Ruixiang GUO

The frequency region from sub-THz to several tens of THz has attracted tremendous attention due to its increasing importance for industrial and basic research fields such as chemical identification, structural imaging, nondestructive testing, and medical diagnosis and so on. We have developed an injection-seeded THz-wave parametric generator (is-TPG) with wide frequency tuning, small size, room-temperature operation. It can emit the monochromatic THz-wave over a wide tunable frequency range from 0.4 THz to 2.8 THz with the narrow linewidth of lower than 100 MHz. The frequency scanning rate was as fast as 3GHz/second for the step resolution of 6 MHz by using the 500 Hz high-repetition-rate Q-switch Nd:YAG laser as a pump source.

Fig 1. The experimental setup for is-TPG.

Figure 1 shows the experimental setup for is-TPG. The system consisted of a pump source, a seed source and nonlinear crystals. Owing to the design of the stationary dispersion-compensated optical arrangement, the seed beam automatically entered the MgO: LiNbO3 crystal at the appropriate phase-matching angle, changing the frequency of the seed beam only.
　　Figure 2 shows the picture of a compact, table-top, high resolution THz-wave spectrometer based on the is-TPG. The system is packaged with three chambers: the chamber (�T) of optics for achromatic phase matching of seeder beam, the chamber (�U) of Nonlinear crystals MgO:LiNbO₃ for THz-wave generation, and the chamber (�V) of gas cells for THz-wave spectroscopy. The two chambers for nonlinear crystals and gas cells were designed under an inflow of pure nitrogen gas to eliminate the water vapor in the chambers.

Figure 2. THz-wave spectrometer.

　　THz-wave radiation from MgO:LiNbO₃ was collimated by a cylindrical lens and steered into the chamber of gas cells. A standing wire-grid beam splitter divided the THz-wave radiation into two paths: Reference beam and reference beam. The signal beam was steered into a gas cell with two gas inlet which was via a UHV leak valve for easily achieving the sample, controlling and monitoring its pressure outside.

Figure 3 shows the measured absorption spectrum of water vapor at low pressure　from 1.2 THz to 1.9 THz. Only four minutes were required to scan this 700 GHz frequency range with 6-MHz frequency step and 50 MHz frequency resolution. Twelve pure rotation transitions were observed over this frequency range. The measured absorption lines agree well with the spectrum data from the NASA database, shown as a stick spectrum in Fig. 3.
　　The pressure-varying absorption spectroscopy of water vapor was observed and the N₂ collision broadening coefficient of water vapor is successfully measured using this THz-wave spectrometer. Fig.4 shows the spectra of the two rotation transitions of 6₂₄-6₁₅ and 7₃₄-7₂₅ of water vapor under the different pressure. Linewidth broadening is observed clearly with the increasing of N₂ pressure.

Fig.4 The spectra of the two rotation transitions of 6₂₄-6₁₅ and 7₃₄-7₂₅ of water vapor under the different pressure.