Discovery of high-gain stimulated polariton scattering near 4 THz from lithium niobate.

Lithium niobate is the most popular material for terahertz wave generation via stimulated polariton scattering (SPS), previously known to have a gain peak near 2 THz. Here we report the discovery of another phase-matched gain peak near 4 THz in lithium niobate, which greatly extends the useful gain spectrum of lithium niobate. Despite the relatively high 4 THz absorption in lithium niobate, the 4 THz SPS becomes dominant over the 2 THz one in an intensely pumped short lithium niobate crystal due to less diffraction-induced absorption and mode-area mismatch. We also demonstrate a signal-seeded OTPO that generates 1.4 nJ at 4.2 THz from lithium niobate with 17.5 mJ pump energy.