Optical generation of terahertz pulses with strong DC precursors

Summary form only given. Optical rectification of ultrashort laser pulses in electro-optic crystals is an established technique of terahertz generation. In this technique, the pump optical pulse produces a nonlinear polarization that moves with the group velocity of the optical pulse and acts as a source of terahertz radiation. The up-to-date generation schemes based on this principle utilize both the standard collinear phase matching and more sophisticated noncollinear geometries, such as tilted-pulse-front pumping and Cherenkov radiation in waveguide structures. At high pump intensity, which is required for generating strong terahertz pulses, two-photon (or multi-photon) absorption becomes essential and limits the optical-to-terahertz conversion efficiency. Multi-photon absorption leads not only to the depletion of the pump but also to the generation of free carriers that absorb the terahertz waves and, therefore, free carrier generation is usually viewed as a detrimental effect. We show here that free carrier generation can give rise to a much less trivial physical effect, compared to free-carrier terahertz absorption, namely, to the generation of strong quasistatic (dc) electric and magnetic precursors ahead of the laser pulse.