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

摘要

We demonstrate the self-phase modulation (SPM) of a single-cycle THz pulse ina semiconductor, using bulk n-GaAs as a model system. The SPM arises from theheating of free electrons in the electric field of the THz pulse, leading to anultrafast reduction of the plasma frequency, and hence to a strong modificationof the THz-range dielectric function of the material. THz SPM is observeddirectly in the time domain. In the frequency domain it corresponds to a strongfrequency-dependent refractive index nonlinearity of n-GaAs, found to be bothpositive and negative within the broad THz pulse spectrum, with thezero-crossing point defined by the electron momentum relaxation rate. We alsoobserved the nonlinear spectral broadening and compression of the THz pulse.