Strong spectral broadening of Cherenkov-type terahertz radiation by free carrier generation

Free carrier generation is commonly viewed as detrimental to the optical-to-terahertz conversion efficiency in electro-optic crystals due to free carrier absorption of terahertz radiation. Contrary to this view, we show by numerical simulation that free carrier generation can have a positive effect on the Cherenkov-type terahertz emission from ultrashort laser pulses by strongly broadening the radiation bandwidth. The broadening is produced by terahertz emission from a surge current of the optically generated carriers, which are accelerated by the rectified electric field. Specific results are obtained for GaP pumped at a $1.7\text{\ensuremath{-}}\textmu{}\mathrm{m}$ wavelength.