-type forming Ed level 0.04 eV below the conduction band, but vacancies are known to be of p-type forming Ea level 0.2 eV above the valence band (Claeys & Simon, 2007).

radiation quantum and appearance of current-voltage characteristics rectification effect we explain in the following way: irradiation of the sample by strongly absorbed laser radiation leads to transformation of the light energy to the thermal one. Heating up the sample by laser radiation increases additional generation of intrinsic defects in crystal - interstitials and vacancies which are quenched in crystal lattice after the end of the laser pulse. The formation of a potential barrier takes place due to separation of vacancies and interstitials in temperature gradient field (Medvid, 2002). Calculations of Tmax on the surface of the sample at threshold intensity of laser radiation using the heat balance equation in approach of adiabatic process show that Tmax1 in the case of fundamental frequency is lower than Tmax2

> Absorption coefficient, x108 m-1

4,66 266 1.60 6.25 9.29 0.36 2,32 532 0.45 22.2 4.16 0.34 1,16 1064 0.05 200 2.14 0.32

It means that gradient of temperature has the main role in formation of the potential barrier, because gradient of T is proportional to the light absorption coefficient in i-Ge crystal which

Decrease of the threshold intensity of laser radiation with increase of laser radiation quantum, as can be seen in Fig. 2, is an evidence of this suggestion. Concentration of interstitials at the irradiated surface and vacancies in the buried layer of the sample leads to formation of n-p junction because interstitials are donors and vacancies are acceptors in Ge (Shaw, 1973).

Schematic illustration of n-p-i structure formed by the laser in i-Ge is shown in Fig. 4.

Fig. 4. Schematic illustration of n-p-i structure after irradiation of i-Ge sample. Interstitial atoms are known to be of n-type forming Ed level 0.04 eV below the conduction band, but vacancies are known to be of p-type forming Ea level 0.2 eV above the valence band (Claeys

Absorption depth, x10-9 m

Rectification ratio

Barrier height, eV

for the second and Tmax4 for the fourth harmonics.

Table 1. i-Ge and p-n junction parameters

Wavelength, nm

increases with energy of laser radiation quantum, as seen in Table 1.

Energy of laser radiation quantum, eV

& Simon, 2007).

Nonmonotonous dependence of rectification ratio as function of the laser intensity and decrease of rectification ratio at maximal intensities of laser irradiation are explained by formation of nanocones on the irradiated surfaces of the samples (Medvid', Onufrijevs, et al., 2011) and therefore a partial destruction of the n-type layer on the irradiated surface of the samples. The current-voltage characteristics measured at a different number of the laser pulses (N) from 350 to 880 at irradiation of the samples by the second laser harmonic and laser intensity 1.5 MW/cm2 are shown in Fig. 3. We can see that rectification ratio of the current-voltage characteristics is a non-monotonous function of N with maximum at 650 pulses, as can be seen in Fig.3. At the initial stage of the function rectification ratio increases with N, but at 650 it sharply decreases. Increase of rectification ratio with N (Fig. 3.) we explain by accumulation effect: after every laser pulse concentration of interstitials at the irradiated surface and concentration of vacancies in the buried layer of the sample increases and, of course, n-p junction barrier height increases, too, as shown in Fig. 3, curve 1.

Fig. 5. Current-voltage characteristics of i-Ge samples before and after Nd:YAG laser irradiation at intensity 1.5 MW/cm2 and quantum energy hν2=2.32 for different number of laser pulses.

To calculate barrier height equation 1 was rearranged to the express barrier height (Gupta, Yakuphanoglu, et al., 2011):

$$\mathbf{I}\_s = \mathbf{A} \mathbf{A}^\* \mathbf{T}^2 \exp\left(-\frac{\phi\_\mathbf{b}}{\mathbf{k} \mathbf{T}}\right),\tag{1}$$

where A is the irradiated area (0.3 × 0.3 cm2) of i-Ge, A is the Richardson constant, which is 40.8 A cm- ² K-² for i-Ge (Chua, Nikolai, et al., 2003), k is the Boltzmann constant, *b* - the barrier height.

