Simultaneously, the “holy grail” for the imaging sensor science are highly efficient, single-photon detectors, since they reach the ultimate sensitivity, i.e., one photon energy resolution, for the images obtained using electromagnetic radiation.
The authors of this Optics Express article have made the first important step toward unifying the two ideas mentioned earlier by incorporating single-photon detectors into a large, multi-pixel, photon imaging array. They developed a 64-pixel (8 x 8) detector matrix operational in the near-infrared radiation spectrum and, what is most important, they tested it at the 1.55-µm wavelength---the industry standard for optical telecommunication; thus, the presented device is compatible with off-the-shelf optoelectronic components.
To assure high quantum efficiency of their pixel elements in the near-infrared radiation range, they implemented what they refer to as superconducting single-photon detectors (SSPDs). Since their introduction in 2001 jointly by groups from Moscow in Russia and Rochester NY in the USA, SSPDs have become the photon detectors of choice for advanced performance applications in the visible-to-infrared optical range. The detection mechanism of SSPDs is based on photon-induced hotspot formation and, subsequently, generation of a voltage transient across a nanostructured superconducting meander structure. The system detection efficiency of the best SSPD devices reaches unity at 1550 nm, with almost negligible dark counts and timing jitter. The SSPDs have already been employed in a number of applications, focusing mainly on advanced quantum optics and quantum key distribution experiments.
The SSPD pixel elements presented in this work are 5 x 5 μm2 meanders, formed of a 5-nm-thick and 100-nm-wide NbTiN nanostripe, and they operate at ~2.3 K temperature. The authors used a standard fabrication process, consisting of magnetron deposition of an ultrathin NbTiN film, followed by e-beam lithography and reactive-ion etching. Their fabrication, however, is very mature and well established–not only did all the pixels in the tested array work, i.e., were sensitive to single photons, but the measured spread of the superconducting critical temperatures and critical currents between the pixels was very small.
As stressed earlier, this paper is just a first step towards realization of a truly single-photon two-dimensional imaging array, so the major success story is that the array operated very uniformly with 60 out of 64 pixels showing a pulse generation probability higher than 90% after photon absorption. Using a conventional biasing and readout scheme, namely, wiring separately each pixel, they were unable to demonstrate any real-time photon imaging operation, but, again, this was just the first fully successful demonstration of a large SSPD array.
In their conclusion, the authors promise to couple, in the near future, their SSPD array to a superconducting flux quantum (SFQ) electronics read-out. If successful, the resulting device would be the first, truly integrated single-photon imaging “camera,” as well as a technological marvel, merging the two most advanced superconducting optoelectronics and electronics systems. Such camera would not only allow for real-time spatially resolved photon detection, but should also enable the performance of simultaneous, both time and space, photon correlations studies of “photon starved” sources and optical signals with ultraweak photon fluxes. This will have a great impact on novel experiments in quantum optics and could lead to novel time-correlated astronomical observations.
Finally, it must be mentioned that although the SSPD multi-pixel array (as well as the SFQ read-out) requires a cryogenic environment, the experiments presented in this work were performed in a liquid-cryogen–free cryocooler, making it, in principle, a portable, operator-friendly system.
You must log in to add comments.