Fast Room Temperature Detection of State of Circular Polarization of Terahertz Radiation
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Fast Room Temperature Detection of State of Circular Polarization of Terahertz Radiation. Sergey D. Ganichev1,2 , Hermann Ketterl1 , and Wilhelm Prettl1 , 1 Institut f¨ ur Experim. und Angew. Physik, Universit¨ at Regensburg, 93040 Regensburg, Germany, 2 A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia
ABSTRACT We report on a room temperature detector which allows to determine and monitor the state of polarization of terahertz radiation with picosecond temporal resolution. The detector is based on the circular photogalvanic effect recently observed in GaAs/AlGaAs quantum wells. The circular photogalvanic effect yields in response to elliptically polarized radiation a current signal proportional to the degree of circular polarization. The peak current signal occurs in unbiased samples for circular polarization, vanishes at linear polarization and changes sign by switching the helicity from right-handed to left-handed. The detector consists of a (113)A MBE grown p-GaAs/AlGaAs multiple quantum well structure. The response has been measured in the wavelength range between 76 µm and 280 µm at normal incidence of the radiation on the sample.
INTRODUCTION Room temperature detection of short THz laser pulses is possible using pyroelectricity [1], bulk semiconductor devices like photon drag detectors [2-5], Schottky-diodes [5], and intraband photoconductivity [6]. Recently low dimensional semiconductor structures and superlattices have been demonstrated to be very efficient for detection THz radiation with high temporal resolution. In particular photovoltaic [7], photon-drag [8], and hot electrons [9] effects in quantum well (QW) structures as well as THz radiation driving superlattices [10] are promising means of large bandwidth detection. Here we report on a new approach to THz detection applying the circular photogalvanic effect which has recently been observed in GaAs based quantum wells (QW) [11,12]. Circularly polarized radiation generates a current in the unbiased sample. Because the photogalvanic effect does not involve any charge separation, space charge regions or gradient-induced drift currents, photogalvanic detectors have the advantages of fast transient signal response at low impedance. The only physical speed limitations result from momentum relaxation times, which are in order of picoseconds at room temperature. The striking feature of this photogalvanic effect is that the current flows perpendicular to the radiation propagation and that the sign of the current reverses by changing the helicity of the radiation from right-handed to left-handed. Therefore the circular photogalvanic effect does not only trace the time dependence of a radiations pulse but it gives also the state of polarization of the electromagnetic field in a very direct way.
PHYSICAL BACKGROUND The photogalvanic effects arise in homogeneous samples of noncentrosymmetric media [13] under homogeneous excitation due to an asymmetry of the interaction of free carriers with photons, phonons, static defects or other carriers. In the THz
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