A sharp decrease of rectification ratio at N more than 650 pulses is explained by nanostructure formation on the irradiated surface of the sample due to relaxation of

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 207

application of radiation detectors and semiconductor devices is a study of the possibility to

Investigation of influence of γ radiation on the work of CdTe detectors is studied in papers (Franks et. al., 1999; Taguchi et. al.,1978; Cavallini et.al., 2000; Cavallini et.al., 2002). It was noted that CdTe possesses a high enough radiation hardness due to high mass of elements

In this work the possibility to increase radiation hardness of CdZnTe crystals using laser

Single crystals of Cd1-xZnxTe with x= 0.1 and sizes 10.0 mm × 10.0 mm × 2.0 mm grown by High-Pressure Vertical Zone Melting method were used in our experiments. After irradiation by pulsed Nd:YAG laser samples were γ-irradiated by a 60Co source (Eγ photons = 1.2 MeV) at room temperature with a dose rate of 5×105 Rad = 5.0 KGy. Nd:YAG laser with the following parameters: wavelength λ = 0.532 μm, pulse duration = 10.0 ns, power P = 1.0 MW and intensity range I = 0.48 - 1.80 MW/cm2 was used. The irradiated surface of crystal was covered with a thin layer of SiO2 in order to avoid material evaporation during laser irradiation. The thickness of SiO2 layer was 0.3 μm and it was transparent for laser radiation. To assess the damage of the crystalline lattice after irradiation by γ-ray, photoluminescence method was used. Photoluminescence spectra were measured at 5K using 632.8 nm line of He-Ne laser with excitation power of less than 200 mW. It is known

Photoluminescence spectra of irradiated and non-irradiated Cd1-xZnxTe crystal by Nd:YAG laser are shown in Figure 7. There are typical photoluminescence spectra of p-type Cd0.9Zn0.1Te containing an intense band at 1.645 eV (A0X) ascribed to excitons bound to shallow acceptors (Cd vacancies - VCd), longitudinal optical phonon replicas at 1.624 eV (A0X-LO) and a weak band at 1.657 eV (D°X) of excitons bound to shallow donors (Cd interstitial - CdI). The PL band around 1.54 eV is caused by recombination of donor-acceptor pairs (D-A) with vacancy impurity complexes (VCd-DCd, D=group III or VII elements) (Yang

This non-monotonic change of the intensity of the A0X line in photoluminescence can be attributed to the effects of the temperature gradient induced by strong absorption of laser radiation (Thermogradient effect). This effect causes intense generation of point defects and

Point defects and vacancies are generated under the influence of the temperature gradient in the surface layer during irradiation by laser: Cd (VCd) vacancies and interstitial atoms of Cd (CdI). The increase in VCd concentration causes an increase in the intensity of the A0X line in the photoluminescence spectrum. The D0X line in the photoluminescence spectrum is not shown, since this semiconductor is initially p-type and therefore the concentration of donors

After irradiation at high laser intensity generation of point defects in the irradiated area increases. In the irradiated area of semiconductor processes of generation and recombination and simultaneous redistribution of defects in the temperature gradient field take place. Due to the temperature gradient Cd vacancies move in to the bulk of semiconductor in the region of lower temperature, but interstitial atoms of Cd - in the opposite direction, to the surface of semiconductor. When the laser irradiation intensity reaches 1.2 MW/cm2 gradient of

that -radiation mainly causes intrinsic defect generation in a semiconductor.

reduce the influence of ionizing radiation on their parameters.

in the compound material (Shapiro, (2002).

radiation has been studied.

et. al., 2006; Suzuki et. al., 2001).

is very small (Figure 7 curves 2 and 3).

their redistribution in the irradiated area (Medvid', 2002).

mechanic compressive stress, as shown in Fig. 6. It arises in the structure due to a high concentration of interstitials in the top layer and vacancies in the buried layer.

#### **Conclusion**

#### **2.2 X- and Y-ray detector with increased radiation hardness**

It was noted that radiation damage occurs in semiconductor radiation detectors during their operation, while measuring ionizing radiation, which impairs the ability of the device (Lindstrom, 2003).The main expressions of radiation damage are the following: the increase of leakage current in a semiconductor detector, the need to increase the bias voltage, reduction of the efficiency of collecting the charge created by ionization. The main objectives of research are not only the elucidation of the causes of degradation, but also the development of methods for increasing the operation time of devices.

The preliminary irradiation with neutrons or charged particles with subsequent annealing also allows increasing radiation hardness of material.

