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**Chapter 8**

**Provisional chapter**

**Graphene-Based Acousto-Optic Sensors with Vibrating**

**Vibrating Resonance Energy Transfer and Applications**

Graphene as a two-dimensional planar material has numerous advantages for realizing highperformance nano-electromechanical systems (NEMS) such as nanoscale sensors including strain sensors, optical modulators or energy harvesters. Large Young's modulus (1 TPa for single layer graphene), ultra-low weight, low residual stress and large breaking strength properties are important properties as two-dimensional (2D) ultrathin resonators. Graphene resonators are recently utilized for low complexity design of nanoscale acousto-optic sensors based on a novel theoretical model describing vibrating Förster resonance energy transfer (VFRET) mechanism. Proposed system combines the advantages of graphene with quantum dots (QDs) as donor and acceptor pairs with broad absorption spectrum, large cross-sections, tunable emission spectra, size-dependent emission wavelength, high photochemical stability and improved quantum yield. Device structure supporting wide-band resonance frequencies including acoustic and ultrasound ranges promises high-performance applications for challenging environments. Remote sensors and acousto-optic communication channels are formed for in-body applications, wireless body area sensor networks (WBASNs), space and interplanetary systems, microfluidics and visible light communication (VLC)-based architectures. **Keywords:** nano acousto-optic sensor, graphene resonator, nanoscale acousto-optic transduction, vibrating Förster resonance energy transfer, energy harvesting, interplanetary communications, in-body sensor network, microfluidics, visible light

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Graphene has important advantages in significantly many application areas of physical, chemical and biological sciences with advanced engineering system designs including sensors,

DOI: 10.5772/intechopen.73618

**Resonance Energy Transfer and Applications**

**Graphene-Based Acousto-Optic Sensors with**

Burhan Gulbahar and Gorkem Memisoglu

Burhan Gulbahar and Gorkem Memisoglu

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73618

**Abstract**

communications

**1. Introduction**

#### **Graphene-Based Acousto-Optic Sensors with Vibrating Resonance Energy Transfer and Applications Graphene-Based Acousto-Optic Sensors with Vibrating Resonance Energy Transfer and Applications**

DOI: 10.5772/intechopen.73618

Burhan Gulbahar and Gorkem Memisoglu Burhan Gulbahar and Gorkem Memisoglu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73618

#### **Abstract**

Graphene as a two-dimensional planar material has numerous advantages for realizing highperformance nano-electromechanical systems (NEMS) such as nanoscale sensors including strain sensors, optical modulators or energy harvesters. Large Young's modulus (1 TPa for single layer graphene), ultra-low weight, low residual stress and large breaking strength properties are important properties as two-dimensional (2D) ultrathin resonators. Graphene resonators are recently utilized for low complexity design of nanoscale acousto-optic sensors based on a novel theoretical model describing vibrating Förster resonance energy transfer (VFRET) mechanism. Proposed system combines the advantages of graphene with quantum dots (QDs) as donor and acceptor pairs with broad absorption spectrum, large cross-sections, tunable emission spectra, size-dependent emission wavelength, high photochemical stability and improved quantum yield. Device structure supporting wide-band resonance frequencies including acoustic and ultrasound ranges promises high-performance applications for challenging environments. Remote sensors and acousto-optic communication channels are formed for in-body applications, wireless body area sensor networks (WBASNs), space and interplanetary systems, microfluidics and visible light communication (VLC)-based architectures.

**Keywords:** nano acousto-optic sensor, graphene resonator, nanoscale acousto-optic transduction, vibrating Förster resonance energy transfer, energy harvesting, interplanetary communications, in-body sensor network, microfluidics, visible light communications

## **1. Introduction**

Graphene has important advantages in significantly many application areas of physical, chemical and biological sciences with advanced engineering system designs including sensors,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

modulators and two-dimensional (2D) ultrathin resonators. Mechanical designs utilizing graphene achieve high-performance nano-electromechanical systems (NEMS) with large Young's modulus (1 TPa for single layer graphene), ultra-low weight, low residual stress and large breaking strength, i.e., graphene strain allowing 25% without breaking [1]. Unique optical, electrical, physical and mechanical properties of graphene allow the designed resonators to be utilized in challenging applications, e.g., strain sensor, mass sensor, nanogenerators, transducers, photodetectors and novel NEMS devices [1–8].

**1. Biomedical applications:**

**2. Space applications:**

environment.

lizing external acoustic excitations.

**a.** In-body: hybrid acousto-optic noninvasive communication channels inside body by uti-

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181

**b.** Wireless body area sensor networks (WBASNs): wearable sensors such as heart rate

**b.** Optical wireless communications: realizing optical channels between devices in space

**4. Visible light communications (VLC):** generating visible light sequences with advanced

In this Chapter, VFRET mechanism and nanoscale acousto-optic sensor design are briefly introduced in Sections 2 and 3, respectively, as fundamental novel tools described in [4]. Then, Section 4 discusses applications in biomedical sensing, interplanetary and space, microfluidics and communication networks. Finally, Section 5 concludes the Chapter with a brief

An illustration describing the basics of VFRET mechanism is given in **Figure 1**. Vibrating multi-layer graphene membrane has height *h*, radius *a*, tunable donor-acceptor (D-A) dis-

phase-b and phase-c. Phase-a corresponds to the position at rest. FRET or VFRET occurs in phase-b at the shortest distance between D-A pairs and phase-c has the largest distance

**Figure 1.** Nanoscale acousto-optic transduction mechanism with VFRET mechanism where mechanical vibration is converted to optical emission (adapted from [4]) with the phases denoted by a, b and c showing the relative positions of

. Three phases of VFRET are denoted by phase-a,

monitoring or any sensor transmitting data with optical sequences.

**a.** Interplanetary remote sensing: sensing atmospheric events remotely.

**3. Microfluidics:** cell and particle monitoring and tracking.

geometrical design to produce modulated data sequences.

summary of VFRET-based sensor design and its applications.

**2. Vibrating FRET mechanism**

tance of *dAD* and resonance amplitude *d*0

acceptors and donors in a single resonance period.

Nanoscale photonic solutions with a novel design for sensing, energy harvesting or communication purposes are recently presented in [3–5, 8] with significant performances and rich set of applications. They utilize unique and unexploited features of graphene and quantum dots (QDs) combined with a special mechanical design. QDs are future promising as donor-acceptor couples in next generation nanoscale devices [3–5, 8, 9]. More specifically, passive acousto-optic nanoscale optical modulator design is presented by exploiting high performance mechanical properties of graphene resonators and radiating photon emission from QDs with a novel method denoted by vibrating Förster Resonance Energy Transfer (VFRET) [4]. It promises high performance applications in biomedical, space and microfluidic monitoring and tracking areas while utilizing energy harvesting, low complexity and all-in-one acousto-optic transducer mechanical design.

VFRET mechanism uniquely exploits important properties of graphene resonators having wideband spectrum covering acoustic and ultrasound frequencies combined with special features of QDs, and FRET which is a nanoscale energy transfer process between two molecules denoted as donors and acceptors [4]. FRET utilized in a rich set of biological, physical and chemical applications such as monitoring cellular activities has also been utilized for nanoscale communication channels [10–12]. Semiconductor nanocrystal QDs form a unique cooperation with graphene resonators by utilizing high performance properties such as sharp and broad absorption spectrum, large absorption cross-sections, tunable emission spectra and wavelength, photochemical stability and photoluminescence quantum yield [13, 14]. In [4], a modulator is designed to be utilized in challenging applications for communications, sensing and energy harvesting with a scalable range including both nanoscale and macroscale dimensions. Ambient light sources with low power levels are utilized instead of laser sources in a nanoscale hybrid acousto-optic platform while periodically modulating nanoscale distance between donors attached on vibrating graphene and acceptors fixed on a support. Therefore, a design combining flexibility without requiring special laser set-ups, energy efficiency with low power ambient sources and wideband resonance frequency including ultrasound levels is presented as a future promising tool. In addition, it is emphasized that graphene vibration can also be generated by thermoacoustic or opto-acoustic methods by proposing the tool beneficial in significantly challenging environments.

VFRET system design promises important application areas as discussed in detail in Section 5 by combining unique properties of QDs and graphene including its biocompatibility. Various application areas are listed as follows presenting biomedical, space, microfluidics and communication purposes:

#### **1. Biomedical applications:**

modulators and two-dimensional (2D) ultrathin resonators. Mechanical designs utilizing graphene achieve high-performance nano-electromechanical systems (NEMS) with large Young's modulus (1 TPa for single layer graphene), ultra-low weight, low residual stress and large breaking strength, i.e., graphene strain allowing 25% without breaking [1]. Unique optical, electrical, physical and mechanical properties of graphene allow the designed resonators to be utilized in challenging applications, e.g., strain sensor, mass sensor, nanogenerators, transduc-

Nanoscale photonic solutions with a novel design for sensing, energy harvesting or communication purposes are recently presented in [3–5, 8] with significant performances and rich set of applications. They utilize unique and unexploited features of graphene and quantum dots (QDs) combined with a special mechanical design. QDs are future promising as donor-acceptor couples in next generation nanoscale devices [3–5, 8, 9]. More specifically, passive acousto-optic nanoscale optical modulator design is presented by exploiting high performance mechanical properties of graphene resonators and radiating photon emission from QDs with a novel method denoted by vibrating Förster Resonance Energy Transfer (VFRET) [4]. It promises high performance applications in biomedical, space and microfluidic monitoring and tracking areas while utilizing energy harvesting, low complexity and all-in-one acousto-optic transducer

VFRET mechanism uniquely exploits important properties of graphene resonators having wideband spectrum covering acoustic and ultrasound frequencies combined with special features of QDs, and FRET which is a nanoscale energy transfer process between two molecules denoted as donors and acceptors [4]. FRET utilized in a rich set of biological, physical and chemical applications such as monitoring cellular activities has also been utilized for nanoscale communication channels [10–12]. Semiconductor nanocrystal QDs form a unique cooperation with graphene resonators by utilizing high performance properties such as sharp and broad absorption spectrum, large absorption cross-sections, tunable emission spectra and wavelength, photochemical stability and photoluminescence quantum yield [13, 14]. In [4], a modulator is designed to be utilized in challenging applications for communications, sensing and energy harvesting with a scalable range including both nanoscale and macroscale dimensions. Ambient light sources with low power levels are utilized instead of laser sources in a nanoscale hybrid acousto-optic platform while periodically modulating nanoscale distance between donors attached on vibrating graphene and acceptors fixed on a support. Therefore, a design combining flexibility without requiring special laser set-ups, energy efficiency with low power ambient sources and wideband resonance frequency including ultrasound levels is presented as a future promising tool. In addition, it is emphasized that graphene vibration can also be generated by thermoacoustic or opto-acoustic methods by proposing the tool beneficial in significantly chal-

VFRET system design promises important application areas as discussed in detail in Section 5 by combining unique properties of QDs and graphene including its biocompatibility. Various application areas are listed as follows presenting biomedical, space, microfluidics and com-

ers, photodetectors and novel NEMS devices [1–8].

180 Two-dimensional Materials for Photodetector

mechanical design.

lenging environments.

munication purposes:

#### **2. Space applications:**

In this Chapter, VFRET mechanism and nanoscale acousto-optic sensor design are briefly introduced in Sections 2 and 3, respectively, as fundamental novel tools described in [4]. Then, Section 4 discusses applications in biomedical sensing, interplanetary and space, microfluidics and communication networks. Finally, Section 5 concludes the Chapter with a brief summary of VFRET-based sensor design and its applications.

