Abstract: The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity =3.9 V/W.

Abstract: We present a new, simple to manufacture superconducting single-photon detector operational in the range from ultraviolet to mid-infrared radiation wavelengths. The detector combines GHz counting rate, high quantum efficiency and very low level of dark (false) counts. At 1.3–1.5 μm wavelength range our detector exhibits a quantum efficiency of 5–10%. The detector photoresponse voltage pulse duration was measured to be about 150 ps with jitter of 35 ps and both of them were limited mostly by our measurement equipment. In terms of quantum efficiency, dark counts level, speed of operation the detector surpasses all semiconductor counterparts and was successfully applied for CMOS integrated circuits diagnostics.

Abstract: We report on our progress in development of superconducting single-photon detectors (SSPDs), specifically designed for secure high-speed quantum communications. The SSPDs consist of NbN-based meander nanostructures and operate at liquid helium temperatures. In general, our devices are capable of GHz-rate photon counting in a spectral range from visible light to mid-infrared. The device jitter is 18 ps and dark counts can reach negligibly small levels. The quantum efficiency (QE) of our best SSPDs for visible-light photons approaches a saturation level of ~30-40%, which is limited by the NbN film absorption. For the infrared range (1.55µm), QE is ~6% at 4.2 K, but it can be significantly improved by reduction of the operation temperature to the 2-K level, when QE reaches ~20% for 1.55-µm photons. In order to further enhance the SSPD efficiency at the wavelength of 1.55 µm, we have integrated our detectors with optical cavities, aiming to increase the effective interaction of the photon with the superconducting meander and, therefore, increase the QE. A successful effort was made to fabricate an advanced SSPD structure with an optical microcavity optimized for absorption of 1.55 µm photons. The design consisted of a quarter-wave dielectric layer, combined with a metallic mirror. Early tests performed on relatively low-QE devices integrated with microcavities, showed that the QE value at the resonator maximum (1.55-µm wavelength) was of the factor 3-to-4 higher than that for a nonresonant SSPD. Independently, we have successfully coupled our SSPDs to single-mode optical fibers. The completed receivers, inserted into a liquid-helium transport dewar, reached ~1% system QE for 1.55 µm photons. The SSPD receivers that are fiber-coupled and, simultaneously, integrated with resonators are expected to be the ultimate photon counters for optical quantum communications.