Third Harmonic Generation in Ge:Ga pumped by FLARE

We observe third-harmonic generation (THG) in gallium-doped germanium (Ge:Ga) upon pumping with picosecond sub-THz (0.6-0.7 THz) pulses from the THz free-electron laser FLARE with a peak electric field strength of up to 4 kV/cm (30kW/cm2). The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 3 as expected.