Terahertz for the Study of Electron carriers

It is of major importance to gain understanding into the fundamental physical processes such as carrier transport and carrier dynamics of semiconductors in order to further develop applications of high-speed devices. 
Terahertz spectroscopy has emerged as a fundamental technique for resolving carrier motion and distribution in semiconductors as it uses ultrashort optical pulses for the excitation of the sample, subsequently probing photo-induced changes in the sample by a short pulse of terahertz radiation. The method is particularly useful for intra-band excitations.

Absorption coefficient and refractive index measured by terahertz pulsed spectroscopy can then be used directly to obtain the frequency dependent complex conductivities of material in the 0.1 – 3  THz (3-100 cm-1) region of the electromagnetic spectrum. Terahertz spectroscopy has the major advantage of being a non-contact method.
Terahertz time-domain spectroscopy is widely used to characterise charge transport properties of wide-band-gap semiconductors (TiO2, ZnO among others) and organic semiconductor polymers. These devices are very important for a number of industrial applications such as photocatalists and solar energy converters where photogenerated carriers are used to trigger chemical reactions to develop the electrical potential for their applications.
TeraView`s TPS Spectra 3000 has been used to measure the complex conductivity and dielectric function of n-type GaN with various carrier concentrations on sapphire substrate. (Terahertz carrier dynamics and dielectric properties of GaN epilayers with different carrier concentrations, H. C. Guo, X. H. Zhang, W. Liu, A. M. Yong, and S. H. Tang, JOURNAL OF APPLIED PHYSICS 106, 2009)

The complex conductivity of two GaN samples S1 and S2. (a) for real part and  (b) for imaginary part.
The theoretical fittings were obtained by using the Drude model.