Advances in far infrared astronomy have been, and will be, defined by instrument capabilities. Especially relevant is the development of imaging spectrometers for the wavelength range of 0.03-3 mm, which are not available at all at this moment. We will discuss recent advances in this field: First, we discuss the development of miniature on-chip spectrometers, that can operate in a 0.09-1 THz by using lossless superconducting circuits to create miniature spectrometers. For higher frequencies this is not possible due to material limitations, moreover instruments have to be operated in space due to the opacity of the atmosphere. Recent proposals for new missions focus on space-based observatories with optics cooled down to 4K, which offer unprecedented spectral imaging speeds, but require large arrays of extremely sensitive detectors. In the second part of this paper we will discuss the development of microwave Kinetic Inductance detectors with a sensitivity of NEP. 3.1.10-20 W/ãHz, sufficient for these applications.@en

This communication proposes a broadband and efficient integrated focal plane array solution based on a near-field focused connected array of slots. The focused aperture provides: 1) broadband and highly efficient illumination of the quasi-optical system and 2) scanning capabilities within a focusing system. The connected array antenna in turn allows for a fully integrated solution that can synthesize a focused aperture while providing broadband impedance matching. Focused connected array antennas enable the coupling to a reflector system over bandwidths in excess of one octave and with aperture efficiencies in excess of 60%. To demonstrate the concept, we present two printed circuit board (PCB) prototypes operating in the band 3-6 GHz and yielding more than 60% reflector aperture efficiency under broadside illumination and allowing to scan one beamwidth at the lowest frequency with a frequency-averaged scan loss of 0.2 dB. The feasibility of scaling this concept to THz frequencies and with dynamic beam-steering capabilities is discussed in the context of a superconducting device.

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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.

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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.

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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

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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.

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The mysteries of the early Universe are largely enshrouded in dust, product of the violent process of star formation. Due to the vast distances of our Universe, infrared light emitted by the heated dust back in those early stages can still be observed today, which has been observed to contribute to about half of the total cosmic background radiation. Gases fueling star-formation also radiate, but in the form of emission lines, which leave distinct spectral signatures that allow the study of the underlying physical processes. Given the expansion of the Universe, the evolutionary information is encoded in the cosmological redshift observed, making the far-infrared or terahertz (THz) regime specially suited for probing star-formation. Superconducting on-chip broadband THz imaging spectrometers with moderate spectral resolution coupled to large telescopes will allow the investigation the early Universe processes over large cosmological volumes. In this dissertation we propose two enabling technologies toward the advancement of this on-chip superconducting instruments: a broadband and moderate spectral resolution channelizing filter-bank, and a broadband phased array antenna as a reflector feed with beam-steering capabilities.

Octave-band THz channelizing filter-banks with moderate spectral resolution of the order R=500 are investigated in this work. These systems allow for a size reduction of several orders of magnitude compared to conventional spectrometers with similar spectral resolution. The proposed filters are half-wavelength resonators, which naturally provide a free-spectral range of an octave. The performance of those filters, both when in isolation and when embedded in a filter-bank, is analyzed using a newly-developed circuit model. This tool also provides design insights such as the required filter ordering and separation within the filter-bank to enable an efficient circuit. The actual implementation of the superconducting filter-bank on a chip is investigated for two of the main on-chip technologies: co-planar waveguide (CPW) and microstrip. Despite the easier manufacturing of co-planar circuitry, that technology is not suited for channelizing THz filter-banks as it suffers from radiation issues. Instead microstrip technology is non-radiative and, although it suffers from the moderate dissipation in deposited dielectrics such as a-Si, it provides a very reliable platform to build THz filter-banks. Half-wavelength I-shaped resonators are proposed as suitable filtering structures with which frequency-sparse filter-banks have been built to test their performance in semi-isolation. The measurements were based on both a frequency response characterization of the filters as well as their optical efficiency, showing good agreement between the two. The measured performance of these filters showed pass-bands with an average peak coupling efficiency of 27% and a spectral resolution R≈940. The coupling is significantly better than earlier results based upon planar technology.