## **2. Vibrating FRET mechanism**

An illustration describing the basics of VFRET mechanism is given in **Figure 1**. Vibrating multi-layer graphene membrane has height *h*, radius *a*, tunable donor-acceptor (D-A) distance of *dAD* and resonance amplitude *d*0 . Three phases of VFRET are denoted by phase-a, phase-b and phase-c. Phase-a corresponds to the position at rest. FRET or VFRET occurs in phase-b at the shortest distance between D-A pairs and phase-c has the largest distance

**Figure 1.** Nanoscale acousto-optic transduction mechanism with VFRET mechanism where mechanical vibration is converted to optical emission (adapted from [4]) with the phases denoted by a, b and c showing the relative positions of acceptors and donors in a single resonance period.

between the pairs without any FRET emission. QD donors are attached on graphene membrane firmly, while acceptor molecules are either the QDs or fluorescent dyes, and attached on top of the modulator frame.

Radiated light is controlled by modulating acoustic pressure on graphene resonator while donors are continuously excited by visible light sources. The number of photons emitted is modeled as follows [4]:

$$N\_{\rm FRET} = \Delta\_{\rm t} I\_{\rm D} D\_{\rm g} \sigma\_{\rm D} \, \oplus\_{\rm A} \to N\_{\rm D} / \left( h\_{\rm p} f\_{\rm D}^{a} \right) \tag{1}$$

where *Δt* is the photon counting time interval which is much larger than the total time *tp* = *tD* + *tFRET* + *tA* for acceptor emissions summing the donor excitation time denoted by *tD*, FRET time *tFRET* and acceptor emission time *tA*, *ID* and *DB* are the light intensity in (W/m2 /nm) and bandwidth in (nm), respectively, for donor excitation, *σD* = *Dext* × 3.825 × 10−23 denotes absorption cross section of donor excitation in (m2 ), *Dext* is donor extinction coefficient in (M−1 cm−1), *ΦA* is the quantum yield of acceptors, E is FRET efficiency, *ND* is the number of donor molecules, *f D a* = *c* /*λD a* is donor excitation frequency, c = 3 × 108 (m/ s), *λD a* is the donor excitation wavelength, and *hp =* 6.62 × 10−34 (J × s) is Planck's constant. FRET efficiency E is defined as follows [4]:

$$E = \frac{k\_A R\_0^s}{d^6 + k\_A R\_0^s} \tag{2}$$

**Figure 2.** Physical structure of the acousto-optic transducer where QD donors on resonating graphene membrane transfer energy to QD acceptors on the support with VRET mechanism [4]. Acceptors have filtering interfaces for incoming and emitted light sources while resonators are vibrated by acoustic waves entering through aperture. Sidewalls and special

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**Figure 3.** Resonators tuned to varying sound pressure level (SPLs) with different colors at the corresponding SPLs and

[13] CdSe/ZnS (2.5 nm) 475 500 Dye (fluorescein27) 505 548 (green) 3.4 0.6 [14] CdSe/ZnS (2.9 nm) 540 555 CdSe/ZnS (3.7 nm) 560 580 (yellow) 6.6 0.6

[14] CdSe/ZnS (3.7 nm) 555 570 Fluorescent protein mCherry 575 615 (red) 5.8 0.6 [14] CdSe/ZnS (3.3 nm) 510 550 CdSe/ZnS (3.3 nm) 555 590 (yellow) 6.1 0.6

*PL* **Acceptor and diameter** *λA*

430 495 Yellow fluorescent protein 500 525 (green) 4.5 0.4

*a λA*

*PL R***0** *ΦA*

geometrical design protect the device and provides durability (adapted from [4]).

*a λD*

**Table 1.** Extended set of D-A pairs to utilize in acousto-optic sensor and modulator.

arrays of resonators enhancing emission intensity.

**Ref # Donor and diameter** *λD*

CdSe/ZnS: cadmium selenide/zinc sulfide.

[14] Cyan fluorescent protein

where *k A* denotes the number of acceptors corresponding to each donor, *d* is the donor-acceptor distance, and *R*0 is Förster distance.

In the next section, proposed model is utilized to design an acousto-optic sensor composed of mainly vibrating multi-layer graphene membrane and D-A molecules.

#### **3. Nanoscale acousto-optic sensor design**

The proposed design is improved by including lenses, supports and special enclosure design to be utilized in challenging environments as shown in **Figure 2** as described in detail in [4]. Donors absorbing incoming light energy transfer the energy to acceptors while received and emitted light are filtered by a combination of optical lens and filter to prevent unintended acceptor emission and to intensify incoming and emitted light.

Graphene layer acoustically vibrates with minor change on the resonance frequency due to the weight of donor molecules. The distance between D-A pairs is tunable with respect to the desired vibration amplitude. An example is shown in **Figure 3** where multi-color emission capability is realized by including an array of resonators with different color emissions tuned to different sound pressure levels (SPLs). The set of D-A pairs in [4] is extended in **Table 1** to include multiple colors, and a diversity of D-A pair selections with different diameters and types, i.e., dyes, QDs and proteins [13, 14]. Excitation wavelengths of donors and acceptors, Graphene-Based Acousto-Optic Sensors with Vibrating Resonance Energy Transfer and… http://dx.doi.org/10.5772/intechopen.73618 183

between the pairs without any FRET emission. QD donors are attached on graphene membrane firmly, while acceptor molecules are either the QDs or fluorescent dyes, and attached

Radiated light is controlled by modulating acoustic pressure on graphene resonator while donors are continuously excited by visible light sources. The number of photons emitted is

where *Δt* is the photon counting time interval which is much larger than the total time *tp* = *tD* + *tFRET* + *tA* for acceptor emissions summing the donor excitation time denoted by *tD*, FRET time *tFRET* and acceptor emission time *tA*, *ID* and *DB* are the light intensity in (W/m2

and bandwidth in (nm), respectively, for donor excitation, *σD* = *Dext* × 3.825 × 10−23 denotes

in (M−1 cm−1), *ΦA* is the quantum yield of acceptors, E is FRET efficiency, *ND* is the number

is donor excitation frequency, c = 3 × 108 (m/ s), *λD*

6 \_\_\_\_\_\_\_ *d*6 + *kA R*0

denotes the number of acceptors corresponding to each donor, *d* is the donor-accep-

In the next section, proposed model is utilized to design an acousto-optic sensor composed of

The proposed design is improved by including lenses, supports and special enclosure design to be utilized in challenging environments as shown in **Figure 2** as described in detail in [4]. Donors absorbing incoming light energy transfer the energy to acceptors while received and emitted light are filtered by a combination of optical lens and filter to prevent unintended

Graphene layer acoustically vibrates with minor change on the resonance frequency due to the weight of donor molecules. The distance between D-A pairs is tunable with respect to the desired vibration amplitude. An example is shown in **Figure 3** where multi-color emission capability is realized by including an array of resonators with different color emissions tuned to different sound pressure levels (SPLs). The set of D-A pairs in [4] is extended in **Table 1** to include multiple colors, and a diversity of D-A pair selections with different diameters and types, i.e., dyes, QDs and proteins [13, 14]. Excitation wavelengths of donors and acceptors,

*D DB σD ΦA E ND*/(*hp f*

*D a*

 *=* 6.62 × 10−34 (J × s) is Planck's constant. FRET efficiency E is

) (1)

), *Dext* is donor extinction coefficient

6 (2)

*a*

/nm)

is the donor

on top of the modulator frame.

182 Two-dimensional Materials for Photodetector

*NFRET* = ∆*t I*

absorption cross section of donor excitation in (m2

*D a* = *c* /*λD a*

*E* = *kA R*0

**3. Nanoscale acousto-optic sensor design**

is Förster distance.

acceptor emission and to intensify incoming and emitted light.

mainly vibrating multi-layer graphene membrane and D-A molecules.

modeled as follows [4]:

of donor molecules, *f*

defined as follows [4]:

tor distance, and *R*0

where *k A*

excitation wavelength, and *hp*

**Figure 2.** Physical structure of the acousto-optic transducer where QD donors on resonating graphene membrane transfer energy to QD acceptors on the support with VRET mechanism [4]. Acceptors have filtering interfaces for incoming and emitted light sources while resonators are vibrated by acoustic waves entering through aperture. Sidewalls and special geometrical design protect the device and provides durability (adapted from [4]).

**Figure 3.** Resonators tuned to varying sound pressure level (SPLs) with different colors at the corresponding SPLs and arrays of resonators enhancing emission intensity.

**Table 1.** Extended set of D-A pairs to utilize in acousto-optic sensor and modulator.

i.e., *λD a* and *λA a* , respectively, and emission wavelengths denoted by *λD PL* and *λA PL*, respectively, are tuned with changing diameters and types of molecules.

Next, various application areas including biomedical sensing, interplanetary remote sensing

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Biomedical sensing is an emerging and future promising topic with various applications including wearable sensors, wireless body area networks (WBAN), and in-body sensing and communications applications [16]. Proposed lightweight and energy efficient device structure is a low complexity and all-in-one alternative to the existing radio frequency (RF)-based WBAN sensor solutions [17]. Potential applications include heart tracking, mobility or any movement related monitoring of body parts. In **Figure 4**, an architecture is provided where acoustic vibrations of heart within specific frequency bands reaching hundreds of Hz are converted to optical emissions with tuned device geometry as indicator of heart rate signaling [18]. It provides a passive heart rate monitoring device without requiring battery and, sepa-

In disaster scenarios, heart rate monitoring device provides a method to track and to monitor the health condition of the people remotely. In addition, multiple people including highly crowded groups can be tracked by monitoring emitted periodic sequences. A simple and

Besides that, it allows to realize communication channels inside body, e.g., for hybrid communication links for intra-bone communication architectures as discussed in [19] and as shown in **Figure 5**. Remote signal transmission and communication with in-body region by using transceivers outside the body are highly difficult with RF and optical signals. On the other hand, ultrasound signals easily penetrate through body. A hybrid communication channel is formed by modulating the device inside the body, e.g., bones, with an external ultrasound excitation while emitting optical signals inside the bone region for optical communication. Therefore, optical communication channels anywhere inside the body are easily formed by

**Figure 4.** Biomedical sensing for heart rate monitoring where acousto-optic transducer device is attached on the surface

and space applications, microfluidics and VLC are discussed in detail.

**4.1. Biomedical sensing and communications applications**

rate modulator and coding blocks with complicated architectures.

remote personal monitoring device is proposed.

exciting externally with ultrasound waves.

of the heart with an adhesive plaster.

In addition to QDs, dyes and proteins are candidates to be utilized in various application environments depending with respect to the requirements. In the next section, applications of the proposed sensor and modulator devices are discussed.

## **4. Applications**

The application areas of the proposed VFRET device include a diverse set of domains with various tasks of the device such as an acousto-optic sensor, modulator and hybrid communication transmitter. In-body hybrid sensors, WBASN applications with wearable sensors and transceivers, microfluidics, space, on-chip communication transceivers and visible light communications (VLC)-based applications are the most promising ones as discussed in the following sections.

Bio-compatibility of graphene and QDs gives opportunity for further improving the proposed design suitable for in-body applications [2, 15, 16]. More promising future applications include biomedical ultra-low power communication networks, acoustic sensors and transducers, and nanophotonics. In addition, array structures utilizing multiple resonators in horizontal and vertical dimensions improve light emission power.

VFRET-based nanoscale sensor/modulator architecture allows high performance future applications by exploiting important features of the special design with graphene and QDs. The proposed device brings unique advantages by exploiting the following features:

Next, various application areas including biomedical sensing, interplanetary remote sensing and space applications, microfluidics and VLC are discussed in detail.

#### **4.1. Biomedical sensing and communications applications**

i.e., *λD a* and *λA a*

**4. Applications**

184 Two-dimensional Materials for Photodetector

• Lightweight design

, respectively, and emission wavelengths denoted by *λD*

In addition to QDs, dyes and proteins are candidates to be utilized in various application environments depending with respect to the requirements. In the next section, applications of

The application areas of the proposed VFRET device include a diverse set of domains with various tasks of the device such as an acousto-optic sensor, modulator and hybrid communication transmitter. In-body hybrid sensors, WBASN applications with wearable sensors and transceivers, microfluidics, space, on-chip communication transceivers and visible light communications (VLC)-based applications are the most promising ones as discussed in the following sections. Bio-compatibility of graphene and QDs gives opportunity for further improving the proposed design suitable for in-body applications [2, 15, 16]. More promising future applications include biomedical ultra-low power communication networks, acoustic sensors and transducers, and nanophotonics. In addition, array structures utilizing multiple resonators in hori-

VFRET-based nanoscale sensor/modulator architecture allows high performance future applications by exploiting important features of the special design with graphene and QDs.

