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.
Using ultrashort pulsed optical excitation and interferometric detection, we image ultrahigh- frequency surface acoustic
waves on two-dimensional (2D) phononic crystals in the time domain. The samples consist of a square lattice of airfilled
holes etched in a silicon substrate. Good agreement with time-domain finite element numerical simulations is
obtained. The dispersion relation is derived and stop bands are revealed by means of Fourier transforms. The wave fields
corresponding to acoustic eigenmodes at specific frequencies are also presented.
A series of experiments have been conducted that microscopically image thermal diffusion and surface acoustic phonon
propagation within a single crystallite of a polycrystalline Si sample. The experimental approach employs ultrashort
optical pulses to generate an electron-hole plasma and a second probe pulse is used to image the evolution of the plasma.
By decomposing the signal into a component that varies with delay time and a steady state component that varies with
pump modulation frequency, the respective influence of carrier recombination and thermal diffusion are identified.
Additionally, the coherent surface acoustic phonon component to the signal is imaged using a Sagnac interferometer to
monitor optical phase.
Picosecond acoustic phonon pulses are generated with ultrashort
laser pulses in a sample containing three GaAs-Al0.3Ga0.7As quantum wells of different thickness. The pump photon energy is tuned through the hh1-e1 transitions of each well (1.44 - 1.64 eV) and the probe photon energy is chosen to allow detection of the phonon pulses at the sample surface (3.06 eV). Transient optical reflectance and phase changes are recorded as a function of the delay time between the pump and probe light pulses using an interferometric technique. The transition between the valence and conduction sublevels of the wells is observed to strongly influence the pump-photon-energy dependence of the acoustic phonon pulse generation. The data are analyzed with a model that relates the carrier wavefunctions in the quantum wells to the acoustic strain through the deformation potential, and the acoustic strain to the transient optical reflectance and phase.