Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals

TL;DR

This paper introduces a hybrid optical method that overcomes the limitations of traditional techniques, enabling the characterization of non-transparent hypersonic phononic crystals with high resolution, even in disordered samples.

Contribution

A novel hybrid pump-probe technique is developed to analyze non-transparent phononic crystals, combining time- and frequency-domain advantages for improved disorder tolerance.

Findings

Successfully characterizes non-transparent PnCs using the hybrid method.

Achieves resolution comparable to advanced asynchronous optical sampling.

Demonstrates effectiveness on metallic Bragg mirror structures.

Abstract

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry-Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately: We inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump-probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples.