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

本项目的目的是研究在超低温下，在微波、太赫兹和光辐射的入射通量下，无序纳米尺寸超导体在相变附近的能量弛豫过程。这些弛豫过程定义了功能性纳米结构中的固有噪声水平，这定义了传感元件的基本灵敏度。纳米岛或纳米桥中准粒子的小热容和相对较长的能量弛豫时间使得这些结构即使在超低光子通量的情况下也对激发非常敏感。潜在的应用可能是在宽频率范围内的超灵敏光子传感器领域。纳米岛的材料是低温无序超导体，如钛（Ti）和铪（Hf），临界温度在100 mK范围内。当纳米岛吸收光子时，准粒子的浓度增加或其能量分布变得基本上非热。可以详细观察这一过程的动态。在非常低的温度下，由于电子-声子相互作用的减少，准粒子的特征能量弛豫时间，即使在Tc附近，也变得足够长。因此，可以通过测量纳米岛的复阻抗的变化来观察该非平衡状态。由于波动或由于光子吸收而引起的准粒子数量的变化将通过其在GHz频率下的复微波阻抗的变化来监测。这可以通过将纳米岛嵌入微波高Q超导谐振器中来实现。这种方法允许精确调查的能量弛豫过程和特征准粒子的寿命在超导纳米结构附近和低于其Tc。因此也是灵敏度的基本极限。

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.