Now we will see even better at night and will remotely detect harmful or polluting agents, by using agile, compact and robust systems. A new generation of ultra-sensitive detectors opens the infrared to an immensity of applications that will impact our daily lives: from autonomous vehicles to communication in free space, from civil protection to the control of the environment.
Far-infrared (that is to say for wavelengths of 20 to 100 times greater than those of the visible) is an “invisible light” to the naked eye that can be measured only with the help of a detector. Well known to scientists, this light is emitted by any object at room temperature. However, it remains today little exploited because of the absence of sources and functional detectors, namely devices that operate at room temperature and sufficiently quickly.
A team of scientists from Paris Diderot University, ETH Zurich and the University of Leeds has recently demonstrated a new structure that amplifies the performance of infrared detectors. To this end novel light propagation concepts have been brilliantly developed and implemented in quantum structures, thanks to a refined work of nano-fabrication.
In these new detectors, the incident light radiation is taken from a very large surface and then concentrated in a volume of nanometric size where it generates an electrical signal. This is possible thanks to a photonic structure that acts as a photonic funnel. The detector is in fact made up of active elements of one micrometer square (see figure) whose area is ten times smaller than the surface where photons are captured.
Reducing the volumes where photons are transformed into electric current is essential to limit the contribution of the intrinsic current that flows even in the absence of light, called the dark current. In current detectors, this dark current can be so large that it completely masks the photo-current and requires the use of heavy and expensive cryogenic cooling. The new detectors developed in this work show, for their part, excellent performance at room temperature and a very fast response.
Moreover, the high speed of response of these detectors makes it possible to achieve an ultrasensitive coherent detection, similar to that which has been developed for radio waves: heterodyne detection. This technique is based on mixing the signal to be detected with a reference signal (the local oscillator). After the demonstration of this proof of principle, it is now a question of working to exceed the limits of the current detection. Today, they are limited by very short integration times, imposed by the temporal fluctuations of the local oscillator. These fluctuations can be suppressed by active stabilization of the local oscillator on a metrological reference. Such a heterodyne system can reach a sensitivity approaching that of the single photon.
The applications of far-infrared radiation or, even further in the electromagnetic spectrum, the terahertz region, are potentially very numerous. This new technology could quickly find applications in the areas of telecommunications, high resolution molecular spectroscopy, remote detection of pollutants or harmful or optical RADAR systems.
The cover depicts a scanning electron microscope image of quantum-well infrared photodetectors created from a metamaterial array of metallic resonators.
Room-temperature nine-μm-wavelength photodetectors and GHz-frequency heterodyne receivers, D. Palaferri, Y. Todorov, A. Bigioli, A. Mottaghizadeh, D. Gacemi, A. Calabrese, A. Vasanelli, L. Li, A. G. Davies, E. H. Linfield, F. Kapsalidis, M. Beck, J. Faist, and C. Sirtori, Nature 556, 85 (2018)
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