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The very short life-time of the excited carriers is a very important intrinsic property of quantum well photodetectors (QWIPs), based on III-V semiconductor materials, that have not been exploited for performances yet. Its typical value is in the order of a ps [7], which leads to two important consequences: the detector frequency response can be up to 100 GHz and its saturation intensity is extremely high in the in the order of 10e7 W/cm2. These two figures are ideal for a heterodyne detection scheme, where a powerful local oscillator (LO) can drive a strong photocurrent, higher than the detector dark current, that can coherently mix with a signal shifted in frequency with respect to the LO. Notably, these unique properties are unmatched in infrared interband detectors based on mercury-cadmium-telluride alloys, which have a much longer carrier lifetime and therefore intrinsic low-speed response. Yet, the performances of all photonic detectors are limited by the high dark current which originates from thermal emission of electrons from the wells, rising exponentially with temperature and imposing cryogenic operation (~ 80K) for high sensitivity.
In the present work we show that the intrinsic limitation of QWIP detectors can be overcome by the use of a photonic metamaterial made of metallic resonators. The absorbing region of the detector consists of a 5 period GaAs/AlGaAs QWIP operating at a wavelength 8.9µm (139meV) which has been designed according to an optimized bound to continuum structure. The absorbing region is inserted in an array of double-metal patch resonators [ref], which provide sub-wavelength electric field confinement and act as antennas. The resonant wavelength is fixed by the patch size s through the expression lambda = 2 s neff, with neff = 3.3 the effective index. This allows us to tune the cavity mode in resonance with the absorption peak of the bare detector for values of s between 1.3µm-1.4µm. The detectors obtained in this way have high detectivity up at room temperature, in the order of 3-4 10e7 cmHz1/2/W. Given this his high detectivity we could calibrate the detector using a black body emitting only few µW. Up to date high sensitivity at room temperature, with values comparable to those we are reporting, have been demonstrated, only in the 3-5µm wavelength range, using quantum cascade detectors (QCDs) [9-11] and with MCT standard detectors.
To the best of our knowledge this is the best results ever reported for a photoconductive detector at 9µm at room temperature. Since the detector has an extremely fast response of several GHz we built an heterodyne setup tp beat two quantum cascade lasers. We obtained heterodyne signal at high frequencies up to 3GHz, with NEP in the pW range at 300K.
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