We designed, fabricated, and characterized the properties of a silicon-based vacuum window suitable for millimeter-wave astrophysical applications. The window, which has a diameter of 124 mm, an optically usable diameter of 68 mm, and a thickness of about 4 mm, gives an average transmittance and reflectance of 99.6 ± 3.5% and 1.3 ± 1.4%, respectively, a fractional bandwidth of 67%, centered at 300 GHz. Absorptive loss is below the detection limit of our measurement. The anti-reflection coating is made with laser-ablated sub-wavelength structures (SWS), and the measured transmittance and reflectance values agree with the modeling based on the measured SWS shapes. The window has been integrated into DESHIMA 2.0, an astrophysics instrument that took year-long observations with the Atacama Submillimeter Telescope Experiment.

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.

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.

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.

Understanding telescope pointing (i.e. line of sight) is important for observing the cosmic microwave background (CMB) and astronomical objects. The Moon is a candidate astronomical source for pointing calibration. Although the visible size of the Moon (30^{`</sup>) is larger than that of the planets, we can frequently observe the Moon once a month with a high signal-to-noise ratio. We developed a method for performing pointing calibration using observational data from the Moon. We considered the tilts of the telescope axes as well as the encoder and collimation offsets for pointing calibration. In addition, we evaluated the effects of the nonuniformity of the brightness temperature of the Moon, which is a dominant systematic error. As a result, we successfully achieved a pointing accuracy of 3.3<sup>`}. This is one order of magnitude smaller than an angular resolution of 36^`. This level of accuracy competes with past achievements in other ground-based CMB experiments using observational data from the planets.

Context. Integrated superconducting spectrometers (ISSs) for wide-band submillimeter (submm) astronomy use quasi-optical systems for coupling radiation from the telescope to the instrument. Misalignment in these systems is detrimental to the system performance. The common method of using an optical laser to align the quasi-optical components requires an accurate alignment of the laser to the submm beam from the instrument, which is not always guaranteed to a sufficient accuracy. Aims. We develop an alignment strategy for wide-band ISSs that directly uses the submm beam of the wide-band ISS. The strategy should be applicable in both telescope and laboratory environments. Moreover, the strategy should deliver similar quality of the alignment across the spectral range of the wide-band ISS. Methods. We measured the misalignment in a quasi-optical system operating at submm wavelengths using a novel phase and amplitude measurement scheme that is capable of simultaneously measuring the complex beam patterns of a direct-detecting ISS across a harmonic range of frequencies. The direct detection nature of the microwave kinetic inductance detectors in our device-under-test, DESHIMA 2.0, necessitates the use of this measurement scheme. Using geometrical optics, the measured misalignment, a mechanical hexapod, and an optimisation algorithm, we followed a numerical approach to optimise the positioning of corrective optics with respect to a given cost function. Laboratory measurements of the complex beam patterns were taken across a harmonic range between 205 and 391 GHz and were simulated through a model of the ASTE telescope in order to assess the performance of the optimisation at the ASTE telescope. Results. Laboratory measurements show that the optimised optical setup corrects for tilts and offsets of the submm beam. Moreover, we find that the simulated telescope aperture efficiency is increased across the frequency range of the ISS after the optimisation.

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Many superconducting on-chip filter-banks suffer from poor coupling to the detectors behind each filter. This is a problem intrinsic to the commonly used half-wavelength filter, which has a maximum theoretical coupling of 50 %. In this paper, we introduce a phase-coherent filter, called a directional filter, which has a theoretical coupling of 100 %. In order to study and compare different types of filter-banks, we first analyze the measured filter frequency scatter, losses, and spectral resolution of a DESHIMA 2.0 filter-bank chip. Based on measured fabrication tolerances and losses, we adapt the input parameters for our circuit simulations, quantitatively reproducing the measurements. We find that the frequency scatter is caused by nanometer-scale line width variations and that variances in the spectral resolution is caused by losses in the dielectric only. Finally, we include these realistic parameters in a full filter-bank model and simulate a wide range of spectral resolutions and oversampling values. For all cases, the directional filter-bank has significantly higher coupling to the detectors than the half-wave resonator filter-bank. The directional filter eliminates the need to use oversampling as a method to improve the total efficiency, instead capturing nearly all the power remaining after dielectric losses.

