Generation and Detection of THz Waves

Abstract

Before discussing the nature of THz waves and their applications, it is suitable to introduce how THz waves are generated and detected. As mentioned in Chapter 1, this book will focus on pulsed THz technologies. A typical pulsed THz wave generation and detection system is a pump and probe setup as presented in Fig. 2.1. The most common way that pulsed systems work is by splitting a beam from a femtosecond (fs) laser into two beams: the probe and the pump beams. The pump beam is used to generate the THz pulse, while the probe beam is used to sample and obtain the pulse profile. Detecting of THz field is performed by modulating the probe pulse with the THz field or by accelerating free carriers induced by the probe pulse with the THz field. A mechanical delay line is used to change the time delay between THz pulse and the probe pulse. The THz waveform can be obtained by scanning this time delay. To increase the sensitivity, the pump beam is modulated by an optical chopper, and the THz-induced modulation on the probe beam is extracted by a lock-in amplifier. This pulse information acquired in the time domain is transformed to the frequency domain with a Fourier transform from which spectral information can be obtained.