Terahertz hyper-Raman time-domain spectroscopy of gallium selenide and its application in terahertz detection

We report the observation of Terahertz (THz) hyper-Raman generation in a gallium selenide crystal. This nonlinear optical process derives from the four- and five-wave-mixing of femtosecond optical pulses and intense, subps, broadband terahertz pulses. The wavelength spectrum of the resulting signal displays two pronounced frequency sidebands close to the optical second-harmonic central frequency 2 ω L, where ωL is the optical central frequency of the fundamental beam. The two sidebands develop around the central frequency at the (anti-) Stokes side of ω s, a = 2 ω L ω T, where ωT is the THz central frequency. This nonlinear optical process is used for the coherent detection of intense and broadband terahertz waves. The proposed technique shows a good linear response of up to 90 kV/cm and a better efficiency in detecting the lowest terahertz frequencies, as compared to the standard electro-optic sampling performed in two different nonlinear crystals.