High-speed single-photon counting detectors have many applications, including optical communications, quantum information, spectroscopy of ultrafast quantum phenomena in biology and solid-state physics, semiconductor processing. Of particular interest would be a detector that exhibits both ultrafast count rates ≥ GHz and high single-photon detection efficiency at near-infrared wavelengths (1550 nm); however, this combination is not available with current technologies. InGaAs avalanche photodiodes are limited to much lower count rates by long reset times and hybrid photomultipliers have low detection efficiencies at these wavelengths.
A promising detector technology emerged recently: the Superconducting Nanowires Single Photon Detector (SNSPD). At the moment SNSPD is an hot topic in the international scenario. Italy is involved in this effort through the participation at the European project SINPHONIA of CNR with the Institute of Cybernetics, Pozzuoli (ICIB) and the Institute of Photonics and Nanotechnology, Rome (IFN), and with Pirelli Labs, Milan.
SNSPDs are "nanowires", i.e. they are very thin films with typical thickness of few nanometers, patterned in 0.5mm long lines with a width of about 100 nm. Any variation in the width and thickness of the wire has dramatic consequences on the device efficiency - meaning that we are working at the limit of state-of-the-art nanomaterial technology. The material of choice for the realization of SNSPD is niobium nitrate (NbN) because of its high critical temperature (Tc=16K) and short electron-phonon characteristic time (12ps) which is responsible of the detector speed.
In many of the above mentioned applications, a detector with large area coverage would be important. This is currently achieved using a very long meandered wire which results in a high detector inductance due to kinetic inductance. The large kinetic inductance has the effect of a delays of the SNSPD current recovery time and reduces the maximum count rate. Furthermore, the low cross-sectional area of the wire limits the superconductive critical current, and thereby the signal pulse amplitude reaches rather low values, typically less then 0.5 mV. This produces low signal to noise ratios, which prohibits reliable high count rate operation.
A new SNSPD configuration which decreases the detector inductance and increases the signal amplitude through an internal avalanche mechanism while keeping the good sensitivity and dark count rate has been developed in a collaboration between the CNR-ICIB and the University of Salerno. The benefits are obtained by rearranging the N wires in series in meandered SNSPDs to N wires in parallel . High quality NbN ultrathin films were fabricated at ICIB, the nano-patterning was made at the EBL facility of CNR-IFN and the measurements were carried out at CNR-ICIB and Salerno University. With a 24 nanowires parallel detector it has been obtained a signal amplitude which is about 24 times larger than a meander with the same number of wires, and a significant improvement of the signal to noise ratio. In a device with 5 parallel nanowires it was observed a pulse duration of 500 ps, with a rise time less of 80 ps, with respect to the intrinsic expected value for NbN that is ≤ 60 ps. The parallel configuration open new perspectives for these detectors because large area coverage could be easily obtained without deteriorating the count rate which is the major merit of the device. Future experiments at high count rates will establish whether the promises will be fully confirmed.
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