Frequency comb generation at THz frequencies by coherent phonon excitation in Si

TL;DR

This paper demonstrates the generation of a >100 THz optical frequency comb in silicon through ultrafast phononic modulation driven by coherent LO phonons at 15.6 THz, enabling new high-bandwidth optical processing.

Contribution

It introduces a novel method for THz frequency comb generation in silicon using coherent phonon excitation, expanding the understanding of nonlinear optical processes in solid-state materials.

Findings

Frequency combs up to 7th order generated in silicon.

Ultrafast phononic modulation of silicon's optical index achieved.

All-optical >100 THz frequency comb demonstrated.

Abstract

High-order nonlinear light-matter interactions in gases enable generation of x-ray and attosecond light pulses, metrology, and spectroscopy. Optical nonlinearities in solid-state materials are particularly interesting for combining optical and electronic functions for high-bandwidth information processing. Third-order nonlinear optical processes in silicon have been used to process optical signals with greater than 1 GHz bandwidths. Fundamental physical processes for a Si-based optical modulator in the THz bandwidth range, however, have not yet been explored. Here we demonstrate ultrafast phononic modulation of the optical index of Si by irradiation with intense few-cycle femtosecond pulses. The anisotropic reflectivity modulation by the resonant Raman susceptibility at the fundamental frequency of the longitudinal optical (LO) phonon of Si (15.6 THz) generates a frequency comb up to 7th-order. All optical >100 THz frequency comb generation is realized by harnessing the coherent atomic motion of the Si crystalline lattice at its highest mechanical frequency.