Search

Kinetic-inductance detectors (KIDs) combine high sensitivity with an intrinsic frequency-division readout capability. This kind of detector is ideal to build large arrays, which can be read out via frequency-division multiplexing. In order to precisely measure the detuning of the resonators, a highly specialized data acquisition system (DAQ) has to be used. We have developed a DAQ system, which is based on a XILINX Kintex 7 FPGA, with additional analog-to-digital/digital-to-analog converters. The analog signal chain is divided in a baseband with a bandwidth of 400 MHz and an RF band of 4–8.5 GHz, which can be adapted to different resonant frequencies of the detector array. To design a system with good linearity and noise performance, carefully selected analog components have to be used. To achieve a readout with video rate and high-frequency resolution, digital signal flow and signal processing are crucial. Therefore, clock synchronization is an important factor to achieve a dual-channel sampling rate of 1 GSPS. In order to obtain the upmost flexibility in signal processing, the digital signal processing is done completely in software on the PC. We will present the electrical characterization of the Kinetic-Inductance Readout Circuit (KIRC). Measurement results of a 20-pixel KID array will be shown and discussed in more detail.

One of the important performance parameters of superconducting nanowire single photon detectors (SNSPD) is the dark count rate. Dark counts are related to thermally-activated hopping of magnetic vortices across the nanowire. We present an experimental analysis of how the thermal coupling of the NbN SNSPD to the heat sink affects the dark count rate. It was found that the rate decreases with the increase in coupling strength while the detection efficiency remains almost constant.

Ultra-fast THz detectors from superconducting YBa2Cu3O7−δ (YBCO) thin films were developed to monitor picosecond THz pulses. YBCO thin films were optimized by the introduction of CeO2 and PrBaCuO buffer layers. The transition temperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge (transition temperature—83 K, critical current density at 77 K—2.4 MA/cm2) embedded in a planar log-spiral antenna was used to detect pulsed THz radiation of the ANKA storage ring. First time resolved measurements of the multi-bunch filling pattern are presented.

Nowadays detection systems based on superconducting nanowire single-photon detectors (SNSPD) are mainly with a single-pixel detector. Multi-pixel systems would extend the field of possible applications. Therefore, new concepts are required for effective readout of multi-pixel SNSPDs. We give an overview of possible concepts, conditions of operation and dedicated effort to optimize the requirements of different applications in the single-photon regime.

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.

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.

High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with improved responsivities were developed for fast time-domain measurements in the THz frequency range. YBCO thin films of 30 nm thickness were patterned to micro- and nanobridges and embedded into planar log-spiral THz antennas. The YBCO thin-film detectors were characterized with continuous wave radiation at 0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO nanobridges. Pulsed measurements in the THz frequency range were performed at the electron storage ring ANKA from the Karlsruhe Institute of Technology (KIT). Due to the high responsivities of the nanobridges no biasing was required for the detection of the coherent synchrotron radiation pulses achieving very good agreement between the measured pulse shapes and simulations.

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.

We have applied an ultrafast superconducting YBa 2 Cu 3 O 7-δ (YBCO) detector to study the time jitter of the propagating electron bunches in a synchrotron. The jitter was determined from the coherent terahertz (THz) synchrotron radiation (CSR).

The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) has 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 more than two orders of magnitude decrease of the dark count rates 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., 15 pages, 6 figures

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.

The recording of coherent synchrotron radiation requires data acquisition systems with a temporal resolution of tens of picosecond. This paper describes a new real-time and high-accuracy data acquisition system suitable for recording individual ultra-short pulses generated by a fast terahertz (THz) detector (e.g. YBCO, NbN, Zero Biased Schottky Diode). The system consists of a fast sampling board combined with a high data throughput readout. The first board is designed for sampling the fast pulse signals with a full width half maximum (FWHM) between a few tens to one hundred picoseconds with a minimum sampling time of 3 ps. The high data throughput board consists of a PCIe-Bus Master DMA architecture used for fast data transfer up to 3 GByte/s. The full readout chain with fast THz detectors and the acquisition system has been successfully tested at the synchrotron ANKA. An overview of the electronics system and preliminary results with multi-bunch filling pattern will be presented.

