电光取样材料性能对THz光谱探测的影响

【作者】 贾新峰；

【导师】 赵国忠；

【作者基本信息】 首都师范大学 ， 光学， 2008， 硕士

【摘要】 理想情况下,通过电光取样直接测量得到的太赫兹(THz)信号就代表了THz脉冲的原始波形。然而,在实验中观察到的THz信号经常会发生变形。这种变形可能是由诸多因素造成的,一个因素是电光取样过程中相位失配造成的影响,另一个因素是THz脉冲在激发晶体和电光取样晶体中传播时色散和晶体界面反射造成的影响。另外,水蒸气的吸收也可以导致THz脉冲波形的改变。本论文主要研究电光晶体的性能对于THz脉冲探测的影响,这些电光晶体的性能主要包括:横光学声子频率,介电常数,折射率,吸收系数等。由于探测晶体本身性能的限制,使得其探测能力受到了很大的影响。我们常用到的ZnTe晶体横光学声子频率在5.3THz处,靠近这个频率的THz脉冲成分由于共振吸收而衰解,因此其可探测的最大频率范围被限制在0-5THz。在国外,用GaP晶体代替ZnTe晶体,可使其可探测频率范围扩展到0-7 THz。本论文根据THz脉冲与电光晶体相互作用的基本理论,通过用Matlab编程计算模拟了电光晶体性能对THz脉冲探测的影响。这项研究有助于THz初始脉冲的重建和探测晶体的选择。另外,对于飞秒超快动力学的研究有助于对THz产生过程的深层了解。载流子寿命是影响THz脉冲频谱宽度的重要原因之一,通过对载流子寿命的测量可以从一个侧面了解THz辐射的谱宽特性。本论文完成了如下研究工作:(1)根据相关理论用Matlab程序模拟了THz脉冲在介质中的传播特性。THz脉冲在介质中色散特性、THz脉冲在晶体界面的反射以及探测脉冲与THz脉冲的相位匹配会对THz脉冲波形产生影响。这些影响已在实验中观察到,通过理论上加以解释和模拟,能够使我们对THz脉冲波形的变化得以深刻地理解和修正。本文得到的一些结果包括:THz脉冲在介质中的色散特性会导致THz脉冲中的高频部分被吸收;THz脉冲晶体表面的反射会导致THz脉冲中的高频分量被反射;相位匹配会影响探测到的THz脉冲波形。(2)模拟了不同晶体对于高频THz脉冲的探测特性。我们发现晶体中的横光学声子频率是影响晶体探测性能的一个重要参数,当THz脉冲的频率分布靠近探测晶体中的横光学声子频率时,由于太赫兹波与晶体横光学声子发生共振而被吸收,使得探测到的THz频谱在横光学声子频率处出现凹陷。通过模拟我们发现,横光学声子频率较高的晶体,如GaP晶体等,对于高频THz脉冲的探测性能会较高。(3)研究了飞秒脉冲激发THz脉冲的超快过程所涉及的物理机制;搭建了测量载流子寿命的装置,得到了半绝缘GaAs的载流子寿命,这一工作对于选择THz发射极材料具有重要的参考价值。在测量过程中我们发现,当泵浦脉冲的强度和探测脉冲的强度为10:1时,测量效果会较好。我们所测得的半绝缘GaAs载流子寿命在0.5ps-0.6ps之间,同样的我们可以研究其它半导体材料的载流子动力学,从而对选择THz发射极材料提供指导。综上所述,本文考虑了影响THz脉冲探测的各个因素,通过理论分析和数值模拟,说明了这些因素对THz脉冲的探测所造成的影响。我们发现,晶体中的横光学声子频率是影响探测晶体性能的一个重要参数,横光学声子频率较高的晶体,如GaP晶体等,可以用于探测包含更高频率成分的宽带THz脉冲。更多还原

【Abstract】 Idealy, the THz pulse shape is that measured directly, but the THz pulse shape obtained is distorted in experiment. This distortion is caused by several factors, one is the phase matching of the electro-optic sampling process, another one is the dispersive and reflection in the process of THz pulse propagation in the which crystal the THz pulses are generated and the electro-optic sampling crystal. The absorption of ambient water vapor can also lead to distortion.This thesis studies the character of the electro-optic crystal which affects the THz pulse detection. The properties of these electro-optic crystal include: transverse-optical- (TO) phonon resonance frequency dielectric constant, refractive index, absorption coefficient, etc. the character of these electro-optic crystal limit abilitivity of detection. We used the ZnTe crystal, in which the transverse optical phonon frequency at 5.3 THz, This makes around the THz pulse frequency of 5.3 THz is absorbed by the resonance, So its detection frequency range of 0-5 THz. In foreign countries, instead of using GaP crystal by ZnTe crystals, the frequency range extended to 0-7 THz.According to the relevant physics formulas .We make a program by Matlab, to simulate the changes of the crystal THz pulse detection property. The research is contributed to the in THz pulse reconstruction and detection crystal choice. In addition, the ultrafast-femtosecond Dynamics of THz contribute to a deeper for understanding of the process. The carrier life related with THz pulse width of the spectrum, by the carrier lifetime measurements can be found out THz pulse properties of THz pulse width of the spectrum. This thesis has already some results: (1)In accordance with the relevant formula using Matlab simulation procedures the THz pulse propagation in the medium, THz pulse dispersion in the medium, THz pulse reflection of the crystal surface as well as the pulse phase matching, which will influence the THz pulse. These effects have already been in the experiment performance, we will now rise to the theory, And get some results, these results include: THz pulse in the medium dispersion characteristics will lead to THz pulse of high-frequency part absorption, crystal surface reflection will lead to high frequency components easily reflection, the detection phase-matched will affect the THz pulse waveform. (2)We simulate in the different detection crystal for the high-frequency THz pulse characteristics. We found the transverse phonon resonance is an important parameter affected the detection properties .THz pulse frequency distribution around the transverse phonon resonance frequency will vanish due to the resonance absorption, Detection THz spectrum will be deficiencies. We found that by using of the higher of transverse phonon frequency in crystal, such as GaP crystal, and so on, the better of high-frequency THz pulse detection performance.(3) The physical mechanism of the THz pulse femtosecond pulses of ultrafast excitation process is studied. We have measurement carrier lifetime of semi-insulating GaAs. By measurement of the pump pulse and probe pulse intensity and the intensity of 10:1, measured results would be better.In summary, the influence the various factors to THz pulse detection, Through simulation and theory analysis we found the crystal transverse phonon resonance frequency crystal is an important parameter which impact detection properties, the higher of transverse phonon resonance frequency in crystal, such as GaP crystal, and so on, the better of high-frequency THz pulse detection performance.更多还原