One of the promising materials for the use in radiation detectors is CdTe and solid solutions based on this material (Fougeres et al., 1999; Niraula et. al., 1999). Due to the wide band gap, detectors based on CdTe can operate at room temperature. One of the important tasks for

mechanic compressive stress, as shown in Fig. 6. It arises in the structure due to a high

Fig. 6. AFM image of i-Ge surface irradiated by the Nd:YAG laser at intensity 7.0 MW/cm2.

 For the first time we have proved that the mechanism of p-n junction formation in intrinsic semiconductor is caused by generation and redistribution of intrinsic point

 Increase of rectification ratio and the potential barrier of p-n junction with the increase of laser radiation intensity and number of the laser pulses are explained by Thermogradient effect, therefore this effect has the main role in formation of p-n

It was noted that radiation damage occurs in semiconductor radiation detectors during their operation, while measuring ionizing radiation, which impairs the ability of the device (Lindstrom, 2003).The main expressions of radiation damage are the following: the increase of leakage current in a semiconductor detector, the need to increase the bias voltage, reduction of the efficiency of collecting the charge created by ionization. The main objectives of research are not only the elucidation of the causes of degradation, but also the

The preliminary irradiation with neutrons or charged particles with subsequent annealing

One of the promising materials for the use in radiation detectors is CdTe and solid solutions based on this material (Fougeres et al., 1999; Niraula et. al., 1999). Due to the wide band gap, detectors based on CdTe can operate at room temperature. One of the important tasks for

**Conclusion**

junction.

defects in temperature gradient field.

**2.2 X- and Y-ray detector with increased radiation hardness**

development of methods for increasing the operation time of devices.

also allows increasing radiation hardness of material.

concentration of interstitials in the top layer and vacancies in the buried layer.

application of radiation detectors and semiconductor devices is a study of the possibility to reduce the influence of ionizing radiation on their parameters.

Investigation of influence of γ radiation on the work of CdTe detectors is studied in papers (Franks et. al., 1999; Taguchi et. al.,1978; Cavallini et.al., 2000; Cavallini et.al., 2002). It was noted that CdTe possesses a high enough radiation hardness due to high mass of elements in the compound material (Shapiro, (2002).

In this work the possibility to increase radiation hardness of CdZnTe crystals using laser radiation has been studied.

Single crystals of Cd1-xZnxTe with x= 0.1 and sizes 10.0 mm × 10.0 mm × 2.0 mm grown by High-Pressure Vertical Zone Melting method were used in our experiments. After irradiation by pulsed Nd:YAG laser samples were γ-irradiated by a 60Co source (Eγ photons = 1.2 MeV) at room temperature with a dose rate of 5×105 Rad = 5.0 KGy. Nd:YAG laser with the following parameters: wavelength λ = 0.532 μm, pulse duration = 10.0 ns, power P = 1.0 MW and intensity range I = 0.48 - 1.80 MW/cm2 was used. The irradiated surface of crystal was covered with a thin layer of SiO2 in order to avoid material evaporation during laser irradiation. The thickness of SiO2 layer was 0.3 μm and it was transparent for laser radiation. To assess the damage of the crystalline lattice after irradiation by γ-ray, photoluminescence method was used. Photoluminescence spectra were measured at 5K using 632.8 nm line of He-Ne laser with excitation power of less than 200 mW. It is known that -radiation mainly causes intrinsic defect generation in a semiconductor.

Photoluminescence spectra of irradiated and non-irradiated Cd1-xZnxTe crystal by Nd:YAG laser are shown in Figure 7. There are typical photoluminescence spectra of p-type Cd0.9Zn0.1Te containing an intense band at 1.645 eV (A0X) ascribed to excitons bound to shallow acceptors (Cd vacancies - VCd), longitudinal optical phonon replicas at 1.624 eV (A0X-LO) and a weak band at 1.657 eV (D°X) of excitons bound to shallow donors (Cd interstitial - CdI). The PL band around 1.54 eV is caused by recombination of donor-acceptor pairs (D-A) with vacancy impurity complexes (VCd-DCd, D=group III or VII elements) (Yang et. al., 2006; Suzuki et. al., 2001).

This non-monotonic change of the intensity of the A0X line in photoluminescence can be attributed to the effects of the temperature gradient induced by strong absorption of laser radiation (Thermogradient effect). This effect causes intense generation of point defects and their redistribution in the irradiated area (Medvid', 2002).

