Dual-frequency multicycle terahertz generation in virtual PPLN

We report the simultaneous generation of multicycle terahertz pulses at two distinct frequencies via optical rectification of a femtosecond laser pulse in a bulk LiNbO3 crystal cut at 65° to its z axis. The nonlinear interaction between the ordinary and extraordinary components of the pump pulse induces an ultrashort pulse of second-order polarization that periodically alternates its polarity during propagation, enabling quasi-phase-matched terahertz generation analogous to that in periodically poled lithium niobate. Using 1.03 μm, 1.2 mJ, 270 fs laser pulses in a forward-emission geometry, we produce sequential terahertz pulses with an equal number of cycles ∼(20–35) and a twofold frequency ratio, achieving an efficiency of ∼3×10−6. By varying the laser incidence angle on the crystal face from −3° to 15°, the generated frequencies are continuously tuned from 1.1 to 0.58 THz for the leading pulse and from 0.55 to 0.29 THz for the trailing pulse. This approach bypasses the fabrication complexities of periodically poled structures and is highly scalable to greater terahertz energies by employing large-diameter LiNbO3 plates and wide-aperture, high-energy pump beams.