Resonantly enhanced terahertz four-wave mixing in fluorides

Converting a THz signal into the optical domain is of great interest for THz sensing and spectroscopy. Here intense broadband THz pulses with a central frequency of $Ω$$_{THz}$ are mixed with an optical pump at $ω$$_p$ and a signal is observed at a wavelength of $ω$$_s$ = 2($ω$$_p$ - $Δω$$_p$) - $Ω_{THz}$ with the detuning $ω$$_p$ - $Δω$$_p$ being due to pump pulse spectral broadening. The observed THz-FWM signal close to 400 nm is shown to result from a resonantly amplified four-wave mixing in CaF$_2$, BaF$_2$ and MgF$_2$.