A horn-coupled millimetre-wave on-chip spectrometer based on lumped-element kinetic inductance detectors

¹ Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 25 rue des Martyrs, 38042 Grenoble, France
e-mail: monfardini@neel.cnrs.fr
² Groupement d’Intérêt Scientifique KID, 38042 Grenoble and 38400 Saint-Martin-d’Hères, France
³ Institut de RadioAstronomie Millimétrique (IRAM), 300 rue de la piscine, 38400 Saint-Martin-d’Hères, France

Received: 5 September 2022
Accepted: 5 February 2023

Abstract

Context. Millimetre-wave astronomy is an important tool for both general astrophysics studies and cosmology. A large number of unidentified sources are being detected by the large field-of-view continuum instruments operating on large telescopes.

Aims. New smart focal planes are needed to bridge the gap between the large bandwidth continuum instruments operating on single-dish telescopes and high spectral and angular resolution interferometers (e.g. ALMA in Chile and NOEMA in France). The aim is to perform low to medium spectral resolution observations and select a lower number of potentially interesting sources (i.e. high-redshift galaxies) for further follow-up.

Methods. We have designed, fabricated, and tested an innovative on-chip spectrometer sensitive in the 85–110 GHz range. It contains 16 channels, each of which covers a frequency band of about 0.2 GHz. A conical horn antenna coupled to a slot in the ground plane collects the radiation and guides it to a millimetre-wave microstrip transmission line placed on the other side of the mono-crystalline substrate. The millimetre-wave line is coupled to a filter-bank spectrometer. Each filter is capacitively coupled to a lumped-element kinetic inductance detector (LEKID). The microstrip configuration provides the benefit of low loss, due to the mono-crystalline substrate, and protects the LEKIDs from illumination by stray un-filtered light.

Results. The prototype spectrometer exhibits a spectral resolution R = λ/Δλ ≈ 300. The optical noise equivalent power is in the low 10⁻¹⁶ W Hz^−1/2 range for an incoming power of about 0.2 pW per channel. The device is polarisation-sensitive, with a cross-polarisation lower than 1% for the best channels.