Fano resonance between coherent acoustic phonon oscillations and electronic states near the bandgap of photoexcited GaAs

M Vinod,V Sivasubramanian,G Raghavan

doi:10.1038/s41598-018-35866-7

Abstract

Impulsive photo-excitation of solids results in a travelling strain pulse which manifests itself as coherent acoustic phonon oscillations. These oscillations have been extensively studied using time-resolved pump-probe spectroscopy. In the present work, we report the generation of extremely long-lived, coherent longitudinal acoustic phonon oscillations in intrinsic GaAs (100), with clear and unambiguous evidence of Fano interference between these oscillations and the continuum of electronic states close to the bandgap. Fano resonance is a widespread phenomenon observed in atomic systems and condensed media that arises from quantum interference between a continuum of quantum states and a discrete quantum state. Among other techniques, Fano resonance has been investigated with respect to optical phonons studied with Raman Spectroscopy. In the present work, we investigate Fano resonance in coherent phonon oscillations generated without the aid of any capping layer, dopants or substrate/interface effects. Since Fano resonance is sensitive to changes in electronic structure, doping and defects, these observations are important to the field of picosecond ultrasonics which is used for non-destructive depth profiling of solids and for carrier diffusion studies.

Highlights

In a classical work, asymmetric non-Lorentzian lineshapes of optical spectra were originally investigated by Fano in the context of autoionization in He atoms[1]
Since the Deformation Potential (DP) process is less efficient than TE, CLAP waves are weaker in semiconductors
For intrinsic GaAs along (100) direction, the relative contributions coming from the DP and TE processes can be obtained from the following considerations: As the pump size is substantially larger than the penetration depth, a one dimensional treatment is adequate to model the stress generation process

Summary

Introduction

In a classical work, asymmetric non-Lorentzian lineshapes of optical spectra were originally investigated by Fano in the context of autoionization in He atoms[1]. Fano resonance have since been reported in the spectroscopy of a wide variety of condensed matter systems like metals, semiconductors, ferroelectrics and superconductors, quantum well structures including those of nanostructured systems[2,3,4,5,6,7,8,9,10] It is fairly well-established that Fano interference occurs due to the excitation of a discrete state and that of a continuum of states that interacts with it[1]. This sharp transient is followed by an oscillatory tail in the probe reflectance as a function of delay time These oscillations result from the travelling strain wave generated by internal stresses produced by the pump pulse.

Methods

Results

Conclusion