# Detection of On-Chip Generated Weak Microwave Radiation Using Superconducting Normal-Metal SET
## 3 Experimental Setup and Results
### Frequency Dependence of the Response  
We observed a dramatic change in the detector output caused by the microwave signal generated on-chip by the Josephson source. Figure 5 shows the detector current as a function of the detector bias and gate voltages when the source is turned off (panel (a)) and when the voltage across the source junctions is tuned to \(V_{\mathrm{JJ}}\approx 200\)\(\mathrm{\SIUnitSymbolMicro\SIUnitSymbolMicro V}\) (\(f_{\mathrm{J}}\approx 100\,\mathrm{GHz}\)) corresponding to the maximum detector signal (panel (b)). Without irradiation, see Figure 5a, the current \(I_{\mathrm{NISIN}}\) is a \(2e\)-periodic function of \(n_{\mathrm{g}}\), which is in accordance with the model described in Section 2.1, since the tunneling is only possible for odd values of \(n_{\mathrm{g}}\). At even values of \(n_{\mathrm{g}}\) the detector current is low and basically reduces to the dark signal appearing due to the background microwave radiation. As a result of the microwave irradiation coming from the source, a significant current appears also for the even settings of \(n_{\mathrm{g}}\) and, for the sufficiently strong microwave signal, approaches the value measured for odd \(n_{\mathrm{g}}\) (see right and upper panels in Figure 5b). We also note that the Andreev reflection does not contribute significantly to the transport mechanism since we do not observe the Andreev peaks in our measurements (in contrast to [25], where these peaks were clearly observed in subgap voltages and at odd values of \(n_{\mathrm{g}}\)).  
Figure 5: Detector current, \(I_{\mathrm{NISIN}}\), measured as a function of the gate and bias voltages at zero magnetic field, (**a**) without irradiation and (**b**) under irradiation, when the source voltage is set to \(V_{\mathrm{JJ}}\approx 200\)\(\mathrm{\SIUnitSymbolMicro V}>E_{t}^{\mathit{econ}}/2e\) generating microwave signal with frequency \(f_{\mathrm{J}}\approx 100\,\mathrm{GHz}\).  
The frequency dependence of the detector response is shown in Figure 6a (green curve) for the subgap Josephson source voltages. The response signal appears above the threshold voltage \(V_{\parallel,A}\approx 120\) uV (\(f_{t}=V_{\parallel,A}/\Phi_{0}\approx 58\) GHz, marked by the blue dashed line in Figure 6a) corresponding to the activation energy \(E_{t}^{even}\approx\)240 ueV calculated for the parameters of the experiment. After achieving a broad maximum in the voltage range 150 uV\(\lesssim V_{\parallel}\lesssim 250\) uV, the detector current gradually decreases as, presumably, a result of reducing the radiation effect at higher photon energies \(2 eV_{\parallel}\) (see Section 4).  
The response was also featured by a set of resonances. This occurs even in spite of the fact that the transmission line is terminated to a matched load. In order to get an insight in the origin of the observed resonances, we compared the detector signal to that measured using a direct capacitive coupling (red dashed curve). As is seen in Figure 6a, the resonances are located approximately at the same frequencies for both types of the coupling, although for capacitive coupling these are rather smooth. We also found out that these locations did not substantially change when using a double thick Si substrate with the purpose of reducing the resonance frequency of the possible standing waves to the metallic sample table. We also noticed some peculiarities occurring almost at corresponding voltages in the autonomous _1-V_ curve of the Josephson source as depicted in Figure 6b. As a possible explanation, we attribute these features to resonances in the on-chip wiring being identical in both layouts.