Development of TIFUUN: Terahertz Integral Field Units with Universal Nanotechnology

Abstract: TIFUUN (THz Integral Field Units with Universal Nanotechnology) is an ultra-wideband mm-submm wave imaging spectrometer that capitalizes on the highly scalable integrated superconducting spectrometer technology. TIFUUN has two slots for integral field units (IFUs), which can jointly be optimized as open-hardware for each astronomical observation in terms of spatial and spectral coverage. These IFUs can have observation frequencies in the range of 90--360 GHz, with spectral resolution up to $R\equiv F/ΔF \le 1,000$, with up to $\sim$18,000 kinetic inductance detectors (shared by the two IFUs with a flexible ratio). The ultra-wide 4:1 (2 octave) bandwidth optics fits in a remarkably compact volume, by means of thin silicon lenses and a high chief ray angle design. The first pair of IFUs are being developed for the SUBLIME (Study of the Universe By Line Intensity Mapping Experiments) experiment that aims to map CII emission at redshift $\sim$6 to trace the cosmic large-scale structure and the buildup of galaxies during reionization, using TIFUUN on the ASTE 10-m telescope. The scalability, flexibility and compactness makes TIFUUN a highly compatible and portable system suited also for upcoming telescope facilities in the vicinity, such as FYST and AtLAST/LST.

• The paper demonstrates a novel integrated superconducting spectrometer approach combining superconducting filterbanks with KIDs to enable scalable terahertz IFU systems.
• It details a compact, configurable design with ultra-wide bandwidth (4:1) and high spatial/spectral resolution tailored for cosmic structure and galaxy surveys.
• Practical implications include rapid instrument adaptation, enhanced low-noise detection, and a testbed for advancing astrophotonic technologies.

Terahertz Integral Field Units with Universal Nanotechnology: Design, Architecture, and Implications

Instrumental Motivation and Technological Context

Spectroscopic imaging in the millimeter and submillimeter (mm-submm) regime remains fundamental for addressing critical astrophysical questions relating to cosmic structure formation, galaxy cluster evolution, and star formation histories. Traditional instrumentation bifurcates between coherent spectrometers (e.g., ALMA, NOEMA) offering high sensitivity and spatial resolution with limited cosmological volume, and direct-detection cameras delivering wide-area mapping of dust-obscured processes but lacking spectroscopic and redshift fidelity. Attempts at quasioptical direct-detection spectrometers have suffered from scalability limitations due to cryogenic optics size constraints.

TIFUUN leverages the integrated superconducting spectrometer (ISS) paradigm, combining superconducting filterbanks and kinetic inductance detectors (KIDs) on a single chip. This architecture enables high scalability and compactness, facilitating 3D spectral imaging (integral field units, IFUs) with flexible spatial, spectral, and bandwidth configurations optimized for diverse science cases (e.g., Sunyaev-Zeldovich cluster studies, THz galaxy surveys, line intensity mapping). The deployment of thin silicon lens arrays and high chief ray angle optics results in ultra-wideband (4:1 bandwidth ratio) coverage within a compact footprint.

System Architecture and IFU Design Principles

TIFUUN targets a parameter space spanning up to 18,000 KIDs shared between two IFUs, configurable in frequency range (90–360 GHz), spectral resolution ($R \leq 1,000$), and spatial pixel (spaxel) layout for maximal adaptability. The integrated hardware design is driven by open-hardware principles:

• Detector Sharing and Readout: The modular division of KIDs between bands enables tailored observational bandwidth allocation. SpaceKIDs electronics efficiently support up to 3,000 KIDs per RF channel over a 2–4 GHz readout window.
• Scalable Filterbanks: Use of microstrip or parallel plate capacitor (PPC) KIDs with amorphous silicon carbide dielectrics supports high $Q_i$ ($\sim 10^{4}$), critical for achieving $R \sim 1,000$, with ongoing material interface optimization.
• Compact Optics: The lens array design (e.g., $\phi$100 mm wafer with hexagonal packing for the ASTE telescope’s $\sim$7.5 arcmin field-of-view) minimizes cryostat size, enabling portability to future facilities (FYST, AtLAST/LST).
• Magnetic Shielding: Nb/mu-metal double-layer shields achieve attenuation factors $\approx 2 \times 10^3$, mitigating terrestrial magnetic interference, essential for low-noise superconducting operation.

