Coherent phonon induced optical modulation in semiconductors at terahertz frequencies

TL;DR

This paper demonstrates how intense laser pulses can coherently modulate the optical properties of silicon and gallium arsenide at terahertz frequencies, producing broad phonon frequency combs through nonlinear phononic interactions.

Contribution

It introduces a method to generate broad phonon frequency combs in semiconductors via coherent phonon excitation near their band gaps, expanding nonlinear optical control in solids.

Findings

Generated phonon frequency combs over 100 THz in Si

Produced phonon frequency combs over 60 THz in GaAs

Achieved amplitude and phase modulation of reflected light

Abstract

The coherent modulation of electronic and vibrational nonlinearities in atoms and molecular gases by intense few-cycle pulses has been used for high-harmonic generation in the soft X-ray and attosecond regime, as well as for Raman frequency combs that span multiple octaves from the Terahertz to Petahertz frequency regions. In principle, similar high-order nonlinear processes can be excited efficiently in solids and liquids on account of their high nonlinear polarizability densities. In this paper, we demonstrate the phononic modulation of the optical index of Si and GaAs for excitation and probing near their direct band gaps, respectively at ~3.4 eV and ~3.0 eV. The large amplitude coherent longitudinal optical polarization due to the excitation of longitudinal optical (LO) phonon of Si (001) and LO phonon-plasmon coupled modes in GaAs (001) excited by 10-fs laser pulses induces effective amplitude and phase modulation of the reflected probe light. The combined action of the amplitude and phase modulation in Si and GaAs generates phonon frequency combs with more than 100 and 60 THz bandwidth, respectively.