## 3 Results  
Figure 2a shows the current-voltage curves (IVCs) at \(T_{\text{b}}\) = 5 K, 50 K and 70 K. The contact resistance was subtracted. The gray arrows in Figure 2a indicate the sweep direction of the bias current across the stack at \(T_{\text{b}}\) = 5 K. For this bath temperature, all junctions switched to their resistive state when the bias current was increased from zero to 60.5 mA. By further increasing the bias current to 72.6 mA and then decreasing it to zero, the S-shaped IVC was obtained. The maximum voltage of the emitter is about 2.88 V, corresponding to about 2.4 mV across each Josephson junction, indicating that the heat exhaust of this device was efficient. As the bath temperature was increased, the back-bending of the IVCs was reduced. It was absent for \(T_{\text{b}}\) = 70 K. The shapes of IVCs at different bath temperatures are similar to those in Ref. [24], where we also performed 3D numerical simulations of the stack dynamics to better understand the observed IVCs. Figure 1: (**a–d**) Schematic diagram of the main steps in the fabrication process. (**e**) Schematic view of the final BSCCO stack mounted on a sapphire lens. The emission power and frequency were also characterized at \(T_{\rm b}\) = 5 K, 50 K and 70 K, as shown in Figure 2. The maximum emission power of about 5.62 uW was detected at point \(E\) (\(I\) = 12.96 mA, \(V\) = 0.992 V) at \(T_{\rm b}\) = 50 K, with a measured emission frequency of 0.35 THz. For this bath temperature, the highest emission frequency was observed to be 0.64 THz, measured at current \(I\) = 30.03 mA (point \(C\) in Figure 2a,b). At \(T_{\rm b}\) = 70 K, the highest emission power of 4.4 uW was found at bias point \(F\) (\(I\) = 31.87 mA, \(V\) = 0.877 V), with an emission frequency of 0.35 THz, corresponding to about 1230 junctions in the stack. This is nearly the same junction number as the value we estimated from the crystal thickness. Furthermore, within experimental accuracy, the junction number did not change with the bath temperature. The lowest emission frequency was measured to be 0.25 THz at \(I\) = 21.17 mA (point \(G\) in Figure 2a,b), indicating the good low-frequency performance of the stack. For \(T_{\rm b}\) = 5 K, the emission occurs mainly in the low-bias region, with a wide voltage range from 2.56 to 0.34 V. For the stable working points, the highest emission power of about 0.47 uW occurs at point \(D\) (\(I\) = 5.96 mA, \(V\) = 1.36 V), with an emission frequency of 0.52 THz. At bias point \(B\) (\(I\) = 9.92 mA, \(V\) = 2.2 V), the emission frequency is measured to be 0.87 THz. The highest emission frequency was observed to be 1.01 THz at \(I\) = 12.28 mA and \(V\) = 2.541 V (point \(A\) in Figure 2a,b), indicating the good cooling of the emitter. In Figure 3, we studied the IVCs and emission characteristics at \(T_{\rm b}\) = 75 K, 80 K and 85 K. For the three bath temperatures, the IVCs exhibit a positive differential resistance at their resistance state. For \(T_{\rm b}\) = 75 K, the maximum emission power of about 3.5 uW was observed at \(I\) = 24.79 mA and \(V\) = 0.578 V. At point \(C\) (\(I\) = 25.46 mA and \(V\) = 0.609 V), the measured emission power was about 2.6 uW, with an emission frequency of 0.23 THz. Spectra for the bias points \(A\) and \(B\) in Figure 3a are also measured, as shown in Figure 3b. The emission frequencies are 0.33 THz and 0.31 THz for points \(A\) and \(B\), respectively. At \(T_{\rm b}\) = 80 K, a maximum emission power of about 4.6 uW was obtained at \(I\) = 28.46 mA and \(V\) = 0.541 V. Low frequencies of 0.23 THz and 0.2 THz were obtained at \(I\) = 29.82 mA and 26.13 mA (point \(D\) and \(E\)), respectively. The lowest emission frequency, \(f\) = 0.15 THz, with an emission power of about 0.18 uW, was found at \(I\) = 23.39 mA (point \(F\) in Figure 3a,b at \(T_{\rm b}\) = 85 K. To our knowledge, 0.15 THz is a record for the low-frequency end of emission of BSCCO emitters.