# Time-domain Brillouin scattering assisted by diffraction gratings

## TL;DR

This paper introduces a novel time-domain Brillouin scattering technique using metallic diffraction gratings to access a wide range of acoustic frequencies and directions in a single setup, enabling advanced material characterization.

## Contribution

The work demonstrates a new reflection-mode scheme that multiplexes acoustic and optical diffraction to measure broad acoustic spectra and material properties simultaneously.

## Key findings

- Access to a wide range of acoustic frequencies in one experiment
- Simultaneous measurement of sound velocity and refractive index
- Ability to analyze samples with limited optical access

## Abstract

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by the delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first order diffracted light. The obtained data contain multiple frequency components which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through the multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of the plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus, the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic grating operating in reflection mode provides access to acoustic frequencies from the minimal to the maximal possible in a single experimental configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring of the backward and forward Brillouin scattering processes in parallel. Applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of the samples with a single direction of possible optical access.