The coupling between the quasi-optical reflector system of a telescope and the on-chip filter-bank requires of a broadband antenna. Currently, broadband integrated anti-reflection-coated lenses are being developed for this purpose, but their manufacturing is specially complicated for cryogenics and require mechanical actuators to perform beam scanning in the case of a multi-object spectrometer. In this dissertation, we propose a broadband phased-array antenna concept with electronic beam-steering that exploits two key properties of superconductors in its feeding network: the negligible conductor loss and the tunable kinetic inductance with a bias current. The focused connected array antenna concept proposed is based on the broadband impedance matching enabled by the connected arrays and the largely frequency-independent far fields of near-field focused apertures. To demonstrate this concept we designed, fabricated and tested two low frequency (3-6 GHz) prototypes in PCB technology: one pointing broadside and another one scanning. The measured fields met the predictions to a large degree and provided with a reflector aperture efficiency in excess of 60% over an octave of bandwidth and allowing to scan one half-power beamwidth at the lowest frequency with a frequency-averaged scan loss of 0.2 dB. Both the directivity and the gain were measured, allowing to report the losses, which chiefly originated from the tin-finished copper lines in the PCB. As a result, we can expect a highly-efficient reflector feed at THz frequencies with beam-steering capabilities in the near future.

The beam-steering concept proposed for the phased-array antenna relies on the current-dependent kinetic inductance of superconducting lines. With this effect, the phase velocity of biased superconducting lines may be modified, allowing thereby an electronic tuning of the phase-shift introduced. Prior to the integration of such phase-shifters with the phased-array antenna, we devised an on-chip platform based a tunable Fabry-Pérot resonator to quantify the phase-shifting capabilities at THz frequencies. In this concept, the dc bias currents are injected in the proximity of the edges of the resonator through 9th order Chebyshev stepped-impedance low-pass filters, whose high rejection mitigates any possible disturbance to the THz resonances. Using a circuit model including the resonator and the low-pass filters, as well as the simulated properties of the superconducting buried microstrip lines used in the designs, we anticipate an expected maximum tuning of dφ/φ=-df/f≈2%. With such tuning range millimeter-long tunable delay lines will be required for THz superconducting phased-array.@en

Aims. Future actively cooled space-borne observatories for the far-infrared, loosely defined as a 1-10 THz band, can potentially reach a sensitivity limited only by background radiation from the Universe. This will result in an increase in observing speed of many orders of magnitude. A spectroscopic instrument on such an observatory requires large arrays of detectors with a sensitivity expressed as a noise equivalent power NEP = 3 × 10-20 W√ p Hz. Methods. We present the design, fabrication, and characterisation of microwave kinetic inductance detectors (MKIDs) for this frequency range reaching the required sensitivity. The devices are based on thin-film NbTiN resonators which use lens-antenna coupling to a submicron-width aluminium transmission line at the shorted end of the resonator where the radiation is absorbed. We optimised the MKID geometry for a low NEP by using a small aluminium volume of ≈1 μm3 and fabricating the aluminium section on a very thin (100 nm) SiN membrane. Both methods of optimisation also reduce the effect of excess noise by increasing the responsivity of the device, which is further increased by reducing the parasitic geometrical inductance of the resonator. Results. We measure the sensitivity of eight MKIDs with respect to the power absorbed in the detector using a thermal calibration source filtered in a narrow band around 1.5 THz. We obtain a NEPexp(Pabs) = 3:1 ± 0:9 × 10-20 W√ p Hz at a modulation frequency of 200 Hz averaged over all measured MKIDs. The NEP is limited by quasiparticle trapping. Conclusions. The measured sensitivity is sufficient for spectroscopic observations from future, actively cooled space-based observatories. Moreover, the presented device design and assembly can be adapted for frequencies up to ≈10 THz and can be readily implemented in kilopixel arrays.

