Widely tunable terahertz wave generation using organic salt DAST

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A dual signal-wave quasi-phase-matched optical parametric oscillator was successfully demonstrated by use of periodically poled LiNbO_3 with cascade gratings. A compact THz-wave or far-infrared source was realized by difference frequency generation in DAST crystal using the DSW-QPM-OPO as a source. Since it is possible to vary the two signal wavelengths widely, a broad tuning range in the THz region is easily obtained. A PPLN with two periods on a pump-path has two energy conservation and phase matching conditions, so that one pump-wave can generate two signal-waves. We applied this light source to THz-wave difference frequency generation usilng a highly efficient organic ionic salt crystal of 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) which we grew. The incident beams were focused by a lens (f=100mm) on a spot about 0.5mmφ in size. The THz-wave generated was monitored with a 4K Si-bolometer. The typical THz-wave output was 10fJ/pulse. The THz wavelength was measured using a scanning Fabry-Perot etalon consisting of two metal mesh plates. The polarization direction of the THz-wave was shown to be parallel to the a-axis of the crystal by using a wire grid polarizer.