To exploit phononic crystals/metamaterials to various applications including functional devices, it is useful to monitor the propagation of acoustic waves in the structures in a spatiotemporal manner. In this talk, our recent development of the time-resolved two-dimensional gigahertz acoustic field imaging technique, which is suitable for such investigations, will be presented. It utilizes the optical pump-probe method in which the acoustic waves/vibrations are generated by the absorption of ultra short light pulses (pump light pulses) with temporal width of the picosecond regime through the thermoelastic effect, and the surface displacement caused by the acoustic waves/vibrations is monitored with delayed light pulses (probe light pulses) using an optical interferometer. By varying the delay time and scanning the probe light spot position across the imaging area, the spatiotemporal evolution of the acoustic field is obtained. The technique is applied to study two-dimensional phononic crystals consist of regularly aligned holes in a silicon (100) substrate. From the spatiotemporal images of the acoustic field, we can retrieve the dispersion relation of the acoustic modes in two-dimensional k-space. The specific phonon-focusing patterns as well as the mode pattern of the nearly zero-group velocity modes around the phononic band gap are observed. We also clarify the details of the dispersion relation of the wave-guide mode for the one-dimensional wave guide formed in the two-dimensional phononic crystal, with the newly developed arbitrary frequency measurement technique. These results show the advantage of applying spatiotemporal imaging technique to investigate the phononic crystals/metamaterials and their derivatives.

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Osamu Matsuda, "Time-resolved two-dimensional imaging of gigahertz acoustic waves in phononic crystals and related structures (Conference Presentation)," Proc. SPIE 10541, Photonic and Phononic Properties of Engineered Nanostructures VIII, 105411V (Presented at SPIE OPTO: February 02, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2297394.5751516194001.