A new generation of quasi-optical (QO) systems with multiple refractive and reflective components are required for several (sub)-millimeter applications such as the next generation of communication systems, sensing and security imagers, and instrumentation for far-infrared astronomy. Currently, there are no numerically efficient and accurate modeling tools available to design such systems tailored to their moderate sizes, in terms of wavelength, and wide operational bandwidths. In this article, a numerically efficient and accurate technique is proposed for designing QO systems containing a cascade of homogeneous dielectric lens components. The methodology is based on sequential geometrical optics (GO) combined with an analysis in reception mode. Thanks to the computational efficiency of the proposed technique, we were able to embed it as the kernel for multiobjective optimizers to design complex multilens QO systems. Here, two indicative examples of such geometries targeting specific requirements and scenarios are showcased. The overall optimization technique is validated against in-house and commercial physical optics (PO) codes, as well as full-wave simulations where possible, with excellent agreement in both efficiency terms and far-field beam patterns. Moreover, the proposed technique achieves orders of magnitude faster execution times with respect to the currently available commercial tools.

Scaling astronomical detectors and quantum devices to larger and larger sizes will require more readout lines. A problem with extra readout lines is the complexity and cost of many coaxial cables in parallel. Furthermore, the thermal loading on the coldest stages via thermal conductivity should be kept at a minimum. Delft circuits provides an eight-channel flexible cabling solution based on stripline technology with Ag or NbTi conductors and polyimide dielectric (flexline) as a solution to these problems. However, a compact and light-tight flex-to-chip interface is lacking, and this would be key to unlocking its full scaling and lightweight potential. Here, we present the design and measurements of a flexline to PCB interface for cryogenic detectors. The flexline is connected securely via a spring-loaded pressing tool, ensuring reliable contact even after thermal contraction. To reduce stray radiation, the chip and connector assembly sits in a light-tight setup. The connector spans a width of 20 mm along one edge of the chip, illustrating the compact design. We show that the flexline connector has an insertion loss of -2 dB at 7 GHz using a probe measurement at room temperature. We also demonstrate the connector with a test chip with kinetic inductance detectors in a dilution refrigerator cooled to 40 mK.

Context. The Deep Spectroscopic HIgh-redshift MApper (DESHIMA) is a broadband integrated superconducting spectrometer for millimeter (mm) and submillimeter (submm) wave astronomy based on kinetic inductance detectors (KIDs). The first generation, DESHIMA 1.0, was successfully tested in 2017. This paper describes the upgrade to DESHIMA 2.0 and presents its characterization in laboratory settings. Aims. We enhanced the performance by increasing the band coverage and efficiency while developing a fast sky-position chopper for the efficient removal of atmospheric fluctuations. Methods. The instrument features NbTiN superconducting microstrip (MS) filters with a low-loss a-SiC:H dielectric and an ultra-wideband leaky-wave antenna. A laboratory setup was designed, incorporating the cryostat housing cryogenic optics and integrated superconducting spectrometer chip comprising 339 KIDs connected to MS filters tuned for (sub)mm wave frequencies. Room-temperature mirrors on a hexapod stage allowed us to precisely position and align the optical elements. The sky-position chopper was positioned on a motor-controlled stage for a fine-tuned control over its position and alignment. The multiplexing capability of KIDs enabled us to simultaneously measure multiple performance metrics across the entire frequency range. Results. DESHIMA 2.0 achieved significant improvements in the performance compared to its predecessor: the measured instantaneous frequency coverage was 200–400 GHz with a mean filter Q_filter of 340 ± 50, the instrument efficiency reached ~8%, indicating a four times wider band coverage and a four times higher sensitivity. The yield rate for the MS filters exceeded 98%. The data loss caused by the sky-position chopper was limited to <20% without beam truncation. The estimated aperture efficiency from measured beam patterns agreed well with the designed value of approximately 70%. The telescope far-field beam patterns calculated from the measured patterns also agree well with the design specifications. We also demonstrated that a new method of absolute frequency calibration is valid using the data from the beam pattern measurement.