The next technological breakthrough in millimeter–submillimeter astronomy is three-dimensional imaging spectrometry with wide instantaneous spectral bandwidths and wide fields of view. The total optimization of the focal-plane instrument, the telescope, the observing strategy, and the signal-processing software must enable efficient removal of foreground emission from the Earth’s atmosphere, which is time-dependent and highly nonlinear in frequency. Here, we present Time-dependent End-to-end Model for Post-process Optimization (TiEMPO) of the DEep Spectroscopic HIgh-redshift MApper (DESHIMA) spectrometer. TiEMPO utilizes a dynamical model of the atmosphere and parameterized models of the astronomical source, the telescope, the instrument, and the detector. The output of TiEMPO is a time stream of sky brightness temperature and detected power, which can be analyzed by standard signal-processing software. We first compare TiEMPO simulations with an on-sky measurement by the wideband DESHIMA spectrometer, and find good agreement in the noise and sensitivity. We then use TiEMPO to simulate the detection of the line emission spectrum of a high-redshift galaxy using the DESHIMA 2.0 spectrometer in development. The TiEMPO model is open source. Its modular and parametrized design enables users to adapt it to optimize the end-to-end performance of spectroscopic and photometric instruments on existing and future telescopes

We present a feasibility study for the high-redshift galaxy part of the Science Verification Campaign with the 220–440 GHz deshima 2.0 integrated superconducting spectrometer on the ASTE telescope. The first version of the deshima 2.0 chip has been recently manufactured and tested in the lab. Based on these realistic performance measurements, we evaluate potential target samples and prospects for detecting the [CII] and CO emission lines. The planned observations comprise two distinct, but complementary objectives: (1) acquiring spectroscopic redshifts for dusty galaxies selected in far-infrared/mm-wave surveys; (2) multi-line observations to infer physical conditions in dusty galaxies.

We present a phase and amplitude beam pattern measurement technique using harmonic mixers. This allows a simultaneous multi-frequency phase sensitive characterization of a low resolution and wideband (220-420GHz) on-chip spectrometer using microwave kinetic inductance detectors. We investigate the beam quality, in particular the beam pointing and inferred telescope coupling and hence aperture efficiency. The measurements match the goal requirements for the DESHIMA-2 instrument for the ASTE telescope. The technique would be of interest for any (direct detector) spectrometer with a wide instantaneous bandwidth, particularly ones with dispersive components.

Integrated superconducting spectrometer (ISS) technology will enable ultra-wideband, integral-field spectroscopy for (sub)millimeter-wave astronomy, in particular, for uncovering the dust-obscured cosmic star formation and galaxy evolution over cosmic time. Here, we present the development of DESHIMA 2.0, an ISS for ultra-wideband spectroscopy toward high-redshift galaxies. DESHIMA 2.0 is designed to observe the 220–440 GHz band in a single shot, corresponding to a redshift range of z = 3.3–7.6 for the ionized carbon emission ([C II] 158 μ m). The first-light experiment of DESHIMA 1.0, using the 332–377 GHz band, has shown an excellent agreement among the on-sky measurements, the laboratory measurements, and the design. As a successor to DESHIMA 1.0, we plan the commissioning and the scientific observation campaign of DESHIMA 2.0 on the ASTE 10-m telescope in 2023. Ongoing upgrades for the full octave-bandwidth system include the wideband 347-channel chip design and the wideband quasi-optical system. For efficient measurements, we also develop the observation strategy using the mechanical fast sky-position chopper and the sky-noise removal technique based on a novel data-scientific approach. In the paper, we show the recent status of the upgrades and the plans for the scientific observation campaign.

Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits, and kinetic inductance parametric amplifiers. Compared with planar structures, multilayer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multilayer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measure the subkelvin and low-power microwave and millimeter-submillimeter-wave dielectric loss of hydrogenated amorphous silicon carbide (a-SiC:H), using superconducting chips with Nb-Ti-N/a-SiC:H/Nb-Ti-N microstrip resonators. We deposit the a-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of 400°C. The a-SiC:H has a millimeter-submillimeter loss tangent ranging from 0.9×10-4 at 270 GHz to 1.5×10-4 at 385 GHz. The microwave loss tangent is 3.1×10-5. These are the lowest low-power subkelvin loss tangents that have been reported for microstrip resonators at millimeter-submillimeter and microwave frequencies. The a-SiC:H films are free of blisters and have low stress: -20 MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.

