A Transient Grating Method to Measure the Dispersion of Elastic Waves in Nanostructures

E Charron,L Lu,B Bonello,E Glushkov,O Boyko,N Gogneau,M Tchernycheva,N Glushkova

doi:10.1088/1742-6596/1461/1/012022

Abstract

We describe the so-called “Transient Grating Method” which is a non-invasive experimental technique well suited to measure the dispersion properties of ordered or disordered nanostructures and thin films, at frequencies up to a few GHz. This pump-and-probe technique involves the interference pattern produced by two in-coming IR optical pulses to set a standing elastic wave on the surface of the sample through photoelastic processes. The wave vector of this elastic wave can be easily tuned by adjusting the angle between the two incident beams. Two continuous visible laser beams in a heterodyne detection scheme are used to detect the vibrations on the surface and in turn the dispersion of the related elastic modes. The achievements of the technique are illustrated by the measurement of the dispersion of surface acoustic waves in piezogenerators consisting of GaN nanowires embedded into a dielectric matrix (HSQ). We then report on the analysis of the experimental data that allowed extracting the elastic parameters of this composite medium.

Highlights

Two-dimensional elastic nanostructures are found in countless industrial products and, with the progresses made in the nano/micro-fabrication techniques, devices featuring mechanical working frequencies of several hundreds of MHz are nowadays almost banal
The non-contact technique that we describe here, namely the Transient Grating Method (TGM), allows for characterizing the elastic properties of almost any kind of materials, transparent or opaque, ordered or disordered, piezoelectric or not... in a frequency range that stretches from a few tens of MHz up to several GHz
We used this technique to investigate the features of waves guided in a fully functional piezogenerator based on GaN nanowires (NWs) embedded into a dielectric matrix [hydrogen silsesquioxane (HSQ)]

Summary

Introduction

Two-dimensional elastic nanostructures are found in countless industrial products and, with the progresses made in the nano/micro-fabrication techniques, devices featuring mechanical working frequencies of several hundreds of MHz are nowadays almost banal. When the device includes piezoelectric elements or electromechanical transducers, network analyzers can be used for characterization but with the disadvantage of a poor tuning of the frequency. The non-contact technique that we describe here, namely the Transient Grating Method (TGM), allows for characterizing the elastic properties of almost any kind of materials, transparent or opaque, ordered or disordered, piezoelectric or not... In a frequency range that stretches from a few tens of MHz up to several GHz. In this work, we used this technique to investigate the features of waves guided in a fully functional piezogenerator based on GaN nanowires (NWs) embedded into a dielectric matrix [hydrogen silsesquioxane (HSQ)].

Methods

Results

Conclusion