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Increasing the area coverage of superconducting nanowire single-photon detectors
A new way of interconnecting nanometric light-sensitive elements allows detectors to span areas up to 40 x 40µm2 without compromising the count rate.
26 May 2011, SPIE Newsroom. DOI: 10.1117/2.1201104.003674
Nanometric control of the physical dimensions of matter enables the realization of devices with highly improved performance. However, it is often difficult to make these nanostructures in the scales necessary to interface them properly in real-world applications without compromising their performance. This challenge exists with near-infrared superconducting nanowire single-photon detectors (SNSPDs),1 which are valuable tools for the characterization of non-classical light sources.
SNSPDs currently use 100nm-wide nanowires. However, because the wavelength of near-infrared light is microns in size, it is necessary to use many closely packed nanowires to efficiently detect the photons. A number of nanowires can be connected together to increase the area of the detector, but this has been found to decrease the maximum count rate—an impediment for use with multimode optical fibers.2
Figure 1. Scanning electron micrograph of an SNSPD. (a) Current SNSPDs are based on serially connected nanowires. (b) The SNSPD inductance is reduced using a cascade switch of parallel nanowires. The blocks of parallel nanowires are presented in different colors to show the serial connection of the blocks more clearly. Both SNSPD types use the same nanowires for photon detection; the difference lies in the interconnection between the nanowires. In both (a) and (b) the light regions are the superconductive nanofilm, and the dark regions are the substrate.
The problem is the inductance of the SNSPD, which is proportional to the device area in today's SNSPD designs because all of the nanowires are connected in series: see Figure 1(a). Some approaches to speed up SNSPD operation based on modifications of the surrounding electrical circuit have been tried,3, 4 mainly focusing on SNSPDs with a 10×10μm2 area. We have approached the problem from a different perspective by connecting the nanowires in parallel, which dramatically reduces the SNSPD inductance. In this way, the SNSPD area coverage can be greatly increased without compromising the maximum count rate.5
The difficulty with using parallel-connected superconducting nanowires is that they will short-circuit the photon detection signal, so the necessary photon detector electronics are not triggered. Our research indicates that this readout problem can be resolved by arranging the parallel nanowires in a way that a small inductance in series triggers the superconducting state to induce a cascade switch of the parallel nanowires when they short-circuit the signal. This allows the photon detection signal to register.6
We achieve the cascade switch in our SNSPD design by connecting blocks of nanowires in series where the nanowires within each block are connected in parallel, as shown in Figure 1(b). (The blocks are in different colors for clarity.7) We found that we could achieve a significant increase in the detector coverage area by increasing the number of parallel connected nanowires within each block (without increasing the number of blocks connected in series). A key point is that the parallel configuration only affects the nanowire interconnection. The nanowires themselves remain the same, leaving key SNSPD performance parameters, such as efficiency and dark count rate, intact. Another advantage is that the cascade switch is an amplification mechanism, so our SNSPDs emit signal pulses with an increased signal-to-noise ratio. This may be of particular interest should the size of the nanowires be further scaled to extend the sensitivity of the SNSPD to far-infrared wavelengths.8
Based on the possibility of achieving fast large-area coverage SNSPDs, we have realized detectors based on 100nm-wide nanowires with a 40% filling factor that covers an area of up to 40×40μm2. The material properties and nanowire dimensions of these SNSPDs show good uniformity, so the detectors function well. We characterized the SNSPDs, and the maximum count rate was in the range of tens of megahertz for the largest detectors, which is comparable to the count rates of conventional SNSPDs with a smaller area of 10×10μm2.
In conclusion, we have demonstrated how parallel nanowires can be used to allow SNSPDs to be realized in scales of tens of micrometers without decreasing the maximum count rate. We believe that further increases in the coverage area of SNSPDs based on parallel nanowires should enable good coupling to multimode optical fibers. Our future work will investigate whether the parallel nanowire configuration can speed up the operation of small SNSPDs, so that count rates in the gigahertz range can be achieved.
Finally, we stress, that our configuration of parallel nanowires can already allow the realization of SNSPDs with the necessary area coverage for interfacing with multimode optical fibers without suffering the speed loss of conventional series-connected SNSPDs. In this way, the parallel configuration is an example of how a macroscopic object can truly exploit the performance of nanostructured materials.
C.N.R. - Institute of Cybernetics
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