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 LiNbO 3 or MgO-doped LiNbO 3 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 100 MHz (0.0033 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.
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Kawase et al. (2002) studied this question.