Power scaling of widely-tunable monochromatic terahertz radiation by stacking high-resistivity GaP plates

A high-resistivity GaP crystal was used to generate monochromatic THz pulses with peak output powers reaching 722 W at $108.1μm$ by mixing two coherent beams at about $1μm$ based on phase-matched difference-frequency generation. By stacking two and three GaP plates with their second-order nonlinear coefficients being switched between the adjacent ones, we have increased the peak power at $120.3μm$ from 433.4 W to 1.36 and 2.36 kW, respectively. 2.36 kW corresponds to the photon conversion efficiency of 25%, which is two orders of magnitude higher than our previous result. In contrast, if they are stacked for having the same sign of the nonlinear coefficients, the wavelength corresponding to the highest peak power is red-shifted to 204.8 and $303.9μm$⁠, respectively. Such a result indicates that there is an optimal interaction length for each specific output wavelength.