Published Online at stacks.iop.org/JPhysD/34

Widely tunable terahertz (THz) wave generation by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals is reviewed. Using parametric oscillation of LiNbO3 or MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized widely tunable coherent THz-wave sources in the range between 0.7 and 3 THz, with a simple configuration. For the efficient coupling of the THz wave, a monolithic grating coupler or an Si-prism array coupler was used. We report the detailed characteristics of the oscillation and the radiation, including tunability, spatial and temporal coherency, uni-directivity, and efficiency. Further, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A linewidth of about 100MHz (0.003 cm−1) was assured by the absorption spectrum measurement of low-pressure water vapour. At the same time, the THz-wave output was increased hundreds of times higher than that of a conventional generator which has no injection seeder. In addition, a wider tunability was observed using a tunable diode laser as the injection seeder. This room-temperature-operated, tabletop system promises to be a new widely tunable THz-wave source that is suited to a variety of applications. 1