• Energy efficient design operating with low level of light sources such as ambient light lev-

• Passive and energy harvesting design by combining vibration and external light sources

• Hybrid acousto-optic architecture allowing utilizing ultrasound and acoustic signals for challenging environments such as creating optical communication links inside body by

• Tuning to different frequencies, SPLs, light levels and applications by varying device ge-

• Flexible system design without requiring complicated laser set-ups having strict orientations • Adaptability to different application areas including in-body communications, wireless body area networks (WBANs), nanoscale and microscale communication networks, microfluidic monitoring and tracking applications, space and aerial applications, and VLC.

The proposed device brings unique advantages by exploiting the following features:

• Broadband spectrum capability supporting ultrasound and acoustic waves

tuned with changing diameters and types of molecules.

the proposed sensor and modulator devices are discussed.

zontal and vertical dimensions improve light emission power.

els for day time and LED sources for night time operation

ometry, graphene resonator and the number of QD molecules

• Planar architecture to be attached on varying objects

using external ultrasound vibrations

*PL* and *λA*

*PL*, respectively, are

Biomedical sensing is an emerging and future promising topic with various applications including wearable sensors, wireless body area networks (WBAN), and in-body sensing and communications applications [16]. Proposed lightweight and energy efficient device structure is a low complexity and all-in-one alternative to the existing radio frequency (RF)-based WBAN sensor solutions [17]. Potential applications include heart tracking, mobility or any movement related monitoring of body parts. In **Figure 4**, an architecture is provided where acoustic vibrations of heart within specific frequency bands reaching hundreds of Hz are converted to optical emissions with tuned device geometry as indicator of heart rate signaling [18]. It provides a passive heart rate monitoring device without requiring battery and, separate modulator and coding blocks with complicated architectures.

In disaster scenarios, heart rate monitoring device provides a method to track and to monitor the health condition of the people remotely. In addition, multiple people including highly crowded groups can be tracked by monitoring emitted periodic sequences. A simple and remote personal monitoring device is proposed.

Besides that, it allows to realize communication channels inside body, e.g., for hybrid communication links for intra-bone communication architectures as discussed in [19] and as shown in **Figure 5**. Remote signal transmission and communication with in-body region by using transceivers outside the body are highly difficult with RF and optical signals. On the other hand, ultrasound signals easily penetrate through body. A hybrid communication channel is formed by modulating the device inside the body, e.g., bones, with an external ultrasound excitation while emitting optical signals inside the bone region for optical communication. Therefore, optical communication channels anywhere inside the body are easily formed by exciting externally with ultrasound waves.

**Figure 4.** Biomedical sensing for heart rate monitoring where acousto-optic transducer device is attached on the surface of the heart with an adhesive plaster.

surfaces with the light shining on the diaphragm limits the practicality of these devices. On the other hand, proposed acousto-optical system architecture allows nanoscale architectures, flexible design with a controlled light source without any reflection-based positioning requirements and multi-color light modulation. Atmospheric events and space-based environmental sensing applications of vibrations are easily tracked with optical signal detection where the light energy is harvested from high power and line-of-sight radiation of the sun as shown in **Figure 6**. Hard-to-reach areas where vibratory events occur are monitored with a sensor network composed of energy harvesting transducers with significantly high lifetimes,

Graphene-Based Acousto-Optic Sensors with Vibrating Resonance Energy Transfer and…

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On the other hand, energy harvesting nature and optical emission capability are utilized for creating optical wireless communications channels in space. Simple device mechanism and array forming capability provide a unique opportunity to utilize in challenging environments in space.

Multi particle tracking (MPT) or single particle tracking (SPT) with labeling by observing fluorescent molecule emission are common tools for monitoring cellular processes for in-vivo nano-biological and in-vitro microfluidic systems [3, 5]. Special fluorescing tags attached on molecules are tracked and digital image processing tools allow nanometer (nm) resolution in positioning. Image processing methods and algorithms are developed in significantly many studies for particle and cell tracking in microscopic platforms [22, 23]. However, there are important challenges for collection capability and analyzing high complexity imaging data. Tags do not have signaling capability to support a signaling-based tracking compared with traditional imaging-based tracking systems like fluorescence lifetime imaging for cell tracking. Recently, a novel nanoscale acousto-optic radar and particle tracking system design using chirp spread spectrum (CSS) sequences with special geometrical design of acousto-optic transducers [5]. The design denoted with the name *CSSTag* promises signaling-based tagging in microfluidic environments by generating unique tags as shown in **Figure 7**. CSSTag system utilizes spread

**Figure 6.** Space application for interplanetary remote sensing where energy is harvested from sun to track acoustic

vibrations of atmospheric effects in space or planetary environments.

durability and low complexity design allowing long duration space travel.

**4.3. Microfluidic applications**

**Figure 5.** Hybrid in-body sensing and communication scenario for realizing optical communication links inside the body with external acoustic excitation. Photodetectors detect acousto-optic signals inside the bones as a challenging medium to realize communication networks.

In-body applications are realized by exploiting acousto-optic transduction properties, energy harvesting, low power operation and unique features of graphene and QDs. Hybrid communication links are formed such that graphene resonator is vibrated remotely without requiring contact by using ultrasound waves for creating optical in-body networks. Noninvasive design promises to improve opto-genetics with capability to operate inside different parts of the body [20]. In-body system design requires low power LEDs to be available with the proposed device. This is feasible compared with high power laser operations requiring strict orientations and complicated set-ups. Scalability of the geometrical design, planar structure, flexible and simple system design, and all-mechanical structure make VFRET design with graphene resonators more promising for challenging optical in-body nanonetworks.

Information is externally generated and transmitted to photodetectors inside the body by using external acoustic modulation and hybrid acousto-optic transduction in the VFRET device as a solution for significant challenges of the optical or RF signals to penetrate inside the body [16].

#### **4.2. Interplanetary remote sensing and space applications**

Long range acoustical sensors are utilized for passive sensing of vibrations, flutter, atmospheric turbulence, and terrestrial and planetary sounds remotely [21]. Utilization of reflective surfaces with the light shining on the diaphragm limits the practicality of these devices. On the other hand, proposed acousto-optical system architecture allows nanoscale architectures, flexible design with a controlled light source without any reflection-based positioning requirements and multi-color light modulation. Atmospheric events and space-based environmental sensing applications of vibrations are easily tracked with optical signal detection where the light energy is harvested from high power and line-of-sight radiation of the sun as shown in **Figure 6**. Hard-to-reach areas where vibratory events occur are monitored with a sensor network composed of energy harvesting transducers with significantly high lifetimes, durability and low complexity design allowing long duration space travel.

On the other hand, energy harvesting nature and optical emission capability are utilized for creating optical wireless communications channels in space. Simple device mechanism and array forming capability provide a unique opportunity to utilize in challenging environments in space.

#### **4.3. Microfluidic applications**

In-body applications are realized by exploiting acousto-optic transduction properties, energy harvesting, low power operation and unique features of graphene and QDs. Hybrid communication links are formed such that graphene resonator is vibrated remotely without requiring contact by using ultrasound waves for creating optical in-body networks. Noninvasive design promises to improve opto-genetics with capability to operate inside different parts of the body [20]. In-body system design requires low power LEDs to be available with the proposed device. This is feasible compared with high power laser operations requiring strict orientations and complicated set-ups. Scalability of the geometrical design, planar structure, flexible and simple system design, and all-mechanical structure make VFRET design with graphene

**Figure 5.** Hybrid in-body sensing and communication scenario for realizing optical communication links inside the body with external acoustic excitation. Photodetectors detect acousto-optic signals inside the bones as a challenging medium

Information is externally generated and transmitted to photodetectors inside the body by using external acoustic modulation and hybrid acousto-optic transduction in the VFRET device as a solution for significant challenges of the optical or RF signals to penetrate inside

Long range acoustical sensors are utilized for passive sensing of vibrations, flutter, atmospheric turbulence, and terrestrial and planetary sounds remotely [21]. Utilization of reflective

resonators more promising for challenging optical in-body nanonetworks.

**4.2. Interplanetary remote sensing and space applications**

the body [16].

to realize communication networks.

186 Two-dimensional Materials for Photodetector

Multi particle tracking (MPT) or single particle tracking (SPT) with labeling by observing fluorescent molecule emission are common tools for monitoring cellular processes for in-vivo nano-biological and in-vitro microfluidic systems [3, 5]. Special fluorescing tags attached on molecules are tracked and digital image processing tools allow nanometer (nm) resolution in positioning. Image processing methods and algorithms are developed in significantly many studies for particle and cell tracking in microscopic platforms [22, 23]. However, there are important challenges for collection capability and analyzing high complexity imaging data. Tags do not have signaling capability to support a signaling-based tracking compared with traditional imaging-based tracking systems like fluorescence lifetime imaging for cell tracking.

Recently, a novel nanoscale acousto-optic radar and particle tracking system design using chirp spread spectrum (CSS) sequences with special geometrical design of acousto-optic transducers [5]. The design denoted with the name *CSSTag* promises signaling-based tagging in microfluidic environments by generating unique tags as shown in **Figure 7**. CSSTag system utilizes spread

**Figure 6.** Space application for interplanetary remote sensing where energy is harvested from sun to track acoustic vibrations of atmospheric effects in space or planetary environments.

**5. Conclusion**

microfluidics and VLC-based systems.

This work is supported by Vestel Electronics Inc., Manisa, Turkey.

\* and Gorkem Memisoglu2

Technology Products, Ozyegin University, Istanbul, Turkey

\*Address all correspondence to<EMAIL_ADDRESS>
2 Vestel Electronics Inc., Organize Sanayi Bolgesi, Manisa, Turkey

1 Department of Electrical and Electronics Engineering, Applied Research Center of

[1] Castallenos-Gomez A, Singh Van der Zant HS, Steele GA. Mechanics of freely suspended ultrathin layered materials. Annalen der Physik. 2015;**527**:27-44. DOI: 10.1002/andp.

[2] Ferrari AC et al. Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale. 2015;**7**:4598-4810. DOI: 10.1039/C4NR01600A [3] Memisoglu G, Gulbahar B. Acousto-optic tagging and identification, PCT/EP409508,

**Acknowledgements**

**Author details**

Burhan Gulbahar1

**References**

201400153

Patent Application. June 2017

In this chapter, VFRET mechanism-based nanoscale acousto-optic sensor, acousto-optic wireless communication modulator and transducer converting the vibrations of multi-layer graphene resonator to multi-color photon emission are described. Graphene resonators achieve vibration frequencies ranging from acoustic to ultrasound ranges while significant advantages of graphene such as lightweight and 2D planar structure, large Young's modulus, low residual stress and large breaking strength. The device structure combined with QDs includes unique advantages of broad absorption spectrum, large cross-sections, tunable emission spectra, size dependent emission wavelength, high photochemical stability and improved quantum yield. Hybrid nanoscale acousto-optic system design promises important applications in sensing and communications for in-body networks and WBANs, space and interplanetary applications,

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189

**Figure 7.** Multiple optical sequence generation for tagging different cells where the first two cells are tagged with the same color but different sequences while Kth cell is tagged with same sequence with the first cell but with different color.

spectrum modulation, time difference of arrival (TDOA)-based positioning and cell specific signaling sequences by exploiting the theoretical design providing nanoscale acousto-optic transduction with VRET and graphene resonators. Ultrasound carrier vibrated graphene resonators with specially placed multi-layer geometrical design to produce different optical emission sequences which are detected by a set of photodetector arrays with color filters.

