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-four-wave mixing (FWM) signal close to 400 nm is shown to result from a resonantly amplified four-wave mixing in CaF₂, BaF₂, and MgF₂.