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A focal plane array of extended-hemispherical silicon lenses coupled to aluminum coplanar-waveguide (CPW) Microwave Kinetic Inductance Detectors (MKIDs) has been designed to operate at 7.8 THz. Low-dispersive leaky-wave radiation has been used to efficiently illuminate the antireflection-coated lenses. To minimize the radiation loss from the antenna feeding lines at these high frequencies, the CPWs have been miniaturized and placed on a dielectric membrane. A test device has been fabricated and its experimental characterization in terms of sensitivity, optical coupling, and beam patterns is ongoing.@en

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.

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A superconducting on-chip microstrip filter bank spectrometer prototype for Far-Infrared (FIR) Astronomy is presented. The measurements showcase its capabilities towards moderate spectral resolution (f/\Delta f\sim 500) broadband FIR spectroscopy. In this sub-mm-wave filter bank, each spectral channel consists of an 'I-shaped' microstrip THz bandpass filter that couples the radiation to a Microwave Kinetic Inductance Detector (MKID) for a background limited detection and a scalable frequency-multiplexed microwave readout.

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DESHIMA 2.0 is a broadband sub-mm wave superconducting on-chip spectrometer for astronomy, targeting an instantaneous octave bandwidth (220 - 440 GHz) sampled with moderate spectral resolution channels (f/df ~ 500). In this work we propose a microstrip filter-bank implementation for DESHIMA 2.0 based on “H-shaped” resonators. These bandpass filters are free from spurious resonances over an octave bandwidth, do not suffer from radiation losses and can be arrayed in large filter-banks thanks to their extremely low reflections off-resonance. The design has been aided by an analytical circuit model that can fast and reliably predict the filter-bank behaviour. Prototype chips have been characterised in terms of frequency response and coupling efficiency.@en

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.@en

Microfabrication of on-chip filterbanks, such as DESHIMA 2.0, would greatly benefit from reliable fabrication with sub-micrometer resolution. This enables smaller devices and reduces scatter in parameters such as filter bandwidth and resonant frequency. Here we present “mix-and-match” processing by combining optical and electron-beam exposures of a single layer of negative ma-N1405 resist from Micro-Resist-Technology GmbH. This allows for minimal features down to 300 nm where needed and large structure exposure with UV, limiting e-beam writing time. Relative alignment is possible to less than 500 nm on a regular basis.@en

DESHIMA 2.0 is a sub-millimetre wave spectrometer based on a single superconducting chip with a large instantaneous bandwidth. The instrument consists of a Quasi-optical (QO) system and an on-chip filter-bank coupled to an array of Kinetic Inductance Detectors (KID). In this work, this broad band QO system, operating at sub-millimetre wavelengths (220 GHz to 720 GHz), will be presented. This design is achieved using a field matching technique and consists of a hyper-hemispherical leaky lens antenna coupled to a series of Dragonian reflectors. The optimized design has an average illumination efficiency over the band of ~70%. This performance is also measured directly through the response of the KIDs.@en

We are developing an ultra-wideband spectroscopic instrument, DESHIMA (DEep Spectroscopic HIgh-redshift MApper), based on the technologies of an on-chip filter bank and microwave kinetic inductance detector (MKID) to investigate dusty starburst galaxies in the distant universe at millimeter and submillimeter wavelengths. An on-site experiment of DESHIMA was performed using the ASTE 10-m telescope. We established a responsivity model that converts frequency responses of the MKIDs to line-of-sight brightness temperature. We estimated two parameters of the responsivity model using a set of skydip data taken under various precipitable water vapor (PWV 0.4–3.0 mm) conditions for each MKID. The line-of-sight brightness temperature of sky is estimated using an atmospheric transmission model and the PWVs. As a result, we obtain an average temperature calibration uncertainty of 1σ=4%, which is smaller than other photometric biases. In addition, the average forward efficiency of 0.88 in our responsivity model is consistent with the value expected from the geometrical support structure of the telescope. We also estimate line-of-sight PWVs of each skydip observation using the frequency response of MKIDs and confirm the consistency with PWVs reported by the Atacama Large Millimeter/submillimeter Array.@en

