Ultrafast Spectroscopy using High-Power Terahertz Electromagnetic Pulses

首先，利用去年底建成的太赫兹脉冲产生装置和基于EO采样的波形测量装置，对太赫兹脉冲的聚焦和自由空间传播进行了研究。这是在控制脉冲波形的情况下实现高电场的必要条件。其次，采用较短焦距的离轴抛物面反射镜制作了新型光学装置，研究了太赫兹脉冲的聚焦特性。从这些研究中，澄清了以下几点：1)脉冲的负尾部，包括我们在内的许多小组已经观察到，被发现是由构成EO晶体支架的金属结构的波导效应的切断引起的。ii)通过在焦平面上的刀口测量，澄清了频率相关的聚焦特性，这与高斯光束理论的结果很好地一致。iii)最大电场达到11 kV/cm，据我们所知，这是用电磁法直接测得的最大值的世界纪录。

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.