Far-infrared space-borne instruments will require high sensitivity focal-plane arrays of densely packed low temperature detectors with noise equivalent power similar to 1 & times;10(-19) W/root Hz for imaging applications and one or more orders of magnitude lower than that for spectrometry. Transition-edge sensor (TES) bolometers, a mature and well-understood detector architecture, have demonstrated background limited performance in both ground-based and sub-orbital astronomical instruments. The fundamental noise source of a bolometer is thermal fluctuation noise, which scales as the square root of the thermal conductance. In order to make a more sensitive detector, thermal conductance must decrease. Aiming to advance TES bolometers to the next level of noise performance for space applications, we have developed a direct-absorber-coupled sensor that utilizes phononically engineered thermally isolated legs to achieve low enough thermal conductance for background limited operation in the 30 mu m to 300 mu m waveband. In practice, simultaneously achieving the required low thermal conductance yet high mechanical yield while maintaining a compact footprint is extremely challenging. We present the fabrication process of thin silicon nitride membrane released sub-arrays of densely packed TES bolometers with short (similar to 50 mu m) legs built with phononic filter structures. The phononic filters are created by etching periodic or semi-periodic high- and low-impedance structures into the silicon nitride legs. The direct-absorption TES bolometer sub-arrays have been fabricated with pixel sizes of 600 mu m, 800 mu m, and 1000 mu m and multiple phononic filter leg designs, all defined with standard optical photolithographic processes and isolated with a deep reactive ion etch process. High mechanical yield has been achieved, allowing for initial dark measurements which show that noise performance similar to 3 & times;10(-19) W/root Hz can be achieved with T-c similar to 120 mK, a notable result given that the thermally isolating bolometer legs are only 50 mu m long. We report on the fabrication details of the TES bolometer detector sub-arrays and discuss future work.

We present the status and goals of the readout electronics system we are developing to support the detector arrays in the coronagraph instrument on the NASA Habitable Worlds Observatory (HWO) mission currently in development. HWO aims to revolutionize exoplanet exploration by performing direct imaging and spectroscopy of 25 or more habitable exoplanets, and to resolve a broad range of astrophysics science questions as well. Since exoplanet yield depends critically on the detector dark count rate, as we show in this article, the ambitious goals of HWO require arrays of single-photon energy-resolving detectors. We argue that Kinetic Inductance Detectors (KIDs) are best suited to meet these requirements. To support the detectors required for HWO and future far-IR missions, at the required power consumption and detector count, we are developing a radiation-tolerant reconfigurable readout system for both imaging and energy-resolving single photon KID detector arrays. We leverage an existing RFSoC-based system we built for NASA balloons that has a power consumption of 30 W and reads out 2000-4000 detectors (i.e., 7-15 mW/pixel), and move to a radiation tolerant Kintex Ultrascale FPGA chip to bring low-power wide bandwidth readout to a space-qualified platform for the first time. This improves significantly over previous spaceflight systems, and delivers what is required for NASA's future needs: 100 000 pixels with less than 1 kW total power consumption. Overall, the system we are developing is a significant step forward in capability, and retires many key risks for the HWO mission.

Heat capacity and magnetic measurements were used to extract superconducting parameters for deposited NbTiN films in the bulk limit, including the critical temperature, the ratio of the superconducting gap to the critical temperature, the phase coherence length, and the upper critical field. We found that the film is a strong coupling superconductor, with a small phase coherence length, consistent with trends reported in the literature. Due to the large thickness, these materials correspond to 3D analogues of bulk NbTiN. The upper critical fields determined from heat capacity and magnetic field measurements are different, emphasizing the sensitivity of magnetic field measurements to defects. The Ginzburg number is much smaller compared to high-Tc superconductors, while magnetic relaxation measurements point towards a Kim-Anderson vortex creep regime.

We present, to our knowledge, the first measurement of the spectral performance of a compact, cryogenically cooled, silicon-based virtually imaged phased array (VIPA) at far-infrared wavelengths. Our cryogenic (4 K) characterization, performed in-house using a quantum-cascade laser (115.7 µm) and a pyroelectric detector, resulted in a spectral resolution of approximately R∼16,300. A far-infrared VIPA is compact, has no moving parts, and delivers an instantaneous multi-element spectrum dispersed in exit angle so that it can be recorded with a detector array. It, therefore, has the potential to be a transformative technology for balloon- and space-borne velocity-resolved astrophysics investigations in the far-infrared.