Point defects and vacancies are generated under the influence of the temperature gradient in the surface layer during irradiation by laser: Cd (VCd) vacancies and interstitial atoms of Cd (CdI). The increase in VCd concentration causes an increase in the intensity of the A0X line in the photoluminescence spectrum. The D0X line in the photoluminescence spectrum is not shown, since this semiconductor is initially p-type and therefore the concentration of donors is very small (Figure 7 curves 2 and 3).

After irradiation at high laser intensity generation of point defects in the irradiated area increases. In the irradiated area of semiconductor processes of generation and recombination and simultaneous redistribution of defects in the temperature gradient field take place. Due to the temperature gradient Cd vacancies move in to the bulk of semiconductor in the region of lower temperature, but interstitial atoms of Cd - in the opposite direction, to the surface of semiconductor. When the laser irradiation intensity reaches 1.2 MW/cm2 gradient of

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 209

1,572 1,584 1,596 1,608 1,620 1,632 1,644 1,656 1,668

Photon energy, eV

0,3 0,6 0,9 1,2 1,5 1,8

)

Laser intensity (MW/cm2

Fig. 9. The photoluminescence intensity of the A0X (curve 1) and D0X (curve 2) bands of

CdZnTe crystal irradiated by the laser as a function of Nd:YAG laser intensity.

Fig. 8. The photoluminescence spectra of Cd1-xZnxTe crystal before (curve 1) and after irradiation by Nd:YAG laser: curve 2 - irradiated by Nd:YAG laser with intensity 1.80 MW/cm2; curve 4 – irradiated by γ- ray with a dose rate of 5.0 KGy; curve 3 – previously irradiated by Nd:YAG laser with intensity 1.80 MW/cm2 and subsequently by γ- ray with a

*A0*

4

1

3

2

*X-LO D0*

Pl intensity increasing

*A0 X*

by 1.6 times

*D0 X*

1

2

by 9 times

0

0

D0 X

A0 X

50

100

150

200

Photoluminescences intensity (arb.un.)

250

300

350

400

170

340

*A0 +eA0 -LO*

*D0 A0 +eA0*

510

680

PL intensity (a.u.)

dose rate of 5.0 KGy

850

1020

1190

1360

temperature partially compensates vacancies of Cd atoms by Zn atoms (Figure 7 curve 4). This is due to the greater bonding energy of Zn and Te atoms in comparison to Cd and Te atoms. At lower intensities of laser irradiation, this process is less likely to happen.

Fig. 7. The photoluminescence intensity as a function of photon energy for Cd1-xZnxTe before and after irradiation by Nd:YAG laser: curve 1 – non irradiated; curve 2 – irradiated with intensity 0.48 MW/cm2; curve 3 - irradiated with intensity 0.84 MW/cm2; curve 4 – irradiated with intensity 1.20 MW/cm2; curve 5 - irradiated with intensity 1.80 MW/cm2.

As a result, the A0X line intensity decreases due compensation of vacancies by Zn atoms as well as localization of VCd in the bulk of semiconductor. Intensive generation of interstitial atoms of Cd and increase of their concentration, and their localization at the surface can be proved by appearance of a the D0X line in the photoluminescence spectrum (Figure 7 curve 5 and Figure 8 curve 2). Thus, in the surface layer of irradiated semiconductor a region highly enriched by point defects VCd and CdI is formed.

Irradiation of Cd1-xZnxTe crystal by γ-ray with a dose rate of 5×105 Rad = 5.0 KGy leads to a strong increase of the A0X band intensity in photoluminescence spectra of Cd1-xZnxTe crystal, as shown in Figure 8. For example, intensity of the A0X band is increased 10 times for curve 3 in comparison with intensity for curve 1. At the same time, the D0X band in photoluminescence spectrum of Cd1-xZnxTe crystal disappears fully, as shown in Figure 9, curve 2 and Figure 10, curve 2.

We explain this effect by Cd vacancies' generation and localization in the excited luminescence thin layer after γ-irradiation of Cd1-xZnxTe crystal.

The main effect observed in the study is suppression of VCd generation and /or localization by γ-radiation at the irradiated surface of Cd1-xZnxTe crystal if the crystal is preliminary irradiated by the laser.

temperature partially compensates vacancies of Cd atoms by Zn atoms (Figure 7 curve 4). This is due to the greater bonding energy of Zn and Te atoms in comparison to Cd and Te atoms.

