Optimizing multi-cycle THz generation in PPLN: the role of central frequency and temperature

We experimentally investigate the impact of central terahertz (THz) frequency and crystal temperature on multi-cycle (MC) THz pulse generation efficiency in periodically poled lithium niobate (PPLN). Through a comprehensive evaluation of six different PPLN samples, we uncover a significant interplay between quadratic scaling with central frequency and absorption effects, which governs optimal THz generation. At cryogenic temperatures (15 K), the peak conversion efficiency is observed near 1 THz, achieving a favorable balance between enhanced quadratic frequency scaling and minimized phonon-induced absorption losses. In contrast, at room temperature (RT, 295 K), the shoulder of the absorption peak, centered around 7.4 THz, becomes strong enough to push the optimal frequency down to 0.5 THz. These findings may offer guidance for upscaling the energy of narrowband THz sources, facilitating advances in nonlinear spectroscopy, tabletop electron acceleration, and ultrafast THz photonics.