In double sideband (DSB) receiver systems, both the desired signals and unwanted image frequencies are downconverted, necessitating effective filtering to enhance system selectivity and sensitivity. This manuscript presents a novel submillimeter-wave bandpass filter developed to suppress the unwanted image frequencies in a spectrometer system that is optimized to detect key atmospheric constituents in the passband, including water vapor (H216O – 559.9 GHz), deuterium oxide (HDO – 539.0 GHz), and oxygen (O3 – 570.1 GHz). The filter design employs compact triangular microstrip patch resonators that achieve sharp roll-off characteristics and wide stopband performance, making it ideal for image rejection applications across submillimeter-wave and THz bands. The filter is fabricated on a 15 μm quartz substrate with 0.5 μm gold metallization, providing a 3 dB bandwidth of approximately 80 GHz with well-defined rejection bands below 530 GHz and above 610 GHz. Signal coupling is accomplished through microstrip waveguide feed probes for input and output. The design methodology offers significant flexibility, allowing straightforward scaling and adjustment of center frequency and bandwidth for implementations ranging from 1 GHz to 2 THz. The versatility of the filter architecture presented has utility in radio astronomy, defense systems, and high-frequency communication network applications.

Superconducting microcalorimeter arrays with hundreds to thousands of pixels are enabling new measurements and capabilities in exotic atom spectroscopy, X-ray astronomy, nuclear materials analysis, and many other fields. We consider the prospects of multiplexing microcalorimeters with the kinetic inductance current sensor (KICS), an alternative to the superconducting quantum interference device (SQUID) historically used to read out cryogenic microcalorimeters. We show that the amplifier-limited noise of the KICS is determined by its dynamic range, which is a free design parameter and tuneable with a DC current bias. We predict that KICS can meet the requirements to read out prototype metallic magnetic calorimeter pixels designed for the Lynx X-ray satellite mission concept with the use of low-latency tone tracking. We propose a KICS design that has both high coupling efficiency and high isolation between the RF and DC components of the circuit. We argue that the KICS provides engineering tradeoffs that are better suited to the readout of microcalorimeters when compared to the analogous microwave SQUID multiplexer.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is a cryogenically cooled 1.8-m space telescope designed to address fundamental questions about the evolution of galactic ecosystems, the origins of planetary atmospheres, and the buildup of dust and metals over cosmic time. PRIMA will achieve unprecedented sensitivity in the 24-261 $\mu$m wavelength range, enabled by background-limited kinetic inductance detectors (KIDs) cooled to 120 mK. For PRIMA's Far-InfraRed Enhanced Survey Spectrometer (FIRESS) instrument, we have developed monolithic kilopixel silicon lenslet arrays to efficiently couple incident radiation from the telescope's fore-optics onto the KID absorber elements. These 3-D lenslet arrays are fabricated using grayscale lithography, followed by deep reactive ion etching, and are antireflection (AR) coated with a quarter-wavelength thick deposition of Parylene-C. The lenslet arrays are aligned and bonded to the KID arrays using a thin layer of epoxy through a flip-chip bonder. In this work, we report on the optimized fabrication, lens design, AR coating, and bonding processes developed for the FIRESS lenslet arrays. We characterize brassboard lenslet arrays fabricated to meet the specifications of the FIRESS low and high spectral bands, demonstrate stepped-thickness AR coatings to achieve high efficiency across broad wavelength ranges, and present spectral transmission measurements of the AR coating and the epoxy bonding layers.

The Astro2020 Decadal Survey recommended a new line of astrophysics observatories intermediate in scale between MIDEXs and Flagship-class observatories. In response, NASA created the Astrophysics Probe Explorer class and solicited proposals for the first generation of Probes. With a larger cost cap, Probes can achieve more ambitious science than SMEXs or MIDEXs and be implemented faster than Flagships-as frequently as one per decade. The PRobe far-Infrared Mission for Astrophysics (PRIMA) is one of two Probe concepts selected by NASA for a concept study in 2024/2025, potentially leading to implementation and launch as early as 2031. PRIMA was designed for a broad range of astrophysics, from how planets assemble their atmospheres, to the coevolution of galaxies and black holes, to the evolving properties of dust and galactic metallicity over cosmic time. Seventy-five percent of PRIMA's observing time will be allocated to guest observer observations and 25% allocated to principal investigator science; however, the principal investigator science data will be available promptly for guest investigator usage. The observatory features a 1.8-m diameter telescope cooled to 4.5 K with two science instruments: the Far-InfraRed Enhanced Survey Spectrometer (FIRESS) and the PRIMA imager (PRIMAger). FIRESS provides continuous spectral coverage from 24 to 235 mu m, in two spectral resolution modes (R >= 85 and R=4400(112 mu m/lambda)), with spectral mapping capability and order-of-magnitude sensitivity improvement over previous observatories. PRIMAger delivers similar sensitivity advances and first-of-its-kind far-infrared hyperspectral imaging for astrophysics with R similar to 8 from 25 to 84 mu m, and polarimetry in four broadband filters from 80 to 261 mu m. PRIMA's science and technical motivation is outlined, its overall architecture is described, and its cryogenic payload and instruments, including the kinetic inductance detector arrays, and operations and observing modes, are summarized. (c) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JATIS.11.3.031628]

