Stefan Wuensch

A Novel Analytical Model of Resonance Effects of Log-Periodic Planar Antennas

In this paper a new analytical model about the calculation of resonance frequencies and bandwidth of log-periodic planar antennas is proposed. The two previous known models and our novel model are compared to numerical simulations and experimental measurements. In the theoretical part analytical calculations are opposed to the results of reflection parameter simulations. Our model shows a very good coincidence according to the number and position of resonance frequencies. We show that this is not the case for the previous models. For the experimental validation of the novel model several devices with different geometries were fabricated and characterized in the 1-8 GHz frequency range. All considered antenna structures exhibit an excellent coincidence between calculations, simulations and measurements.

TES Bolometers With High-Frequency Readout Circuit

In order to improve the frequency-division multiplexing (FDM) in transition edge sensor (TES) imaging arrays, it is suggested to replace commonly used SQUID amplifiers with a semiconductor high-frequency cooled amplifier. This would result in a single 10-GHz bandwidth amplifier serving the array of more than 1000 detectors. The basic idea is to implement an antenna-coupled TES as a load for a high-Q resonator, weakly coupled to a microwave transmission line. This high-frequency scheme substitutes the traditional wire connections to the TES. The NEP as low as 2×10-19 W/Hz0.5 is estimated at ambient temperature of 300 mK for submicron-size TES absorber made of Ti; the NEP is limited by 3 K noise temperature of the amplifier. To verify the new concept, prototype TES devices made of Nb are developed and tested above 4 K. The NEP of about 1.5×10-15 W/Hz0.5 is estimated for the experimental micron-size prototype devices made of Nb at 4.5 K. The IV -curves of the TES at different temperatures are recovered using the RF and heat balance models along with the experimental R(T) data; presence of the negative electrothermal feedback is verified.

Superconductor-to-Semiconductor Interface Circuit for High Data Rates

We present a new kind of rapid-single-flux-quantum (RSFQ) output driver together with a pseudomorphic high electron mobility transistor (p-HEMT) amplifier both operating at liquid helium temperature. The passive interconnect including the interchip connection between the RSFQ output driver and the first transistor stage of the semiconductor amplifier is the key element for signal matching and was optimized for minimizing the reflections to the RSFQ circuit. The RSFQ output driver is based on a single-flux-quantum to dc converter and a voltage doubler. The circuit is realized in the Niobium based 1 kA/cm2 process of FLUXONICS Foundry and provides up to 438-muV output voltage. We demonstrate high-speed experiments of the output driver in combination with two different semiconductor amplifier circuits at liquid helium temperature. The output voltage of a 2-Gb/s data stream was measured to be about 3.5 mV.

An ultra-fast data acquisition system for coherent synchrotron radiation with terahertz detectors

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.

Transient Analysis of THz-QCL Pulses Using NbN and YBCO Superconducting Detectors

We report the time-domain analysis of fast pulses emitted by a quantum cascade laser (QCL) operating at ~ 3.1 THz using superconducting THz detectors made from either NbN or YBa2Cu3O7-δ (YBCO) thin films. The ultrafast response from these detectors allows resolution of emission features occurring on a nanosecond time-scale, which is not possible with commercially available Ge or InSb bolometers owing to their much larger time constants. We demonstrate that the time-dependent emission can be strongly affected by relatively small variations in the driving pulse. The QCL output power-current relationship was determined, based on correlation of the time-dependent emission of radiation with current flow in the QCL, under different QCL bias conditions. We show that this relationship differs from that obtained using bolometric detectors that respond only to the integrated pulse energy. The linearity of the detectors, and their agreement with measurements using a Ge bolometer, was also established by studying the QCL emission as a function of bias voltage and excitation pulse length. This measurement scheme could be readily applied to the study of ultrafast modulation and mode-locking of THz-QCLs.

Operation of Superconducting Nanowire Single-Photon Detectors Embedded in Lumped-Element Resonant Circuits

We propose a new operation principle for superconducting nanowire single-photon detectors (SNSPDs) enabling an efficient frequency-division multiplexing of the bias and readout signals of a large multipixel detector array. The SNSPD is part of a resonant circuit, and the microwave signal inside the device is used to bias the detector close to the critical state. Hence, an n×m pixel SNSPD array requires only two coaxial cables from room temperature to the detector ambient temperature of 4.2 K. We show the microwave characterization and single-photon response of such devices and compare the detector efficiencies with conventional dc-biased SNSPDs. Furthermore, we demonstrate the feasibility of array applications with the new operation mode in a two-pixel proof-of-principle device.