Studying the polarization and spectral distortion of the cosmic microwave background (CMB) in tandem with intensity fluctuations of the cosmic infrared background allows us to verify our assumptions on cosmic inflation and investigate the dynamics and evolution of galaxy clusters in the past 10 billion years. Because of its broadband emission and being an all-sky extended source, observing the entire CMB in detail is a very time-consuming and expensive exercise. Fortunately, in the past few years, the on-chip superconducting spectrometer technology has moved out of the lab and into the telescope. With its compact size and background-limited sensitivity, this family of instruments is particularly well-suited for fast and large area observations in a relatively unexplored range of the electromagnetic spectrum. However, recent examples of this technology do not yet reach the requirements needed for large spectroscopic and polarimetric surveys of the CMB. We formulate several of these requirements and introduce novel on-chip components and fabrication techniques. We introduce a crossover to enable distinguishing signal polarization, minimize signal loss by locally optimized lithography of a coplanar waveguide, lower the spectral resolution of microstrip filters by deposition of a dielectric layer, and increase the yield of the spectrometer array by removing individual line shorts. These together have culminated in the successful fabrication of a 14-spaxel integral field unit.

We developed, characterized, and verified an alignment procedure for the DESHIMA 2.0 instrument, an ultra-wide-band spectrometer operating between 200 and 400 GHz, at the ASTE telescope. To this end, we mounted the warm optics, consisting of a modified Dragonian dual reflector system, on a motor-controlled hexapod. Crucial in the alignment procedure is our sky chopper, which allows fast beam switching. It has a small entrance and exit aperture coupling to the (cold) sky, which creates a measurable signal with respect to the warm cabin environment. By scanning the instrument beam across the entrance aperture of the sky chopper using the hexapod, we found the hexapod configuration that produced the lowest signal on our detectors, implying that the beam is coupled fully to the cold sky and not the warm cabin. We first characterized the alignment procedure in the laboratory, where we used a vat containing liquid nitrogen as the cold source behind the sky chopper. Then, we applied the alignment procedure to DESHIMA 2.0 at ASTE. We found that the alignment procedure significantly improved the aperture efficiency compared with previously reported values of the aperture efficiency of DESHIMA at ASTE, which indicates the veracity of the alignment procedure.

Low-loss deposited dielectrics are beneficial for the advancement of superconducting integrated circuits for astronomy. In the microwave band (approximately 1-10 GHz) the dielectric loss at cryogenic temperatures and low electric field strengths is dominated by two-level systems. However, the origin of the loss in the millimeter-submillimeter band (approximately 0.1-1 THz) is not understood. We measured the loss of hydrogenated-amorphous-SiC films in the 0.27-100-THz range using superconducting-microstrip resonators and Fourier-transform spectroscopy. The agreement between the loss data and a Maxwell-Helmholtz-Drude dispersion model suggests that vibrational modes above 10 THz dominate the loss in hydrogenated amorphous SiC above 200 GHz.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an astrophysics mission concept currently under study. The instrument comprises four focal plane arrays (FPAs), each with over 1000 pixels, consisting of lens-coupled kinetic inductance detectors designed for ultra-high sensitivity spectroscopy and imaging. PRIMA covers the far-infrared band from 24 to 250 μm. We present the fundamental properties of lens-coupled absorbers, including aperture efficiency, throughput, and beam patterns. Compared with bare absorbers, lens-coupled absorbers exhibit reduced sensitivity to noise from both the instrument enclosure and the sky background. We analyze FPA sampling strategies in terms of relative observing speed, considering both detector noise–limited and background noise–limited scenarios. In the background noise–limited case, near-maximum-gain sampling is optimal only when the point spread function (PSF) in the reflector focal plane is aligned with the lens. For arbitrary PSF incidence, a near-Nyquist sampling configuration provides the best average performance. These calculations are based on a computationally efficient quasi-analytical technique and assume a hex-packed array of circular lenses, recently developed for PRIMA.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an infrared observatory for the next decade, currently in Phase A, with a 1.8 m telescope actively cooled to 4.5 K. On board, an infrared camera, PRIMAger, equipped with ultra-sensitive kinetic inductance detector arrays, will provide observers with a coverage of mid-infrared to far-infrared wavelengths from 24 to 264 μm. PRIMAger will offer two imaging modes: the hyperspectral mode will cover the 24 to 84 μm wavelength range with a spectral resolution R ≥ 8, whereas the polarimetric mode will provide polarimetric imaging in four broadbands from 80 to 264 μm. These observational capabilities have been tailored to answer fundamental astrophysical questions such as black hole and star-formation co-evolution in galaxies, the evolution of small dust grains over a wide range of redshifts, and the effects of interstellar magnetic fields in various environments, as well as to open a vast discovery space with versatile photometric and polarimetric capabilities. PRIMAger is being developed by an international collaboration bringing together French institutes (Laboratoire d’Astrophysique de Marseille and CEA) through the Center National d’Etudes Spatiales (CNES, Paris, France), the Netherlands Institute for Space Research (SRON, Leiden, Netherlands), and the Cardiff University (Cardiff, UK) in Europe, as well as the Jet Propulsion Laboratory and Goddard Space Flight Center in the United States.

