Ultrafast Spectroscopy using High-Power Terahertz Electromagnetic Pulses

首先，利用去年年底建成的基于电光采样的太赫兹脉冲产生装置和波形测量装置，研究了太赫兹脉冲的聚焦和自由空间传输特性。这对于在控制脉冲波形的情况下实现高电场是必要的。第二，利用离轴抛物面镜制作了一种新的短焦距光学系统，并对太赫兹脉冲的聚焦特性进行了研究。从这些研究中，已经澄清了以下几点：i）已经被包括我们在内的许多小组观察到的脉冲的负拖尾部分，被发现是由构成EO晶体保持器的金属结构的波导效应的截止引起的。ii）通过焦平面刀口测量，阐明了频率依赖的聚焦特性，并与高斯光束理论的结果相符合。iii）已经实现了11 kV/cm的最大场，据我们所知，这是通过EO方法直接测量的值的世界纪录。

Firstly, using the equipment for the terahertz pulse generation and waveform measurement setup based on the EO sampling, both have been constructed by the end of last year, focusing and free-space propagation of terahertz pulses were studied. This is necessary to achieve high electric field with the waveform of the pulses under control. Secondly, new optical setup using off-axis parabolic mirrors with a shorter focal length was newly made, and focusing properties of terahertz pulses were studies. From these studies, the following has been clarified : i) The negative trailing part of the pulse, which has been observed by many groups including us, was found to be caused by the cutoff of the waveguide effects of metal structures constituting the EO crystal holder. ii) By knife-edge measurements at the focal plane, frequency-dependent focusing properties has been clarified, which has been shown to be in good agreement with the results of the Gaussian beam theory. iii) Maximum field of 11 kV/cm has been achieved, which is, to our knowledge, is the world record of the values directly measured by the EO method.