Abstract
Random frequency accessible, ultra-broad-band (1.5–37 THz) THz-wave generation was demonstrated using difference frequency generation (DFG) in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. Such DAST crystals are promising materials for efficient and high-power THz-wave generation because of their very high nonlinearity and low refractive index dispersion between the near-infrared region and the THz-wave region. We can use the highest nonlinear component of DAST, d11 (about 230 pm/V), to generate THz waves using DFG because the collinear phase-matching condition of the Type 0 configuration is satisfied. We constructed a dual-wavelength optical parametric oscillator (OPO) with two KTP crystals pumped by a frequency-doubled Nd:YAG laser. Each KTP crystal was set on a galvano scanner. The angle of each crystal was controlled independently. The OPO is tunable at 1300–1900 nm, giving an ultra-broad tunable range of the THz wave. We generated an ultra-broad tunable THz wave using only one DAST crystal without any change of the experimental setup aside from the computer-controlled galvano-scanner angle change. The highest THz-wave energy of 10 nJ was obtained at around the 26 THz region under 2 mJ of pumping energy. Also, the THz-wave source can access a desired THz frequency at every pulse (50 Hz at present). The galvano scanner has 1 kHz of response, with 1-ms frequency access speed.