Detection of shorter-than-skin-depth acoustic pulses in a metal film via transient reflectivity

TL;DR

This paper demonstrates a method to detect ultrashort acoustic pulses in a metal film using transient reflectivity, revealing that pulse shape resolution depends on the photoelastic constant ratio and introducing a Fourier transform algorithm for strain profile reconstruction.

Contribution

It introduces a novel approach to detect and analyze acoustic pulses shorter than the optical skin depth in metals, utilizing a parameter related to the photoelastic constant and a Fourier transform-based reconstruction algorithm.

Findings

Detection of sub-skin-depth acoustic pulses is feasible via transient reflectivity.

The shape of the reflectivity waveform depends on the photoelastic constant ratio.

A Fourier transform algorithm effectively reconstructs acoustic strain profiles.

Abstract

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO$_{3}$ layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.