Broadband GaP contact-grating terahertz source pumped at 3.9 µm

A gallium phosphide (GaP) semiconductor contact-grating THz source with trapezoidal grating groove profile, pumped at a mid-infrared wavelength of 3.9 μm, was demonstrated. The source can be scaled to high electric fields beyond 1 MV/cm due to the plane-parallel geometry, the availability of large GaP single crystals of good quality, and the large bandwidth extending from 0.2 THz to 6 THz. Due to the long pump wavelength, the lowest-order effective multiphoton absorption was 9-photon absorption, which enabled exceptionally high usable pump intensities up to 310 GW/cm². Saturation of THz generation efficiency occurred at a high pump intensity of about 160 GW/cm², caused by pump-induced free-carrier generation which can be explained by multiphoton and Keldysh tunnel ionization models.