: P.H. Otsuka, R. Chinbe, M. Tomoda, O. Matsuda, Y. Tanaka, D.M. Profunser, S. Kim, H. Jeon, I.A. Veres, A.A. Maznev, O.B. Wright

: Photoacoustics, 2023, 100481, ISSN 2213-5979

DOI: https://doi.org/10.1016/j.pacs.2023.100481 We extend gigahertz time-domain imaging to a wideband investigation of the eigenstates of a phononic crystal cavity. Using omnidirectionally excited phonon wave vectors, we implement an ultrafast technique to experimentally probe the two-dimensional acoustic field inside and outside a hexagonal cavity in a honeycomb-lattice phononic crystal formed in a microscopic crystalline silicon slab, thereby revealing the confinement and mode volumes of phonon eigenstates—some which are clearly hexapole in character—lying both inside and outside the phononic-crystal band gap. This allows us to obtain a quantitative measure of the spatial acoustic energy storage characteristics of a phononic crystal cavity. We also introduce a numerical approach involving toneburst excitation and the monitoring of the acoustic energy decay together with the integral of the Poynting vector to calculate the Q factor of the principal in-gap eigenmode, showing it to be limited by ultrasonic attenuation rather than by phonon leakage to the surrounding region.