1.50 1.52 1.54 1.56 1.58 1.60 1.62 1.64 1.66

Fig. 7. The photoluminescence intensity as a function of photon energy for Cd1-xZnxTe before and after irradiation by Nd:YAG laser: curve 1 – non irradiated; curve 2 – irradiated with intensity 0.48 MW/cm2; curve 3 - irradiated with intensity 0.84 MW/cm2; curve 4 – irradiated with intensity 1.20 MW/cm2; curve 5 - irradiated with intensity 1.80 MW/cm2.

*D0 A0 +eA0 -LO*

Photon energy (eV)

As a result, the A0X line intensity decreases due compensation of vacancies by Zn atoms as well as localization of VCd in the bulk of semiconductor. Intensive generation of interstitial atoms of Cd and increase of their concentration, and their localization at the surface can be proved by appearance of a the D0X line in the photoluminescence spectrum (Figure 7 curve 5 and Figure 8 curve 2). Thus, in the surface layer of irradiated semiconductor a region

Irradiation of Cd1-xZnxTe crystal by γ-ray with a dose rate of 5×105 Rad = 5.0 KGy leads to a strong increase of the A0X band intensity in photoluminescence spectra of Cd1-xZnxTe crystal, as shown in Figure 8. For example, intensity of the A0X band is increased 10 times for curve 3 in comparison with intensity for curve 1. At the same time, the D0X band in photoluminescence spectrum of Cd1-xZnxTe crystal disappears fully, as shown in Figure 9,

We explain this effect by Cd vacancies' generation and localization in the excited

The main effect observed in the study is suppression of VCd generation and /or localization by γ-radiation at the irradiated surface of Cd1-xZnxTe crystal if the crystal is preliminary

*D0 A0 +eA0*

0

50

100

*D0 X* 150

200

250

300

*4*

5

*2*

*1*

*A0 X-LO*

*3*

*A0 X*

350

400

450

0

curve 2 and Figure 10, curve 2.

irradiated by the laser.

50

100

150

200

Photoluminescences intensity (arb.un.)

250

300

350

400

450

At lower intensities of laser irradiation, this process is less likely to happen.

*DAP*

highly enriched by point defects VCd and CdI is formed.

luminescence thin layer after γ-irradiation of Cd1-xZnxTe crystal.

Fig. 8. The photoluminescence spectra of Cd1-xZnxTe crystal before (curve 1) and after irradiation by Nd:YAG laser: curve 2 - irradiated by Nd:YAG laser with intensity 1.80 MW/cm2; curve 4 – irradiated by γ- ray with a dose rate of 5.0 KGy; curve 3 – previously irradiated by Nd:YAG laser with intensity 1.80 MW/cm2 and subsequently by γ- ray with a dose rate of 5.0 KGy

Fig. 9. The photoluminescence intensity of the A0X (curve 1) and D0X (curve 2) bands of CdZnTe crystal irradiated by the laser as a function of Nd:YAG laser intensity.

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 211

*et al*., 2002), surface uniformity and process compatibility (Blasundaraprabhu *et al*., 2009). ITO is a perspective material for elaboration of new generation solar cells using nanotechnology. We investigated the ITO/p-Si/Al structure irradiated by Nd:YAG laser with the aim to form

ITO/p-Si/Al structures, with ITO top layer thickness of 70 nm, Si layer thicknes of 500 μm and an Al back layer thickness of 100 nm in the experiments were studied. The structures were irradiated from the ITO side by Nd:YAG laser second harmonic with a wavelength of λ = 512 nm and a pulse duration of τ = 10 ns. The diameter of the laser beam spot was 0.9

a) b)

c) d)

Fig. 11. Atomic force microscope images of ITO/p-Si structure: a) non-irradiated, irradiated by Nd:YAG laser at intensities of b) 1.13 MW/cm2; c) 2.83 MW/cm2 and d) boarder of nonirradiated (smooth surface) and irradiated by Nd:YAG laser at intensity 2.83 MW/cm2

(surface with nanocones).

a p-n junction and to grow nanocones on the interface of ITO/Si.

