Asymmetric NbN–Nb–Al–AlOx-Al–Nb superconducting tunnel junctions have been investigated as photon counting detectors at x-ray and ultraviolet (UV)-visible wavelengths. The inclusion of a thin NbN passivation layer on the top electrode of the devices in place of the natural niobium oxides has reduced the quasiparticle loss rates, thereby enhancing the probability of multiple tunnel processes. As a consequence, the detector responsivity has increased from 900e−/eV, up to values in excess of 2000e−/eV in the temperature range 0.3–0.8 K. Such a responsivity level has allowed single photon counting performance at wavelengths as long as 700 nm and at operating temperatures as high as 830 mK. The devices show a linear response in the UV-visible range, while at 6 keV the expected nonlinearities in the energy response and moderate energy resolution similar to that found in Nb–Al junctions are observed.
Asymmetric NbN–Nb–Al–AlOx-Al–Nb superconducting tunnel junctions have been investigated as photon counting detectors at x-ray and ultraviolet (UV)-visible wavelengths. The inclusion of a thin NbN passivation layer on the top electrode of the devices in place of the natural niobium oxides has reduced the quasiparticle loss rates, thereby enhancing the probability of multiple tunnel processes. As a consequence, the detector responsivity has increased from 900e−/eV, up to values in excess of 2000e−/eV in the temperature range 0.3–0.8 K. Such a responsivity level has allowed single photon counting performance at wavelengths as long as 700 nm and at operating temperatures as high as 830 mK. The devices show a linear response in the UV-visible range, while at 6 keV the expected nonlinearities in the energy response and moderate energy resolution similar to that found in Nb–Al junctions are observed.
We have measured the quantum efficiency (QE), GHz counting rate, jitter, and noise-equivalent power (NEP) of nanostructured NbN superconducting single-photon detectors (SSPDs) in the visible to infrared radiation range. Our 3.5-nm-thick and 100- to 200-nm-wide meander-type devices (total area 10×10μm2), operating at 4.2K, exhibit an experimental QE of up to 20% in the visible range and ∼10% at 1.3 to 1.55μm wavelength and are potentially sensitive up to midinfrared (∼10μm) radiation. The SSPD counting rate was measured to be above 2GHz with jitter <18ps, independent of the wavelength. The devices’ NEP varies from ∼10−17W∕Hz1∕2 for 1.55μm photons to ∼10−20W∕Hz1∕2 for visible radiation. Lowering the SSPD operating temperature to 2.3K significantly enhanced its performance, by increasing the QE to ∼20% and lowering the NEP level to ∼3×10−22W∕Hz1∕2, both measured at 1.26μm wavelength.
We present a new class of single-photon devices for counting of both visible and infrared photons. Our superconducting single-photon detectors (SSPDs) are characterized by the intrinsic quantum efficiency (QE) reaching up to 100%, above 10 GHz counting rate, and negligible dark counts. The detection mechanism is based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The devices are fabricated from 3.5-nm-thick NbN films and operate at 4.2 K, well below the NbN superconducting transition temperature. Various continuous and pulsed laser sources in the wavelength range from 0.4 μm up to >3 μm were implemented in our experiments, enabling us to determine the detector QE in the photon-counting mode, response time, and jitter. For our best 3.5-nm-thick, 10×10 μm2-area devices, QE was found to reach almost 100% for any wavelength shorter than about 800 nm. For longer-wavelength (infrared) radiation, QE decreased exponentially with the photon wavelength increase. Time-resolved measurements of our SSPDs showed that the system-limited detector response pulse width was below 150 ps. The system jitter was measured to be 35 ps. In terms of the counting rate, jitter, and dark counts, the NbN SSPDs significantly outperform their semiconductor counterparts. Already identifeid and implemented applications of our devices range from noninvasive testing of semiconductor VLSI circuits to free-space quantum communications and quantum cryptography.
We report on fabrication and photon detection experiments of Nb/Al and Ta/Al superconducting tunnel junctions (STJs). 5-layer STJ thin-films were fabricated using UV photolithography, DC magnetron sputtering, reactive ion etching, and chemical vapor deposition techniques. STJs with 4 different sizes (20, 40, 60 and 80 μm) were deposited on sapphire substrates and tested in a two stage adiabatic demagnetization refrigerator with an operating temperature ~ 50 mK. Photons from different light sources are injected into the junctions via an optical fiber in combination with a monochromator which can produce photons from 30 nm to 550 nm with 0.1 nm resolution. The junction is read out through a charge-sensitive preamplifier followed by a shaping stage. We have measured some performance indicators and quality factors of the junctions from resultant I-V curves.
Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.
We present our studies on ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs are patterned by electron beam lithography from 4-nm thick NbN film into meander-shaped strips covering square area of 10/spl times/10 /spl mu/m/sup 2/. The advances in the fabrication technology allowed us to produce highly uniform 100-120-nm-wide strips with meander filling factor close to 0.6. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, an avalanche of excited hot electrons and the biasing supercurrent, jointly produce a picosecond voltage transient response across the superconducting nanostrip. The SSPDs are typically operated at 4.2 K, but they have shown that their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by optical absorption of our 4-nm-thick NbN film. For 1.55 /spl mu/m photons, QE was /spl sim/20% and decreases exponentially with the increase of the optical wavelength, but even at the wavelength of 6 /spl mu/m the detector remains sensitive to single photons and exhibits QE of about 10/sup -2/%. The dark (false) count rate at 2 K is as low as 2 /spl times/ 10/sup -4/ s/sup -1/, what makes our detector essentially a background-limited sensor. The very low dark-count rate results in the noise equivalent power (NEP) as low as 10/sup -18/ WHz/sup -1/2/ for the mid-infrared range (6 /spl mu/m). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for the other, lower-T/sub c/ superconductors with the narrow superconducting gap and low quasiparticle diffusivity. The use of such materials will shift the cutoff wavelength towards the values even longer than 6 /spl mu/m.
SUMMARY We report our studies on the spectral sensitivity of superconducting NbN thin-film single-photon detectors (SPD’s)capable of GHz counting rates of visible and nearinfrared photons. In particular, it has been shown that a NbN SPD is sensitive to 1.55-µm wavelength radiation and can be used for quantum communication. Our SPD’s exhibit experimentally measured intrinsic quantum efficiencies from 20% at 800 nm up to ∼1% at 1.55-µm wavelength. The devices demonstrate picosecond response time (< 100 ps, limited by our readout system)and negligibly low dark counts. Spectral dependencies of photon counting of continuous-wave, 0.4-µm to 3.5-µm radiation, and 0.63-µm, 1.33-µm, and 1.55-µm laser-pulsed radiations are presented for the single-stripe-type and meander-type devices.