Cascaded third-harmonic generation of ultrashort optical pulses in two-dimensional quasi-phase-matching gratings

Cascaded third-harmonic generation (THG) of femtosecond pulses in a two-dimensional periodically poled lithium niobate is demonstrated experimentally. The poling pattern satisfies quasi-phase-matching conditions for both second-harmonic generation (SHG) and the following sum-frequency generation. The pulse-front tilt in noncollinear interactions compensates for group-velocity mismatch among fundamental, second-harmonic (SH), and third-harmonic (TH) pulses, thereby achieving broad acceptance bandwidths in SHG and THG. A 2 mm thick device generates SH and TH pulses of 106 and 110 fs, respectively, from fundamental pulses with temporal duration of 118 fs and a 1568 nm center wavelength. The frequency conversion properties are studied theoretically, and the optimal focusing condition for maximum conversion efficiency is derived. (c) 2007 Optical Society of America.