【作者】 左志高；

【导师】 姚建铨； 凌福日；

【作者基本信息】 华中科技大学 ， 光学工程， 2013， 博士

【摘要】 太赫兹成像技术及其功能器件的研究是目前太赫兹研究领域的热门课题,在太赫兹检测及通信的应用中具有极其重要的研究价值。本论文主要针对太赫兹成像技术及调制技术进行了实验和理论研究,尤其是对太赫兹相干层析成像技术进行了系统的介绍与分析。通过与现有的太赫兹三维成像技术相对比,我们所提出的太赫兹相干层析成像技术的纵向分辨率有了明显的提高,达到1001μm。此外,我们对掺镁近化学计量比铌酸锂晶体和石墨烯材料在太赫兹波段的光学性能进行了研究；通过光泵浦的方法,我们还研究了铌酸锂晶体及石墨烯材料在太赫兹波段的调制特性,为太赫兹调制器及探测器的研究提供了一定的实验依据。本论文主要研究内容如下：(1)研究了CO2激光泵浦CH3OH气体太赫兹激光器的超精细输出光谱。实验结果中在中心频率两侧均匀的分布其它频率,相邻两个频率之间的间隔为0.15THz。通过对受激辐射过程的理论分析,得出能级分裂导致是导致这一实验现象的直接原因这一结论。(2)分析了不同焦距的离轴抛物面镜对太赫兹光束的聚焦效果。结果表明,2.52THz频率的太赫兹激光经过一寸有效焦距的离轴抛物面镜聚焦后在焦点处的光斑直径为0.33mm,小于两寸抛物面镜的聚焦效果,在太赫兹成像应用中也能得到更高的横向分辨率。(3)与光学相干层析成像技术相结合,我们提出了一种太赫兹相干层析成像技术。该技术的纵向分辨率可达100μm以下,这一实验结果高于太赫兹飞行时间成像技术和合成孔径成像技术。此外,该技术具有系统结构简单、紧凑等特点,在高精度的材料无损探伤领域具有及其巨大的应用前景。(4)使用中心波长为532nm的连续激光泵浦掺镁近化学计量比铌酸锂晶体,通过太赫兹时域光谱系统测量了晶体吸收系数在太赫兹波段的调制特性,调制深度达15%。进一步的理论分析指出,外光场泵浦对铌酸锂晶体在太赫兹波段吸收系数的调制特性与光泵过程中OH-离子的吸收及光致畴反转过程有关。铌酸锂晶体的光电导损耗会随着泵浦光强的增加而增加,而OH-对太赫兹的吸收及光致畴反转过程会使得铌酸锂晶体的吸收系数在一定泵浦光强范围内出现下降。这一实验结果对基于铌酸锂的太赫兹调制器件研究具有一定参考价值。(5)使用中心波长为1064nm的激光泵浦,实现了石墨烯电导率在太赫兹波段的调制特性。根据实验测量结果,计算了光泵过程中载流子动量弛豫时间随泵浦光强的变化曲线。结果显示出载流子动量弛豫时间随泵浦光强的增加而减小,并且变化速度先快后慢,呈现出阈值现象。进一步的实验研究表明,这一阈值与石墨烯的层数有关,并且单层石墨烯的阂值小于六层石墨烯样品的阈值。该实验结果为研究石墨烯在太赫兹波段的放大及调制特性奠定了良好的实验及理论基础。更多还原

【Abstract】 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.更多还原