#### **4.4. Visible light communications**

The capability to generate modulated optical signals in visible frequency band allows to realize nanoscale VLC transmitter at the resonance frequency of the membrane utilizing various modulation structures [24]. Multi-color emission capability and spatial diversity support generation of more advanced modulation methods such as color shift keying (CSK) as one of the important standard methods in VLC. The proposed device is embedded either into on-chip platforms for intra-chip hybrid communications or can be utilized externally in free space communication channel.

Future works promise novel modulation/demodulation methods for generating data by optimizing geometry and material selection with specific colors and donor-acceptor pair arrays.

In addition, nanophotonics is another important area where the proposed device as a VLC modulator promises novel optical devices such as in applications utilizing single photon devices, quantum communications, plasmonics and electro-optic modulator structures. A simple, nanoscale and hybrid acousto-optic VLC modulator is designed without complicated device architecture and potential with high speed modulation capability [25]. The proposed system design is a future promising VLC transmitter with significantly many application areas.

## **5. Conclusion**

In this chapter, VFRET mechanism-based nanoscale acousto-optic sensor, acousto-optic wireless communication modulator and transducer converting the vibrations of multi-layer graphene resonator to multi-color photon emission are described. Graphene resonators achieve vibration frequencies ranging from acoustic to ultrasound ranges while significant advantages of graphene such as lightweight and 2D planar structure, large Young's modulus, low residual stress and large breaking strength. The device structure combined with QDs includes unique advantages of broad absorption spectrum, large cross-sections, tunable emission spectra, size dependent emission wavelength, high photochemical stability and improved quantum yield. Hybrid nanoscale acousto-optic system design promises important applications in sensing and communications for in-body networks and WBANs, space and interplanetary applications, microfluidics and VLC-based systems.

## **Acknowledgements**

This work is supported by Vestel Electronics Inc., Manisa, Turkey.

## **Author details**

spectrum modulation, time difference of arrival (TDOA)-based positioning and cell specific signaling sequences by exploiting the theoretical design providing nanoscale acousto-optic transduction with VRET and graphene resonators. Ultrasound carrier vibrated graphene resonators with specially placed multi-layer geometrical design to produce different optical emis-

**Figure 7.** Multiple optical sequence generation for tagging different cells where the first two cells are tagged with the same color but different sequences while Kth cell is tagged with same sequence with the first cell but with different color.

The capability to generate modulated optical signals in visible frequency band allows to realize nanoscale VLC transmitter at the resonance frequency of the membrane utilizing various modulation structures [24]. Multi-color emission capability and spatial diversity support generation of more advanced modulation methods such as color shift keying (CSK) as one of the important standard methods in VLC. The proposed device is embedded either into on-chip platforms for intra-chip hybrid communications or can be utilized externally in free space

Future works promise novel modulation/demodulation methods for generating data by optimizing geometry and material selection with specific colors and donor-acceptor pair arrays. In addition, nanophotonics is another important area where the proposed device as a VLC modulator promises novel optical devices such as in applications utilizing single photon devices, quantum communications, plasmonics and electro-optic modulator structures. A simple, nanoscale and hybrid acousto-optic VLC modulator is designed without complicated device architecture and potential with high speed modulation capability [25]. The proposed system design is a future promising VLC transmitter with significantly many application

sion sequences which are detected by a set of photodetector arrays with color filters.

**4.4. Visible light communications**

188 Two-dimensional Materials for Photodetector

communication channel.

areas.

Burhan Gulbahar1 \* and Gorkem Memisoglu2

\*Address all correspondence to<EMAIL_ADDRESS>
1 Department of Electrical and Electronics Engineering, Applied Research Center of Technology Products, Ozyegin University, Istanbul, Turkey

2 Vestel Electronics Inc., Organize Sanayi Bolgesi, Manisa, Turkey

## **References**

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[24] Rajagopal S, Roberts RD, Lim SK. IEEE 802.15.7 visible light communication: Modulation schemes and dimming support. IEEE Communications Magazine. 2012;**50**(3):72-82. DOI:

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[5] Gulbahar B, Memisoglu G. CSSTag: Optical nanoscale radar and particle tracking for inbody and microfluidic systems with vibrating graphene and resonance energy transfer. IEEE Transactions on NanoBioscience. 2017;**16**:905-916. DOI: 10.1109/TNB.2017.2785226

[6] Gulbahar B, Akan OB. A communication theoretical modeling of single-layer graphene photodetectors and efficient multireceiver diversity combining. IEEE Transactions on

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**Chapter 9**

**Provisional chapter**

**Increasing Light Absorption and Collection Using**

**Increasing Light Absorption and Collection Using**

DOI: 10.5772/intechopen.73079

In recent years we have witnessed an explosion of interest in two dimensional (2D) materials, due to their unique physical properties. Excitement surrounds the promise of replacing conventional bulk photodetectors with devices based on 2D materials, allowing better integration, flexibility and potentially improving performance. However, the low inherent light absorption of 2D materials is an outstanding issue to be solved. In this chapter we review two independent approaches to tackling this problem, which have the potential to be combined to find a robust solution. The first approach involves patterning the substrate with a rod-type photonic crystal (PhC) cavity structure, which is shown to increase the light absorption into a 2D material flake coupled spatially to the cavity mode. Secondly, we review 2D–compatible solid immersion lenses (SILs) and their ability to increase both the optical magnification of the structures they encapsulate, and the longevity of the material. SILs have been shown to reduce the requirements for complex optics in the implementation of 2D materials in optoelectronic devices, and also in preserving the photodetector's optical performance over long periods of time. Finally, we show how by combining rod-type PhC cavities with SILs, we can improve the perfor-

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Two-dimensional transition metal dichalcogenides (TMDs) are a class of semiconducting materials, which can be exploited for a range of diverse applications [1]. Their ultra-thin dimensions and novel properties can provide unique advantages in fields such as energy [2, 3], computing [4–6] optoelectronics [7, 8], sensing [9, 10] and security [11, 12]. Current

**Keywords:** rod-type photonic crystals, quality factor, solid immersion lenses, epoxy

**Engineered Structures**

**Engineered Structures**

http://dx.doi.org/10.5772/intechopen.73079

Robert J. Young

**Abstract**

**1. Introduction**

Robert J. Young

Yasir J. Noori, Christopher S. Woodhead and

Yasir J. Noori, Christopher S. Woodhead and

Additional information is available at the end of the chapter

mance of 2D material-based photodetectors.

Additional information is available at the end of the chapter

**Provisional chapter**

## **Increasing Light Absorption and Collection Using Engineered Structures Engineered Structures**

**Increasing Light Absorption and Collection Using**

DOI: 10.5772/intechopen.73079

Yasir J. Noori, Christopher S. Woodhead and Robert J. Young Robert J. Young Additional information is available at the end of the chapter

Yasir J. Noori, Christopher S. Woodhead and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73079

#### **Abstract**

In recent years we have witnessed an explosion of interest in two dimensional (2D) materials, due to their unique physical properties. Excitement surrounds the promise of replacing conventional bulk photodetectors with devices based on 2D materials, allowing better integration, flexibility and potentially improving performance. However, the low inherent light absorption of 2D materials is an outstanding issue to be solved. In this chapter we review two independent approaches to tackling this problem, which have the potential to be combined to find a robust solution. The first approach involves patterning the substrate with a rod-type photonic crystal (PhC) cavity structure, which is shown to increase the light absorption into a 2D material flake coupled spatially to the cavity mode. Secondly, we review 2D–compatible solid immersion lenses (SILs) and their ability to increase both the optical magnification of the structures they encapsulate, and the longevity of the material. SILs have been shown to reduce the requirements for complex optics in the implementation of 2D materials in optoelectronic devices, and also in preserving the photodetector's optical performance over long periods of time. Finally, we show how by combining rod-type PhC cavities with SILs, we can improve the performance of 2D material-based photodetectors.

**Keywords:** rod-type photonic crystals, quality factor, solid immersion lenses, epoxy

### **1. Introduction**

Two-dimensional transition metal dichalcogenides (TMDs) are a class of semiconducting materials, which can be exploited for a range of diverse applications [1]. Their ultra-thin dimensions and novel properties can provide unique advantages in fields such as energy [2, 3], computing [4–6] optoelectronics [7, 8], sensing [9, 10] and security [11, 12]. Current

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

bottlenecks in commercialising devices formed from TMDs are their relatively poor quantum yield, low optical absorption and high rate of degradation in ambient conditions. The quantum yield of TMDs, namely the number of excitons generated divided by the number of photons impinged on the device, is typically between 0.01 and 6% [13], which is exceptionally low in comparison to other emitters such as III-V based structures that can have quantum yields reaching close to 100% [14, 15].

greater than 60% [32]. Combining graphene with hole-type PhC cavities, has been shown to increase light reflectivity from graphene by 4.0x [33]. A photodetector can be realized by fabricating metal contacts above sheets, through which a circuit current can be amplified and

Increasing Light Absorption and Collection Using Engineered Structures

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195

An alternative method for enhancing monolayer light absorption using PhCs was proposed by Noori et al. in [31]. In this method, a rod-type PhC is used to achieve an increased light to monolayer coupling. This PhC can be combined with a high refractive index solid immersion lens (SIL) [34, 35], which is placed directly above the cavity. This has two effects: the first is to act as a reflective interface to redirect any leaking light back into the cavity, increasing the chance of light absorption by the monolayer. The second is to increase the coupling of light into/out of the cavity, increasing the probability that an applied photon gets into the cavity, which SILs are considered especially effective at [36, 37]. SILs are commonly made out of glass [36], which can pose a problem when integrating into delicate structures such as PhCs and 2D materials. Recent work forming SILs out of photopolymers, has shown progress in solving

For the rest of this chapter, we will focus on the concept of a rod-SIL photodetector. We will review the rod-based PhC structure and compare it to the more common method of coupling monolayers to uncapped hole-type PhCs. We will then look at photopolymer based solid immersion lenses, and their effectiveness at both coupling light into/out of a material, as well as protecting it from the ambient environment. Finally, we discuss how the combination of

In 1987, Yablonovitch proposed the theoretical concept of using periodic optical structures to create photonic bandgap systems, now popularly known as photonic crystals [41]. A PhC structure comprises of a series of periodic changes in refractive index that creates a range of disallowed states for a photon, effectively forming a photonic bandgap. This effect is very similar to electronic bandgaps experienced by electrons in an atomic lattice, thus PhCs are

Experimental realization of manmade PhCs occurred a decade later [42, 43]. Ever since that time, PhCs became a potential platform for making integrated photonic components such as cavities, waveguides, mirrors and wavelength/polarization multiplexers [44, 45]. This allows the realization of technologies such as single photon sources, lasers, filters, interferometers,

Coupling 2D materials to photonic cavities can lead to substantial improvements in the material's absorption efficiency, opening the doors for their use in the development of ultrathin but highly efficient detectors. TMD monolayers have also been coupled to hole-type PhC cavities, resulting in lasing and enhancement in the spontaneous emission rate for light emitted

the two structures can provide an efficient optical package for use in photodetection.

**2. Enhancing light absorption using photonic crystal cavities**

**2.1. Coupling 2D materials to photonic crystals**

sometimes describes as 'optical lattices'.

modulators and slow light waveguides.

measured when the device is illuminated.

this issue [38–40].

TMD monolayers such as MoS2 have excellent absorption characteristics [16], and high photoresponsivity [9] relative to their dimensions. However, their extreme thinness of just 0.65 nm means that the absolute absorption is usually relatively low (typically less than 6% at 450 nm) [17] severely limiting the efficiency of TMDs in real implementations such as detectors and solar cells. This value can be increased by using multiple monolayers, [17] or by careful preparation of the incident light [18]; however such methods may not be practical for specific optoelectronic applications.