Cryogenic Semiconductor Amplifier for RSFQ-Circuits With High Data Rates at 4.2 K

The connection of RSFQ circuits with commercial room temperature electronics and amplifying SFQ pulses with high data rates require cryogenic interface amplifiers. Such amplifiers should provide extremely high bandwidth, low noise and low power consumption at the same time. Various hybrid amplifiers based on commercial p-HEMT transistors in an embedded microwave design were designed and characterized. Towards the p-HEMT transistor characterization at cryogenic temperature the biasing settings according to an optimum between voltage gain and low power consumption were determined. Thus a total power consumption of 2 mW and a voltage gain of 12 dB per single stage were achieved. For a preamplifier and an amplifier concept a multiple number of these stages were implemented in a microstrip and a coplanar transmission line design with a special matched interconnect taper towards the RSFQ components. Measurements of the amplifiers, the combination of an amplifier with a RSFQ circuit at 4.2 K showed their good performances without any disturbance of the RSFQ circuit. Due to the losses between the stages a total gain of 25 dB with a four stage amplifier was achieved; pulse rates went up to 3 Gb/s. The total power consumption was in the range of 8 mW. Further increase of data rates will be achieved by improving the matching between RSFQ output stage and amplifier.

Frequency-multiplexed bias and readout of a 16-pixel superconducting nanowire single-photon detector array

We demonstrate a 16-pixel array of microwave-current driven superconducting nanowire single-photon detectors with an integrated and scalable frequency-division multiplexing architecture, which reduces the required number of bias and readout lines to a single microwave feed line. The electrical behavior of the photon-sensitive nanowires, embedded in a resonant circuit, as well as the optical performance and timing jitter of the single detectors is discussed. Besides the single pixel measurements, we also demonstrate the operation of a 16-pixel array with a temporal, spatial, and photon-number resolution.

Optimized Microwave LEKID Arrays for High-Resolution Applications

High-resolution applications demand a strong increase in number of pixels of detector arrays. In terms of the lumped-element kinetic inductance detector, this requires a detailed knowledge of the microwave properties of each single pixel; moreover, the unwanted interaction (cross-talk) between them has to be examined and eliminated. In this paper, the basic microwave properties of lumped-element kinetic inductance detector structures were investigated. Simulations demonstrate the influence of the embedding design parameters, e.g., orientation and coupling distances, in respect of the microwave behavior. In addition, different array arrangements to increase the number of pixels and to identify possible reasons causing unwanted cross-talk were examined. For this purposes several samples in niobium thin film technology on silicon substrates were designed, fabricated and characterized. Loaded quality factors QL up to 3 000 at 4.2 K were achieved. The results of the simulations and measurements of small and large arrays will be presented and discussed.

Design and development of a cryogenic semiconductor amplifier for interfacing RSFQ circuits at 4.2 K

There is a great deal of interest in amplifiers with extremely high bandwidth, low noise and low power consumption which interface RSFQ circuits with commercial room temperature electronics or amplifying SFQ pulses. In this special application a hybrid amplifier should interface an RSFQ circuitry with a Josephson junction array quantizer chip (JA-Q) at 4.2 K. The input signal for the amplifier is generated by an RSFQ signal generator and the output signal fed-in to a Josephson array quantizer. A voltage gain of about 104 with a pattern frequency of 2 GHz for a good transfer of the SFQ pulses is required. Additionally, the JA-Q requires a high output current at low power consumption of the amplifier at the same time. Various hybrid amplifiers based on commercially available p-HEMT transistors in an embedded microwave design were designed and characterized. For the p-HEMT transistor characterization at cryogenic temperatures the biasing settings according to an optimum between voltage gain AV and low power consumption PV were determined. Thus a power consumption of PV = 2 mW and a voltage gain of about AV = 4 per stage were achieved. For a preamplifier and an amplifier concept a number of these stages were implemented in a microstrip and a coplanar transmission line design with a special matched interconnect taper towards the RSFQ components. Measurements of the amplifiers and the combination of the amplifiers and the JA-Q at 4.2 K showed their good performance without any disturbances of the quantized voltage steps.