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 $\mathrm{\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$ - $\mathrm{\Delta }\omega$${}_p$) - ${\mathrm{\Omega }}_{THz}$ with the detuning $\omega$${}_p$ - $\mathrm{\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${}_2$.