Thin-film radiation detector

A method of operating a thin-film radiation detector such as a superconducting single photon detector (SSPD) comprises the steps of selecting a first polarization state of an incident beam of radiation; and directing the resulting first polarized beam of radiation at the thin-film detector surface such that the beam axis is oblique to the plane of the detector surface and the first polarized beam has an s-polarization state relative to the absorption surface of the detector. The thin-film radiation detector comprises an absorbing thin film material 3 defining the detector surface disposed between a substrate 2 and a second material 12 such as air. Coupling optics 4 includes a polarizer 6 and prism 7. The angle of incidence of beam 8 is close to the critical angle for total internal reflection. The absorbing thin film may comprise a meander of superconducting material (Fig-2). The detector may comprise a two-stage detector for capturing both polarization states of incident light on two thin-film detectors (Fig.9).