Multiple existing approaches have been developed to circumvent these limitations. One method involves the use of superacid treatment to obtain near a 100% photoluminescence quantum yield from these materials [13, 19]. Currently, reports of superacid treatment have all involved using Bis(trifluoromethane) sulfonimide as the superacid material for treating MoS2 . The photoluminescence intensity of MoS2 monolayers have been shown to increase up to 190-fold after treatment [13]. While the explanation of the enhancement in light emission as a result of superacid treatment is still not very clear, recent studies demonstrated that enhancement could be caused by the transformation of trions into neutral excitons combined with a reduction in the density of mid-gap trap states for CVD grown monolayers [20].

Improved monolayer absorption can be achieved by coupling TMD monolayers to an engineered structure that confines plasmonic modes such as gold nanostructures [21, 22], nanoparticles [23] and nanorods [24, 25]. This phenomenon stems from the fact that plasmonic resonance couples to both excitation and emission fields, hence boosting the light-matter interaction at the nanoscale. For example, Johnson et al. have observed a giant enhancement in the luminescence intensity of tungsten diselenide (WSe2 ) by coupling it to plasmonic structures, resulting in an increased light absorption [26]. They report achieving an enhancement factor of up to 200 using silver nanotriangle arrays coupled to the monolayers. Sobhani et al. in [23] have observed that by tuning plasmonic core-shell nanoparticles to the direct bandgap of monolayer MoS2 and depositing them sparsely onto the monolayer's surface, the photocurrent achieved through the monolayer increases 3-fold, hence promising a model for more sensitive TMD based photodetectors. Other structures such as nanocubes [27] and bowtie antennae [28, 29] have also been utilized for enhancing light emission, resulting in up to 2000 fold increase in material's absorption.

Absorption of TMD monolayers can also be increased when they are coupled to photonic crystal (PhC) structures such as cavities, which can greatly aid in improving the efficiency of photodetectors [30, 31]. Like plasmonic structures, the strong localization of electromagnetic radiation in photonic cavities increases the light-matter interaction, leading to higher absorption efficiency. For example, by monolithically integrating graphene with a Fabry-Perot microcavity, the optical absorption can be enhanced by 26-fold, reaching absorption values greater than 60% [32]. Combining graphene with hole-type PhC cavities, has been shown to increase light reflectivity from graphene by 4.0x [33]. A photodetector can be realized by fabricating metal contacts above sheets, through which a circuit current can be amplified and measured when the device is illuminated.

An alternative method for enhancing monolayer light absorption using PhCs was proposed by Noori et al. in [31]. In this method, a rod-type PhC is used to achieve an increased light to monolayer coupling. This PhC can be combined with a high refractive index solid immersion lens (SIL) [34, 35], which is placed directly above the cavity. This has two effects: the first is to act as a reflective interface to redirect any leaking light back into the cavity, increasing the chance of light absorption by the monolayer. The second is to increase the coupling of light into/out of the cavity, increasing the probability that an applied photon gets into the cavity, which SILs are considered especially effective at [36, 37]. SILs are commonly made out of glass [36], which can pose a problem when integrating into delicate structures such as PhCs and 2D materials. Recent work forming SILs out of photopolymers, has shown progress in solving this issue [38–40].

For the rest of this chapter, we will focus on the concept of a rod-SIL photodetector. We will review the rod-based PhC structure and compare it to the more common method of coupling monolayers to uncapped hole-type PhCs. We will then look at photopolymer based solid immersion lenses, and their effectiveness at both coupling light into/out of a material, as well as protecting it from the ambient environment. Finally, we discuss how the combination of the two structures can provide an efficient optical package for use in photodetection.

## **2. Enhancing light absorption using photonic crystal cavities**

### **2.1. Coupling 2D materials to photonic crystals**

bottlenecks in commercialising devices formed from TMDs are their relatively poor quantum yield, low optical absorption and high rate of degradation in ambient conditions. The quantum yield of TMDs, namely the number of excitons generated divided by the number of photons impinged on the device, is typically between 0.01 and 6% [13], which is exceptionally low in comparison to other emitters such as III-V based structures that can have quantum yields

responsivity [9] relative to their dimensions. However, their extreme thinness of just 0.65 nm means that the absolute absorption is usually relatively low (typically less than 6% at 450 nm) [17] severely limiting the efficiency of TMDs in real implementations such as detectors and solar cells. This value can be increased by using multiple monolayers, [17] or by careful preparation of the incident light [18]; however such methods may not be practical for specific opto-

Multiple existing approaches have been developed to circumvent these limitations. One method involves the use of superacid treatment to obtain near a 100% photoluminescence quantum yield from these materials [13, 19]. Currently, reports of superacid treatment have all involved using Bis(trifluoromethane) sulfonimide as the superacid material for treating

up to 190-fold after treatment [13]. While the explanation of the enhancement in light emission as a result of superacid treatment is still not very clear, recent studies demonstrated that enhancement could be caused by the transformation of trions into neutral excitons combined with a reduction in the density of mid-gap trap states for CVD grown monolayers [20].

Improved monolayer absorption can be achieved by coupling TMD monolayers to an engineered structure that confines plasmonic modes such as gold nanostructures [21, 22], nanoparticles [23] and nanorods [24, 25]. This phenomenon stems from the fact that plasmonic resonance couples to both excitation and emission fields, hence boosting the light-matter interaction at the nanoscale. For example, Johnson et al. have observed a giant enhancement

tures, resulting in an increased light absorption [26]. They report achieving an enhancement factor of up to 200 using silver nanotriangle arrays coupled to the monolayers. Sobhani et al. in [23] have observed that by tuning plasmonic core-shell nanoparticles to the direct bandgap

rent achieved through the monolayer increases 3-fold, hence promising a model for more sensitive TMD based photodetectors. Other structures such as nanocubes [27] and bowtie antennae [28, 29] have also been utilized for enhancing light emission, resulting in up to 2000

Absorption of TMD monolayers can also be increased when they are coupled to photonic crystal (PhC) structures such as cavities, which can greatly aid in improving the efficiency of photodetectors [30, 31]. Like plasmonic structures, the strong localization of electromagnetic radiation in photonic cavities increases the light-matter interaction, leading to higher absorption efficiency. For example, by monolithically integrating graphene with a Fabry-Perot microcavity, the optical absorption can be enhanced by 26-fold, reaching absorption values

and depositing them sparsely onto the monolayer's surface, the photocur-

have excellent absorption characteristics [16], and high photo-

monolayers have been shown to increase

) by coupling it to plasmonic struc-

reaching close to 100% [14, 15].

194 Two-dimensional Materials for Photodetector

TMD monolayers such as MoS2

. The photoluminescence intensity of MoS2

in the luminescence intensity of tungsten diselenide (WSe2

electronic applications.

of monolayer MoS2

fold increase in material's absorption.

MoS2

In 1987, Yablonovitch proposed the theoretical concept of using periodic optical structures to create photonic bandgap systems, now popularly known as photonic crystals [41]. A PhC structure comprises of a series of periodic changes in refractive index that creates a range of disallowed states for a photon, effectively forming a photonic bandgap. This effect is very similar to electronic bandgaps experienced by electrons in an atomic lattice, thus PhCs are sometimes describes as 'optical lattices'.

Experimental realization of manmade PhCs occurred a decade later [42, 43]. Ever since that time, PhCs became a potential platform for making integrated photonic components such as cavities, waveguides, mirrors and wavelength/polarization multiplexers [44, 45]. This allows the realization of technologies such as single photon sources, lasers, filters, interferometers, modulators and slow light waveguides.

Coupling 2D materials to photonic cavities can lead to substantial improvements in the material's absorption efficiency, opening the doors for their use in the development of ultrathin but highly efficient detectors. TMD monolayers have also been coupled to hole-type PhC cavities, resulting in lasing and enhancement in the spontaneous emission rate for light emitted from these 2D materials [46–50]. Hole-type PhC cavities can have high Q-factors due to their reduced mode losses in the vertical direction due to the refractive index contrast at the bridgeair interface. However, the hole-type PhCs have distinct disadvantages when being used with non-embedded emitters such as 2D materials, because the cavity mode is confined within the bridge structure; a region where the TMD monolayer cannot realistically be placed. This causes reduced coupling between the photo-absorber monolayer and the cavity mode's maximum, leading to a reduction in light-matter interactions. Secondly, the large dielectric-to-air volume ratio that exists in the structures of hole-type PhCs results in undesirable light absorption by the dielectric material. This becomes critical when a high absorption material, such as GaAs, is used with an operating wavelength that lies in the visible regime. However, a lot of these issues can be solved by changing the PhC structure from a hole-type to a rod-type [31].

topological minimum spatially matches the cavity mode's antinode. Optimized alignment in both position and energy optimizes absorption by the 2D material. The PhC hexagonal lattice

this work because of its large neutral and charged exciton binding energies, making it a great candidate for room temperature photodetection. Nevertheless, the PhC design is universal, as the normalized parameters may be scaled and adjusted to match any light absorptive TMD. It consists of rods with a lattice constant, *a* = 1, and a radius, *r* = 0.165*a*, where the rods are made

2D PWE simulations of the PhC structure (**Figure 2a**), have shown that a bandgap exists in the frequency range 1.05*a* < *λ* < 1.17*a*. Once the photonic band diagram was simulated, a H1 cavity was formed in the centre of the array. Subsequently, FDTD is commonly used to test the cavity's performance. 2D simulations can confirm the presence of the cavity mode and used to analyze its central wavelength. Performing 2D simulations at this point can usually save a lot of computation time and power that may be spent with exhaustive 3D simulations. Q-factor measurements from 2D simulations typically show astronomical values since leakage in the

3D FDTD simulations of the cavity mode are shown in **Figure 2b** and **c**. Initially, an Ex

rised source was placed at the center of the cavity, with a central wavelength λ of 1.1*a* and a full-width at half maximum (FWHM) of *d*λ = 0.1*a*. Using Meep, a visual image of the light propagation can be produced. After allowing enough time to pass following initialization of the dipole, to allow edge states and propagating modes to leak, the cavity standing mode's field was observed on its own, as shown in **Figure 3a**. The cavity is later simulated with an Ey

**Figure 1.** Cross-sectional illustration of a silicon rod PhC cavity with a monolayer transferred on top. Declining of the

monolayer within the cavity region can increase coupling between excitons and the cavity modes.

is the material considered here for

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Increasing Light Absorption and Collection Using Engineered Structures

pola-

197

is surrounded by air for optimum index contrast. MoS2

by etching the silicon substrate.

vertical direction is not included.

**2.3. Cavity performance**

#### **2.2. Photonic crystal cavity design**

PhC structures are commonly formed from a hexagonal array structure, rather than a square array because, they are easier to fabricate in practice [51]. For rod-type cavities, square lattice arrays exhibit a transverse magnetic (TM) bandgap with Q-factors that may exceed 1000. However, confining light in the visible wavelength range using the square lattice PhC requires the structure to have feature sizes that may be difficult to fabricate using conventional lithography techniques. For example, assume a square lattice PhC is designed for the optical bandgap of monolayer molybdenum disulfide (MoS2 ), i.e. approximately 660 nm [52]; Villeneuve et al. in [51] showed that for a rod-type PhC lattice with material index *n* = 3.4, *a* = 1 and *r* = 0.2*a*, a H1 cavity would have its cavity mode at *λ* = 2.56*a*. In other words, a PhC cavity with a mode at *λ* = 660 *nm*, requires its lattice constant to be, *a* = 258 *nm* and the radius to be, *r* = 52 *nm*, with rod heights of at least 660 nm. Designing rods with such dimensions is challenging and requires extreme controllability of the fabrication process. Furthermore, small fabrication uncertainties that change the rods' radius, the lattice constant, sidewall roughness and/or their vertical height can shift the cavity mode and change its Q-factor. An H1 hexagonal lattice PhC cavity of similar dimensions has its cavity mode at a much smaller wavelength. Hence, a cavity designed to have a mode wavelength at 660 nm will have larger dimensions, making it less dependent on fabrication limitations. The cavity that will be discussed in this chapter can achieve this goal using *a* = 595 *nm* and *r* = 95 *nm*, which are easier to achieve in practice.