DESHIMA is a spectrometer for astronomical applications targeting sources at sub-mm wavelengths from 240GHz to 720GHz that will operate in the ASTE telescope in Atacama Desert, Chile. In this work, a quasi-optical system based on a hyper-hemispherical leaky lens antenna and a series of Dragonian reflectors is presented as the coupling chain for the EM radiation captured by the telescope into the detector. The design procedure is based on a field matching technique in reception. The achieved average illumination efficiency over the band is approximately 70%. The directivity patterns in the sky are also estimated. The side lobe, and cross-polarization levels, over the whole frequency band, are below-16dB, and-18dB, respectively. The measurement of the system is on-going, and will be presented at the conference.

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Ultra-wideband, three-dimensional (3D) imaging spectrometry in the millimeter–submillimeter (mm–submm) band is an essential tool for uncovering the dust-enshrouded portion of the cosmic history of star formation and galaxy evolution^1–3. However, it is challenging to scale up conventional coherent heterodyne receivers⁴ or free-space diffraction techniques⁵ to sufficient bandwidths (≥1 octave) and numbers of spatial pixels^2,3 (>10²). Here, we present the design and astronomical spectra of an intrinsically scalable, integrated superconducting spectrometer⁶, which covers 332–377 GHz with a spectral resolution of F/ΔF ~ 380. It combines the multiplexing advantage of microwave kinetic inductance detectors (MKIDs)⁷ with planar superconducting filters for dispersing the signal in a single, small superconducting integrated circuit. We demonstrate the two key applications for an instrument of this type: as an efficient redshift machine and as a fast multi-line spectral mapper of extended areas. The line detection sensitivity is in excellent agreement with the instrument design and laboratory performance, reaching the atmospheric foreground photon noise limit on-sky. The design can be scaled to bandwidths in excess of an octave, spectral resolution up to a few thousand and frequencies up to ~1.1 THz. The miniature chip footprint of a few cm² allows for compact multi-pixel spectral imagers, which would enable spectroscopic direct imaging and large-volume spectroscopic surveys that are several orders of magnitude faster than what is currently possible^1–3.

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The design of an octave bandwidth sub-mm wave superconducting on-chip filter-bank spectrometer for Astronomy is presented. An array of THz band-pass filters subdivides the bandwidth 220-440 GHz into channels with a spectral resolution of 400 and an average maximum coupling strength f/ Delta {f} of 40%. The filter-bank performance is assessed by means of a transmission line formalism that approximates its behavior. The chip is under fabrication and its measurements will follow.

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Terahertz spectrometers with a wide instantaneous frequency coverage for passive remote sensing are enormously attractive for many terahertz applications, such as astronomy, atmospheric science, and security. Here we demonstrate a wide-band terahertz spectrometer based on a single superconducting chip. The chip consists of an antenna coupled to a transmission line filterbank, with a microwave kinetic inductance detector behind each filter. Using frequency division multiplexing, all detectors are read-out simultaneously, creating a wide-band spectrometer with an instantaneous bandwidth of 45 GHz centered around 350 GHz. The spectrometer has a spectral resolution of F/ΔF =380 and reaches photon-noise limited sensitivity. We discuss the chip design and fabrication, as well as the system integration and testing. We confirm full system operation by the detection of an emission line spectrum of methanol gas. The proposed concept allows for spectroscopic radiation detection over large bandwidths and resolutions up to F/ΔF ∼ 1000, all using a chip area of a few cm². This will allow the construction of medium resolution imaging spectrometers with unprecedented speed and sensitivity.

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