Speaker
Description
Among the main challenges in modern fundamental physics, low-energy phenomena related to dark matter searches, cosmic microwave background (CMB), and gravitational-wave astronomy require ultra-sensitive detectors. Transition Edge Sensors (TESs) and Josephson Escape Sensors (JESs) [Phys. Rev. Applied 14, 2020] are promising candidates for the development of single-photon calorimeters and ultra-sensitive bolometers operating from the infrared to the THz regime. TESs currently achieve noise-equivalent power (NEP) of the order of $10^{-19}$ $W/\sqrt{Hz}$ and energy resolution of a few meV. Within the STEEP project of INFN, we aim to develop bolometers with $NEP\simeq 10^{-20}$ $W/\sqrt{Hz}$ and calorimeters with energy resolution $\delta E \simeq 400$ $\mu$eV, improving current performance by approximately one order of magnitude.
To optimise the detectors’ performance, which share a common structure, each component is studied separately. A gold shunt resistor is developed and characterised through temperature-dependent resistivity measurements to ensure stable electrothermal feedback. The electrodes require large-gap superconductors to minimise resistance and thermal leaks. Niobium electrodes were investigated, allowing operation over a wider temperature range, but fabrication challenges led to the selection of aluminium for the final device. The active region consists of a 15 nm Al / 12 nm Cu bilayer, with $T_c \simeq$ $400 mK$. The superconducting properties and their dependence on the dimensions, bias current, and temperature were studied via 4-terminal cryogenic measurements. Decrease of the width revealed a crossover from two-dimensional to one-dimensional behaviour, described by the 2D BKT and 1D Ivanchenko–Zil'berman models, respectively, while preliminary results indicate a NEP of $10^{-18}$ $W/\sqrt{Hz}$ in TES configuration and approaching $10^{-24}$ $W/\sqrt{Hz}$ in JES configuration. These results establish the material and device parameters required for the future implementation of frequency-domain multiplexed TES/JES focal-plane arrays.