Superconducting nanowire single photon detectors (SNSPDs) are distinguished by a very high sensitivity, a broad spectral range and a sparse dark count rate. Therefore, arrays of SNSPDs enable novel ultrasensitive imaging applications. The goal is to realize large arrays in the near future. Hence, in order to improve detector systems, there is the necessity to develop multiplexing techniques to keep the number of wires in the cryogenic environment as low as possible. We realized a readout circuit with rapid single flux quantum electronics. All detector output signals were merged to a single output channel. This leads to a loss of spatial resolution of the information provided by the single pixels. In order to overcome this effect, this work describes a theoretical analysis of time and code multiplexing technique for arrays of SNSPDs and a comparison concerning their applicability in different cases. We developed rules for the usability of each multiplexing technique and verified the results with measurements of a 4-pixel SNSPD array.

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.

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 um wide and 5 um long microbridges by electron-beam lithography and ion-milling technique. The microbridges show Tc up to 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 case of MgAl2O4 characterized by lattice matching to pnictide film in comparison to MgO substrate., ASC 2012 submission

The temporal evolution of picosecond THz pulses generated at ANKA, the electron storage ring of the Karlsruhe Institute of Technology, has been measured in real-time using an ultra-fast YBa2Cu3O7−δ detection system. YBa2Cu3O7−δ thin-film detectors with 30 nm thickness were patterned to microbridges (2 μm long, 4.5 μm wide) and embedded into a planar log-spiral THz antenna. The detectors were glued on a silicon lens and installed in an ultra-fast readout system with a temporal resolution of 15 ps (full width at half maximum). Detector responses as short as 17 ps were recorded showing very good agreement with the expected storage ring bunch lengths.

Materials with a small superconducting energy gap are expected to favor a high detection efficiency of low-energy photons in superconducting nanowire single-photon detectors. We developed a TaN detector with smaller gap and lower density of states at the Fermi energy than in comparable NbN devices, while other relevant parameters remain essentially unchanged. The observed reduction of the minimum photon energy required for direct detection is in line with model predictions of $\approx1/3$ as compared to NbN., 9 pages incl. 1 table and 2 figures; corrected typos in table 1

We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.

Thin YBa 2 Cu 3 O 7 − δ (YBCO) film detectors embedded into a log-spiral planar antenna were implemented for the detection and analysis of ultrashort terahertz pulses emitted by electron bunches in a synchrotron storage ring. In the direct detection mode terahertz radiation pulses from single electron bunches were resolved. A response time of 45 ps was determined as the full width at half maximum of the voltage transient at the output of the detection system. The sensitivity of the YBCOdetector to pulsed terahertz radiation was 70 mV/pJ along with a sensitivity of 30 V/W for continuous radiation at 0.8 THz and a very broad dynamic range of over 30 dB. We found experimental evidences of a nonbolometric nature of the detection mechanism.

For a systematic study of energy relaxation processes in thin NbN and YBCO films on sapphire substrates, a frequency domain technique has been set up and employed. The magnetron sputtered NbN films of 3 nm to 22 nm thickness and pulsed-laser deposited YBCO films with thicknesses between 20 nm and 45 nm were excited by amplitude-modulated optical radiation (? = 850 nm). The response spectra were analyzed on basis of the two-temperature model of the energy dynamics in the interacting electron and phonon subsystems at quasi-equilibrium conditions. An increase of the energy relaxation time with increasing film thickness has been obtained for both NbN and YBCO thin film samples.

The quantum efficiency of superconducting nanowire single-photon detectors has been systematically studied in a broad spectral range from near ultra-violet to near infrared radiation. Detectors were made from NbN films on sapphire substrates and had thicknesses between 4 and 12 nm. The gradual reduction of the quantum efficiency with increasing wavelength is caused by a crossover from the "hot-spot" to the "vortex" detection scenario. For thicker films the wavelength demarcating the two regimes shifts to shorter wavelengths.

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., Comment: 5 pages, 3 figures, presented at ASC 2012, accepted for publication in special issue of IEEE Trans. Appl. Supercond. Small changes in text and figures in response to reviewer comments