As superconducting kinetic inductance detectors (KIDs) continue to grow in popularity for sensitive submillimeter detection and other applications, there is a drive to advance toward lower-loss devices. We present measurements of diagnostic thin-film aluminum coplanar waveguide (CPW) resonators designed to inform ongoing KID development at NASA Goddard Space Flight Center. The resonance frequencies span f_0 = 3.5-4 GHz and include quarter-wave and half-wave resonators with varying coupling capacitor designs. We present measurements of the device film properties and an analysis of the dominant mechanisms of loss in the resonators measured in a dark environment, demonstrating quality factors of Q_i^-1≈ 3.64-8.57 × 10^-8. We observe an enhanced level of suppression in the loss contributions from two-level systems (TLS) at intermediate-to-high read powers, and a regime at these powers and low temperatures where contributions from intrinsic processes Q_i^-1,other dominate the total loss. We also observe deviations from the standard TLS loss model at low powers and temperatures below 60 mK, and use a modified model to describe this behavior.

Superconducting aluminum thin films are integral to many astrophysics detector applications. Using x-ray absorption spectroscopy (XAS), we have studied the residues and adsorbates created during various standard lithography and etch steps, which are commonly used to pattern thin aluminum films into device structures. We have observed the formation of aluminum oxide as alpha-Al2O3 and aluminum fluoride as beta-AlF3. We have observed correlations between these XAS signatures and the Al film's microwave loss due to two-level systems. This study, which guides the way for future device optimization, further explores the chemical impact of different process steps, including standard silicon substrate wafer cleaning processes, sulfur-hexa-fluoride plasma etching, passivation with a fluorocarbon, and exposure to photoresist adhesion promoters during the lithography process with the help of control samples. (c) 2024 Author(s).

We present the design and optical characterization of a novel highly compact three-dimensional microwave kinetic inductance detector (3D-MKID). At short wavelengths, such as the far- and mid-infrared, the pixel density of MKID arrays is often limited by the size and geometry of each resonator's capacitor. Sending the non-optically active component of the resonator into the third dimension minimizes the footprint of each pixel, allowing much higher array densities to be achieved. In our 3D-MKID design, we compactify the resonator by conformally coating deep-etched holes in the silicon substrate with superconducting films formed by atomic layer deposition. The resulting geometry consists of three-dimensional coaxial transmission lines, which are then connected to a meandered absorber on the substrate surface. An array of these resonators are capacitively coupled to a microstrip feedline. We present the characterization of a prototype 3D-MKID array and compare it to simulate resonator properties. We additionally describe the detector sensitivity performance when illuminated by a far-infrared blackbody source.

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) is a balloon-borne telescope designed to survey star formation over cosmological time scales using intensity mapping in the 420 - 540 GHz frequency range. EXCLAIM uses a fully cryogenic telescope coupled to six on-chip spectrometers featuring kinetic inductance detectors (KIDs) to achieve high sensitivity, allowing for fast integration in dark atmospheric windows. The telescope receiver is cooled to approximate to 1.7 K by immersion in a superfluid helium bath and enclosed in a superfluid-tight shell with a meta-material anti-reflection coated silicon window. In addition to the optics and the spectrometer package, the receiver contains the magnetic shielding, the cryogenic segment of the spectrometer readout, and the sub-Kelvin cooling system. A three-stage continuous adiabatic demagnetization refrigerator (CADR) keeps the detectors at 100 mK while a He-4 sorption cooler provides a 900 mK thermal intercept for mechanical suspensions and coaxial cables. We present the design of the EXCLAIM receiver and report on the flight-like testing of major receiver components, including the superfluid-tight receiver window and the sub-Kelvin coolers.

