Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor

TL;DR

This paper demonstrates self-phase modulation of a single-cycle terahertz pulse in n-GaAs caused by free-carrier heating, resulting in spectral broadening and compression, and reveals a strong, frequency-dependent nonlinear dielectric response.

Contribution

It provides the first direct observation of THz self-phase modulation in a semiconductor due to free-carrier response, highlighting ultrafast dielectric modifications.

Findings

Observation of THz SPM directly in time domain

Identification of positive and negative refractive index nonlinearities

Spectral broadening and compression of THz pulses

Abstract

We demonstrate the self-phase modulation (SPM) of a single-cycle THz pulse in a semiconductor, using bulk n-GaAs as a model system. The SPM arises from the heating of free electrons in the electric field of the THz pulse, leading to an ultrafast reduction of the plasma frequency, and hence to a strong modification of the THz-range dielectric function of the material. THz SPM is observed directly in the time domain. In the frequency domain it corresponds to a strong frequency-dependent refractive index nonlinearity of n-GaAs, found to be both positive and negative within the broad THz pulse spectrum, with the zero-crossing point defined by the electron momentum relaxation rate. We also observed the nonlinear spectral broadening and compression of the THz pulse.