Yasumatsu Naoya, Watanabe Shinichi
Department of Physics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan.
Rev Sci Instrum. 2012 Feb;83(2):023104. doi: 10.1063/1.3683570.
We propose and develop a method to quickly and precisely determine the polarization direction of coherent terahertz electromagnetic waves generated by femtosecond laser pulses. The measurement system consists of a conventional terahertz time-domain spectroscopy system with the electro-optic (EO) sampling method, but we add a new functionality in the EO crystal which is continuously rotating with the angular frequency ω. We find a simple yet useful formulation of the EO signal as a function of the crystal orientation, which enables a lock-in-like detection of both the electric-field amplitude and the absolute polarization direction of the terahertz waves with respect to the probe laser pulse polarization direction at the same time. The single measurement finishes around two periods of the crystal rotations (∼21 ms), and we experimentally prove that the accuracy of the polarization measurement does not suffer from the long-term amplitude fluctuation of the terahertz pulses. Distribution of the measured polarization directions by repeating the measurements is excellently fitted by a gaussian distribution function with a standard deviation of σ = 0.56°. The developed technique is useful for the fast direct determination of the polarization state of the terahertz electromagnetic waves for polarization imaging applications as well as the precise terahertz Faraday or Kerr rotation spectroscopy.
我们提出并开发了一种方法，用于快速、精确地确定飞秒激光脉冲产生的相干太赫兹电磁波的偏振方向。该测量系统由一个采用电光（EO）采样方法的传统太赫兹时域光谱系统组成，但我们在以角频率ω连续旋转的EO晶体中添加了一项新功能。我们找到了一种简单而有用的电光信号公式，它是晶体取向的函数，这使得能够同时对太赫兹波的电场幅度和相对于探测激光脉冲偏振方向的绝对偏振方向进行类似锁相的检测。单次测量在晶体旋转大约两个周期（约21毫秒）时完成，并且我们通过实验证明，偏振测量的精度不受太赫兹脉冲长期幅度波动的影响。通过重复测量得到的测量偏振方向分布，由标准差为σ = 0.56°的高斯分布函数很好地拟合。所开发的技术对于偏振成像应用中太赫兹电磁波偏振态的快速直接测定以及精确的太赫兹法拉第或克尔旋转光谱学很有用。
