Multicycle terahertz generation in virtual PPLN

Multicycle terahertz pulses are in urgent demand for spectroscopy, manipulation of condensed matter, and particle acceleration. We experimentally demonstrate generating terahertz pulses with tens-of-cycles duration by optical rectification of femtosecond laser pulses in a bulk LiNbO₃ crystal in the regime of nonlinear mixing of ordinary and extraordinary waves. In this regime, the pulse of second-order nonlinear polarization is alternating its polarity during propagation in the crystal, thus providing quasi-phase-matched terahertz generation, very much as in periodically poled LiNbO₃ (PPLN). In the backward-emission geometry, terahertz radiation with a bandwidth of 8 GHz at 0.47 THz was generated by 600 fs, 1.2 mJ, 1.03 μm laser pulses with the efficiency ∼4×10^-6. The technique offers easy and wide frequency tunability. By varying the incidence angle of the laser beam on the crystal surface from 9^∘ to -15^∘, the central frequency of the radiation was tuned from 0.37 to 0.76 THz. The terahertz yield can be scaled up by using large-diameter LiNbO₃ plates and wide-aperture high-energy pump laser beams.