We present measurements of large-scale cosmic microwave background E -mode polarization from the Cosmology Large Angular Scale Surveyor 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of 82 μ K arcmin , comparable to Planck at similar frequencies (100 and 143 GHz ). The analysis demonstrates effective mitigation of systematic errors and addresses challenges to large-angular-scale power recovery posed by time-domain filtering in maximum-likelihood map-making. A novel implementation of the pixel-space transfer matrix is introduced, which enables efficient filtering simulations and bias correction in the power spectrum using the quadratic cross-spectrum estimator. Overall, we achieved an unbiased time-domain filtering correction to recover the largest angular scale polarization, with the only power deficit, arising from map-making nonlinearity, being characterized as <3%. Through cross-correlation with Planck, we detected the cosmic reionization at 99.4% significance and measured the reionization optical depth τ = 0.05 3 − 0.019 + 0.018 , marking the first ground-based attempt at such a measurement. At intermediate angular scales ( ℓ > 30), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck’s measurements.

We present the in-lab and on-sky performance for the upgraded 90 GHz focal plane of the Cosmology Large Angular Scale Surveyor, which had four of its seven detector wafers updated during the austral winter of 2022. The update aimed to improve the transition-edge-sensor (TES) stability and bias range and to realize the high optical efficiency of the sensor design. Modifications included revised circuit terminations, electrical contact between the TES superconductor and the normal metal providing the bulk of the bolometer heat capacity, and additional filtering on the TES bias lines. The upgrade was successful: 94% of detectors are stable down to 15% of the normal resistance, providing a wide overlapping range of bias voltages for all TESs on a wafer. The median telescope efficiency improved from 0.4 2 − 0.22 + 0.15 to 0.6 0 − 0.32 + 0.10 (68% quantiles). For the four upgraded wafers alone, median telescope efficiency increased to 0.6 5 − 0.06 + 0.06 . Given our efficiency estimate for the receiver optics, this telescope efficiency implies a detector efficiency exceeding 0.90. The overall noise-equivalent temperature of the 90 GHz focal plane improved from 19 μ K s to 9.7 μ K s .

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 MaxwellHelmholtz-Drude dispersion model suggests that vibrational modes above 10 THz dominate the loss in hydrogenated amorphous SiC above 200 GHz.

Low-loss deposited dielectrics are beneficial for the advancement of superconducting integrated circuits for astronomy. In the microwave band (∼1x201310 GHz) the cryogenic and low-power dielectric loss is dominated by two-level systems. However, the origin of the loss in the millimeter-submillimeter band (∼0.1x20131 THz) is not understood. We measured the loss of hydrogenated amorphous SiC (a-SiC:H) films in the 0.27x2013100 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 the a-SiC:H above 200 GHz.

We present recent progress in the development of ultra-low-noise Transition-Edge Sensors (TESs) pixels designed for far-infrared (~30-300µm) astronomical instruments. The TES sensitivity is maximized using phononic filters, which are sub-wavelength coherent filters that provide broadband rejection of thermal phonons emitted at the TES critical temperature, Tc~100mK. The phononic filter isolation legs are compact, ~50 µm. In a absorber-coupled bolometer suspended by four legs, the thermal conductance is reduced to achieve an NEP of less than 0.3 aW/rtHz, which is sufficient for balloon- and space-based imaging and low-resolution spectrometer instruments with cold optics. We discuss the phononic filter and TES design, the performance of the phononic-isolated TES pixels, and the advantage of these highly-sensitive absorber-coupled TES bolometers for astronomical instruments.

Low-loss deposited dielectrics are beneficial for the advancement of superconducting integrated circuits for astronomy. In the microwave band ($\mathrm{\sim}$1-10 GHz) the cryogenic and low-power dielectric loss is dominated by two-level systems. However, the origin of the loss in the millimeter-submillimeter band ($\mathrm{\sim}$0.1-1 THz) is not understood. We measured the loss of hydrogenated amorphous SiC (a-SiC:H) 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 the a-SiC:H above 200 GHz.

We present the design and cryogenic characterization of highly sensitive 7 THz lens-antenna-coupled MKIDs for future actively cooled far-infrared space telescopes. This is the highest operating frequency ever demonstrated for antenna-coupled MKIDs. The detector is based on a broadband leaky-wave lens-antenna coupled to a hybrid (Al/NbTiN) CPW MKID. Both the antenna and the photosensitive Al section of the MKID lay on a thin dielectric membrane, improving both the antenna efficiency and the detector sensitivity. The high operating frequency requires the definition of sub-micron features with electron-beam lithography, pristine laser-ablated lenses, and very accurate alignments during assembly. We have tested a prototype chip and have obtained a detector noise equivalent power of 3e-20W/sqrt(Hz) with a high coupling efficiency. Additionally, we have measured the antenna beam pattern. With these measurements we demonstrate a detector system suitable for highly-sensitive (imaging) spectrometers.

