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We present terahertz photoconductivity measurements on GaAs/ AlGaAs and HgTe/ HgCdTe heterostructures. The photoresponse is investigated time and spectrally resolved under quantum Hall conditions. The samples are excited by a pulsed p-Ge laser, which emits photons of frequencies around 2 THz ͑corresponding to photon energies around 10 meV͒. Corbino-shaped GaAs/ AlGaAs samples show relaxation times down to 10 ns. The dependence of on the applied source-drain voltage is explained by a two-level picture after normalizing the data. All spectrally resolved measurements show contributions of the cyclotron resonance and the bolometric effect. These results are compared to numerical calculations based on a self-consistent Born approximation method. The measurements on HgTe/ HgCdTe samples show comparable results. However, the effective mass in these samples is only m c = 0.026m 0 ͑approximately 1 / 3 of the mass in GaAs/ AlGaAs͒. Thus the cyclotron resonance is shifted to smaller magnetic fields around 2 T. This fact makes HgTe/ HgCdTe systems especially interesting for terahertz detector applications.

We present THz photoconductivity measurements on Corbino-shaped GaAs∕AlGaAs heterostructures. The THz source is a pulsed p-Ge laser, which provides photon frequencies of 1.7THzto2.5THz (corresponding to wavelengths of 180–120μm). We investigate the relaxation process from the dissipative state to the quantum Hall state time-resolved and find that the relaxation time depends on the applied voltage and on the mobility of the sample. Relaxation times of approximately 10ns to over 200ns are observed. A simple picture is suggested to explain the results. In addition, spectrally resolved measurements are discussed. The short response time and the useful spectral selectivity together with the high sensitivity make QH devices promising for high-performance THz detectors.

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