The goal of this study was to establish conditions under which a magnetic-field-tunable InSb detector can be used to characterize the radiation spectrum of terahertz (THz) sources. The response of a bulk InSb detector to a monochromatic radiation with the frequency 0.1 < f < 4.6 THz was studied at liquid helium temperatures and magnetic fields B up to 5 T. A constant voltage (measurements of photocurrent) or a constant current (measurements of photoresistance) biasing condition was tested, and both signals were found to be strongly influenced by magnetoresistance of the detector. At f0.5 THz, a photoresponse was observed in the whole range of B, which reflected a non-resonant absorption by free electrons and had no relation to cyclotron-resonance-related transitions. Resonant transitions developed gradually at f0.5 THz and dominated the spectra only at f1.0 THz. Thus, we show that only at this latter frequency range, the detector can be used for a spectral characterization of unknown sources. To avoid false interpretation of measured signals, a detector should be supplied with a 1-THz high-pass filter.

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