A millimetre-wave superconducting hyper-spectral device

Abstract

Millimetre-wave observations represent an important tool for cosmology studies. The line intensity mapping technique has been proposed to map in three dimensions the specific intensity due to line (e.g. [C ii] and CO) emission, e.g. 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- to 90-GHz range and contains 19 horns for 16 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 bi-layer Lumped Element Kinetic Inductance Detector is then in charge of dissipating and sensing the supercurrent established in the resonant absorber. The prototype is fabricated with only two photolithography 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 three-dimensional simulations, is polarization-sensitive, paving the way to spectro-polarimetry measurements over very large instantaneous field of views.