Pulsed terahertz radiation

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

E. Pickwell, B. E. Cole, A. J. Fitzgerald, M. Pepper, and V. P. Wallace, In vivo study of human skin using pulsed terahertz radiation, Physics in Medicine and Biology, vol. 49, pp. 1595-1607, 2004. 

Mapping with Pulsed Terahertz Radiation 59 Beamsplitter... 

Hangyo, M., Nakashima, T, and Nashima, S. (2002) Spectroscopy by pulsed terahertz radiation. Meas. Sci. Technol., 13,1727. 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

A PCA is effective also as a detector for the terahertz radiation. As shown in Figure 19.2, the terahertz radiation that has passed through the sample is focused onto the PCA in the detection system. The femtosecond laser pulse is synchronously superimposed onto the terahertz radiation at the same PCA. Since the density of the electric current j t) occurring between the electrodes is proportional to the number of carriers formed by photoexcitation A/exc(0. the current formed is also proportional to the complex electric field thz(0 of the terahertz radiation. Therefore, by measuring the electric current between the electrodes as illustrated in Figure 19.2, it is possible to determine the electric field of the terahertz radiation and its phase (t). 

However, it is not possible to determine directly Ej (t) and 4> t) of the terahertz radiation, because the terahertz radiation is emitted as an extremely short pulse. For this reason, as illustrated schematically in Figure 19.4, time delays are instigated between the laser pulses for generating the terahertz radiation and those for detecting the terahertz radiation. The shape of the terahertz radiation on the time axis is determined by scanning the delay time of the laser pulses. 

These relationships mean that, in order to expand the wavenumber region of the terahertz radiation, it is necessary to make the width of the laser pulse for generating the terahertz radiation narrower and the response of the photogenerated carrier faster in order to have a shorter lifetime. At present, LT-GaAs is widely used for the PCA as a material appropriate for the above-mentioned requirement. 

In theory, the full wavenumber region of the terahertz radiation generated by the PCA is inversely proportional to the pulse width of the exciting femtosecond pulse, so that it would be possible for this THz-TDS instrument to cover an extended wavenumber region... 

Terahertz radiation can be generated with the aid of certain polymer systems that are irradiated with 800 nm pulses (duration of less than 200 fs) emitted from a Ti sapphire laser system. A prerequisite for the generation of THz radiation is the... 

Terahertz (THz) spectroscopy systems utilize far-infrared radiation to extract molecular spectral information in an otherwise inaccessible region of the electromagnetic spectrum where various rotational, vibrational, and translational modes of molecules are located, 0.1-10 THz (Fig. 1). As the wavenumber range is narrowed, THz-radiation can yield more specific information about a particular chemical component within the system. Unlike most spectroscopic techniques, THz instrument measures the wave temporal electric field, which can be Fourier transformed to yield THz pulse amplitude and phase. This added capability allows precise... 

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