**5. Acknowledgments**

This work has been financially supported by the *Ministerio de Ciencia e Innovación* and the *Generalitat Valenciana* of Spain (projects TEC2008-05490 and PROMETEO/2009/077, respectively). C. Cuadrado-Laborde acknowledges the *Secretaría de Estado de Universidades e Investigación del Ministerio de Ciencia e Innovación* (Spain) and ANPCyT (project PICT 2008- 1506, Argentina).

### **6. References**

632 Acoustic Waves – From Microdevices to Helioseismology

Perot cavity. Fully modulated *Q*-switched mode-locked trains of optical pulses were obtained for a wide range of pump powers and repetition rates. For a *Q*-switched repetition rate of 500 Hz and a pump power of 100 mW, the laser generates trains of 12–14 modelocked pulses of about 1 ns each, within an envelope of 550 ns, an overall energy of 0.65 μJ, and a peak power higher than 250 W for the central pulses of the train. Then, we discussed the construction of a mode-locked laser when the mode-locker is driven by travelling acoustic waves. In this case, the modulation frequency is half the frequency obtained when standing acoustic waves are used. Optical pulses were obtained of 530 mW peak power, 700 ps pulse width, at a repetition rate of 4.1 MHz. The variation of the pulses parameters under frequency detuning and applied voltage was also studied. Finally, we demonstrated that it is not necessary to modify the setup in order to reach double active *Q*-switching and modelocking, when travelling acoustic waves were used to drive the mode-locker. In this case the commutation between mode-locking and *Q*-switching mode-locking is remarkably simple; it just needs use of a different electrical signal to drive the piezoelectric of the mode-locker, i.e. from a sinusoidal to a burst-sinusoidal electrical signal. In this case, fully modulated 10–25 modelocked pulses around 700 ps each within a *Q*-switching envelope around 1 μs and a

In subsection 3.3 we shown another example of application of acousto-optic devices based on the interaction of longitudinal acoustic waves with fiber Bragg gratings in the shortwavelength regime. Thus, we carried out an experimental and theoretical study of the phase and group delay response of the acousto-optic supper-lattice modulator. The phase properties of the first sidebands permit the implementation of electrically-tuned photonic true-time delay line controlled by the AC voltage applied to the piezoelectric transducer that generates the acoustic wave. The proposed photonic true-time delay line permits to vary the

Finally, when the acoustic perturbation is not a harmonic wave but a single pulse, its passage through the FBG creates a defect which can be used to control the *Q*-factor in a DFB all-fiber laser, subsection 3.4. Thus, we shown a single-mode, transform-limited, actively *Q*switched distributed-feedback fiber laser. Optical pulses of 800 mW peak power, 32 ns temporal width, and up to 20 kHz repetition rates were obtained. The measured linewidth demonstrates that these pulses are transform limited: 6 MHz for a train of pulses of 10 kHz repetition rate, 80 ns temporal width, and 60 mW peak power. Efficient excitation of

In summary, photonic devices can benefit highly of a strictly all-fiber configuration which provides them a series of attractive advantages. Among all the proposed in-fiber solutions, devices controlled by acoustic waves have been by far the most employed, especially in mode-locked lasers, providing a broad range of alternatives. The recent advances in acoustically controlled photonic systems positioned them as a promising candidate for

This work has been financially supported by the *Ministerio de Ciencia e Innovación* and the *Generalitat Valenciana* of Spain (projects TEC2008-05490 and PROMETEO/2009/077, respectively). C. Cuadrado-Laborde acknowledges the *Secretaría de Estado de Universidades e Investigación del Ministerio de Ciencia e Innovación* (Spain) and ANPCyT (project PICT 2008-

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**5. Acknowledgments**

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**28**

*U.S.A.*

**Surface Acoustic Waves and**

*Department of Electrical and Computer Engineering*

Dustin J. Kreft and Robert H. Blick *University of Wisconsin – Madison*

**Nano–Electromechanical Systems**

Surface acoustic waves (SAW) follow the industrial trend of reducing the size, enhancing the speed, while enhancing the efficiency of energy coupling. Integrating this with microelectromechanical systems (MEMS) and nano-electromechanical systems (NEMS) offers a wide variety of applications such as touch screens, gas and biological sensors, and embedded RFID devices. With modern lithographic techniques, allowing the fabrication of smaller SAW devices, we now use SAWs to probe the mechanical interactions of nano structures. In particular, SAWs can be used to actuate NEMS which gives rise to many interesting phenomena including anomalous acoustoelectric currents, shock waves in suspended devices, and few electron transport, to only name a few, (Beil et al., 2008; Talyanskii et al., 1997). Today, SAWs are also used to generate a quantized current for use as a current standard. In practice two counter-propagating SAWs are used to observe a quantized acoustoelectric current. This leads to population and depopulation of discrete states (Kataoka et al., 2007). In the following we want to give an overview of the state of the art of applying SAWs to nanomechanical devices. We will also give a brief introduction to recent nanoelectromechanical systems with integrated low-dimensional electron gases,

which have the potential to reveal insights into quantized acoustoelectric states.

acoustic impedance matching of IDTs to nanomechanical devices.

The main equation to consider when designing an IDT is:

**2.1 Interdigitated Transducer Design**

The focus for generating SAWs in this chapter will involve the fabrication of interdigitated transducers (IDT). An IDT is simple in concept but can be very involved when fine tuning a structure for engineering applications. Such topics as electronic impedance matching to RF lines, effects of bulk waves in contrast to SAWs, and increasing bandwidth will not be covered; though, this is simply a shortened list of things to consider when designing a proper IDT for engineering applications, they do fall outside the scope of this chapter. Nevertheless, another fabrication step we will consider is the use of acoustic waveguides for

**1. Introduction**

**2. Fabrication**