Future far-infrared astrophysics observatories will require focal plane arrays containing thousands of ultrasensitive, superconducting detectors, each of which require efficient optical coupling to the telescope fore-optics. At longer wavelengths, many approaches have been developed, including feedhorn arrays and macroscopic arrays of lenslets. However, with wavelengths as short as 25 µm, optical coupling in the far infrared remains challenging. In this paper, we present an approach to fabricate far-infrared monolithic silicon microlens arrays using grayscale lithography and deep reactive ion etching. The fabricated microlens arrays presented here are designed for two different wavebands: 25–40 µm and 135–240 µm. The microlens arrays have sags as deep as 150 µm, are hexagonally packed with a pixel pitch of 900 µm, and have an overall size as large as 80 by 15 mm. We compare an as-fabricated lens profile to the design profile and calculate that the fabricated lenses would achieve 84% encircled power for the designed detector, which is only 3% less than the designed performance. We also present methods developed for antireflection coating microlens arrays and for a silicon-to-silicon die bonding process to hybridize microlens arrays with detector arrays.

Kinetic Inductance Detectors (KIDs) are an emerging technology useful for a wide variety of astronomy applications, including the Habitable Exoplanet Imaging Mission (HabEx), the Origins Space Telescope (OST), the Probe of Inflation and Cosmic Origins (PICO), and more. KIDs operate at cryogenic temperatures and can detect photons with high accuracy, sensitivity, and over a wide range of wavelengths. Though many KID models describe their performance well under certain operating conditions, some important pieces of physics related to quasiparticle dynamics are not yet either well understood or integrated into these models and can strongly affect device performance. In this paper we describe our framework for building an extended KID model, present the results of a quasiparticle diffusion simulation that incorporates scattering, cooling and diffusion, and discuss plans for the experimental testing of the model. We also discuss additional features to be added into future models that aim to capture a wide variety of potential scenarios encountered by researchers.

The optical coupling of detectors to telescope optics is a challenge for future far-infrared astrophysics observatories. The PRobe far-Infrared Mission for Astrophysics (PRIMA) collaboration has developed monolithic silicon microlens arrays for superconducting detector arrays spanning wavelengths from 24 to 261 microns. These custom-microfabricated lens arrays meet the stringent surface accuracy and roughness requirements that are necessary at PRIMA's shortest wavelengths. Grayscale lithography is used in combination with deep silicon plasma etching to create arrays of three-dimensional lens profiles. This fabrication process generates highly uniform and accurate microlenses across a roughly 80 by 10 millimeter 1008-element array. The kilopixel microlens arrays are bonded to matching PRIMA detector arrays with a thin and uniform layer of epoxy. In this presentation, we report on the status and performance of PRIMA's microlens fabrication and microlens-detector array hybridization processes.

We describe a testbed to characterize the optical response of compact superconducting on-chip spectrometers in development for the Experiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) mission. EXCLAIM is a balloonborne far-infrared experiment to probe the CO and CII emission lines in galaxies from redshift 3.5 to the present. The spectrometer, called u-Spec, comprises a diffraction grating on a silicon chip coupled to kinetic inductance detectors (KIDs) read out via a single microwave feedline. We use a prototype spectrometer for EXCLAIM to demonstrate our ability to characterize the spectrometers spectral response using a photomixer source. We utilize an on-chip reference detector to normalize relative to spectral structure from the off-chip optics and a silicon etalon to calibrate the absolute frequency.

We report measurements characterizing the performance of a kinetic inductance detector array designed for a wavelength of 25 microns and very low optical background level suitable for applications such as a far-infrared instrument on a cryogenically cooled space telescope. In a pulse counting mode of operation at low optical flux, the detectors can resolve individual 25-micron photons. In an integrating mode, the detectors remain photon noise limited over more than six orders of magnitude in absorbed power from 70 zW to 200 fW, with a limiting NEP of 4.6 x 10^-20 W/rtHz at 1 Hz. In addition, the detectors are highly stable with flat power spectra under optical load down to 1 mHz. Operational parameters of the detector are determined including the efficiency of conversion of the incident optical power into quasiparticles in the aluminum absorbing element and the quasiparticle self-recombination constant.

Future far-infrared astrophysics observatories will require focal plane arrays containing thousands of ultra-sensitive, superconducting detectors, each of which needs to be optically coupled to the telescope. At longer wavelengths, many approaches have been developed including feedhorn arrays and macroscopic arrays of lenslets. However, with wavelengths as short as 25 microns, optical coupling in the far-infrared remains challenging. In this paper, we present a novel approach for fabricating far-infrared monolithic silicon microlens arrays using grayscale lithography and deep reactive ion etching. The design, fabrication, and characterization of the microlens arrays are discussed. We compare the designed and fabricated lens profile, and calculate that the fabricated lenses will achieve 84% encircled power for the designed detector, which is only 3% less than the designed performance. We also present methods developed for anti-reflection coating microlens arrays and for a silicon-to-silicon die bonding process to hybridize microlens arrays with detector arrays.

