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 $\Omega$$_{THz}$ are mixed with an optical pump at $\omega$$_p$ and a signal is observed at a wavelength of $\omega$$_s$ = 2($\omega$$_p$ - $\Delta\omega$$_p$) - $\Omega_{THz}$ with the detuning $\omega$$_p$ - $\Delta\omega$$_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$_2$.