The presence of quasiparticles typically degrades the performance of superconducting microwave circuits. The readout signal can generate nonequilibrium quasiparticles, which lead to excess microwave loss and decoherence. To understand this effect quantitatively, we measure quasiparticle fluctuations and extract the quasiparticle density across different temperatures, readout powers, and resonator volumes. We find that microwave power generates a higher quasiparticle density as the active resonator volume is reduced and show that this effect sets a sensitivity limit on kinetic inductance detectors. We compare our results with theoretical models of direct microwave photon absorption by quasiparticles and conclude that an unknown, indirect mechanism plays a dominant role in quasiparticle generation. These results provide a route to mitigate quasiparticle generation due to readout power in superconducting devices.

We present measurements and simulations of the polarization purity of leaky lens-antenna coupled microwave kinetic inductance detectors (KIDs) at 1.5 THz. From polarized phase and amplitude beam pattern measurements we find the integrated cross-polarization ratio to be at –21.5 dB for 1 f#λ spatial sampling. The measurements agree well with the theoretical description which is based on a combination of in-transmission simulation of the antenna feed, and an in-reception analysis of the antenna-KID system. A neutral density filter limited the power per detector to around 500 fW, enabling these measurements to be taken on detectors that in a low background have a measured noise equivalent power of 5–7×10⁻²⁰ W/√Hz. These combined measurements show that these detectors are excellent candidates for large scale and high-performance imaging polarimetric instruments.

Disordered superconductors offer new impedance regimes for quantum circuits, enable a pathway to protected qubits, and can improve superconducting detectors due to their high kinetic inductance and sheet resistance. The performance of these devices can be limited, however, by quasiparticles—the fundamental excitations of a superconductor. While experiments have shown that disorder affects the relaxation of quasiparticles drastically, the microscopic mechanisms are still not understood. We address this issue by measuring quasiparticle relaxation in a disordered β-Ta film, which we pattern as the inductor of a microwave resonator. We observe that quasiparticle recombination is governed by the phonon scattering time, which is faster than conventional recombination in ordered superconductors. We interpret the results as recombination of localized quasiparticles, induced by disorder, which first delocalize via phonon absorption. We analyze quasiparticle relaxation measurements on superconductors with different degrees of disorder and conclude that this phenomenon is inherent to disordered superconductors.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is under study as a potential far-IR space mission, featuring actively cooled optics, and both imaging and spectroscopic instrumentation. To fully take advantage of the low background afforded by a cold telescope, spectroscopy with PRIMA requires detectors with a noise equivalent power (NEP) better than 1 × 10 ^-19 W Hz ^-12. To meet this goal, we are developing large format arrays of kinetic inductance detectors (KIDs) to work across the 25–250 μm range. Here, we present the design and characterization of a single pixel prototype detector optimized for 210 μm. The KID consists of a lens-coupled aluminum inductor-absorber connected to a niobium interdigitated capacitor to form a 2 GHz resonator. We have fabricated a small array with 28 KIDs, and we measure the performance of one of these detectors with an optical loading in the 0.01–300 aW range. At low loading, the detector achieves an NEP of 9 × 10 ^-20 W Hz ^-1/2 at a 10 Hz readout frequency. An extrapolation of these measurements suggests this detector may remain photon noise limited at up to 20 fW of loading, offering a high dynamic range for PRIMA observations of bright astronomical sources.

In this article, the wrong figure appeared as Figures 1 and 2; the figure should have appeared as shown below Figure 1 (right side of the figure only): (Figure presented.) Figure 2: (Figure presented.)