Dagmar Henrich

Geometry-induced reduction of the critical current in superconducting nanowires

Reduction of the critical current in narrow superconducting NbN lines with sharp and rounded bends with respect to the critical current in straight lines was studied at different temperatures.We compare our experimental results with the reduction expected in the framework of the London model and the Ginsburg-Landau model. We have experimentally found that the reduction is significantly less than either model predicts. We also show that in our NbN lines the bends mostly contribute to the reduction of the critical current at temperatures well below the superconducting transition temperature.

Broadening of hot-spot response spectrum of superconducting NbN nanowire single-photon detector with reduced nitrogen content

The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) have been studied systematically on detectors made from thin NbN films with different chemical compositions. Reduction of the nitrogen content in the 4 nm thick NbN films results in a decrease of the dark count rates more than two orders of magnitude and in a red shift of the cut-off wavelength of the hot-spot SNSPD response. The observed phenomena are explained by an improvement of uniformity of NbN films that has been confirmed by a decrease of resistivity and an increase of the ratio of the measured critical current to the depairing current. The latter factor is considered as the most crucial for both the cut-off wavelength and the dark count rates of SNSPD. Based on our results we propose a set of criteria for material properties to optimize SNSPD in the infrared spectral region.

Orthogonal sequencing multiplexer for superconducting nanowire single-photon detectors with RSFQ electronics readout circuit

We propose an efficient multiplexing technique for superconducting nanowire single-photon detectors based on an orthogonal detector bias switching method enabling the extraction of the average count rate of a set of detectors by one readout line. We implemented a system prototype where the SNSPDs are connected to an integrated cryogenic readout and a pulse merger system based on rapid single flux quantum (RSFQ) electronics. We discuss the general scalability of this concept, analyze the environmental requirements which define the resolvability and the accuracy and demonstrate the feasibility of this approach with experimental results for a SNSPD array with four pixels.

Time-Tagged Multiplexing of Serially Biased Superconducting Nanowire Single-Photon Detectors

We present a concept for a time-tagged multiplexed readout of several superconducting nanowire single-photon detector elements for small arrays in ultra-short pulsed laser applications. The detector elements were coupled in an array by a superconducting delay line giving each detector element a temporal signature. The complete detector chain is biased by one bias supply. The patterning concept and the first experimental proof of principle are demonstrated on two-detector element arrays with delay times of 86 and 156 ps each made from a 5-nm NbN film on sapphire. We discuss the propagation delay of a delay line taking the geometric and kinetic inductance into account. We show that mainly the normal conducting propagation velocity defines the characteristic time of the delay line and that the inductance dependent response pulse width currently limits the maximum number of detector elements.

Temperature-Dependence of Detection Efficiency in NbN and TaN SNSPD

We present systematic measurements of the temperature dependence of detection efficiencies in TaN and NbN superconducting nanowire single-photon detectors. We have observed a clear increase of the cut-off wavelength with decreasing temperature that we can qualitatively describe with a temperature-dependent diffusion coefficient of the quasi-particles created after photon absorption. Furthermore, the detection efficiency at wavelengths shorter than the cut-off wavelength as well as at longer wavelengths exhibit distinct temperature dependencies. The underlying causes and possible consequences for microscopic detection models are discussed.

Superconducting single-photon counting system for optical experiments requiring time-resolution in the picosecond range

We have developed a cryogenic measurement system for single-photon counting, which can be used in optical experiments requiring high time resolution in the picosecond range. The system utilizes niobium nitride superconducting nanowire single-photon detectors which are integrated in a time-correlated single-photon counting (TCSPC) setup. In this work, we describe details of the mechanical design, the electrical setup, and the cryogenic optical components. The performance of the complete system in TCSPC mode is tentatively benchmarked using 140 fs long laser pulses at a repetition frequency of 75 MHz. Due to the high temporal stability of these pulses, the measured time resolution of 35 ps (FWHM) is limited by the timing jitter of the measurement system. The result was cross-checked in a Coherent Anti-stokes Raman Scattering (CARS) setup, where scattered pulses from a β-barium borate crystal have been detected with the same time resolution.

Detection Efficiency of a Spiral-Nanowire Superconducting Single-Photon Detector

We investigate the detection efficiency of a spiral layout of a superconducting nanowire single-photon detector. The design is less susceptible to the critical current reduction in sharp turns of the nanowire than the conventional meander design. Detector samples with different nanowire widths from 100 to 300 nm are patterned from a 4-nm-thick NbN film deposited on sapphire substrates. The critical current IC at 4.2 K for spiral, meander, and single-bridge structures is measured and compared. On the 100-nm-wide samples, the detection efficiency is measured in the wavelength range 400-1700 nm and the spectral bandwidth of the intrinsic detection efficiency is determined. In the optical range, the spiral detector reaches a detection efficiency of 27.6%, which is ~ 1.5 times the value of the meander. In the infrared range the detection efficiency of the spiral superconducting nanowire single-photon detector is more than doubled.

Infrared Photo-Response of Fe-Shunted Ba-122 Thin Film Microstructures

We present a study of the response to pulsed infrared radiation of Fe layer shunted pnictide thin film microstructures. The thin film multilayer consisting of 20-nm-thick Fe buffer, 50-nm-thick Ba(Fe, Co)₂As₂ film, and gold protection layer were deposited on heated MgO and MgAl₂O₄ substrates by pulsed-laser deposition. The multilayers were patterned into 5- to 8-μm-wide and 5-μm-long microbridges by electron-beam lithography and ion-milling technique. The microbridges show T_c ≈ 20 K and a critical current density up to 2.56 MA/cm² at T = 10 K. The photo-response of Fe-shunted Ba(Fe, Co)₂As₂ thin film microbridges to infrared radiation was studied in a wide range of incident optical power, operation temperature, and bias current. We have found that the electron energy relaxation in studied multilayers is dependent on substrate material and is 1.75 times faster in the case of MgAl₂O₄ characterized by lattice matching to the pnictide film in comparison to the MgO substrate.

Superconducting nanowire single-photon detectors for picosecond time resolved spectroscopic applications

Raman scattering spectroscopy allows the direct and fast study of molecules by analysis of their vibrational normal modes. However, for certain materials the scattered signal is superimposed by fluorescence, which - if present - overwhelms the intrinsically weak Raman signal by orders of magnitude. An approved method to resolve the instantaneous Raman signal of interest from the delayed fluorescence background is time-correlated single-photon counting (TCSPC). For that, a single-photon detector with fast dynamics is required. The, so-called, superconducting nanowire single-photon detector (SNSPD) is a promising candidate for TCSPC. We have developed an optical instrument using such a SNSPD for the TCSPC method. The detector is made from a 5 nm thick NbN film, patterned by electron-beam lithography in a meander line with a width of 100 nm and a filling-factor of 50 %, covering an active area of 4 × 4 μm2. As a proof of concept we have shown that it is possible to resolve low power optical signals (λ between 520 and 630 nm) with a timing jitter of about 35 ps. Based on our experimental results we will discuss perspectives and limits of SNSPD application for spectroscopy.