Fig. 10. The photoluminescence intensity of A0X (curve 1) and the D0X (curve 2) bands of CdZnTe crystal irradiated by the laser and after subsequent irradiation by γ-rays with a dose rate of 5.0 KGy as a function of Nd:YAG laser intensity.

The effect increases with intensity of the laser in region of the laser intensity up to 0.50 - 2.0 MW/cm2, as shown in Figure 8, curve 3. This effect is clearly observed in Fig. 3 and Fig.4. Intensity of the A0X band in photoluminescence spectrum of Cd1-xZnxTe crystal increases only 1.7 times (for comparison, non-irradiated by the laser: 9.3 times) after γ-radiation if the crystal preliminary was irradiated by the laser at intensity 1.2 WM/cm2.

Gamma radiation causes the generation of additional Cd vacancies and interstitial Cd atoms, which causes the growth of the intensity of the A0X line in the photoluminescence spectrum. Generation and recombination process takes place simultaneously, when the existing interstitial atoms or atoms generated under the influence of gamma radiation move and fill the existing vacancies or newly generated vacancies. As a result, the rate of increase in the concentration of vacancies and interstitial atoms is slowed down because of their partial recombination. These processes can be explained by a decrease of the A0X line intensity growth, which also means an increase in radiation resistance of semiconductor.

It should be noted that such mechanism of "defect healing" is valid only up to certain values of radiation doses at which the number of donors formed by laser irradiation may still restrain the growth of the number of vacancies due to the effects of gamma radiation.

#### **3. Application of selective laser annealing**

#### **3.1 Third generation solar cells on the base of ITO/Si/Al structure**

Indium–tin-oxide (ITO) thin films are widely used as a transparent conductive oxide in optoelectronics devices such as solar cells (Bruk *et al*., 2009), liquid crystal displays and plasma display panels. This material has high transmittance in the visible region of spectra (Adurodija

0,3 0,6 0,9 1,2 1,5 1,8

)

1

2

Laser intensity (MW/cm2

The effect increases with intensity of the laser in region of the laser intensity up to 0.50 - 2.0 MW/cm2, as shown in Figure 8, curve 3. This effect is clearly observed in Fig. 3 and Fig.4. Intensity of the A0X band in photoluminescence spectrum of Cd1-xZnxTe crystal increases only 1.7 times (for comparison, non-irradiated by the laser: 9.3 times) after γ-radiation if the

Gamma radiation causes the generation of additional Cd vacancies and interstitial Cd atoms, which causes the growth of the intensity of the A0X line in the photoluminescence spectrum. Generation and recombination process takes place simultaneously, when the existing interstitial atoms or atoms generated under the influence of gamma radiation move and fill the existing vacancies or newly generated vacancies. As a result, the rate of increase in the concentration of vacancies and interstitial atoms is slowed down because of their partial recombination. These processes can be explained by a decrease of the A0X line intensity growth, which also means an increase in radiation resistance of semiconductor.

It should be noted that such mechanism of "defect healing" is valid only up to certain values of radiation doses at which the number of donors formed by laser irradiation may still restrain the growth of the number of vacancies due to the effects of gamma radiation.

Indium–tin-oxide (ITO) thin films are widely used as a transparent conductive oxide in optoelectronics devices such as solar cells (Bruk *et al*., 2009), liquid crystal displays and plasma display panels. This material has high transmittance in the visible region of spectra (Adurodija

Fig. 10. The photoluminescence intensity of A0X (curve 1) and the D0X (curve 2) bands of CdZnTe crystal irradiated by the laser and after subsequent irradiation by γ-rays with a

0

D0 X

dose rate of 5.0 KGy as a function of Nd:YAG laser intensity.

**3. Application of selective laser annealing**

crystal preliminary was irradiated by the laser at intensity 1.2 WM/cm2.

**3.1 Third generation solar cells on the base of ITO/Si/Al structure**

A0 X

170

340

510

680

Photoluminescences intensity (arb.un.)

850

1020

1190

1360

*et al*., 2002), surface uniformity and process compatibility (Blasundaraprabhu *et al*., 2009). ITO is a perspective material for elaboration of new generation solar cells using nanotechnology.

We investigated the ITO/p-Si/Al structure irradiated by Nd:YAG laser with the aim to form a p-n junction and to grow nanocones on the interface of ITO/Si.