Software packages (e.g., RAIMAD for mask and IFU design, gateau for time-domain observation simulation) promote user-driven IFU optimization and survey planning, establishing an ecosystem for open instrument development.

SUBLIME Experiment: Science Drivers and Implementation

The first deployment of TIFUUN is for SUBLIME, targeting LIM of [CII] $158~\mu$m (1.9 THz) at $z \sim 6$ to trace reionization-era cosmic structure and galaxy build-up using the ASTE 10 m telescope. SUBLIME implements two IFUs:

• H-band IFU: 195–319 GHz, 61 spaxels, 15,000 KIDs, $R = 500$, optimized for [CII] LIM at $Q_i$0.
• L-band IFU: 130–178 GHz, 19 spaxels, 3,000 KIDs, dual-purpose for CO interloper rejection and stacked detection.

LIM quantifies aggregate line emission fluctuations for clustering and luminosity density analyses, while blind line surveys extract redshifted galaxy catalogues. The dual-band configuration enables simultaneous detection of multiple CO transitions for cross-correlation and interloper removal. This strategy leverages the high mapping speed and spectral resolving power demonstrated by DESHIMA ($Q_i$1) and achieves unbiased THz line emitter surveys, in contrast to continuum-preselected spectroscopic follow-ups.

Intermediate configurations ($Q_i$2 IFU prototypes, 3,000 KIDs per band) have validated detector readout scalability.

Engineering and Optical Design

The mechanical and optical integration centers around compactness, robustness, and exchangeability. A vertical optical axis aligns the cryostat with limited cabin space and pulse-tube cooler orientation. Silicon lenses (up to $Q_i$3290 mm) manufactured via Czochralski and float zone methods, with AR structures created by laser ablation or dicing, handle broad frequency coverage with minimal losses.

The lens array arrangement for IFUs maximizes mapping speed and spatial coverage, optimized via geometric optics simulation. The modular cryostat design allows rapid IFU exchange and easy access to critical components (LNAs, filters).

Practical and Theoretical Implications

TIFUUN’s deployment establishes a highly configurable spectral imaging platform with the following implications:

• Survey Flexibility: The open-hardware paradigm directly empowers astronomers to tailor IFUs to new science cases, facilitating rapid instrument adaptation for emerging observational needs.
• Scalability and Portability: The high-density KID integration and compact optics open avenues for widespread implementation in multiple telescope platforms, with compatibility forecasts for FYST and AtLAST/LST.
• Innovative Materials Development: Ongoing research into a-SiC dielectrics—balancing mm/submm losses and TLS noise—will drive further progress in ISS and KID engineering, potentially extending performance envelopes.
• Enhanced Statistical Cosmology: The dual-mode acquisition (LIM and blind line detection) uniquely enables cross-validation and joint constraints on luminosity functions and cosmic star-formation metrics.
• Testbed for Astrophotonics: TIFUUN’s modular and extensible architecture positions it as a platform for fielding new IFU designs as astrophotonics technology advances, accelerating translation from lab to observatory.

Future developments are expected to refine detector densities, expand bandwidth/readout capabilities, and further miniaturize optical chains, facilitating deeper, broader, and higher-resolution mm-submm surveys.

Conclusion

TIFUUN operationalizes integrated superconducting spectrometer technology for mm-submm astronomy in a robust, user-configurable platform enabling up to 18,000 KIDs over 90–360 GHz. Its open-hardware philosophy and compact cryo-mechanical and optical design allow rapid adaptation to evolving science objectives and telescope platforms. The initial application to SUBLIME demonstrates unprecedented flexibility and performance for large-scale structure and THz line emitter mapping. As a scalable testbed, TIFUUN will catalyze further developments in superconducting astrophotonics and advance the theoretical and practical frontiers of spectral imaging in cosmology and galaxy evolution (2606.21543).