Time-resolved tera-hertz spectroscopy. Ultrafast dynamics probed in the far infrared

The time-domain tera-hertz spectroscopy makes use of ultrashort tera-hertz pulses with a length comparable to their carrier frequency period. This property offers the possibility to carry out experiments in the far infrared with a broad spectral bandwidth and with a sub-picosecond time resolution. From this idea the time-resolved tera-hertz spectroscopy was developed and it has become an essential tool in physics and chemistry for the investigation of ultrafast dynamics of charges and polar excitations. In this paper we explain the basis of the technique and describe - in terms of the transient photoconductivity - some applications in semiconductor systems. The concept of tera-hertz modulators controlled by optical pulses is also briefly introduced. (author)