Fifth-harmonic generation in Si:B pumped with intense terahertz pulses

We observe third-harmonic generation (THG) and fifth-harmonic generation (FHG) of terahertz free-electron laser (FEL) pulses in Si:B at cryogenic temperatures. The intensities of the harmonics increase as a function of pulse energy following power laws with exponents of 4.2 and 6.2, respectively. We attribute the generation of the fifth harmonic mainly to the valence band non-parabolicity. The values of χ(3) and χ(5) increase with the density of mobile carriers, which are excited via impact ionization of the boron dopants under irradiation by the intense FEL pulses.