Electrical switching between terahertz second and third harmonic generation in photo-doped GaAs

We investigate the competing terahertz (THz) nonlinear harmonic generations in photo-doped GaAs with variation of bias voltages. Due to the intrinsic third-order nonlinearity, THz third harmonic generation (THG) is observed in the photo-doped GaAs without bias voltages. While such THG decreases as the bias increases, the second harmonic generation (SHG) is observed by the induced second-order nonlinear polarization, and unlike THG, the efficiency of SHG increases as the bias increases. This electrical switching behavior between THG and SHG could be understood from the surface current saturation in GaAs.