The PhC studied here consists of a hexagonal lattice of rods surrounded by air for, optimum index contrast. Since MoS2 has an absorption wavelength at 660 nm, due to its direct bandgap, rods made from common substrates such as silicon or GaAs can have a refractive index that exceeds 3.5 at this wavelength. To make the photonic cavity, a rod is omitted from the PhC lattice allowing a photonic state to be created within the lattice's photonic bandgap. The confined wavelength of the cavity is also known as the cavity mode. In previous work by Kay et al. [53], 2D materials have been shown to dip when transferred onto hollowed regions such as etched trenches on a substrate. Through exploiting the flexibility of TMD monolayers, spatial coupling between the cavity and the material is possible by suspending the monolayer over the rods; **Figure 1** illustrates this concept. Having a sufficient lattice separation, the cavity region, can allow the suspended flake to sag within the cavity such that the flake's topological minimum spatially matches the cavity mode's antinode. Optimized alignment in both position and energy optimizes absorption by the 2D material. The PhC hexagonal lattice is surrounded by air for optimum index contrast. MoS2 is the material considered here for this work because of its large neutral and charged exciton binding energies, making it a great candidate for room temperature photodetection. Nevertheless, the PhC design is universal, as the normalized parameters may be scaled and adjusted to match any light absorptive TMD. It consists of rods with a lattice constant, *a* = 1, and a radius, *r* = 0.165*a*, where the rods are made by etching the silicon substrate.

#### **2.3. Cavity performance**

from these 2D materials [46–50]. Hole-type PhC cavities can have high Q-factors due to their reduced mode losses in the vertical direction due to the refractive index contrast at the bridgeair interface. However, the hole-type PhCs have distinct disadvantages when being used with non-embedded emitters such as 2D materials, because the cavity mode is confined within the bridge structure; a region where the TMD monolayer cannot realistically be placed. This causes reduced coupling between the photo-absorber monolayer and the cavity mode's maximum, leading to a reduction in light-matter interactions. Secondly, the large dielectric-to-air volume ratio that exists in the structures of hole-type PhCs results in undesirable light absorption by the dielectric material. This becomes critical when a high absorption material, such as GaAs, is used with an operating wavelength that lies in the visible regime. However, a lot of these issues can be solved by changing the PhC structure from a hole-type to a rod-type [31].

PhC structures are commonly formed from a hexagonal array structure, rather than a square array because, they are easier to fabricate in practice [51]. For rod-type cavities, square lattice arrays exhibit a transverse magnetic (TM) bandgap with Q-factors that may exceed 1000. However, confining light in the visible wavelength range using the square lattice PhC requires the structure to have feature sizes that may be difficult to fabricate using conventional lithography techniques. For example, assume a square lattice PhC is designed for the optical band-

et al. in [51] showed that for a rod-type PhC lattice with material index *n* = 3.4, *a* = 1 and *r* = 0.2*a*, a H1 cavity would have its cavity mode at *λ* = 2.56*a*. In other words, a PhC cavity with a mode at *λ* = 660 *nm*, requires its lattice constant to be, *a* = 258 *nm* and the radius to be, *r* = 52 *nm*, with rod heights of at least 660 nm. Designing rods with such dimensions is challenging and requires extreme controllability of the fabrication process. Furthermore, small fabrication uncertainties that change the rods' radius, the lattice constant, sidewall roughness and/or their vertical height can shift the cavity mode and change its Q-factor. An H1 hexagonal lattice PhC cavity of similar dimensions has its cavity mode at a much smaller wavelength. Hence, a cavity designed to have a mode wavelength at 660 nm will have larger dimensions, making it less dependent on fabrication limitations. The cavity that will be discussed in this chapter can

achieve this goal using *a* = 595 *nm* and *r* = 95 *nm*, which are easier to achieve in practice.

The PhC studied here consists of a hexagonal lattice of rods surrounded by air for, optimum

gap, rods made from common substrates such as silicon or GaAs can have a refractive index that exceeds 3.5 at this wavelength. To make the photonic cavity, a rod is omitted from the PhC lattice allowing a photonic state to be created within the lattice's photonic bandgap. The confined wavelength of the cavity is also known as the cavity mode. In previous work by Kay et al. [53], 2D materials have been shown to dip when transferred onto hollowed regions such as etched trenches on a substrate. Through exploiting the flexibility of TMD monolayers, spatial coupling between the cavity and the material is possible by suspending the monolayer over the rods; **Figure 1** illustrates this concept. Having a sufficient lattice separation, the cavity region, can allow the suspended flake to sag within the cavity such that the flake's

has an absorption wavelength at 660 nm, due to its direct band-

), i.e. approximately 660 nm [52]; Villeneuve

**2.2. Photonic crystal cavity design**

196 Two-dimensional Materials for Photodetector

index contrast. Since MoS2

gap of monolayer molybdenum disulfide (MoS2

2D PWE simulations of the PhC structure (**Figure 2a**), have shown that a bandgap exists in the frequency range 1.05*a* < *λ* < 1.17*a*. Once the photonic band diagram was simulated, a H1 cavity was formed in the centre of the array. Subsequently, FDTD is commonly used to test the cavity's performance. 2D simulations can confirm the presence of the cavity mode and used to analyze its central wavelength. Performing 2D simulations at this point can usually save a lot of computation time and power that may be spent with exhaustive 3D simulations. Q-factor measurements from 2D simulations typically show astronomical values since leakage in the vertical direction is not included.

3D FDTD simulations of the cavity mode are shown in **Figure 2b** and **c**. Initially, an Ex polarised source was placed at the center of the cavity, with a central wavelength λ of 1.1*a* and a full-width at half maximum (FWHM) of *d*λ = 0.1*a*. Using Meep, a visual image of the light propagation can be produced. After allowing enough time to pass following initialization of the dipole, to allow edge states and propagating modes to leak, the cavity standing mode's field was observed on its own, as shown in **Figure 3a**. The cavity is later simulated with an Ey

**Figure 1.** Cross-sectional illustration of a silicon rod PhC cavity with a monolayer transferred on top. Declining of the monolayer within the cavity region can increase coupling between excitons and the cavity modes.

*αc* = 1 − (*αm*)

a monolayer that is not coupled to a cavity and *Qc*

away from the cavity's centre will be discussed.

tion for optimized photodetection.

where *αc*

*Qc* (1)

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199

Increasing Light Absorption and Collection Using Engineered Structures

is the cavity's Q-factor. Note that in this

is the absorption of a monolayer that is coupled to a cavity, *αm* is the absorption of

approximation, absorption due to the dielectric host material (in this case considered silicon) was not taken into account. It is expected that absorption due to the material can reduce the cavity's Q, however this becomes less critical for cavities designed to have low-mid Q-factors such as the ones shown here where light is allowed to leak into the cavity to enhance absorp-

A series of simulations were carried out with a source inside a PhC cavity containing rods with different radii. The aim is primarily to investigate the robustness of this design to fabrication imperfections that are likely to occur due to the relatively small structure. Causes of shifts in the radius of the fabricated rods could be due to the inaccurate selection of exposure dosage in the electron-beam (e-beam) writing process. Other reasons could be due to non-anisotropic sidewall profile development of the exposed regions of the resist, due to temperature variations of the developing solution and/or the samples themselves. Hence, the radius was varied between 0.155*a* and 0.170*a*. **Figure 2b** and **c** show screenshots taken directly from Lumerical's user interface for the simulation cell. The mesh resolution used in this simulation is 20 elements per one lattice constant, *a*, in every dimension. Corresponding to approximately one element per 30 nm in real units. To measure the cavity mode, a flux region was setup above the cavity, collecting light from the cavity mode radiated vertically upward. A maximum Q-factor of 341 was achieved for a PhC cavity having rods with a radius, *r* = 0.161*a*, where the cavity mode's central wavelength is *λ* = 1.104*a*, as shown in **Figure 4**. As the radius deviates from *r* = 0.161*a*, the collected power flux starts to decrease, which results from a decrease in the Q-factor, recording a minimum of 155 at *λ* = 1.138*a* for *r* = 0.170*a*. It is clear from **Figure 4** to note that reducing the radius of the rods tends to blue-shift the cavity mode's energy, hence reducing its wavelength. This is expected, as the relationship between the PhC's cell radius (hole-type or rod-type) and the cavity mode's wavelength was anticipated through previous studies [54]. **Figure 4** plots a comparison between the obtained fluxes for two cases. The first case is when a flake is exfoliated directly onto a flat substrate. The second case is when the flake is transferred on top of the PhC structure, dipping to half the height of the rods where the cavity's field maximum exists for rods with *r* = 0.161*a*. Later, the effect of having the source

Throughout the scope of this work, the simulations were made with the assumption that the flake is dipped to about half the height of the rods. This is expected to be the ideal case since the flake will be coupled to the field's antinode, creating maximal spatial coupling to the cavity. Performing simulations to anticipate the amount of dipping of a monolayer over a typical structure depends on many factors. These include the monolayer material, the rods' surface roughness, temperature, etc. These typically require setting up finite element simulations while taking into account the van der Waals, Coulombic and gravitational forces, which is very exhaustive. Hence, a series of simulations were run to investigate the effect of dipping of the monolayer emitter on the monolayer-cavity coupling. These simulations are done

**Figure 2.** (a) An x-y cross-section of the simulated PhC cavity structure. x-y (b) and y-z (c) screenshots of the simulated structure from Lumerical FDTD package showing the PM layers and flux region.

**Figure 3.** 3D FDTD simulation of the photonic cavity mode showing time slice images of the confined Ex (a) and Ey (b) components. (c) Cross section of the Ey field within the microcavity. Red (blue) represents positive (negative) components of the electric field.

polarised dipole, which showed an electric field distribution for the cavity mode, as shown in **Figure 3b**. Using the harmonic inversion (harminv) tool embedded in Meep, the cavity mode was found to have a wavelength λ = 1.12*a* with a Q-factor of 300. On the other hand, **Figure 3c** shows the field distribution for a cross-sectional slice along the x-direction through the cavity. Unsurprisingly, the PhC rod's height also influences the mode's confinement. If the rods are too short, the cavity mode's shape can extend into the air above the PhC. This results in a reduction in the spatial interface between the mode and the rods, lowering the light collection ratio. Conversely, if the rods are too long, higher order and propagating modes can form within the cavity structure. Furthermore, etching high aspect ratio rods will always require complex dry etching techniques to achieve the desired degree of anisotropy. It has been revealed that maximum gap size for a square-lattice rod-type PhC is achieved for rod heights of approximately 2.3*a*, corresponding to approximately two cavity modes wavelengths [51].

The Q-factor of a cavity can be defined as the ratio of the energy stored in the resonating cavity to the energy dissipated per cycle. For a cavity with a Q-factor of 300, such as the rod-type PhC cavity discussed here, a resonating photonic mode inside a cavity is expected to oscillate 300 times before it leaks half of its energy outside the cavity. For a MoS2 monolayer with an absorption coefficient of 0.05, coupled to a rod-type PhC cavity the absorption of the monolayer can be enhanced to almost unity [52]. This can be calculated using the following equation:

$$a\_{\iota} = 1 - \langle a\_{\iota} \rangle^{Q} \tag{1}$$

where *αc* is the absorption of a monolayer that is coupled to a cavity, *αm* is the absorption of a monolayer that is not coupled to a cavity and *Qc* is the cavity's Q-factor. Note that in this approximation, absorption due to the dielectric host material (in this case considered silicon) was not taken into account. It is expected that absorption due to the material can reduce the cavity's Q, however this becomes less critical for cavities designed to have low-mid Q-factors such as the ones shown here where light is allowed to leak into the cavity to enhance absorption for optimized photodetection.

