Cherenkov Phase Matched Monochromatic Tunable Terahertz Wave Generation

Terahertz (THz) waves present attractive possibilities in advanced applications including biomedical analysis and stand-off detection for hazardous materials. The development of monochromatic and tunable coherent THz-wave sources is of great interest for use in these applications. Recently, a parametric process based on second-order nonlinearities was used to generate tunable monochromatic coherent THz waves using nonlinear optical crystals (Boyd et al., 1972; Rice et al., 1994; Shi et al., 2002; Tanabe et al. 2003). In general, however, nonlinear optical materials have high absorption coefficients in the THz-wave region, which inhibits efficient THz-wave generation. Avetisyan et al. proposed surface-emitting THz-wave generation using the difference frequency generation (DFG) technique in a periodically poled lithium niobate (PPLN) waveguide to overcome these problems (Avetisyan et al., 2002). A surface-emitted THz wave radiates from the surface of the PPLN and propagates perpendicular to the direction of the pump beam. The absorption loss is minimized because the THz wave is generated from the PPLN surface. Moreover, the phase-matching condition can be designed using PPLN with an appropriate grating period (Sasaki et al., 2002). Surface-emitted THz-wave devices have the potential for high conversion efficiency, and continuous wave THz-wave generation has been successfully demonstrated (Sasaki et al., 2005). Unfortunately, the tuning range of the THz waves is limited to about 100 GHz by the nature of PPLN, and a wide tuning range cannot be realized using the quasi-phase–matching method. We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses (Auston et al., 1984; Kleinman et al., 1984; Hebling et al., 2002; Wahlstrand, 2003; Badrov et al., 2009), producing very high peak power (Yeh et al., 2007), these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz waves using a nanosecond pulsed laser source.