Time-domain Brillouin scattering theory for probe light and acoustic beams propagating at an angle and acousto-optic interaction at material interfaces

Vitalyi E Gusev; Théo Thréard; David H Hurley; Samuel Raetz

doi:10.1016/j.pacs.2023.100563

Time-domain Brillouin scattering theory for probe light and acoustic beams propagating at an angle and acousto-optic interaction at material interfaces

Photoacoustics. 2023 Oct 13:33:100563. doi: 10.1016/j.pacs.2023.100563. eCollection 2023 Oct.

Authors

Vitalyi E Gusev¹, Théo Thréard¹, David H Hurley², Samuel Raetz¹

Affiliations

¹ Laboratoire d'Acoustique de l'Université du Mans (LAUM), UMR 6613, Institut d'Acoustique - Graduate School (IA-GS), CNRS, Le Mans Université, France.
² Idaho National Laboratory, P.O. Box 1625, Idaho Falls, 83415, ID, USA.

Abstract

A theory has been developed to interpret time-domain Brillouin scattering (TDBS) experiments involving coherent acoustic pulse (CAP) and light pulse beams propagating at an angle to each other. It predicts the influence of the directivity pattern of their acousto-optic interaction on TDBS signals when heterodyne detection of acoustically scattered light is in backward direction to incident light. The theory reveals relationships between the carrier frequency, amplitude and duration of acoustically induced "wave packets" in light transient reflectivity signals, and factors such as CAP duration, widths of light and sound beams, and their interaction angle. It describes the transient dynamics of these wave packets when the light and CAP encounter material interfaces, and how the light scattering by the incident CAP transforms into scattering by the reflected and transmitted CAPs. The theory suggests that single-point TDBS experiments can determine not only depth positions of buried interfaces but also their local inclinations/orientations.

Keywords: Material interface; Non-collinear interaction; Picosecond laser ultrasonics; Time-domain Brillouin scattering; Ultrafast photoacoustics.