Kinetic-inductance-limited reset time of superconducting nanowire photon counters

Abstract

We investigate the recovery of superconducting NbN-nanowire photon counters after detection of an optical pulse at a wavelength of 1550 nm, and present a model that quantitatively accounts for our observations. The reset time is found to be limited by the large kinetic inductance of these nanowires, which forces a tradeoff between counting rate and either detection efficiency or active area. Devices of usable size and high detection efficiency are found to have reset times orders of magnitude longer than their intrinsic photoresponse time. . Of particular interest would be a detector that combines ultrafast count rates (≥ GHz) with high single-photon detection efficiency at near-infrared wavelengths; however, current near-infrared photon-counting technologies such as avalanche photodiodes [6] and photomultiplier tubes [7] are limited to much lower count rates by long reset times.A promising detector technology was reported recently, in which ultrathin superconducting NbN wires are biased with a DC current I bias slightly below the critical value I C [8]. An incident photon of sufficient energy can produce a resistive "hotspot" which in turn disrupts the superconductivity across the wire, resulting in a voltage pulse. Observations of this photoresponse showed promise for high counting rates, with measured intrinsic response times as low as ∼30 ps [9], and counting rates in the GHz regime [10,11]. In this Letter, we present our own investigation into the counting-rate limitation of these devices, in which we directly observe the recovery of the detection efficiency as the device resets (after a detection event), and develop a quantitative model of this process. We find that detectors having both high detection efficiency and usable active area are limited to much lower count rates than studies of their intrinsic response time had suggested [9].We fabricated our nanowires using a newly developed process [12], on ultrathin (3 − 5 nm) NbN films [13]. We used several geometries, including straight nanowires having widths from 20−400 nm and lengths from 0.5−50 µm, as well as large-area "meander" structures [8,10] (e.g., Fig. 1(b)) having active-area aspect ratios from 1 − 50, fill factors from 25 − 50%, and sizes up to 10-µm square. The devices had critical temperatures T C ∼ 9 − 10 K, and critical current densities J C ∼ 2 − 5 × 10 10

Rise time of voltage pulses in NbN superconducting single photon detectors

“…In previous work [4], we investigated the voltage pulse and represented the SSPD as a combination of several electrical elements, two of which have kinetic inductance Lk and resistance Rn that occur in a superconducting strip when a photon is absorbed (Fig. 1).…”

Experimental investigation of the detection mechanism in WSi nanowire superconducting single photon detectors

“…In order to avoid latching [38], the nanowire is defined together with an additional meander (total length 2.8mm and width 300nm) which provides an extra series inductance of approximately 720nH (see figure 1). The sample is mounted in a VeriCold cryocooler equipped with a final Joule-Thomson stage and is kept at a temperature of 1.6K.…”

Electrothermal feedback in superconducting nanowire single-photon detectors

“…The current I d then drops nearly to zero, turning off the heating and allowing the nanowire to quickly cool down and return to the superconducting state, after which I d recovers with a time constant e ϵ L / R L . 2 If one attempts to shorten e too much, the negative feedback becomes fast enough to counterbalance the Joule heating before it runs away, resulting in a stable resistive domain, known as a self-heating hotspot. 9,10 In a standard treatment of these hotspots, 9 solutions to a one-dimensional heat equation are found in which a normalsuperconducting ͑NS͒ boundary propagates at constant velocity v NS for fixed device current I d .…”

“…5,6 Full use of this electrical bandwidth is limited, however, by the fact that the maximum count rates of these devices are much smaller ͑a few hundred MHz for 10 m 2 active area and decreasing as the area is increased 2 ͒, limited by their large kinetic inductance and the input impedance of the readout circuit. 2,7 To increase the count rate, therefore, one must either reduce the kinetic inductance ͑by using a smaller active area or different materials or substrates͒ or increase the load impedance. 7 However, either of these approaches causes the wire to "latch" into a stable resistive state where it no longer detects photons.…”

“…DOI: 10.1103/PhysRevB.79.100509 PACS number͑s͒: 85.25.Oj, 74.78.Ϫw, 85.60.Gz Superconducting nanowire single-photon detectors ͑SNSPDs͒ combine high speed, high detection efficiency ͑DE͒ over a wide range of wavelengths, and low dark counts. [1][2][3][4] Of particular importance is their high singlephoton timing resolution of ϳ30 ps, 4 which permits extremely high data rates in photon-counting communications applications. 5,6 Full use of this electrical bandwidth is limited, however, by the fact that the maximum count rates of these devices are much smaller ͑a few hundred MHz for 10 m 2 active area and decreasing as the area is increased 2 ͒, limited by their large kinetic inductance and the input impedance of the readout circuit.…”