This article presents a high-rejection, thin-film high-temperature superconductor, microstrip bandstop filter to prevent a local, and high-power radio-frequency interference (RFI) source from interfering with NASA Goddard Geophysical Astronomical Observatory (GGAO)’s very long baseline interferometry (VLBI) global observing system (VGOS) cryogenic receiver. This filter has an excellent 2.7%, 50-dB-fractional-bandwidth, and a center stopband frequency of 9.41 GHz. It does not contain any narrow or interdigital features as found in some designs, which reduces the fringing electric fields and improves its power handling capability. The YBCO films were grown on 435- $\mu$ m-thick R-plane sapphire substrate and the anisotropic behavior was modeled and simulated with a high degree of accuracy. The device was tested while cooled to 77 K and the measurements agree well with simulation.

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) will constrain star formation over cosmic time by carrying out a blind and complete census of redshifted carbon monoxide (CO) and ionized carbon ([CII]) emission in cross-correlation with galaxy survey data in redshift windows from the present to z=3.5 with a fully cryogenic, balloon-borne telescope. EXCLAIM will carry out extragalactic and Galactic surveys in a conventional balloon flight planned for 2023. EXCLAIM will be the first instrument to deploy µ-Spec silicon integrated spectrometers with a spectral resolving power R=512 covering 420-540 GHz. We summarize the design, science goals, and status of EXCLAIM.

The current state of far-infrared astronomy drives the need to develop compact, sensitive spectrometers for future space and ground-based instruments. Here we present details of the μ-Spec spectrometers currently in development for the far-infrared balloon mission EXCLAIM. The spectrometers are designed to cover the 555 – 714 μm range with a resolution of R = λ/Δλ = 512 at the 638 μm band center. The spectrometer design incorporates a Rowland grating spectrometer implemented in a parallel plate waveguide on a low-loss single-crystal Si chip, employing Nb microstrip planar transmission lines and thin-film Al kinetic inductance detectors (KIDs). The EXCLAIM μ-Spec design is an advancement upon a successful R = 64 μ-Spec prototype, and can be considered a sub-mm superconducting photonic integrated circuit (PIC) that combines spectral dispersion and detection. The design operates in a single M=2 grating order, allowing one spectrometer to cover the full EXCLAIM band without requiring a multi-order focal plane. The EXCLAIM instrument will fly six spectrometers, which are fabricated on a single 150 mm diameter Si wafer. Fabrication involves a flipwafer-bonding process with patterning of the superconducting layers on both sides of the Si dielectric. The spectrometers are designed to operate at 100 mK, and will include 355 Al KID detectors targeting a goal of NEP ∼8 × 10−19 W/√ Hz. We summarize the design, fabrication, and ongoing development of these μ-Spec spectrometers for EXCLAIM.

This paper describes a cryogenic optical testbed developed to characterize u-Spec spectrometers in a dedicated dilution refrigerator (DR) system. u-Spec is a far-infrared integrated spectrometer that is an analog to a Rowland-type grating spectrometer. It employs a single-crystal silicon substrate with niobium microstrip lines and aluminum kinetic inductance detectors (KIDs). Current designs with a resolution of 512 are in fabrication for the EXCLAIM (Experiment for Cryogenic Large Aperture Intensity Mapping) balloon mission. The primary spectrometer performance and design parameters are efficiency, NEP, inter-channel isolation, spectral resolution, and frequency response for each channel. Here we present the development and design of an optical characterization facility and preliminary validation of that facility with earlier prototype R=64 devices. We have conducted and describe initial optical measurements of R = 64 devices using a swept photomixer line source. We also discuss the test plan for optical characterization of the EXCLAIM R = 512 u-Spec devices in this new testbed.

While the mass differences between neutrino mass states are known, their absolute masses and mass hierarchy have not yet been determined. Determining the mass of neutrinos provides access to physics beyond the Standard Model and the resulting value has implications for the growth of large-scale structure in the universe over cosmic history. Because of the importance of the topic, a number of efforts are already underway to determine the mass of neutrinos including direct kinematic measurements and indirect measurements of astrophysical phenomena that constrain the sum of the mass eigenstates through models of cosmic evolution. Here, we advocate for a collaborative international effort to perform a kinematic determination of the effective electron neutrino mass using calorimetric measurements of the decay of 163Ho. This effort is justified by the success of current experiments using the technique, its high benefit-to-cost ratio, the value of approaches with different systematic errors, and the value of measuring the electron neutrino mass rather than the electron anti-neutrino mass.