Research on Coherent Tunable Terahertz Source by using Nonlinear Optical Effect

点击翻译按钮获取中文摘要

We have investigated a continuously tunable THz-wave generation by using an optical parametric oscillation (OPO) in LiNbO_3 pumped by a Q-switched Nd : YAG laser. The process involved is the OPO by stimulated polariton scattering from the soft phonon mode of A_1-symmetry in LiNbO_3. As noncollinear phase matching condition is satisfied, a widely tunable THz-wave can be obtained by slightly changing the angle between pump and Stokes wavevectors. We proposed a new coupling method such as a monolithic grating coupler and a prism coupler onto a LiNbO_3 crystal. By introducing the prism coupler, we found that the change the radiation angle of THz-wave is only 0.03ﾟ for Si prism coupler, providing this method to be far more convenient than the conventional angled surface coupler. The measured output power of THz-wave was 3mW when pumping energy of Nd : YAG laser was 34.5mJ,and an efficient tunable THz-wave radiation was obtained in the range of 140-310 mu m (0.97-2.15THz).We also experimentally studied cryogenic characteristics of THz-wave generation. As the temperature decreased to 78K,the absorption coefficient alpha was reduced from 30.6cm^<-1> to 6.9cm^<-1> at 1.6THz. We obtained 125 times higher THz-wave output and the threshold was reduced by 32% at 78K,compared to that obtained at room temperature.This THz-wave source has significand advantages in compactness, tunability and case of handling. It can be expected to play an important role in many applications such as ranging, imaging and spectroscopy in THz-wave region. We have applied the THz-wave radiation to the absorption spectroscopy of water vapor by using 1m Iong metal hollow tube and demonstrated for the first time that differential imaging between two wavelengths emphasized the image of a tagged object with an absorption coefficient dependent on THz wavelength.