随着高速半导体器件的发展，高电子迁移率晶体管（HEMT）的工作频率已经达到亚太赫兹波段，所以无法简单地通过传统电学方法进行检测。鉴于此，必须采用超快光学方法测定HEMT器件的截止工作频率。利用持续时间更短的飞秒脉冲激光瞬时关断处于饱和工作状态下的HEMT器件，并且采用太赫兹时域光谱技术测量在器件被关断后电流的变化情况（时间是皮秒量级），最后，利用所测量到的太赫兹波形（即源漏电流随时间变化的曲线）和截止工作频率的关系，直接推算出可达到亚太赫兹波段的HEMT器件的截止工作频率。

With the development of high-speed semiconductor devices, the cutoff frequency of high electron mobility transistor (HEMT) can achieve sub-terahertz level, which cannot be simply measured by traditional electrical method. Therefore, the ultrafast optical method is employed to measure the cutoff frequency of HEMT. The femtosecond laser pulse, which has extremely short duration, is used to suddenly turn off the HEMT that is under saturated condition. Furthermore, the terahertz time-domain spectroscopy technology is applied to measure the change of operation current, which can persist even less than 1 ps after the HEMT is switched off. Finally, the cutoff frequency of HEMT can be directly calculated by using the relationship between the terahertz traces (i.e., the curve of the source-drain current versus time) and the cutoff frequency of HEMT which can achieve sub-terahertz level.