Response of Indium Antimonide at Sub-Terahertz and Terahertz Frequencies

Responsivity of a bulk InSb detector at liquid helium temperature was studied in the frequency range 0.1 THz < f < 2.5 THz. The detector was exposed to a monochromatic radiation and its response was registered as a function of magnetic field up to 7 T. Due to a very strong magnetoresistance of InSb, a dependence of the photocurrent on magnetic field shows a totally different shape than that of the photoresistance. Thus, photoconductivity was found to be the only reliable way of data presentation. The detector develops a resonant response, related to the cyclotron resonance absorption, at f greater than or similar to 0.5 THz but this response dominates the spectrum only at f greater than or similar to 1 THz. At f less than or similar to 1 THz the response is mainly non-resonant and is caused by intra-band transitions. We conclude that an InSb detector can be used as a magnetic-field-tunable spectroscopic element only if one can assure that the frequency of photons which reach the detector is higher than about 1 THz.