Cherenkov-type terahertz generation by optical rectification in KD2PO4 (DKDP) crystal

Abstract

The potential of KD2PO4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼200−300 μJ energy were converted to broadband (∼4 THz bandwidth) terahertz transients with the efficiency of ∼8×10−6. The transients produced electro-optic signals of a ∼0.15 modulation depth and ∼103 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d36≈3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation.