Development of kinetic inductance detectors for CMB experiments

The Cosmic Microwave Background (CMB) radiation is the oldest light we can observe in the universe, dating back to the epoch of recombination. It exhibits a nearly perfect blackbody spectrum at about 3 K, with small temperature anisotropies of the order of ∆T /T ≈ 10−5. Together with its faint linear polarization imprinted by Thomson scattering, these features provide precision tests of the ΛCDM model, the physics of inflation and reionization, and cosmic structure formation. Achieving the next leap in sensitivity–particularly for large-scale primordial B-mode polarization, spectral distortions, fine-scale lensing, spectral characterization of foregrounds– demands detector arrays with kilo-pixel scale, low noise equivalent power (NEP), and robust and highly multiplexed readout. Kinetic Inductance Detectors (KIDs) meet these requirements with a compelling combination of scalability, simplicity, and performance. KIDs are superconducting resonators whose resonant frequency and quality factor change when incident photons break Cooper pairs, changing the kinetic inductance. Their operation at sub-K temperatures enables photon-noise-limited performance across several CMB spectral bands. A key advantage is intrinsic frequency-domain multiplexing: thousands of high-Q resonators can be coupled to a single feedline, reducing cryogenic wiring complexity and thermal loads, while enabling densely- packed, wafer-scale focal planes. Furthermore, KIDs are compatible with standard thin-film microfabrication, allowing rapid iteration of materials, geometries, and optical coupling schemes. In this three-year PhD project, we have developed several arrays of lumped- element KIDs (LEKIDs), which directly absorb radiation in a patterned inductive meander, for experiments and activities concerning the study of the CMB through its main observables: spectrum, temperature anisotropies and polarization. Regarding the spectrum, we present the electrical characterization results of the second iteration detector design of the COSMO experiment, which aims to measure the isotropic spec- tral distortions of the CMB from Antarctica in two frequency bands, centered at 150 and 250 GHz, using two arrays of multi-mode LEKIDs. Temperature anisotropies are observed by MISTRAL, a high-angular resolution W-band camera that operates at the Gregorian focus of the Sardinia Radio Telescope, and by OLIMPO, a multi-band, balloon-borne telescope that observes galaxy clusters via the Sunyaev-Zel’dovich ef- fect. We present the electrical and optical characterization of the MISTRAL detector array and the 360 GHz OLIMPO array. We also discuss the MISTRAL integration tests at the telescope, including its first light–a major milestone demonstrating the proper operation of the instrument–and introduce a method for focal plane mapping using a cryogenic LED matrix. Concerning polarization, we developed an array of polarization-sensitive detectors for the ASI/PREMIALE research project, presenting the electrical and optical characterization for the 240 GHz array, the electrical charac- terization for the 350 GHz array, and the electrical characterization of a preliminary prototype for the new W-band receiver of the QUIJOTE telescope, that will be operated at the Teide Observatory. After discussing the general characteristics of KIDs, such as their superconducting properties, circuit modeling, noise sources, and design, we showcase the procedure of design optimization through electromagnetic simulations and demonstrate their electrical and optical performance in terms of responsivity, noise level, NEP and optical efficiency.