Intracavity third-harmonic generation in Si:B pumped by intense terahertz pulses
Fanqi Meng, Mark D. Thomson, Qamar ul-Islam, Bernhard Klug, Alexej Pashkin, H. Schneider, Hartmut G. Roskos
Abstract
We observe third-harmonic generation (THG) in boron-doped silicon (Si:B) upon pumping with picosecond 1.56-THz pulses from a free-electron laser with a peak electric field strength of up to 12 kV/cm. 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 4. The observations can be explained by THz emission by free holes accelerated in the nonparabolic valence band, under the assumption that the density of free holes increases with the pump intensity. A quantitative treatment supports that these carriers are generated by impact ionization, initiated by the population of thermally ionized carriers, as opposed to direct tunneling ionization. In addition, we also observe intracavity THG by embedding the Si:B in a one-dimensional photonic crystal cavity. The THG efficiency is increased by a factor of eight due to the field enhancement in the cavity, with the potential to reach a factor of more than 100 for pump pulses with a spectrum narrower than the linewidth of the cavity resonance.