Maria Bayard Dühring - Solid Mechanics

Chapter 6
Design of acousto-optical interaction [P3]-[P7]
An acoustic and an optical wave problem have been studied, respectively, in the two
previous chapters and structures have been optimized by the method of topology
optimization. In this chapter the last and most complicated wave problem is considered
where the acousto-optical interaction between surface acoustic waves and
optical waves in channel waveguides is investigated. The chapter gives an overview
of the work presented in publication [P3]-[P7].
6.1 Introduction
The propagation of surface acoustic waves (SAW) is confined to a material surface
and two main types of waves exist. The Rayleigh wave is polarized in the longitudinal
and transverse, vertical directions and the other type is mainly polarized in the
shear horizontal direction such as the Bleustein-Gulyaev wave [7]. In piezoelectric
materials SAWs can be generated by the inverse piezoelectric effect by interdigital
transducers (IDTs), which are arrays of electrode fingers deposited on the surface.
The electrode fingers determine the wavelength and the width of the beam as indicated
in figure 6.1(a). When the electrode height is small the wave properties of the
excited waves are similar to the waves propagating on a free surface. Conventional
SAW devices as filters, resonators and sensors as well as the new acousto-optical
devices are driven by IDTs with thin electrodes. However, when the aspect ratio of
Figure 6.1 Interdigital transducers to generate surface acoustic waves. (a): Double-finger
IDT with thin electrodes. The interferometer setup to measure the wave amplitude by a
laser is sketched. (M.M. de Lima Jr. et al. [21]). (b) and (c): Scanning electron microscope
image of IDT with high aspect ratio electrodes. The substrate is lithium niobate and the
electrodes consist of nickel. (V. Laude et al. [81]).
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