Ultra broad band (from 1.5 to 37 THz) THz-wave generation using difference frequency generation (DFG) in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal was demonstrated. A DAST crystal is one of the promising materials for efficient and high power THz-wave generation, because of its very high nonlinearity and low refractive index dispersion between the near infrared region and the THz-wave region. We can use DAST's highest nonlinear component of, d11 (about 230pm/V), to generate THz-wave with by means of DFG, because the co-linear phase matching condition of the Type0 configuration is satisfied. We constructed a dual-wavelength optical parametric oscillator (OPO) with two KTP crystals pumped by frequency doubled Nd:YAG laser. Each KTP crystal was set on a Galvano scanner and the angle of each crystal was controlled independently. The OPO has a tunable range from 1300 - 1900 nm, results in an ultra broad tunable range of the THz-wave. We successfully generated ultra broad tunable THz-wave just using only one DAST crystal without any change of the experimental setup, except the computer controlled Galvano scanner angle change. The Hhighest THz-wave energy of 10 nJ was obtained at around 26 THz region under 2 mJ of pumping energy. And also, the THz-wave source can access an arbitral THz frequency at for every pulse (50 Hz at present). The Galvano scanner has a 1 kHz of responsibility response and we would obtain 1 msec of frequency access speed.
A continuous and tunable operation from 1.6 to 4.5 THz has been demonstrated using a difference frequency generation (DFG) scheme by using the nonlinear optical crystal 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) as the DFG crystal. The scheme was based on DFG between two seeded optical parametric generators. The
THz bandwidth was 2.4 GHz. A high-resolution transmission spectrum of water vapor in air was used to demonstrate the utility of this source over the above THz spectral range.
Organic nonlinear optical crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate (DAST) can be used for new optical devices such as high frequency electro-optical sampling wavelength conversion, submillimeter wave generation, and terahertz-wave generation. The crystal is soft, brittle and hygroscopic so that it is very difficult to get optical surfaces by using conventional optical polishing process. This paper deals with single-point diamond turning for getting optical surfaces on DAST crystals. Three typical planes on DAST crystals were finished by single-point diamond turning. The quality of single-point diamond turned surface depends upon the crystallographic plane, cutting direction, cutting speed, depth of cut, feed rate, rake angle, nose radius of diamond tool, tool clearance angle, tool wear, lubricant and its supplying method. The turned surface was measured with a Nomarski interference microscope, atomic force microscope, and a three-dimensional optical profiler. By optimizing the machining conditions, 0.33 nm rms surface roughness and 8.7 nm p-v flatness in 1 mm square were obtained on a b-plane of DAST crystal.
Spectroscopic method was used to observe pulse laser induced ablation plume of graphite. Time-integrated emission spectrum of ablation plume of graphite was measured for laser wavelengths of 1064 nm, 532 nm and 308 nm. TOF measurements were also made to determine both the species of plume constituents and their velocity as they move from the target surface. By analyzing the obtained spectrum, several species were identified, i.e., at least a carbon ion, a carbon atom and a C2 molecule. Dynamic vibrational temperature was estimated for the obtained spectra of swan bands based on the statistical mechanics. The ablation mechanisms were discussed considering the non-equilibrium nature of the plume.
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