THz Acoustic Spectroscopy by using Double Quantum Wells and Ultrafast Optical Spectroscopy.

Fan Jun Wei,Kung-Hsuan Lin,Jinn-Kong Sheu,Yu-Hsiang Yeh

doi:10.1038/srep28577

Abstract

GaN is a pivotal material for acoustic transducers and acoustic spectroscopy in the THz regime, but its THz phonon properties have not been experimentally and comprehensively studied. In this report, we demonstrate how to use double quantum wells as a THz acoustic transducer for measuring generated acoustic phonons and deriving a broadband acoustic spectrum with continuous frequencies. We experimentally investigated the sub-THz frequency dependence of acoustic attenuation (i.e., phonon mean-free paths) in GaN, in addition to its physical origins such as anharmonic scattering, defect scattering, and boundary scattering. A new upper limit of attenuation caused by anharmonic scattering, which is lower than previously reported values, was obtained. Our results should be noteworthy for THz acoustic spectroscopy and for gaining a fundamental understanding of heat conduction.

Highlights

Acoustic properties in the THz regime are fundamental to the understanding of materials
Previous research has been stymied by the problem that an initiated acoustic signal is completely overwhelmed by the strong transient electronic signals near zero time delay; in this report, we explain how to circumvent this problem
We investigated the frequency dependence of acoustic attenuation in GaN in the THz regime

Summary

Introduction

Acoustic properties in the THz regime are fundamental to the understanding of materials. Applications such as waveform synthesis[20,21], acoustic spot modulation[22], nondestructive images[23], and phononic device characterization[24] have been demonstrated This technique has been used to study fundamental problems in materials such as GaN25, silica[2,26], ice[27], and water[28]. Our demonstration can characterize the acoustic properties of the transducer, which is crucial to THz acoustic spectroscopy This technique can be applied to measure frequency-dependent phonon MFPs for studies of heat conduction[3,4,5,6,7,8] or the Boson peak in glass[1,11]

Methods

Results

Conclusion