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题名	基于GaSe晶体和金属薄膜的太赫兹辐射源
作者	姜可
学位类别	博士
答辩日期	2015-05
授予单位	中国科学院大学
导师	谢京江,谢冀江
关键词	GaSe晶体 差频 光整流 金属薄膜 太赫兹辐射源
其他题名	Terahertz sources based on GaSe crystal and metal films
学位专业	光学工程
中文摘要	太赫兹辐射因其特殊的性质，在材料光谱、生物医学、空间通信、国际反恐等基础研究和技术应用方面有广阔的前景，而太赫兹辐射源的发展是推动太赫兹应用技术进步的关键。本文基于GaSe晶体这一实现太赫兹辐射最有前景的非线性晶体，对差频输出窄带太赫兹辐射的相位匹配类型进行了理论上的模拟计算和分析比较，通过超快脉冲的光整流获得宽带太赫兹输出并研究了太赫兹辐射对泵浦光强的依赖关系。着眼于新型材料和结构，本文在金属薄膜中同样获得宽带太赫兹输出，并对其太赫兹辐射机理进行了研究与分析。该工作为实现高功率输出和多样化的太赫兹辐射源提供了理论基础和初步的实验结果。本文主要研究内容如下： 1. 利用小型调Q可调谐CO2激光器，基于GaSe晶体，实现了基频9.2-10.7μm的可调谐倍频输出。根据相位匹配角与基频波长的关系，在众多已报道的色散方程组中，确定了GaSe晶体在红外波段的最佳色散方程组； 2. 基于非线性差频原理，以可调谐CO2激光器泵浦GaSe晶体产生窄带太赫兹辐射为例，在三波共线情况下，对oe-e、oe-o和新型oo-e三种相位匹配方式的角度调谐特性、太赫兹输出光强以及相位匹配的允许参量进行了理论计算。计算结果表明，在对GaSe晶体进行适当切割后，oo-e相位匹配方式可以实现太赫兹输出，而且在角度调谐、泵浦光的允许参量和实验操作方面具有明显优势。 3. 基于超快脉冲的光整流，在GaSe晶体中实现0-3THz的宽带太赫兹辐射。随着泵浦光强的增加，太赫兹输出电场强度受到泵浦光双光子吸收的影响，与泵浦光强呈亚线性关系。双光子吸收不仅增加了泵浦光的损耗，抑制了太赫兹辐射的输出，同时也激发了自由载流子，导致晶体对太赫兹辐射吸收的增加。利用Drude-Lorentz模型，以及GaSe晶体自由载流子的平均弛豫时间τ=0.38fs，对太赫兹输出和泵浦光强之间的关系进行了拟合。测量了掺杂Er和In元素GaSe晶体的双光子吸收系数，提出采用元素掺杂的方法来降低双光子吸收，以提高太赫兹转换效率。 4. 在超快脉冲的激发下，利用光滑的金薄膜及其纳米孔结构实现0-3THz的宽带太赫兹辐射。对太赫兹输出与入射角、泵浦波长和光强的关系进行了系统地实验研究，并分析了太赫兹辐射的产生机理。结果表明，在激光脉冲的激发下，电子从金属表面逸出，电子的运动形成瞬态电流，从而向外辐射太赫兹电场。在该过程中，表面等离子激元对太赫兹输出强度进行调制。随着泵浦光强的增加，电子的逸出机制表现出从多光子电离到隧穿电离的过渡，最终导致太赫兹输出与泵浦光强之间的非线性关系。当泵浦光的波长发生改变时，表面等离子体激元的寿命随之改变，太赫兹输出频谱宽度受到影响。因此，通过表面等离子体激元可以实现对太赫兹输出强度和频谱宽度的主动调制。
英文摘要	Terahertz (THz) radiation, benefiting from its unique property, has a promising application on basic and applied science, such as spectroscopy, material science, biology, communication and international anti-terrorism. THz sources play a key role in THz application. In this thesis, the phase-matching types in THz difference frequency generation (DFG) process are calculated and compared based on the most potential nonlinear crystal on THz source - GaSe. The broadband THz radiation is obtained from GaSe based on the optical rectification and the pump intensity dependence of THz output is studied. Besides, the metal films are used to emission broadband THz radiation and the mechanism is discussed. This work applys the theoretical and experiment basis for the high power and varies THz sources. Following is the summary of main research work: 1. Second harmonic generation and dispersion property of GaSe crystal are researched by a polarized and tunable, acousto-optically Q switched CO2 laser. Among the available GaSe dispersion equations, the most suitable dispersion equations in IR range for GaSe is determined, according to the comparison between measured and calculated external phase-matching angle results in varies fundamental wavelengths from 9.2-10.7μm. 2. Based on CO2 laser and GaSe crystal, the phase-matching parameters in the THz-DFG process, including the phase-matching angle, THz output intensity, acceptance parameters, are calculated and compared under collinear propagation situation. The result shows oo-e phase-matching type can realize THz generation with a phase-matching angle cut GaSe crystals. Besides, the oo-e phase-matching type has the optimum angle-tunable property, acceptance parameters and operation feasibility. 3. The 0-3THz THz radiation is realized in GaSe crystal based on the optical rectification of ultrafast laser pulses. Owing to the two-photon absorption the pump intensity dependence of THz output changes from linear at lower pump intensity to sub-linear at higher pump intensity. The two-photon absorption can deplete the pump intensity to reduce the THz radiation; at the same time, it excites the free carriers that increase the THz absorption. To simulate the sub-linear pump intensity dependence of THz output, the two-photon absorption coefficient and the dielectric function of GaSe at THz range are measured. With Drude-Lorentz model, the relaxation time of free carriers is obtained to be 0.38fs and the pump intensity dependence of THz output is simulated which agrees with the experiment result. At last, the Er and In doped GaSe crystals are prompted to reduce the effect of two-photon absorption. 4. The gold film and nanohole structures are studied as THz emitters excited by ultrafast laser pulses and 0-3THz radiation is detected in far-field. The relationship between THz output and incident angle, pump intensity and pump wavelengths is studied with a seirs of experiments. With laser excitation, the ionization and motion of photoelectrons results in a femtosecond current surge in near-field and terahertz electromagnetic radiation in far-field. The laser pulses can also excite the surface plasmon polaritons that enhance the THz emission through the ponder motive acceleration of photoelectrons. This leads to the strongest THz emission in the angle that surface plasmon is optimally coupled. With the increasing pump intensity, the change of relationship between pump intensity and THz emission shows the transformation of electron ionization from mulitphoton ionization to tunnel ionization. Using different pump wavelengths, the lifetime of surface plasmon polaritons can modulate the THz output spectrum range. In conclusion, surface Plasmon polaritons can modulate the THz output enlectric amplitude and spectrum range.
公开日期	2015-12-24
内容类型	学位论文
源URL	[http://ir.ciomp.ac.cn/handle/181722/48847]
专题	长春光学精密机械与物理研究所_中科院长春光机所知识产出
推荐引用方式 GB/T 7714	姜可. 基于GaSe晶体和金属薄膜的太赫兹辐射源[D]. 中国科学院大学. 2015.

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