Self-phase modulation of a single-cycle THz pulse

Abstract

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. Self-phase modulation (SPM), a fundamental nonlinear-optical effect, leads to intensity- dependent temporal and spectral reshaping of optical signals as they propagate through a nonlinear medium. Here we demonstrate the SPM in a new mode - at THz frequencies, using a doped semiconductor, n-GaAs, as an efficient nonlinear medium. However, any other semiconductor with complex band structure will also show SPM along the lines described in this work. Further, we demonstrate the SPM occurring in a single-cycle regime, with the signal spectrum covering many octaves of frequencies. As will be shown below, single-cycle THz pulses can be used as accessible model tools for direct observation of general nonlinear optical effects occurring in the single-cycle regime, and the findings obtained in the THz range can be generalized to other spectral ranges. The THz-range optical nonlinearity of a semiconductor, such as n-GaAs used in this work, arises from the nonlinear response of the free-carrier plasma to the THz ponderomotive excitation. Semiconductors with free carriers are good absorbers of THz radiation, and their THz-range complex-valued dielectric function ߝ is well described by the basic Drude plasma model (1,2) ߝ ሺ ሻ ൌሺ ൅ ߙ/2ሻ