DESHIMA 2.0: A 200-400 GHz Ultra-wideband Integrated Superconducting Spectrometer

DESHIMA (Deep Spectroscopic HIgh-redshift MApper) is a broadband integrated superconducting spectrometer (ISS) for millimeter (mm) / sub-millimeter (sub-mm) wave astronomy based on Kinetic Inductance Detectors (KIDs). This paper describes characterization of DESHIMA 2.0 in laboratory settings. The instrument features NbTiN superconducting microstrip (MS) filters with low-loss a-SiC:H dielectric and an ultra-wideband leaky-wave antenna. A laboratory setup was designed, incorporating the cryostat housing cryogenic optics and ISS chip comprising 339 KIDs connected to MS filters tuned for (sub-)mm wave frequencies. Room-temperature mirrors on a hexapod stage allowed precise positioning and alignment of optical elements. The sky-position chopper was positioned on a motor-controlled stage for fine-tuned control over its position and alignment. Thanks to the multiplexing capability of KIDs, we could simultaneously measure multiple performance metrics across the entire frequency range. We showed that DESHIMA 2.0 achieved significant improvements in performance compared to its predecessor (DESHIMA 1.0): measured instantaneous frequency coverage was 200$-$400 GHz with a mean filter $Q_{filter}$ of $340 \pm 50$; instrument efficiency reached $\sim8$ %, indicating 4 times wider band coverage and 4 times higher sensitivity. The yield rate for MS filters exceeded 98 %. The estimated aperture efficiency from measured beam patterns agreed well with the designed value of approximately 70 %. The telescope far-field beam patterns calculated from measured beam patterns also exhibited good agreement with design specifications. We also demonstrated validity of a new method of absolute frequency calibration using the data from beam pattern measurement.

💡 Research Summary

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The manuscript presents the design, fabrication, and laboratory characterization of DESHIMA 2.0, an ultra‑wideband integrated superconducting spectrometer (ISS) operating from 200 GHz to 400 GHz. Building on the first‑generation DESHIMA 1.0, which covered 332–377 GHz with a modest instrument efficiency of ~2 %, the authors set out to quadruple the instantaneous bandwidth and improve sensitivity by a factor of four. The core of DESHIMA 2.0 is a filter‑bank composed of 339 NbTiN micro‑strip (MS) resonators fabricated on a low‑loss a‑SiC:H dielectric. Each resonator is coupled weakly (≈ ‑29 dB) to a common signal line, delivering an average filter quality factor Q₍filter₎ = 340 ± 50 and a fabrication yield exceeding 98 %.

Radiation is collected by a 500 µm wide leaky‑wave antenna made from NbTiN coplanar waveguide (CPW). The antenna feeds a 20 mm Si lens (AR‑coated with a Stycast mixture) placed only 10 µm away, ensuring efficient broadband coupling across the full 200–400 GHz range. The lens‑antenna assembly, together with a polarizer, is housed inside a cryogenic optics chain that includes a 6 mm thick silicon vacuum window (AR‑structured), four infrared‑blocking low‑pass filters (1.5 THz, 650 GHz, 550 GHz, 450 GHz), and a set of cold mirrors mounted at 4 K. The total optical transmission of this chain averages ~70 % over the band, more than double the transmission of the DESHIMA 1.0 optics.

The detector chip resides at 120 mK inside a light‑tight box, thermally anchored to a two‑stage adiabatic demagnetization refrigerator (ADR). Magnetic shielding (Nb + Cryoperm) and thermal isolation via Vespel rods and intermediate 0.8 K stage reduce parasitic loading to ~2 µW, allowing >24 h hold time. Readout is performed with a single frequency‑division multiplexing (FDM) line spanning 2–4 GHz, enabling simultaneous monitoring of all 339 KIDs. Low‑noise amplifiers at 4 K (+26 dB gain, ~2 K noise temperature) and at room temperature (+20 dB) provide sufficient signal‑to‑noise for both broadband and narrowband measurements. Sampling rates of 160 Hz or 1.3 kHz are selectable depending on the experiment.

A fast sky‑position chopper, mounted on a motor‑controlled stage, provides two beam positions separated by ~100 mm, corresponding to a ±117 arcsec pointing offset on the sky. The wheel rotates at 5 Hz, yielding a 10 Hz chopping frequency that suppresses atmospheric fluctuations (< 1 Hz) and mitigates TLS‑induced 1/f noise in the KIDs. Data loss due to chopping is kept below 20 % and no beam truncation is observed. Precise alignment of the chopper and the warm mirrors is achieved with a six‑axis hexapod (±50 mm travel, ±15° rotation, 4 µm/7.5 µrad precision).

Laboratory measurements confirm the instrument’s performance. Beam pattern scans show an aperture efficiency of ~70 %, matching design expectations, and far‑field patterns agree with the modified Dragonian optics model. The filter bank delivers the targeted Q and yields, resulting in an overall instrument efficiency of ~8 %—four times higher than DESHIMA 1.0. Importantly, the authors introduce a novel absolute frequency calibration method that uses the spatial information from beam‑pattern measurements together with the known response of an internal harmonic mixer, eliminating the need for external calibration sources.

Scientifically, DESHIMA 2.0 enables simultaneous detection of multiple molecular and atomic lines (e.g., CO,