ITO/p-Si/Al structures, with ITO top layer thickness of 70 nm, Si layer thicknes of 500 μm and an Al back layer thickness of 100 nm in the experiments were studied. The structures were irradiated from the ITO side by Nd:YAG laser second harmonic with a wavelength of λ = 512 nm and a pulse duration of τ = 10 ns. The diameter of the laser beam spot was 0.9

Fig. 11. Atomic force microscope images of ITO/p-Si structure: a) non-irradiated, irradiated by Nd:YAG laser at intensities of b) 1.13 MW/cm2; c) 2.83 MW/cm2 and d) boarder of nonirradiated (smooth surface) and irradiated by Nd:YAG laser at intensity 2.83 MW/cm2 (surface with nanocones).

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 213

times in comparison to the nonirradiated structure. This effect is explained by increase of absorption coefficient due to quantum confinement effect in nanocones. That means Si band

Fig. 13. Photocurrent-voltage characteristic of ITO/p-Si/Al structure before and after

Non-irradiated I1= 1.13 MW/cm2

c

I2= 2.83 MW/cm2

b

a

Fig. 14. Dark current-voltage characteristic of ITO/p-Si/Al structure: a) non-irradiated;

It is found that the dark current-voltage characteristic becomes diode-like with rectification coefficient K = 105 at 5 V. The result is caused by the laser irradiation with intensity I2 = 2.83

irradiated by Nd:YAG laser at intensities of b) 1.13 MW/cm2; c) 2.83 MW/cm2.

gap is increasing with the increase of laser radiation intensity.

irradiation by laser with different intensities.

b c

a

mm. Irradiation of the samples in the scanning regime using a two coordinate manipulator with 20 μm step was carried out. Experiments in ambient atmosphere at pressure of 1 atm, T = 200C, and 60% humidity were performed.

The irradiated structures were studied using atomic force microscope and photoluminescence spectroscopy method. The measurements of current - voltage characteristic to observe the changes of electrical parameters for ITO/p-Si/Al structure after irradiation by laser were carried out. The n-type Si layer was formed on p-type Si, due to drift of interstitial Si atoms towards the irradiated surface, as a result of huge gradient of temperature induced by laser radiation, that so called Thermogradient effect (Medvid', 2002).

The two-dimensional surface topography of ITO/p-Si structure was studied by atomic force microscope, non-irradiated and irradiated by the Nd:YAG laser at intensities of 1.13 MW/cm2 and 2.83 MW/cm2 (figures 11a, 11b, 11c, respectively). Nanocones were observed on the irradiated Si surface with the average height 12 nm and 26 nm formed by laser radiation at the intensity of 1.13 MW/cm2, and 2.83 MW/cm2, respectively (e.g. see figures 11d).

Photoluminescence spectra of the ITO/p-Si structures with the maxima at 575 nm and 490 nm obtained after laser irradiation at intensities of 1.13 MW/cm2 and 2.83 MW/cm2 are shown in figure 12. Position of the observed photoluminescence maximum compared with the bulk Si shows a significant "blue shift". The maxima of the photoluminescence band at 575nm and 490 nm are explained by presence of the Quantum confinement effect (Brus, 1984) on the top of nanocones.

Fig. 12. Photoluminescence spectra of ITO/p-Si structure: before and after irradiation by the Nd:YAG laser.

After irradiation of ITO/p-Si/Al structure by the laser, photocurrent - voltage characteristic measurements (see figure 13) have shown, that electrical power output increased by two

mm. Irradiation of the samples in the scanning regime using a two coordinate manipulator with 20 μm step was carried out. Experiments in ambient atmosphere at pressure of 1 atm, T

The irradiated structures were studied using atomic force microscope and photoluminescence spectroscopy method. The measurements of current - voltage characteristic to observe the changes of electrical parameters for ITO/p-Si/Al structure after irradiation by laser were carried out. The n-type Si layer was formed on p-type Si, due to drift of interstitial Si atoms towards the irradiated surface, as a result of huge gradient of temperature induced by laser

The two-dimensional surface topography of ITO/p-Si structure was studied by atomic force microscope, non-irradiated and irradiated by the Nd:YAG laser at intensities of 1.13 MW/cm2 and 2.83 MW/cm2 (figures 11a, 11b, 11c, respectively). Nanocones were observed on the irradiated Si surface with the average height 12 nm and 26 nm formed by laser radiation at the

