Self-phase modulation of a single-cycle THz pulse

We demonstrate 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. Electron heating leads to an ultrafast reduction of the plasma frequency, which results in a strong modification of the THz-range dielectric function of the material. THz SPM is observed directly in the time domain as a characteristic reshaping of single-cycle THz pulse. In the frequency domain, it corresponds to a strong frequency-dependent refractive index nonlinearity of n-GaAs, which is found to be both positive and negative within the broad spectrum of the THz pulse. The spectral position of zero nonlinearity is defined by the electron momentum relaxation rate. Nonlinear spectral broadening and compression of the single-cycle THz pulse was also observed.