Tuning to a particular acoustic whispering-gallery mode in the GHz range

Abstract

Surface Acoustic Waves (SAWs) are commonly used in non-destructive testing and GHz filtering. Typical setups to study SAWs in the GHz range in the time domain make use of subpicosecond light pulses. The absorption of pump light by the medium generates surface acoustic waves. The latter are detected by delayed probe light pulses through strain-induced variations in the optical phase. The spatiotemporal evolution of the SAWs is accessible by scanning the focusing position and time delay of the probe light pulses. In general, the laser repetition rate frep (typically ~80 MHz) limits such a setup to measurement frequencies that are integral multiples of frep. Commonly used setups focus pump light to a circular spot of a few microns in size, thus generating SAWs propagating in all directions. In the case of whisperinggallery modes (WGM) on a disk, that is, modes propagating around the disk rim, two counterpropagating modes are thereby excited with the same intensity. Here we overcome these two limitations. To access any arbitrary frequency, we modulate in intensity both the pump and the probe beams and then carry out appropriate analysis on lock-in detected probe-beam intensity variations. This opens the way to determine the acoustic dispersion curve of samples with arbitrary resonance frequencies as well as the quality factor of any chosen mode. In order to select only WGMs propagating in one direction, we make use of a spatial light modulator (SLM) programmed with the use of computer-generated holograms. In the particular case of WGMs, a windmill-shaped surface source pattern is chosen to produce acoustic WGMs with one rotation sense or the other. These new results extend the possibilities of SAW imaging by allowing fine control of excited surface acoustic modes.