Room temperature 9 $mu$m photodetectors and GHz heterodyne receivers

Room temperature 9m photodetectors and GHz heterodyne receivers
 Daniele Palaferri1, Yanko Todorov1, Azzurra Bigioli1, Alireza Mottaghizadeh1, Djamal Gacemi1,
Allegra Calabrese1, Angela Vasanelli1, Lianhe Li2, A. Giles Davies2, Edmund H. Linfield2, Filippos Kapsalidis3, Mattias Beck3, Jérôme Faist3 and Carlo Sirtori1

1 Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot, Sorbonne Paris Cité, CNRS- UMS 7162, 75013 Paris, France 2 School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom 3 ETH Zurich, Institute of Quantum Electronics, Auguste-Piccard-Hof 1, Zurich 8093, Switzerland

Room temperature operation is mandatory for any optoelectronics technology which aims to provide low-cost compact systems for widespread applications. In recent years, an important technological effort in this direction has been made in bolometric detection for thermal imaging1, which has delivered relatively high sensitivity and video rate performance (60 Hz). However, room temperature operation is still beyond reach for semiconductor photodetectors in the 8–12 µm wavelength band2, and all developments for applications such as imaging, environmental remote sensing and laser-based free-space communication3-5 have therefore had to be realised at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high temperature operation6, 7. Here, we show that a 9 µm quantum well infrared photodetector8, implemented in a metamaterial made of subwavelength metallic resonators9-12, has strongly enhanced performances up to room temperature. This occurs because the photonic collection area is increased with respect to the electrical area for each resonator, thus significantly reducing the dark current of the device13. Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature14, 15, which constrains conventional geometries at cryogenic operation6. Finally, the reduced physical area of the device and its increased responsivity allows us, for the first time, to take advantage of the intrinsic high frequency response of the quantum detector7 at room temperature. By beating two quantum cascade lasers16 we have measured the heterodyne signal at high frequencies up to 4 GHz. 2

An important intrinsic property of inter-subband (ISB) quantum well infrared photodetectors (QWIPs) based on III-V semiconductor materials that has not yet been exploited is the very short lifetime of the excited carriers. The typical lifetime is of the order of one picosecond7, which leads to two important consequences: the detector frequency response can reach up to 100 GHz, and the saturation intensity is extremely high (107 W/cm2)17. These 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 unobtainable in infrared inter-band detectors based on mercury-cadmium-telluride (MCT) alloys, which have a much longer carrier lifetime and therefore an intrinsic lower speed response2,18. Yet, the performance of all photonic detectors is limited by the high dark current that originates from thermal emission of electrons from the wells, and rises exponentially with temperature, thus imposing cryogenic operation ( 80 K) for high sensitivity measurements. Although highly doped (1x1012 cm-2) 10 µm QWIPs have been observed to operate up room temperature, tens of mW incident power from a CO2 laser was required to measure the signal19,20.
In the present work, we show that this intrinsic limitation in QWIP detectors can be overcome through use of a photonic metamaterial. We are able to calibrate our detector at room temperature using a black body emitting only hundreds of nW, orders of magnitude smaller than that required previously. To date, room temperature performance with values comparable to those that we report here has only been demonstrated in the 3–5 µm wavelength range, using quantum cascade detectors (QCDs)21-23 and MCT standard detectors24.
The photonic metamaterial structure is shown in Fig. 1a. The GaAs/AlGaAs QWIP8 contains Nqw = 5 quantum wells absorbing at 8.9 µm wavelength (139 meV) that has been designed according to an optimized bound-to-continuum structure from ref. 7. The absorbing region is inserted in an array of double-metal patch resonators9-12, which provide sub-wavelength electric field confinement and act as antennas. The resonant wavelength is fixed by the patch size s through the expression = 2sneff, where neff = 3.3 is the effective index9. As shown in the reflectivity measurement in Fig. 1b, the cavity mode is in close resonance with the peak

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