Optical generation and detection of gigahertz shear acoustic waves in solids assisted by a metallic diffraction grating

Abstract

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches in the material both compression/dilatation (longitudinal) and shear coherent acoustic pulses in directions of different orders of acoustic diffraction. The propagation of the emitted acoustic pulses can be monitored by measuring the variation of the optical reflectivity of the time-delayed ultrashort probe laser pulses. The direction of probe light incidence and its polarization relative to the sample surface as well as the orientation of the metallic grating should be specifically chosen for efficient Brillouin scattering of the probe light from shear phonons propagating in the elastically isotropic materials. As theoretically predicted, the obtained experimental data contain multiple frequency components which are due to a variety of possible Brillouin scattering angles for both shear and compression/dilatation coherent acoustic waves. All these different frequency components are explained through multiplexing the propagation directions of probe light and coherent sound by the metallic diffraction grating. Our experimental scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to monitoring the shear acoustic waves launched in the direction of the first diffraction order by backward Brillouin scattering process. Applications include simultaneous determination of several different acoustic mode velocities and optical refractive index and, potentially, measurements of the acoustic dispersion of samples with a single direction of possible optical access.

Optically Controlled Nano-Transducers Based on Cleaved Superlattices for Monitoring Gigahertz Surface Acoustic Vibrations

“…Theoretically, only the transverse mode that is mode-converted from the longitudinal mode, either incident at the surface or propagating along the surface, could contribute to shear surface displacement. Based on accumulated experimental , and theoretical , knowledge, it is anticipated that the structures in which either the substrate, the grating, or both exhibit elastic anisotropy, are better suited for controlling T waves. In the absence of elastic anisotropy, gR and L waves are expected to significantly dominate the acoustic signals in transient reflectivity measurements.…”

Optical Metasurfaces for Energy Conversion

“…By using the same experimental approach of the previously described work and simply rotating the elongated pump beam interference pattern on the metasurface plane, the dispersion surfaces for the lowest three acoustic modes of the 1D periodic structure could be obtained. A similar 1D metasurface (a grating made of aluminum rods 190 nm wide, 40 nm high and periodically spaced by 380 nm, deposited on a 1.0 mm silica substrate and with a total area of 100 × 100 mm 2 ) was used for generating longitudinal and shear coherent acoustic waves after the absorption of ultrafast laser pulses, in a pump-and-probe like configuration for exciting and detecting the acoustic waves . This time-resolved Brillouin scattering experimental scheme allowed researchers to generate and detect both kinds of acoustic waves diffracted along different orders by the grating.…”

Sound velocity mapping from GHz Brillouin oscillations in transparent materials by optical incidence from the side of the sample

“…The original technique relies on returning picosecond sound pulses modulating either the optical reflectance or phase of the probe light near the surface of opaque samples [1] , [2] , [3] . It is also possible to measure GHz Brillouin oscillations in optical reflectance arising from the photoelastic effect inside transparent samples, which allows depth profiling of the sound velocity on the nanoscale [4] , [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] , [13] , [14] . This method, known as picosecond ultrasonic interferometry or TDBS (Time-Domain Brillouin Scattering), has been used for depth profiling of sound velocity in samples of transparent inorganic materials that are homogeneous in the lateral direction [10] , [15] , [16] and for three-dimensional (3D) imaging of the sound velocity in biological cells, assuming a constant refractive index [17] , [18] , [19] , [20] , [21] .…”