We present a "mix-and-match"process to create large structures with submicrometer features by combining UV contact lithography and 100 kV electron-beam lithography in a single layer of negative-tone resist: Micro-Resist-Technology ma-N1405. The resist is successfully applied for the fabrication of an on-chip terahertz spectrometer, where the design requires 450 nm wide lines and 300 nm wide trenches in a 150 nm thick niobium-titanium-nitride layer, tolerating errors of ± 30 nm. We use a resist thickness of 500 nm, optimized to allow reliable SF 6/O 2-based reactive ion etching of structures with 30 nm accuracy. We find that resist requires an electron-beam cross-linking dose of 1100 μ C / c m 2 for an acceleration voltage of 100 kV in combination with a 180 s 100 °C bake on a hot plate and 45 s development. The smallest resist bars made with our dedicated recipe are 100 nm wide, with the smallest gaps about 300 nm. The difference between the designed and realized feature size is between 2 and 30 nm for structures up to 700 nm wide. The optical exposure dose is 300 m J / c m 2 for the same development time and is optimized to produce a positive sloped edge profile allowing good step coverage for subsequent layers. The resist can be applied, shipped, and processed in a time span of a couple of days without notable deterioration of patterning quality.

Superconducting resonators and transmission lines are fundamental building blocks of integrated circuits for millimeter-submillimeter astronomy. Accurate simulation of radiation loss from the circuit is crucial for the design of these circuits because radiation loss increases with frequency, and can thereby deteriorate the system performance. Here we show a stratification for a 2.5-dimensional method-of-moment simulator Sonnet EM that enables accurate simulations of the radiative resonant behavior of submillimeter-wave coplanar resonators and straight coplanar waveguides (CPWs). The Sonnet simulation agrees well with the measurement of the transmission through a coplanar resonant filter at 374.6 GHz. Our Sonnet stratification utilizes artificial lossy layers below the lossless substrate to absorb the radiation, and we use co-calibrated internal ports for de-embedding. With this type of stratification, Sonnet can be used to model superconducting millimeter-submillimeter wave circuits even when radiation loss is a potential concern.

A superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy. The proposed filter geometry has a free spectral range of an octave of bandwidth without introducing spurious resonances, reaches a high coupling efficiency in the pass-band and shows very high rejection in the stop-band to minimize reflections and cross-talk with other filters. A spectrally sparse prototype filter-bank in the band 300400 GHz has been developed employing these filters as well as an equivalent circuit model to anticipate systematic errors. The fabricated chip has been characterized in terms of frequency response, reporting an average peak coupling efficiency of 27% with an average spectral resolution of 940.

This paper discusses the science case for a sensitive spectro-polarimetric survey of the microwave sky. Such a survey would provide a tomographic and dynamic census of the three-dimensional distribution of hot gas, velocity flows, early metals, dust, and mass distribution in the entire Hubble volume, exploit CMB temperature and polarisation anisotropies down to fundamental limits, and track energy injection and absorption into the radiation background across cosmic times by measuring spectral distortions of the CMB blackbody emission. In addition to its exceptional capability for cosmology and fundamental physics, such a survey would provide an unprecedented view of microwave emissions at sub-arcminute to few-arcminute angular resolution in hundreds of frequency channels, a data set that would be of immense legacy value for many branches of astrophysics. We propose that this survey be carried out with a large space mission featuring a broad-band polarised imager and a moderate resolution spectro-imager at the focus of a 3.5 m aperture telescope actively cooled to about 8K, complemented with absolutely-calibrated Fourier Transform Spectrometer modules observing at degree-scale angular resolution in the 10–2000 GHz frequency range. We propose two observing modes: a survey mode to map the entire sky as well as a few selected wide fields, and an observatory mode for deeper observations of regions of specific interest.