A millimetre-wave superconducting hyper-spectral device

Abstract

Millimetre-wave observations represent an important tool for Cosmology studies. The Line Intensity Mapping (LIM) technique has been proposed to map in three dimensions the specific intensity due to line (e.g. [C ii], CO) emission, for example from the primordial galaxies, as a function of redshift. Hyper-spectral integrated devices have the potential to replace the current Fourier transform, or the planned Fabry-Perot-based instruments operating at millimetre and sub-millimetre wavelengths. The aim is to perform hyper-spectral mapping, with a spectral resolution R = λ/Δλ = 100–1000, over large, i.e. thousands of beams, instantaneous patches of the Sky. The innovative integrated device that we have developed allows avoiding moving parts, complicated and/or dispersive optics or tunable filters to be operated at cryogenic temperatures. The prototype hyper-spectral focal plane is sensitive in the 75-90 GHz range and contains nineteen horns for sixteen spectral-imaging channels, each selecting a frequency band of about 0.1 GHz. For each channel a conical horn antenna, coupled to a planar superconducting resonant absorber made of thin aluminium, collects the radiation. A capacitively coupled titanium-aluminium bilayer Lumped Element Kinetic Inductance Detector (LEKID) is then in charge of dissipating and sensing the super-current established in the resonant absorber. The prototype is fabricated with only two photo-lithography steps over a commercial mono-crystalline sapphire substrate. It exhibits a spectral resolution R = λ/Δλ ≈ 800. The optical noise equivalent power of the best channels is in the observational relevant $4\cdot 10^{-17} W/\sqrt{Hz}$ range. The average sensitivity of all the channels is around $1\cdot 10^{-16} W/\sqrt{Hz}$. The device, as expected from 3-D simulations, is polarisation-sensitive, paving the way to spectro-polarimetry measurements over very large instantaneous field-of-views.