## 3 Results
Figure 2: (a) Transport and terahertz emission characteristics of the BSCCO stack at \(T_{\rm b}\) = 5 K, 50 K and 70 K. The gray arrows indicate the sweep direction of bias current for the 5-K curve. Colors indicate the emission power (multiplied by a factor of 9 for the \(T_{\rm b}\) = 5 K data) detected by the Golay cell. (b) Fourier spectra of the emission for the bias points \(A\)–\(G\) indicated in (a); the numbers next to the labels are the values of emission peak frequencies and emission power. The data in (b) are rescaled to yield similar heights of the emission peaks. Figure 4 displays the radiation properties of the BSCCO emitter at \(T_{\rm b}\) from 5 to 85 K. Over the whole range of operating temperatures, the emission frequencies are between 0.15 and 1.01 THz. A high emission power is obtained mainly in the frequency range between 0.2 and 0.52 THz, as shown in Figure 4a, indicating the desired good low-frequency performance of the emitter. Four obvious emission peaks near 0.21 THz, 0.32 THz, 0.42 THz and 0.5 THz appear on the radiation intensity spectrum, as indicated by the dashed lines in Figure 4a. At these peaks, it is likely that cavity resonances are excited. For a square IJJ stack, the resonances occur at frequencies \(f_{r}=(c_{0}/2n_{r}L)\sqrt{k^{2}+l^{2}}\), where \(k\) and \(l\) are integers, \(L=240~{}\upmu\)m is the length of the square stack, \(c_{0}\) is the velocity of light in a vacuum, and \(n_{r}\approx 4.2\) is the refractive index of BSCCO crystals [7, 8]. According to the above equation, the cavity frequency of TM (1, 1) resonance modes is about 0.21 THz, which has a good match with the emission peak near 0.21 THz in Figure 4a. The highest emission peak with measured frequencies of about 0.32 THz, as shown in Figure 4a, could match the predicted TM (2, 0) or TM (0, 2) resonance mode with a cavity frequency of about 0.3 THz. The predicted TM (2, 2) resonance modes with a cavity frequency of about 0.42 THz also match well with the peak near 0.42 THz. For the peak near 0.5 THz, the TM (1, 3) or TM (3, 1) resonance mode with the predicted cavity frequency of about 0.47 THz may have been excited to enhance the terahertz radiation. We note that, according to Ref. [34], degenerate cavity modes should not lead to an enhancement of emission, at least for homogeneous stacks. However, we operate our device at high bath temperatures. Here, even small gradients in the stack temperature could lead to gradients in the Josephson critical current density that are large enough to break this symmetry. Figure 3: (**a**) Transport and terahertz emission characteristics of the BSCCO stack at \(T_{\rm b}=75\) K, 80 K and 85 K. Colors indicate the emission power (multiplied by a factor of 10 for the \(T_{\rm b}=85\) K data) detected by the Golay cell. (**b**) Fourier spectra of the emission for the bias points \(A\)–\(F\) indicated in (**a**); the numbers next to the labels are the values of emission peak frequencies and emission power. The data in (**b**) are rescaled to yield similar heights of the emission peaks. Figure 4b shows the observed emission \(f\) versus \(T_{\rm b}\) between 5 and 85 K, using the emission power as color code. At low bath temperatures, the emitter exhibits a wide frequency tunability. At \(T_{\rm b}\) = 15 K, the emission \(f\) was observed, ranging from 0.374 to 1.004 THz. A clear cutoff emission frequency of about 0.37 THz in Figure 4b can be found for \(T_{\rm b}\)\(\leq\) 45 K, which is mainly caused by the saturated \(f_{\rm pl}\). As \(T_{\rm b}\) is increased, the measured lowest emission frequency of each bath temperature decreases due to the decreasing \(f_{\rm pl}\). For \(T_{\rm b}\)\(\geq\) 80 K, the measured emission frequencies are lower than 0.25 THz, with the lowest emission frequency being 0.158 THz and 0.15 THz at \(T_{\rm b}\) = 82 K and 85 K, respectively.