| Terahertz imaging technology and functional device attract great interest in the past decades, and have extremely important research value in the application of terahertz detection and communication. In this thesis we experimentally and theoretically studied the terahertz imaging and modulation technology, and especially introduced the terahertz coherence tomography system in details. The vertical resolution of terahertz coherence tomography is100μm, which is obviously improved compared with existing terahertz3D imaging technology. Furthermore, we studied the optical properties of Magnesium-Doped Near-Stoichiometric Lithium Niobate and graphene in terahertz frequency range. By applying an external optical pump field, we researched the modulation properties of this Lithium Niobate crystal and graphene in terahertz range, and should be helpful for the research of terahertz modulator and detector. The main content is listed as follows:(1) We studied the hyperfine spectrum of the terahertz laser which was generated by the CO2laser to pump the CH3OH gas. From the results we could found that there are additional frequencies on two sides of the center frequency, and the interval of two adjacent frequencies is0.15THz. The theoretical analysis of stimulated emission process revealed that this experimental result was caused by the reason of energy level splitting.(2) We experimentally analysed the spot size of terahertz after focused by different effective focal length of off-axis parabolic mirror. The results indicated that the2.52THz laser can be focused to the diameter of0.33mm by the off-axis parabolic mirror with effective focal length of one inch, and it is smaller than the result of the off-axis parabolic mirror with effective focal length of two inch. This analysis applied an approach to improve the lateral resolution of terahertz imaging result.(3) According to optical coherence tomography, we proposed a new terahertz imaging technology-terahertz coherence tomography. Using the medium-pressure mercury Arc lamp as the terahertz source, this imaging technology’s vertical resolution could be smaller than100μm. The result is much smaller than the time-of-flight imaging method and synthetic aperture imaging method in terahertz. In addition, the system based on this technology is very simple and compact, and it has great prospect in the high-precision nondestructive detection application field.(4) By applying an external laser with center wavelength of532nm to pump Magnesium-Doped Near-Stoichiometric Lithium niobate crystal, we measured the modulation properties of its absorption coefficient in terahertz range by using the terahertz time-domain spectroscopy system, and the modulation depth reached to15%. Further theoretical analysis pointed out that the modulation properties were relation to the OH-absorption of terahertz and the light-induced domain reversal in crystal. The Lithium niobate crystal’s conduction loss increased with optical pump intensity increasing, however, the OH-absorption of terahertz and light-induced domain reversal would decrease the absorption coefficients in certain pump intensity range. This experiment result was valuable for the research of Lithium niobate-based terahertz modulator.(5) Using the laser with center wavelength of1064nm as pumping source, we studied the graphene’s conduction modulation properties. We calculated the functional curve between the carrier’s momentum relaxation time and the external optical pump intensity. The result presented a threshold behavior. Further experiment studies demonstrated that the threshold value is dependent on the layer numbers of graphene, and the monolayer graphene’s result was smaller than six layer graphene’s, which satisfied very well with theoretical prediction. This result laid a good experimental and theoretical foundation for the study of terahertz amplifier and modulator. |
PDF Full Text Request