Physical limits for sensitivity of a monolithic Terahertz superconducting sensor based on a galvanically isolated nanobridge.

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The aim of this project is the investigation of energy relaxation processes in disordered nanometer-sized superconductors near its phase transition at ultra-low temperatures under incident flux of microwave, terahertz and optical irradiation. These relaxation processes define the intrinsic noise level in functional nanostructures, which are defining the fundamental sensitivity of sensing elements. The small heat capacity of quasiparticles in a nano-island or nanobridge and the relatively long energy-relaxation times makes these structures very sensitive to excitations even with ultra-low photon fluxes. The potential application could be in the field of ultrasensitive photon sensors in a wide frequency range. The materials for nano-islands are low-temperature disordered superconductors like titanium (Ti) and hafnium (Hf) with critical temperatures in the 100-mK range. When the nano-island absorbs a photon, the concentration of quasiparticles increases or their energy distribution becomes essentially non-thermal. It is possible to observe in detail the dynamics of this process. At very low temperatures, due to reduced electron-phonon interaction, the characteristic energy-relaxation times of quasiparticles, even near Tc, become sufficiently long. Thus this non-equilibrium state can be observed by measuring a change of the complex impedance of the nano-island. The change in the number of quasiparticles due to fluctuations or due to photon absorption will be monitored via the change of its complex microwave impedance at GHz frequencies. This is possible by embedding the nano-island in a microwave high-Q superconducting resonator. This approach allows for precise investigation of the energy-relaxation processes and characteristic quasiparticles lifetimes in a superconducting nanostructure near and below its Tc. and thus the fundamental limits for sensitivity.