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

Abstract

Surface Acoustic Waves (SAWs) generated with sub-picosecond light pulses are commonly used in non-destructive testing. The absorption of pump light by the medium generates SAWs that are detected by delayed probe light pulses. The spatiotemporal evolution of the SAWs can be imaged by scanning the position and time delay of the focused probe light pulses. Commonly used setups show two main constraints: the pump light is focused to a circular spot that generates SAWs in all directions, which does not allow control of directionality, and the laser repetition rate frep limits accessible frequencies to nfrep (n = 1,2,…). In the case of laser excitation and detection of whispering-gallery modes (WGM) on a disc, only certain frequencies are detectable, and counter-propagating modes cannot be separately excited. Here, we overcome these limitations in experimental GHz SAW imaging of WGMs. To access arbitrary frequencies, we periodically modulate in intensity of the pump and probe beams and make use of both in-phase and in-quadrature lock-in detection [1]-[3]. To generate WGMs propagating in a single direction, we make use of a spatial light modulator and computer-generated holograms. These new results extend the possibilities of SAW imaging by allowing fine control of excited surface acoustic modes. [1] S. Kaneko, M. Tomoda, and O. Matsuda, AIP Adv. 4, 017124 (2014); [2] O. Matsuda et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 62, 584-595 (2015); [3] S. Mezil et al., Opt. Lett., 40, 2157-2160 (2015).