C. F. J. Lodewijk

Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

High critical current-density (10?to?420?kA/cm2) superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10?3–10?1?mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlOx barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.

The ALMA Band 9 receiver - Design, construction, characterization, and first light

Aims. We describe the design, construction, and characterization of the Band 9 heterodyne receivers (600–720 GHz) for the Atacama Large Millimeter/submillimeter Array (ALMA). First-light Band 9 data, obtained during ALMA commissioning and science verification phases, are presented as well. Methods. The ALMA Band 9 receiver units (so-called “cartridges”), which are installed in the telescope’s front end, have been designed to detect and down-convert two orthogonal linear polarization components of the light collected by the ALMA antennas. The light entering the front end is refocused with a compact arrangement of mirrors, which is fully contained within the cartridge. The arrangement contains a grid to separate the polarizations and two beam splitters to combine each resulting beam with a local oscillator signal. The combined beams are fed into independent double-sideband mixers, each with a corrugated feedhorn coupling the radiation by way of a waveguide with backshort cavity into an impedance-tuned superconductor-insulator-superconductor (SIS) junction that performs the heterodyne down-conversion. Finally, the generated intermediate frequency (IF) signals are amplified by cryogenic and room-temperature HEMT amplifiers and exported to the telescope’s IF back end for further processing and, finally, correlation. Results. The receivers have been constructed and tested in the laboratory and they show an excellent performance, complying with ALMA requirements. Performance statistics on all 73 Band 9 receivers are reported. Importantly, two di_erent tunnel-barrier technologies (necessitating di_erent tuning circuits) for the SIS junctions have been used, namely conventional AlOx barriers and the more recent high-current-density AlN barriers. On-sky characterization and tests of the performance of the Band 9 cartridges are presented using commissioning data. Continuum and line images of the low-mass protobinary IRAS 16293-2422 are presented which were obtained as part of the ALMA science verification program. An 8 GHz wide Band 9 spectrum extracted over a 0:300 _0:300 region near source B, containing more than 100 emission lines, illustrates the quality of the data.

Optimizing Superconducting Matching Circuits for Nb SIS Mixers Operating Around the Gap Frequency

The bandwidth and noise temperature of superconducting-insulating-superconducting (SIS) mixers are strongly determined by the matching between antenna and tunnel-junction. Optimal scientific benefit from Band 9 (600 to 720 GHz) of the Atacama large millimeter array requires a proper design of the matching circuit by taking into account the energy gap of the niobium (680 GHz). In preliminary experiments we have found that tapered structures lead to significant improvements, which suggests that a systematic analysis of optimized geometrical shapes might be beneficial. The tuning circuits are analysed using a model, which includes the Mattis-Bardeen theory for the surface impedance of Nb and which allows arbitrary shapes. We will present simulations and experimental results focusing on the transmission efficiency, which can conveniently be evaluated experimentally by using a Fourier transform spectrometer. We will show how the shape influences the transmission efficiency and relate it to observed noise temperatures of the mixers.

Design and performance of the 600-720 GHz ALMA band 9 cartridge

The ALMA band 9 cartridge is a compact receiver unit for the Atacama large millimeter array (ALMA) containing the core of a 600-720 GHz heterodyne front end, including a solid-state local oscillator; broadband SIS mixers; and a 4-12 GHz IF amplifier chain. This paper describes the design and performance of the first band 9 cartridge. The experience gained with this unit is being used to optimize the design in preparation for the production of sufficient cartridges to fully populate the ALMA and ALMA compact arrays (ultimately requiring roughly 80 cartridges).

Bandwidth Limitations of Nb/AlN/Nb SIS Mixers Around 700 GHz

We study, using niobium-technology, the bandwidth of SIS mixers operating at frequencies close to the energy-gap frequency. Microstriplines of niobium-silicon dioxide-niobium have different properties for the top and bottom superconductor, which we find to depend on the used fabrication process. Replacing the AlOx tunnel barrier by AlN, the bandwidth increases by 53%. The measurements, using a Fourier Transform Spectrometer (FTS) and performed in ambient air, demonstrate that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. Excellent noise temperatures are found over a full band of 600 to 720 GHz.

Design of a side-band-separating heterodyne mixer for band 9 of ALMA

A side-band-separating (SBS) heterodyne mixer has been designed for the Atacama large millimeter array (ALMA) 602-720 GHz band, as it presents a great improvement over the current double-side-band configuration under development at the moment. Here we present design details and the results of extensive computer simulations of its performance. The designed SBS mixer exploits waveguide technology. At its core it consists of a quadrature hybrid, two LO injectors, and three dumping loads. The entire structure has been analyzed in a linear circuit simulator with custom code written to accurately (verified by HFSS finite element simulations) model the hybrid structures. This technique permitted an optimization of the dimensions and the study of the consequences of deviations from the ideal situation. It is estimated that the tolerances in several of the components should be kept at less than 3 /spl mu/m.

An SIS-based sideband-separating heterodyne mixer optimized for the 600 to 720 GHz band.

The Atacama Large Millimeter Array (ALMA) is the largest radio astronomical enterprise ever proposed. When completed, each of its 64 constituting radio-telescopes will be able to hold 10 heterodyne receivers covering the spectroscopic windows allowed by the atmospheric transmission at the construction site, the altiplanos of the northern Chilean Andes. In contrast to the sideband-separating (2SB) receivers being developed at low frequencies, double-side-band (DSB) receivers are being developed for the highest two spectroscopic windows (bands 9 and 10). Despite of the well known advantages of 2SB mixers over their DSB counterparts, they have not been implemented at the highest-frequency bands as the involved dimensions for some of the radio frequency components are prohibitory small. However, the current state-of-the-art micromachining technology has proved that the structures necessary for this development are attainable. Here we report the design, modeling, realization, and characterization of a 2SB mixer for band 9 of ALMA (600 to 720 GHz). At the heart of the mixer, two superconductor-insulator-superconductor (SIS) junctions are used as mixing elements. The constructed instrument presents an excellent performance as shown by two important figures of merit: noise temperature of the system and side band ratio, both of them within ALMA specifications.

A Waveguide NbTiN SIS Mixer for THz Array Applications

In this paper we will present the design and realization of a double sideband SIS heterodyne mixer for the frequency range of 780-950 GHz. A NbTiN high Tc superconductor in combination with aluminum is used for a SIS junction tuning circuit. We use a double side waveguide probe with a fixed backshort tuner design which was optimized by means of a 3D EM-modeling software. Compact mechanical envelope of the mixer allows for a combination of several units in a focal plane array. We will present results of instantaneous bandwidth measurement for seven mixer units and compare it with theoretical predictions. We also present a heterodyne sensitivity test results for these mixers with the best achieved DSB mixer noise temperature of about 250 K (physical temperature 4.2 K) which is only several times higher than a quantum noise limit for these frequencies.

Side-band-separating heterodyne mixer for band 9 of ALMA.

Here we present the realization of a side-band-separating (2SB) heterodyne mixer for the frequency range from 602 to 720 GHz (corresponding to ALMA band 9). The mixer, in brief, consists of a quadrature hybrid, two LO injectors, two SIS junctions, and three dumping loads. All the parts were modeled and optimized prior construction. The fabricated 2SB mixer exploits waveguide technology and has been constructed in the split-block technique using state-of-the-art CNC micromachining. We also present the performance of the SIS junctions that will be used in the 2SB mixer.