Photoluminescence spectra of the ITO/p-Si structures with the maxima at 575 nm and 490 nm obtained after laser irradiation at intensities of 1.13 MW/cm2 and 2.83 MW/cm2 are shown in figure 12. Position of the observed photoluminescence maximum compared with the bulk Si shows a significant "blue shift". The maxima of the photoluminescence band at 575nm and 490 nm are explained by presence of the Quantum confinement effect (Brus,

Fig. 12. Photoluminescence spectra of ITO/p-Si structure: before and after irradiation by the

After irradiation of ITO/p-Si/Al structure by the laser, photocurrent - voltage characteristic measurements (see figure 13) have shown, that electrical power output increased by two

intensity of 1.13 MW/cm2, and 2.83 MW/cm2, respectively (e.g. see figures 11d).

= 200C, and 60% humidity were performed.

1984) on the top of nanocones.

Nd:YAG laser.

radiation, that so called Thermogradient effect (Medvid', 2002).

times in comparison to the nonirradiated structure. This effect is explained by increase of absorption coefficient due to quantum confinement effect in nanocones. That means Si band gap is increasing with the increase of laser radiation intensity.

Fig. 13. Photocurrent-voltage characteristic of ITO/p-Si/Al structure before and after irradiation by laser with different intensities.

Fig. 14. Dark current-voltage characteristic of ITO/p-Si/Al structure: a) non-irradiated; irradiated by Nd:YAG laser at intensities of b) 1.13 MW/cm2; c) 2.83 MW/cm2.

It is found that the dark current-voltage characteristic becomes diode-like with rectification coefficient K = 105 at 5 V. The result is caused by the laser irradiation with intensity I2 = 2.83

Application of Nd:YAG Laser in Semiconductors' Nanotechnology 215

Fig. 16. Scanning electron microscope image of Si nanocones created by laser irradiation.

The electron field emission from such nanocones has some peculiarities, namely: (i) decrease of the threshold field from Eth=4×104 V/cm at the first measurement to Eth=3.5×104 V/cm in subsequent measurements, (ii) two slopes of Fowler-Nordheim curves (higher slope at low fields and lower slope at high fields) (figure 17). Analysis of the scanning electron microscope micrographs and electron field emission curves allows us to estimate (i) the electron field enhancement coefficient, 100, (ii) work functions (Φ1=6.8 eV at the first

Fig. 17. Current-voltage characteristics (a) and corresponding Fowler-Nordheim plots (b) of Si nanocones electron field emission. Nanocones have been formed by second harmonic of

Nd:YAG laser with intensity 2.0 MW/cm2.

MW/cm2 (see figure 14). The improvements of ITO/p-Si/Al structure as a solar cell can be explained by the increase of potential barrier between ITO an p-Si layers.

#### **Conclusion**

#### **3.2 Electron field emitter based on Si nanocones**

Silicon electron field emitters may be attractive for numerous applications, mostly due to their compatibility with the dominating Si-based solid state micro- and nanoelectronics.

Nanocones have been formed on the surface of p-Si crystal after irradiation by the second harmonic of Nd:YAG laser with an intensity of 2.0 MW/cm2 (Evtukh et al., 2010). The measurements of electron field emissions were performed with a flat diode configuration with a glass spacer (figure 15). Distance between the cathode (silicon wafer) and the anode was 0.8 mm. The applied field emission setup allows achieving a vacuum of 1× 10-5 Pa. The applied voltage varied between 1500-3600 V.

Fig. 15. Cell for electron field emission measurements.

Nanocones with a nanosphere on top of each cone were formed after irradiation by the laser (figure 16). A decrease in the nanosphere diameter from 600 nm to 20 nm with an increase of intensity of laser pulse from 2.0MW/cm2 to 20.0 MW/cm2 was observed. Nanocones have been formed on the irradiated surface of p-Si crystal by second harmonic of Nd:YAG laser with intensity 2.0 MW/cm2.

MW/cm2 (see figure 14). The improvements of ITO/p-Si/Al structure as a solar cell can be

The possibility of p-n junction and Si nanocones' formation by laser irradiation in

 The photoluminescence spectra from irradiated ITO/p-Si/Al structure by laser irradiation have been found in visible range of spectra and explained by presence of