Picosecond x-ray strain rosette reveals direct laser excitation of coherent transverse acoustic phonons.

Sooheyong Lee,G Jackson Williams,Maria I Campana,Eric C Landahl,Donald A Walko

doi:10.1038/srep19140

Abstract

Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz.

Highlights

Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide
The uniaxial strain model is known to break down under certain situations such as when single crystals are subjected to 100 GPa shock compression[4], in asymmetrically cut crystals with an asymmetric strain tensor[5], when the laser is focused to small lateral spot sizes comparable to the laser penetration depth[6], or within individual nanocrystals[7]
We show for the first time that large-amplitude coherent transverse lattice displacements can be directly generated in bulk semiconductors where the dominant generation mechanism can be attributed to a universal phenomenon: the Poisson effect

Summary

Introduction

Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We show for the first time that large-amplitude coherent transverse lattice displacements can be directly generated in bulk semiconductors where the dominant generation mechanism can be attributed to a universal phenomenon: the Poisson effect Both longitudinal and shear strain waves are permitted by continuum elasticity theory[12], ultrasonic measurements in many contexts are often limited to the longitudinal polarization. If the illuminated area of the pump laser beam on the sample surface is much larger than the optical penetration depth, the strain gradient along the surface normal is expected to be much steeper than along the lateral direction, so the strain may be treated one-dimensionally These diffuse scattering results suggest revisiting the uniaxial continuum model to incorporate longitudinal-to-transverse strain coupling on a macroscopic scale (i.e., the Poisson effect). The magnitude of this motion was consistent with a lateral contraction of the top layer’s ferroelectric mosaic blocks driven by the Poisson effect

Methods

Results

Conclusion