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

TL;DR

This paper develops a theoretical framework for time-domain Brillouin scattering involving angled light and acoustic beams, elucidating their interaction dynamics at interfaces and enabling depth and orientation measurements of buried structures.

Contribution

It introduces a comprehensive theory for angled TDBS experiments, linking acoustic and optical parameters to scattering signals and interface characterization.

Findings

Predicts influence of beam directivity on TDBS signals

Relates acoustic pulse properties to light reflectivity changes

Enables determination of interface depth and orientation

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 the 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 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 the depth positions of buried interfaces but also their inclinations/orientations.