Investigation of terahertz pulse generation in semiconductors pumped at long infrared wavelengths

Abstract

We present designs of semiconductor contact grating high energy terahertz pulse sources pumped by femtosecond pulses in the 1 to 5 µm wavelength range. Nearly wavelength-independent diffraction efficiencies as high as 69% and 75% in the ± 1 st diffraction orders in the transverse electric field polarization state were predicted in GaAs and GaP, respectively, based on a rectangular grating. Numerical simulations—including, for the first time, to our knowledge, the effects of both a nonlinear refractive index and free carrier absorption—were performed to investigate the possible advantage of using longer pumping wavelengths to suppress the two- to seven-photon absorption. Conversion efficiency larger than 1.0% is predicted for both crystals. We also recognized that the nonlinear refractive index and the wavelength-dependent optical parametric amplifier efficiency can significantly reduce the overall terahertz generation efficiency; thus, optimum pump wavelengths exist for the highest conversion efficiency, which are 2 and 3 µ m for GaP and GaAs, respectively.