A series of simulations were carried out with a source inside a PhC cavity containing rods with different radii. The aim is primarily to investigate the robustness of this design to fabrication imperfections that are likely to occur due to the relatively small structure. Causes of shifts in the radius of the fabricated rods could be due to the inaccurate selection of exposure dosage in the electron-beam (e-beam) writing process. Other reasons could be due to non-anisotropic sidewall profile development of the exposed regions of the resist, due to temperature variations of the developing solution and/or the samples themselves. Hence, the radius was varied between 0.155*a* and 0.170*a*. **Figure 2b** and **c** show screenshots taken directly from Lumerical's user interface for the simulation cell. The mesh resolution used in this simulation is 20 elements per one lattice constant, *a*, in every dimension. Corresponding to approximately one element per 30 nm in real units. To measure the cavity mode, a flux region was setup above the cavity, collecting light from the cavity mode radiated vertically upward. A maximum Q-factor of 341 was achieved for a PhC cavity having rods with a radius, *r* = 0.161*a*, where the cavity mode's central wavelength is *λ* = 1.104*a*, as shown in **Figure 4**. As the radius deviates from *r* = 0.161*a*, the collected power flux starts to decrease, which results from a decrease in the Q-factor, recording a minimum of 155 at *λ* = 1.138*a* for *r* = 0.170*a*. It is clear from **Figure 4** to note that reducing the radius of the rods tends to blue-shift the cavity mode's energy, hence reducing its wavelength. This is expected, as the relationship between the PhC's cell radius (hole-type or rod-type) and the cavity mode's wavelength was anticipated through previous studies [54]. **Figure 4** plots a comparison between the obtained fluxes for two cases. The first case is when a flake is exfoliated directly onto a flat substrate. The second case is when the flake is transferred on top of the PhC structure, dipping to half the height of the rods where the cavity's field maximum exists for rods with *r* = 0.161*a*. Later, the effect of having the source away from the cavity's centre will be discussed.

polarised dipole, which showed an electric field distribution for the cavity mode, as shown in **Figure 3b**. Using the harmonic inversion (harminv) tool embedded in Meep, the cavity mode was found to have a wavelength λ = 1.12*a* with a Q-factor of 300. On the other hand, **Figure 3c** shows the field distribution for a cross-sectional slice along the x-direction through the cavity. Unsurprisingly, the PhC rod's height also influences the mode's confinement. If the rods are too short, the cavity mode's shape can extend into the air above the PhC. This results in a reduction in the spatial interface between the mode and the rods, lowering the light collection ratio. Conversely, if the rods are too long, higher order and propagating modes can form within the cavity structure. Furthermore, etching high aspect ratio rods will always require complex dry etching techniques to achieve the desired degree of anisotropy. It has been revealed that maximum gap size for a square-lattice rod-type PhC is achieved for rod heights of approximately 2.3*a*, corresponding to approximately two cavity modes wavelengths [51].

field within the microcavity. Red (blue) represents positive (negative) components

**Figure 3.** 3D FDTD simulation of the photonic cavity mode showing time slice images of the confined Ex

**Figure 2.** (a) An x-y cross-section of the simulated PhC cavity structure. x-y (b) and y-z (c) screenshots of the simulated

structure from Lumerical FDTD package showing the PM layers and flux region.

components. (c) Cross section of the Ey

198 Two-dimensional Materials for Photodetector

of the electric field.

The Q-factor of a cavity can be defined as the ratio of the energy stored in the resonating cavity to the energy dissipated per cycle. For a cavity with a Q-factor of 300, such as the rod-type PhC cavity discussed here, a resonating photonic mode inside a cavity is expected to oscillate 300

tion coefficient of 0.05, coupled to a rod-type PhC cavity the absorption of the monolayer can

be enhanced to almost unity [52]. This can be calculated using the following equation:

monolayer with an absorp-

(a) and Ey

(b)

times before it leaks half of its energy outside the cavity. For a MoS2

Throughout the scope of this work, the simulations were made with the assumption that the flake is dipped to about half the height of the rods. This is expected to be the ideal case since the flake will be coupled to the field's antinode, creating maximal spatial coupling to the cavity. Performing simulations to anticipate the amount of dipping of a monolayer over a typical structure depends on many factors. These include the monolayer material, the rods' surface roughness, temperature, etc. These typically require setting up finite element simulations while taking into account the van der Waals, Coulombic and gravitational forces, which is very exhaustive. Hence, a series of simulations were run to investigate the effect of dipping of the monolayer emitter on the monolayer-cavity coupling. These simulations are done

Compared to H1 cavities, L3 cavities are usually favored for better coupling to PhC waveguides and tend to have lower confinement quality factors of roughly 150. In L3 cavities, the Q-factor is expected to reduce due to increased coupling of the cavity mode to radiative modes. Having an engineered structure that can improve absorption of monolayers is essential, in the next section we will describe how a different structure such as a micro-lens can help increase

Increasing Light Absorption and Collection Using Engineered Structures

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201

A solid immersion lens (SIL) is an optical element with a high refractive index that can be placed on the surface of a semiconductor to increase the optical extraction efficiency of a surface/subsurface emitter. SILs are typically formed from high index glass [55] and are placed directly on the surface of an optical structure to increase the light being coupled into and/ or out from it. SILs can also be formed from photopolymers such as UV-curable epoxy [40], which gives the SIL unique properties such as easy tunability and mounting. The rest of this

Two different geometries of SIL have been predominantly studied; those with a hemispherical shape (h-SILs) and those with a Weierstraß shape (s-SILs) [55], as shown **Figure 6**. S-SILs have a higher magnification than h-SILs, scaling as the refractive index of the SIL squared as opposed to a direct linear relationship for a h-SIL. In subsurface emitters such as quantum dots (QD's), s-SILs have a higher input/output coupling efficiency over h-SILs, due to them being able to refract a higher number of rays at the SIL-air boundary, thus collecting/delivering a greater solid angle of light (*s*) to and from a device. In the case of a surface emitter such as a 2D material this SIL-air boundary refraction in the s-SIL is essential to increase the input/output coupling, as there is no contribution to light coupling from the substrate-SIL

**Figure 6.** Diagram showing the difference in shape between a s-SIL and h-SIL on a sample with embedded emitters (quantum dots) in the above case (yellow features). The dashed lines highlight how a larger angle of emitted light is

the light collection efficiency to these monolayers/cavity systems.

section will focus on these types of SILs.

collected in the s-SIL relative to the h-SIL.

boundary.

**3. Enhancing light collection using photopolymer lenses**

**Figure 4.** The effect of changing the rods' radii r from 0.155a to 0.170a on the cavity mode, showing a maximum Q for r = 0.161a and an increase in the cavity's central wavelength when r is increased, and vice versa.

**Figure 5.** Comparison of the flux spectra for emission from the cavity as the dipping of the material is varied along the z-axis. The drawing on the right illustrates the degrees of bowing of the 2D material from the surface of the structure into the cavity for each of the cases shown on the left.

by placing the dipole at different heights along the vertical direction inside the cavity. Even though the source position in the vertical direction was changed along the height of the cavity, it is important to mention that the flux region height above the photonic structure was not changed. The cavity spectra for the different heights are shown in **Figure 5**. It is expected that maximum absorption of the monolayer should be achieved when the source is spatially aligned with the cavity mode's electric field maximum. This exists at the center of the cavity, one wavelength above the substrate, as was previously shown in **Figure 3c**. The intensity of the cavity mode was found to decrease as the source is moved toward the top of the rods due to reduced spatial coupling between the dipole source and the cavity mode's antinode, which exists in the center of the cavity.

Compared to H1 cavities, L3 cavities are usually favored for better coupling to PhC waveguides and tend to have lower confinement quality factors of roughly 150. In L3 cavities, the Q-factor is expected to reduce due to increased coupling of the cavity mode to radiative modes.

Having an engineered structure that can improve absorption of monolayers is essential, in the next section we will describe how a different structure such as a micro-lens can help increase the light collection efficiency to these monolayers/cavity systems.

## **3. Enhancing light collection using photopolymer lenses**

A solid immersion lens (SIL) is an optical element with a high refractive index that can be placed on the surface of a semiconductor to increase the optical extraction efficiency of a surface/subsurface emitter. SILs are typically formed from high index glass [55] and are placed directly on the surface of an optical structure to increase the light being coupled into and/ or out from it. SILs can also be formed from photopolymers such as UV-curable epoxy [40], which gives the SIL unique properties such as easy tunability and mounting. The rest of this section will focus on these types of SILs.

Two different geometries of SIL have been predominantly studied; those with a hemispherical shape (h-SILs) and those with a Weierstraß shape (s-SILs) [55], as shown **Figure 6**. S-SILs have a higher magnification than h-SILs, scaling as the refractive index of the SIL squared as opposed to a direct linear relationship for a h-SIL. In subsurface emitters such as quantum dots (QD's), s-SILs have a higher input/output coupling efficiency over h-SILs, due to them being able to refract a higher number of rays at the SIL-air boundary, thus collecting/delivering a greater solid angle of light (*s*) to and from a device. In the case of a surface emitter such as a 2D material this SIL-air boundary refraction in the s-SIL is essential to increase the input/output coupling, as there is no contribution to light coupling from the substrate-SIL boundary.

by placing the dipole at different heights along the vertical direction inside the cavity. Even though the source position in the vertical direction was changed along the height of the cavity, it is important to mention that the flux region height above the photonic structure was not changed. The cavity spectra for the different heights are shown in **Figure 5**. It is expected that maximum absorption of the monolayer should be achieved when the source is spatially aligned with the cavity mode's electric field maximum. This exists at the center of the cavity, one wavelength above the substrate, as was previously shown in **Figure 3c**. The intensity of the cavity mode was found to decrease as the source is moved toward the top of the rods due to reduced spatial coupling between the dipole source and the cavity mode's antinode, which

**Figure 5.** Comparison of the flux spectra for emission from the cavity as the dipping of the material is varied along the z-axis. The drawing on the right illustrates the degrees of bowing of the 2D material from the surface of the structure into

**Figure 4.** The effect of changing the rods' radii r from 0.155a to 0.170a on the cavity mode, showing a maximum Q for

r = 0.161a and an increase in the cavity's central wavelength when r is increased, and vice versa.

exists in the center of the cavity.

the cavity for each of the cases shown on the left.

200 Two-dimensional Materials for Photodetector

**Figure 6.** Diagram showing the difference in shape between a s-SIL and h-SIL on a sample with embedded emitters (quantum dots) in the above case (yellow features). The dashed lines highlight how a larger angle of emitted light is collected in the s-SIL relative to the h-SIL.

#### **3.1. SIL fabrication**

The process of creating an epoxy SIL onto a 2D material is described in this chapter. The sample containing the 2D material is immersed into a glycerol bath, which provides the liquid phase medium needed to enable the formation of droplets with high contact angles, such as the SILs those shown in **Figure 6**. This arises due to a modification of the surface tension experienced by the droplet, and can be explained by considering the Young equation given in (2) [56] and illustrated in **Figure 7**.

$$\cos \Theta\_y = \frac{\mathcal{V}\_y - \mathcal{V}\_d}{\mathcal{V}\_y} \tag{2}$$

**3.2. Enhanced light coupling**

ended red arrow).

ment of 2.0 × .

and after (d) being magnified by a SIL.

