Monochromatic-Tunable Terahertz-Wave Sources Based on Nonlinear Frequency Conversion Using Lithium Niobate Crystal

We review widely tunable terahertz (THz)-wave generation by optical parametric processes using lithium niobate crystal. Applying the parametric oscillation of or a MgO-doped crystal pumped by a nanosecond Q-switched Nd:YAG laser, we realized coherent THz-wave sources with a simple configuration widely tunable in the range of 0.7-3 THz. For efficient coupling of the THz wave, we used a monolithic grating coupler or a Si-prism array coupler. In addition, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A line width of about 110 MHz (0.003 cm^-1) was assured by measuring the absorption spectrum of low-pressure water vapor. Using the difference frequency generating method with a periodically poled crystal, we achieved higher conversion efficiency and realized continuous THz-wave generation. This room temperature operated, tabletop system promises to be a widely tunable THz-wave source suited to a variety of applications.