Wide Spectrum Terahertz-Wave Generation From Nonlinear Waveguides

We have obtained efficient terahertz (THz)-wave generation from surface-emitting or waveguide propagation using Cherenkov-type radiation with nonlinear optical (NLO) crystals. This approach presented the following advantages: 1) many crystals can be used as THz-wave emitters; 2) it is not necessary to satisfy the phase-matching condition inside the crystal; and 3) THz-wave generation is not suppressed by the absorption in the crystal. We demonstrated that the THz-wave generation was enhanced 50× by suppressing the phase mismatch with a surfing configuration for bulk lithium niobate (LiNbO₃) crystals. Using the prism-coupled Cherenkov phase-matching method with organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate, we produced tunable THz radiation within ~0.1-10 THz. There was no evidence of significant absorption in the crystal. To show the advantages of the waveguide emitter, we demonstrated THz generation using a ridge-type LiNbO₃ waveguide with a thickness of 3.8 μm. We obtained THz-wave generation with an ideal temporal half-cycle pulse and a wide tuning range, which is applied to reflection tomography imaging.