# Fully Superconducting Josephson Bolometers for Gigahertz Astronomy
## 4 Experimental Realization of JES and Nano-TES
### Density of States and Thermal Properties of the Active Region  
The false-color scanning electron microscope (SEM) picture of a typical auxiliary device is shown in Figure 4a. The 1D-JJ is formed by 1.5 \(\upmu\)m-long (\(l\)), 100 nm-wide (\(w\)) and 25 nm-thick (\(t\)) Al/Cu bilayer nanowire-like active region (purple) sandwiched between the two Al electrodes (blue). To characterize both the energy gap and the thermal properties of \(A\), the device is equipped with two additional Al tunnel probes (green). Figure 4a schematically represents the experimental set up employed to carry out the spectral characterization of the active region in a filtered He\({}^{3}\)-He\({}^{4}\) dry dilution refrigerator. The \(IV\) tunnel characteristics of \(A\) are performed by applying a voltage (\(V\)) and measuring the current (\(I\)) flowing between one lateral electrode and a tunnel probe. For \(T<0.4T_{C}\), the energy gap of a superconductor follows \(\Delta(T)=\Delta_{0}\), where \(\Delta_{0}\) is its zero-temperature value [38]. Typically, aluminum thin films show a \(T_{C}\geq 1.2\) K, that is the bulk Al critical temperature [52]. Therefore, the superconducting gap of the superconducting tunnel probes is temperature independent up to at least 500 mK. On the contrary, superconductivity in \(A\) is expected to be strongly suppressed due to inverse proximity effect [38]. Therefore, this structure allows to determine the zero-temperature superconducting energy gap of the active region (\(\Delta_{0,A}\)). The \(IV\) characteristics were measured at the base temperature (\(T=20\) mK) and well above the expected critical temperature of \(A\), as shown in Figure 4b. At the base temperature, the JJ switches to the normal-state when the voltage bias reaches \(V=\pm(\Delta_{A,0}+\Delta_{P,0})/e\)[44], where \(\Delta_{0,P}\) is the zero-temperature gap the Al probe. Instead, at \(T_{bath}=250\) mK the transition occurs at \(V=\pm\Delta_{P,0}/e\), since \(A\) is in the normal-state. In ad  
Figure 4: Energy gap and thermal properties of the nano-sensor active region. (**a**) False-color scanning electron micrograph of a typical auxiliary device. The core of the device is made of an Al/Cu bilayer nanowire (purple) sandwiched between two Al leads (blue). To perform tunnel spectroscopy a voltage (\(V\)) is applied between \(A\) and an Al tunnel probe (green) while recording the current (\(I\)). (**b**) Tunneling current (\(I\)) versus voltage (\(V\)) characteristics recorded at \(T_{bath}=20\) mK (blue) and \(T_{bath}=250\) mK (purple). (**c**) Zoom of the \(IV\) characteristics in correspondence of the transition to the normal-state. It is possible to extract \(\Delta_{A,0}\simeq 23\)\(\upmu\)eV and \(\Delta_{P,0}\simeq 200\)\(\upmu\)eV. (**d**) Schematic representation of the set up employed for the thermal measurements. The left tunnel junction serving as thermometer is biased with a constant current (\(I_{t}\)) while the voltage (\(V_{t}\)) is measured. The right junction is voltage biased (\(V_{h}\)) and operated as local electronic heater. (**e**) Thermal model describing the main heat exchange mechanisms involved in the experiment. The power injected by the local heater (\(P_{h}\)) relaxes by the exchange with phonons (\(P_{e\text{-}ph}\)) and outflows through the thermometer tunnel junction (\(P_{l}\)). (**f**) Electronic temperature of the active region (\(T_{A}\)) versus the injected power by the heater (\(P_{h}\)). The dots represent 5 different sets of measurements, while the back line depicts the fit performed by employing Equation (22). dition, the tunnel resistance of the JJ takes the values \(R_{I}\simeq 12\) k\(\Omega\). The difference between the curves recorded at 20 mK and 250 mK is highlighted by Figure 4c.