Terahertz (THz) physics and technology are on the frontiers of physics holding great promise for progress in various fields of science such as solid state physics, astrophysics, plasma physics and others. Furthermore, THz physics holds a great potential for applications in medicine, environmental monitoring, high-speed communication, security, spectroscopy of different materials, including explosives, etc. Areas of THz physics of high current interest are the development and application of coherent semiconductor sources, molecular gas lasers, ultrafast time-domain spectroscopy based on femtosecond near-infrared radiation pulses, as well as the development of novel detectors of laser radiation. An important characteristic of THz laser radiation is its state of polarization. The detection of the polarization state, in particular the orientation of the electric field vector of linear polarized radiation and/or the ellipticity of transmitted, reflected or scattered, light represents a powerful technique for analyzing the optical anisotropy of various media such as solids, solid surfaces, plasmas, and biological tissues. The established way to gain information about the polarization state is the use of optical elements, which allow to determine optical path differences.

Here we review a new technique which allows one all-electric fast room temperature detection of the Stokes parameters of laser radiation, with a bandwidth from the infrared to the terahertz range [1-3]. The method is based on the simultaneous measurements of circular and linear photogalvanic effects in semiconductor quantum well (QWs) structures of suitably low symmetry and provides the full characterization of laser beams. The time constant of photogalvanic currents is determined by the momentum relaxation time of electrons, which is in the range of picoseconds at room temperature. This allows to measure the ellipticity of THz laser radiation with sub-nanosecond time resolution. The detection principle has been shown applying doped GaAs-, SiGe- and, most recently, HgTe-based QWs at room temperature. The detection of radiation polarization state is demonstrated for various low power and high power laser systems. A large spectral range has been covered applying cw and pulsed optically pumped molecular terahertz lasers, as well as low-pressure cw, pulsed, and Q-switched CO2 lasers and high-power pulsed transverse excited atmospheric pressure (TEA) CO2 lasers. In addition measurements were carried out with a free-electron-laser making use of its tunability and short pulses.

[1] S.D. Ganichev, J. Kiermaier, W. Weber, S.N. Danilov, D. Schuh, Ch. Gerl, W. Wegscheider, D. Bougeard, G. Abstreiter, and W. Prettl, "Subnanosecond Ellipticity Detector for Laser Radiation", Appl. Phys. Lett. 91, 091101 (2007).

[2] S. D. Ganichev, W. Weber, J. Kiermaier, S. N. Danilov, D. Schuh, W. Wegscheider, Ch. Gerl, D. Bougeard, G. Abstreiter, and W. Prettl, "All-electric detection of the polarization state of terahertz laser radiation", J. Appl. Phys. 103, 114504 (2008)

[3] S.N. Danilov, B. Wittmann, P. Olbrich, W. Eder, W. Prettl, L.E. Golub, E.V. Beregulin, Z.D. Kvon, N.N. Mikhailov, S.A. Dvoretsky, V.A. Shalygin, N.Q. Vinh, A.F.G. van der Meer, B. Murdin, S.D. .  Ganichev, "Fast detector of the ellipticity of infrared and terahertz radiation based on HgTe quantum well structures", J. Appl. Physics 105, 013106 (2009).

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