Injection-seeded backward terahertz-wave parametric oscillators at 0.3 and 0.5 THz bands for nondestructive imaging applications

We have demonstrated injection-seeded backward terahertz (THz)-wave parametric oscillators (BW-TPOs) based on a slant-strip-type periodically poled lithium niobate (PPLN) crystal with two different poling periods. The BW-TPOs were pumped by sub-nanosecond pump pulses at 1064 nm and designed for generating backward-propagating THz waves around 0.30 and 0.46 THz with PPLN poling periods of 53 and 35 mu m, respectively. As a result of an optical injection seeding for the forward-propagating idler wavelength in the BW-TPO process, we achieved over a 1000-fold enhancement in backward-propagating THz-wave output energy, a 63% reduction of the oscillation threshold, and long-term stable operation compared to the unseeded case. Furthermore, we demonstrated that the oscillation frequency of backward-propagating THz waves is continuously tunable in the range of 0.27-0.35 and 0.41-0.52 THz for the poling periods of 53 and 35 mu m, respectively, by angle tuning of the PPLN crystal in parallel with seed wavelength tuning. Using the developed injection-seeded BW-TPOs, we also performed the THz-wave imaging test in transmission geometry for various materials, including glass, wood, and liquids.