Abstract
Stimulated polariton scattering in lithium niobate is an effective means to generate the THz radiations. All the previously reported THz SPS from LN shows a stand-alone gain peak about 2 THz, resulting from the lowest Ai-symmetry vibration mode near 250 cm -1 . Generation of THz radiation from lithium niobate with frequencies higher than 2.5 THz has been considered difficult due to sharply increased material absorption. Since lithium niobate is relatively low-cost and high-quality, it is desirable to extend the useful THz radiation bandwidth of LN to higher frequencies. By using the so-called off-axis THz parametric oscillator, we report the discovery of a phase-matched gain peak near 4 THz from lithium niobate. We also demonstrated a signal-seeded lithium-niobate off-axis THz parametric oscillator that generates 1.4 nJ pulse energy at 4 THz with 17.5 mJ pump energy at 1064 nm.