**Figure 8a** shows how a SIL enhances the coupling of light out of a TMD based device. The SIL on the RHS of the blue dashed line refracts the light rays produced by the TMD at the SILair boundary closer to the normal leading to more rays entering the lens (shown as a double

Increasing Light Absorption and Collection Using Engineered Structures

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The increase in the coupling of light can be used to increase the light output of a TMD by increasing the solid angle of light that can be observed rather than lost to the environment. **Figure 9a** shows the measured photoluminescence (PL) spectra for a monolayer at 10 μW of excitation power. Comparing the integrated intensity of the flake both before and after the application of a SIL, an increase in PL intensity of 4.0× is observed. This enhancement arises from the SIL refracting light at the SIL-air boundary. A SIL with the dimensions shown in **Figure 8b** should increase the solid angle of light emitted vertically by 1.33×, the SiO2

underneath it will also reflect light back, creating a virtual source which will, in turn be enhanced by the SIL. Calculating these values, we find that the theoretical solid angle of the reflected light would be increased by 3.15×, which when scaled to take account of the percentage of light that would be reflected and added to the vertical emission we get a total enhance-

The theoretical enhancement value calculated is only half the power of the experimental results, however, this only considers the coupling of light out of the SIL, and does not consider what happens to the excitation source entering the SIL. A beam of light travelling in

**Figure 8.** (a) A ray trace simulation demonstrating how the SIL increases the coupling of light, (b): A render of the SIL in c and d from the side with dimensions, (c and d): Microscope images showing an isolated TMD flake both before (c),

layer

203

where *γsf*  , *γsl* and *γlf* are the solid-filler, solid–liquid and liquid-filler surface tensions, respectively. When the filler solution is air, *γsf* is greater than both *γsl*  , and *γlf*  . This makes *cos*(*y*) > 0, resulting in a small equilibrium contact angle (*y*). However, when a filler solution such as glycerol is used, *γsf* reduces dramatically, allowing *cos*(*y*) < 0; this allows droplets with a contact angle over 90 degrees to form. Glycerol is an ideal filler solution due to it being relatively inert with respect to the epoxy. Other filler solutions such as water are less ideal as they are known to be absorbed by the epoxy, leading to a significant reduction in the SIL's transparency [40].

The dispensed UV-curable polymer can be tuned, using one of two methods. The first and most simple is to exploit the polymer's relatively strong attraction to the substrate to create a larger than required SIL with the correct sized base. The SIL can then be tuned by withdrawing epoxy from the center of the dispensed droplet, allowing the volume to reduce whilst the base's dimensions stay relatively constant. Another method of tuning the shape involves applying a bias between the needle and sample, and using the principle of electro-wetting to change the droplets shape [38–40]. Once the desired shape is obtained the SIL can be cured by exposure to UV light, permanently fixing its location, and shape. The sample can then be removed from the glycerol bath and washed with deionized water to remove any remaining glycerol. Further details on the fabrication process of an epoxy SIL, can be found from [39].

**Figure 7.** Diagram illustrating Young's equilibrium contact angle (*θ y*), due to the balancing of the solid–liquid (*γ sl*), liquid-filler (*γ lf*) and solid-filler (*γ sf*) surface tensions.

#### **3.2. Enhanced light coupling**

**3.1. SIL fabrication**

202 Two-dimensional Materials for Photodetector

where *γsf*

ency [40].

 , *γsl*

glycerol is used, *γsf*

(2) [56] and illustrated in **Figure 7**.

and *γlf*

tively. When the filler solution is air, *γsf*

liquid-filler (*γ lf*) and solid-filler (*γ sf*) surface tensions.

cos*θy* = *γsf* − *γ* \_\_\_\_\_*sl*

The process of creating an epoxy SIL onto a 2D material is described in this chapter. The sample containing the 2D material is immersed into a glycerol bath, which provides the liquid phase medium needed to enable the formation of droplets with high contact angles, such as the SILs those shown in **Figure 6**. This arises due to a modification of the surface tension experienced by the droplet, and can be explained by considering the Young equation given in

*γlf*

is greater than both *γsl*

resulting in a small equilibrium contact angle (*y*). However, when a filler solution such as

tact angle over 90 degrees to form. Glycerol is an ideal filler solution due to it being relatively inert with respect to the epoxy. Other filler solutions such as water are less ideal as they are known to be absorbed by the epoxy, leading to a significant reduction in the SIL's transpar-

The dispensed UV-curable polymer can be tuned, using one of two methods. The first and most simple is to exploit the polymer's relatively strong attraction to the substrate to create a larger than required SIL with the correct sized base. The SIL can then be tuned by withdrawing epoxy from the center of the dispensed droplet, allowing the volume to reduce whilst the base's dimensions stay relatively constant. Another method of tuning the shape involves applying a bias between the needle and sample, and using the principle of electro-wetting to change the droplets shape [38–40]. Once the desired shape is obtained the SIL can be cured by exposure to UV light, permanently fixing its location, and shape. The sample can then be removed from the glycerol bath and washed with deionized water to remove any remaining glycerol. Further details on the fabrication process of an epoxy SIL, can be found from [39].

**Figure 7.** Diagram illustrating Young's equilibrium contact angle (*θ y*), due to the balancing of the solid–liquid (*γ sl*),

are the solid-filler, solid–liquid and liquid-filler surface tensions, respec-

reduces dramatically, allowing *cos*(*y*) < 0; this allows droplets with a con-

 , and *γlf*

(2)

 . This makes *cos*(*y*) > 0,

**Figure 8a** shows how a SIL enhances the coupling of light out of a TMD based device. The SIL on the RHS of the blue dashed line refracts the light rays produced by the TMD at the SILair boundary closer to the normal leading to more rays entering the lens (shown as a double ended red arrow).

The increase in the coupling of light can be used to increase the light output of a TMD by increasing the solid angle of light that can be observed rather than lost to the environment. **Figure 9a** shows the measured photoluminescence (PL) spectra for a monolayer at 10 μW of excitation power. Comparing the integrated intensity of the flake both before and after the application of a SIL, an increase in PL intensity of 4.0× is observed. This enhancement arises from the SIL refracting light at the SIL-air boundary. A SIL with the dimensions shown in **Figure 8b** should increase the solid angle of light emitted vertically by 1.33×, the SiO2 layer underneath it will also reflect light back, creating a virtual source which will, in turn be enhanced by the SIL. Calculating these values, we find that the theoretical solid angle of the reflected light would be increased by 3.15×, which when scaled to take account of the percentage of light that would be reflected and added to the vertical emission we get a total enhancement of 2.0 × .

The theoretical enhancement value calculated is only half the power of the experimental results, however, this only considers the coupling of light out of the SIL, and does not consider what happens to the excitation source entering the SIL. A beam of light travelling in

**Figure 8.** (a) A ray trace simulation demonstrating how the SIL increases the coupling of light, (b): A render of the SIL in c and d from the side with dimensions, (c and d): Microscope images showing an isolated TMD flake both before (c), and after (d) being magnified by a SIL.

**Figure 9.** (a) Graph showing the room temperature photoluminescence of a WSe2 monolayer both with and without a SIL, demonstrating the influence of the SIL in improving light collection efficiency. (b and c) Photoluminescence map of the same flake as (a) both without a SIL (b) and with a SIL (c), the intensity scale in these maps are identical, and the orange square in b shows the area in c that has been magnified by the SIL.

air is normally diffraction limited in terms of its size, and can be described by the Rayleigh criterion. When considering a beam being focused through a lens onto a surface we can use the following equation to describe the half-width at half-maximum (FWHM) of the resultant airy pattern:

$$F\text{WHM} = \frac{0.52\,\text{h}}{n\,\text{NA}\_{\text{obj}}}\tag{3}$$

is especially important, when pushing the limits of micro-photoluminescence, and can have many optoelectronic applications. In the example of a photodetector, the increased magnification can enable the size of the detector to be reduced, whilst maintaining the same collection

Increasing Light Absorption and Collection Using Engineered Structures

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205

The magnification increase from **Figure 8c**–**d** was found to be 1.8×, indicating that the SIL has a shape between that of a hemisphere (linear dependence with n giving 1.56×) and an ideal super-sphere (quadratic dependence with n giving 2.43×). This result shows that SILs in between the h and s-SIL geometries (shown in **Figure 6**), can give optical properties that are a combination of the two, with the studied SIL showing a greater magnification than an h-SIL

We have up to this point considered the properties of an epoxy SIL in terms of its optical performance, which although considerable is limited by the epoxy's refractive index. SILs formed from materials like glass can have higher refractive indices, and thus can cause better optical enhancement, and indeed are well suited to coupling light into/out of embedded structures with a flat surface over them. However, due to being solid and having a large hardness, glass SILs are unsuitable for coupling light into/out of sensitive photo-absorbers/ emitters like 2D materials, as their presence risks damage to the flake and is likely to leave an air gap that will significantly degrade its optical performance. Epoxy formed SILs do not suffer from this problem as the SIL can be formed over the sensitive structure as a liquid, then hardened into a solid to fully encapsulate the emitter. This hardening process causes no damage to the emitter and fully seals the emitter away from any physical or chemical harm, giving

Encapsulation can be essential for materials that have finite lifetimes in an ambient environment. The photoluminescence of emitters such as colloidal quantum dots (CQD's) [61, 62] and monolayer TMD's are strongly suppressed when exposed to oxidants and organic contaminants present in the air [63–65]. In the case of TMD's this strong suppression of PL is accompanied by a large change in the flake's structure and morphology especially at grain boundaries [66]. Strategies such as encapsulation with hexagonal boron nitride (h-BN) have been shown to be effective at preventing this oxidization [67] however, this form of encapsulation adds complexity and cost to the potential manufacturing processes. Photopolymers such as UV cured epoxies are commonly used in the mass-manufacture of products [68] and should also

within a SIL and its appearance and PL intensity measured over several months in ambient conditions (a time period known to cause a reduction in PL [69]). The results of this experiment are shown in **Figure 10**, the total integrated PL of the peak was found to stay constant within error over 5 months. In addition, the inset microscope images show that there are no visible changes in the monolayers appearance, suggesting that the SIL encapsulation helps to

was encapsulated

area, giving potential reductions in response times and jitter.

the epoxy SILs a significant advantage over other types of SILs.

seal the TMD from oxygen. To test this hypothesis, a monolayer of WSe2

prolong the emissive lifetime of a monolayer in ambient conditions.

without introducing strong chromatic aberrations.

**4. Improved device longevity**

where *n* is refractive index of the medium above the TMDC, *NAobj* is the numerical aperture of the μPL system and *λ* is the excitation wavelength [57]. For a 532 nm laser travelling through the air we get a FWHM of 0.42 nm; in contrast a SIL with refractive index of 1.56 (such as the one demonstrated in **Figure 8**) has a FWHM of 2.7 nm. Assuming light is not lost due to scattering/reflection from the SIL then the incident optical power is unchanged, meaning that the power density of the laser spot increases due to the same optical power being focused into a smaller area. This change in power density for the aforementioned SIL translates to a 2.4× increase in light per unit area. Unfortunately, this will not lead to a 2.4× increase in light output of the monolayer WSe2 , as the quantum efficiency of WSe2 is typically low [13, 58, 59]. However, the increased excitation will lead to an increase in PL intensity (providing the excitonic ground state does not become saturated), and may explain the difference observed between theory and experimental observations.

**Figure 8c** and **d** show a 2D flake before and after being magnified through the application of a SIL. This magnification increase arises from the SIL creating an optical lever effect [60] (i.e. moving the focal position laterally across the SIL produces a smaller lateral movement under the SIL). The magnification allows maps to be made with a higher number of measurements per unit area. This can be easily observed from **Figure 9b** and **c** which show PL maps of the emission of a flake without and with a SIL, respectively. Note that both maps have the same number of pixels despite **Figure 9c** showing a map of a smaller area. This increased resolution is especially important, when pushing the limits of micro-photoluminescence, and can have many optoelectronic applications. In the example of a photodetector, the increased magnification can enable the size of the detector to be reduced, whilst maintaining the same collection area, giving potential reductions in response times and jitter.

The magnification increase from **Figure 8c**–**d** was found to be 1.8×, indicating that the SIL has a shape between that of a hemisphere (linear dependence with n giving 1.56×) and an ideal super-sphere (quadratic dependence with n giving 2.43×). This result shows that SILs in between the h and s-SIL geometries (shown in **Figure 6**), can give optical properties that are a combination of the two, with the studied SIL showing a greater magnification than an h-SIL without introducing strong chromatic aberrations.

## **4. Improved device longevity**

air is normally diffraction limited in terms of its size, and can be described by the Rayleigh criterion. When considering a beam being focused through a lens onto a surface we can use the following equation to describe the half-width at half-maximum (FWHM) of the resultant