Improved polarization measurements at frequencies below 70 GHz with degree-level angular resolution are crucial for advancing our understanding of the Galactic synchrotron radiation and the potential polarized anomalous microwave emission and ultimately benefiting the detection of primordial $B$ modes. In this study, we present sensitivity-improved 40 GHz polarization maps obtained by combining the CLASS 40 GHz and WMAP $Q$-band data through a weighted average in the harmonic domain. The decision to include WMAP $Q$-band data stems from similarities in the bandpasses. Leveraging the accurate large-scale measurements from WMAP $Q$ band and the high-sensitivity information from CLASS 40 GHz band at intermediate scales, the noise level at $\ell\in[30, 100]$ is reduced by a factor of $2-3$ in the map space. A pixel domain analysis of the polarized synchrotron spectral index ($\beta_s$) using WMAP $K$ band and the combined maps (mean and 16/84th percentile across the $\beta_s$ map: $-3.08_{-0.20}^{+0.20}$) reveals a stronger preference for spatial variation (PTE for a uniform $\beta_s$ hypothesis smaller than 0.001) than the results obtained using WMAP $K$ and $Ka$ bands ($-3.08_{-0.14}^{+0.14}$). The cross-power spectra of the combined maps follow the same trend as other low-frequency data, and validation through simulations indicates negligible bias introduced by the combination method (sub-percent level in the power spectra). The products of this work are publicly available on $\mathtt{LAMBDA}$.

We have created and demonstrated a Virtually Imaged Phased Array (VIPA) device for velocity resolved spectroscopy in the far-IR and will present the first cryogenic (LHe) measurements of the spectral profile of a prototype VIPA at 115.7 mu m. A VIPA is a compact spectral filter without moving parts that consists of a resonating cavity that generates angular dispersion due to constructive interference, and that can deliver an instantaneous spectrum with a resolving power of 100,000 or more. Coupled with superconducting direct detection detectors, the VIPA promises unsurpassed sensitivity at velocity resolutions > 3 km/s, making it a prime choice for spectroscopic instruments to observe e.g. protoplanetary disks to trace the gaseous building blocks out of which planets form. Due to their compactness and absence of moving parts, VIPAs are optimal for balloon and space-borne astronomical instruments like the NASA Pioneer Mission POEMM and the proposed NASA Probe Mission FIRSST. The prototype we have measured was made of a 3 x 5 x 1 cm block of high-purity float-zone silicon. The entrance and exit sides of the VIPA are highly parallel, and the entrance side was fully metallized (gold-coated) except for a narrow entrance slit resulting in a reflectivity of 100%, while the exit side was metallized with an inductive mesh resulting in a reflectivity of about 88%. The measurement was done using a custom testbed that included separate cryostats for a cryogenically cooled THz Quantum Cascade Laser (QCL) and the VIPA, as well as a warm pyroelectric detector. The QCL was tuned to a number of different frequencies near 2.59 THz and the detector was scanned along the dispersion direction. Our VIPA prototype achieved the designed resolving power of 15,000 and the spectral profile matched extremely well with the expectations from our simulations. We are now building a full testbed to straightforwardly measure and characterize VIPAs and other GHz/THz filters with resolving powers of order 100,000 using tuneable THz lasers.

The Direct Detection Spectrometer Instrument (DDSI) is one of two instruments designed for the Far-IR Spectroscopy Space Telescope (FIRSST) recently proposed to NASA in response to the Astrophysics Probe Explorer call. The DDSI consists of two modules: HR delivering spectra at R~20,000 to 100,000 in three select bands (HR1-3) across 56-184μm, and LR providing broadband spectral coverage at R~100 in four bands (LR1-4) across 35-260 µm. The dispersive element of the HR bands is a compact optical resonator known as a virtually imaged phase array. All DDSI bands use microwave kinetic inductance detector (MKID) arrays cooled to 120mK. The total DDSI MKID pixel count is 2612 pixels.

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over ∼75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale CMB polarization to constrain the tensor-to-scalar ratio and the optical depth to last scattering. This paper presents the optical characterization of the 90 GHz telescope. Observations of the Moon establish the pointing while dedicated observations of Jupiter are used for beam calibration. The standard deviations of the pointing error in azimuth, elevation, and boresight angle are 1.′3, 2.′1, and 2.′0, respectively, over the first 3 yr of observations. This corresponds to a pointing uncertainty ∼7% of the beam’s full width at half-maximum (FWHM). The effective azimuthally symmetrized instrument 1D beam estimated at 90 GHz has an FWHM of 0.°620 ± 0.°003 and a solid angle of 138.7 ± 0.6(stats.) ± 1.1(sys.) μ sr integrated to a radius of 4°. The corresponding beam window function drops to b ℓ 2 = 0.93 , 0.71 , 0.14 at ℓ = 30, 100, 300, respectively. Far-sidelobes are studied using detector-centered intensity maps of the Moon and measured to be at a level of 10 −3 or below relative to the peak. The polarization angle of Tau A estimated from preliminary survey maps is 149°.6 ± 0°.2(stats.) in equatorial coordinates. The instrumental temperature-to-polarization ( T → P ) leakage fraction, inferred from per-detector demodulated Jupiter scan data, has a monopole component at the level of 1.7 × 10 −3 , a dipole component with an amplitude of 4.3 × 10 −3 , with no evidence of quadrupolar leakage.