Alexander Scheuring

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.

From Superconducting to Semiconducting YBCO Thin Film Bolometers: Sensitivity and Crosstalk Investigations for Future THz Imagers

YBa2Cu3O6+x compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting properties for lower oxygen content. Superconducting YBCO was obtained commercially; the semiconducting material was deposited by sputtering close to room temperature (100degC, typically). In order to migrate from superconducting to uncooled semiconducting far-infrared bolometer technologies, we have compared the performance of 2 times 2 pixel arrays. Pixels were in the shape of meanders, embedded in an area of about 1 mm2. Pixel detectivity and thermal crosstalk were studied in the 1 Hz to 100 kHz modulation frequency range by using a 850 nm solid state laser. Specific THz antenna concepts were also considered.

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.

Real-time measurement of picosecond THz pulses by an ultra-fast YBa2Cu3O7−d detection system

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.

High-Speed Y–Ba–Cu–O Direct Detection System for Monitoring Picosecond THz Pulses

A high-speed YBa₂Cu₃O_7-δ direct detection system was developed to monitor terahertz picosecond pulses in the time domain. High-T_C superconducting thin-film YBa₂Cu₃O_7-δ microbridges with critical temperatures of T_C = 85 K were embedded into a planar log-spiral antenna to couple the broadband terahertz radiation (0.1 -2 THz) of several picosecond pulsed sources. The YBa₂Cu₃O_7-δ detectors were installed in a liquid nitrogen cryostat equipped with 18 GHz effective bandwidth readout electronics. THz pulses generated at the electron storage rings ANKA and UVSOR-II have been resolved with a temporal resolution of 30 ps (full width at half maximum) limited by the readout electronics bandwidth. Beam dynamic effects of bursting coherent synchrotron radiation were successfully monitored.

Thin Pr–Ba–Cu–O Film Antenna-Coupled THz Bolometers for Room Temperature Operation

We report on the development of room temperature THz bolometers made from semiconducting PrBa₂Cu₃O_7-δ (PBCO) thin films on MgO substrate. PBCO thin films show a high temperature coefficient of resistance (TCR ≈ 1-2%/K) at low resistivity values (ρ ≈ 2000 μΩcm) in comparison to other semiconducting materials like amorphous silicon or vanadium oxide. A low resistivity enables efficient coupling to an integrated planar antenna required to couple THz radiation to micrometer sized detector elements. A detailed electrical characterization as well as radiation measurements at 0.65 THz of the 70-100 nm thick PBCO film microbridges embedded into log-spiral planar antennas have been performed. An electrical responsivity up to <i>S</i>=33 V/W and a noise equivalent power of NEP=1.52&times;10^-10 W/√Hz at a modulation frequency of 10 kHz limited by the measurement setup have been achieved.

Highly Responsive Y–Ba–Cu–O Thin Film THz Detectors With Picosecond Time Resolution

High-temperature superconducting YBa2Cu3O7 - δ (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.

Technology and Performance of THz Hot-Electron Bolometer Mixers

Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.

Dielectric RF properties of CVD diamond disks from sub-mm wave to THz frequencies

ITER torus windows with CVD diamond disks for high power heating applications (170 GHz, 1–2 MW) are being investigated by different low- and high power measurement setups in the frequency range of 90 to 170 GHz [1,2]. To understand the loss mechanisms in diamond material the determination of the frequency dependence of dielectric constant (ε = ε′ −jε″) and loss tangent (tan δ) at higher frequencies up to several THz is essential. It is well known from the experience with other window materials for high power fusion applications (ECRH) like silicon or sapphire electrons and phonons are responsible for microwave losses. In diamond the sp2-carbon content and surface roughness determines surface losses. Additionally, the electronic surface states for different chemical finishing of the diamond disks can be studied in the THz region.