A single-photon detector in the far-infrared range

The far-infrared region (wavelengths in the range 10 mu m-1 mm) is one of t he richest areas of spectroscopic research(1), encompassing the rotational spectra of molecules and vibrational spectra of solids, liquids and gases. But studies in this spectral region are hampered by the absence of sensitiv e detectors(2-5)-despite recent efforts to improve superconducting bolomete rs(6), attainable sensitivities are currently far below the level of single -photon detection. This is in marked contrast to the visible and near-infra red regions (wavelengths shorter than about 1.5 mu m), in which single-phot on counting is possible using photomultiplier tubes. Here we report the det ection of single far-infrared photons in the wavelength range 175-210 mu m (6.0-7.1 meV), using a single-electron transistor consisting of a semicondu ctor quantum dot in high magnetic field. We detect, with a time resolution of a millisecond, an incident flux of 0.1 photons per second on an effectiv e detector area of 0.1 mm(2)-a sensitivity that exceeds previously reported values by a factor of more than 10(4). The sensitivity is a consequence of the unconventional detection mechanism, in which one absorbed photon leads to a current of 10(6)-10(12) electrons through the quantum dot By contrast , mechanisms of conventional detectors(2-6) or photon assisted tunnelling(7 ) in single-electron transistors produce only a few electrons per incident photon.