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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Superconducting single photon detectors based on multiple cascade switches of parallel NbN nanowires

Roberto Cristiano, Mikkel Ejrnaes, A. Casaburi, S. Pagano, F. Mattioli, Alessandro Gaggero, Roberto Leoni

Published Apr 30, 2011• 2 citations
Publisher

Abstract

Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade switch principle, showing correct operation.

DOI
10.1117/12.888055

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Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade switch principle, showing correct operation.
Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade switch principle, showing correct operation, increased operational bias range and increased signal pulse amplitudes.
The authors present superconducting single photon detectors (SSPDs) based on parallel nanostrips with an area up to 40 × 40 μm2. The SSPDs presented here are based on 100 nm wide ultrathin NbN nanostrips with a filling factor of 40%. The devices are fabricated by extending the standard electron beam lithography (EBL) patterning process to those densely structured large areas. By a thorough characterization it is shown that the electrical properties of the parallel SSPDs are comparable with those of smaller devices, as expected, proving in this way that the extended EBL process results in uniform nanostrips also in large area detectors. Furthermore, the estimated maximum count rate of the 40 × 40 μm2 parallel SSPDs was 33 MHz, showing that the parallel nanostrip configuration is much faster when compared with standard meandered serial SSPDs. The successful extension of parallel SSPDs to a large area coverage opens a new route to the use of such detectors also with multimode fibers.
Superconducting single-photon detector (SSPD) is patterned from 4-nm-thick NbN film deposited on sapphire substrate as a 100-nm-wide strip. Due to its high detection efficiency, low dark counts, and picosecond timing jitter SSPD has become a competitor to the InGaAs avalanche photodiodes at 1550 nm and longer wavelengths. Although the SSPD is operated at liquid helium temperature its efficient single-mode fibre coupling enabled its usage in many applications ranging from single-photon sources research to quantum cryptography. In our strive to increase the detection efficiency at 1550 nm and longer wavelengths we developed and fabricated SSPD with the strip almost twice narrower compared to the standard 100 nm. To increase the voltage response of the device we utilized cascade switching mechanism: we connected 50-nm-wide and 10-μm-long strips in parallel covering the area of 10 μmx10 μm. Absorption of a photon breaks the superconductivity in a strip leading to the bias current redistribution between other strips followed their cascade switching. As the total current of all the strips about is 1 mA by the order of magnitude the response voltage of such an SSPD is several times higher compared to the traditional meander-shaped SSPDs. In middle infrared (about 3 μm wavelength) these devices have the detection efficiency several times higher compared to the traditional SSPDs.
We have developed a novel geometrical configuration for NbN-based superconducting single photon optical detector (SSPD) that achieves two goals: a much lower intrinsic impedance, and a consequently greater bandwidth, and a much larger signal amplitude compared to the standard meandered configuration. This has been obtained by implementing a properly designed parallel stripline structure where a cascade switching mechanism occurs when one of the striplines is hit by an optical photon. The overall switching occurs synchronously and in a very short time, giving rise to a strong and fast voltage pulse. The SSPD have been realized using state of the art NbN deposition technology and e-beam lithography. The strips are 100 nm wide and 5 μm long and have been realized with 4 nm NbN film on sapphire and Si substrate. We report on experimental characterization of such novel devices. The performances of the proposed novel type of SSPD are compared with standard SSPD design and results in terms of signal amplitude, risetime and effective detection area.
We present our latest generation of superconducting single-photon detectors (SSPDs) patterned from 4-nm-thick NbN films, as meander-shaped ~0.5-mm-long and ~100-nm-wide stripes. The SSPDs exhibit excellent performance parameters in the visible-to-near-infrared radiation wavelengths: quantum efficiency (QE) of our best devices approaches a saturation level of ~30% even at 4.2 K (limited by the NbN film optical absorption) and dark counts as low as 2x10-4 Hz. The presented SSPDs were designed to maintain the QE of large-active-area devices, but, unless our earlier SSPDs, hampered by a significant kinetic inductance and a nanosecond response time, they are characterized by a low inductance and GHz counting rates. We have designed, simulated, and tested the structures consisting of several, connected in parallel, meander sections, each having a resistor connected in series. Such new, multi-element geometry led to a significant decrease of the device kinetic inductance without the decrease of its active area and QE. The presented improvement in the SSPD performance makes our detectors most attractive for high-speed quantum communications and quantum cryptography applications.