Research on Coherent Tunable Terahertz Source by using Nonlinear Optical Effect

利用调q Nd: YAG激光器泵浦LiNbO_3的光参量振荡（OPO），研究了连续可调谐太赫兹波的产生。所涉及的过程是LiNbO_3中a_1对称软声子模式的受激极化子散射的OPO。在满足非共线相位匹配条件的情况下，通过稍微改变泵浦波与斯托克斯波之间的夹角，可以获得可广泛调谐的太赫兹波。提出了在LiNbO_3晶体上采用单片光栅耦合器和棱镜耦合器的新型耦合方法。通过引入棱镜耦合器，我们发现硅棱镜耦合器对太赫兹波的辐射角变化仅为0.03‰，比传统的角度表面耦合器方便得多。当Nd: YAG激光器抽运能量为34.5mJ时，测量到的太赫兹输出功率为3mW，在140 ~ 310 μ m （0.97 ~ 2.15 thz）范围内获得了有效的可调谐太赫兹辐射。我们还实验研究了太赫兹波产生的低温特性。当温度降至78K时，1.6THz下的吸收系数α从30.6cm^<-1>降至6.9cm^<-1>。在78K下，我们获得了125倍高的太赫兹波输出，与室温下相比，阈值降低了32%。该太赫兹波源在结构紧凑、可调性好、操作方便等方面具有显著的优势。它将在太赫兹波段的测距、成像和光谱等诸多应用中发挥重要作用。我们利用1m长的金属空心管将太赫兹辐射应用于水蒸气的吸收光谱，首次证明了两个波长之间的差分成像强调了标记物体的图像，并且吸收系数依赖于太赫兹波长。

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.