Pulsewidth and chirp dependence of THz generation efficiency in PPLN at different central frequencies

We report a systematic study of multi-cycle (MC) terahertz (THz) generation efficiency in periodically poled lithium niobate (PPLN) via optical rectification of a broadband laser pulse. By varying pump pulsewidth (65 fs to 5 ps), chirp, central THz frequency (0.25–1.55 THz), fluence, and crystal temperature (15–300 K), we show that negatively chirped pulses achieve up to two-fold higher efficiency than positively chirped ones, especially at higher THz frequencies. This gain arises from temporal compression by the positive group velocity dispersion (GVD) of lithium niobate (LN) and from reduced self-phase modulation (SPM) and Kerr-induced distortions. The optimal pulsewidth increases with fluence but decreases with THz frequency and is generally longer than bandwidth-overlap predictions, except when higher-order quasi-phase-matched (QPM) emission favors shorter durations. Cryogenic cooling further enhances the efficiency and sharpens the pulsewidth dependence. Together, these results provide quantitative design rules for efficient, tunable narrowband THz sources in PPLN.