# A Bunch of YBCO Josephson Generators for the Analysis of Resonant Cold-Electron Bolometers
## 2 Experimental Setup  
The used experimental setup consists of long YBCO Josephson junctions as tunable THz sources, bolometers with direct electron cooling of the absorber as sensitive receivers and various holders and horns, feeding the radiation from a source to a receiver. In Figure 1, the photographs of YBCO JJs inside the modified dipole antennas are demonstrated. The distributed Josephson junctions on the basis of the YBa\({}_{2}\)Cu\({}_{3}\)O\({}_{7-\delta}\) thin films were fabricated by the method of the preliminary topology mask [16, 17] using magnetron sputtering on 24\({}^{\circ}\)[001]-tilt Zr\({}_{1-x}\)Y\({}_{x}\)O\({}_{2}\) bicrystal substrate. This technology assumes the preliminary deposition of cerium dioxide (CeO\({}_{2}\)) on the heated Zr\({}_{1-x}\)Y\({}_{x}\)O\({}_{2}\) (or any other suitable) substrate as a buffer layer. Due to the high temperature, this buffer CeO\({}_{2}\) layer provides an epitaxially clean surface; thus, the further deposition of theYBCO layer will lead to a superconducting film. Using the photoresist mask, a structure of Josephson junctions with electrodes is formed, and the further deposition of thick CeO\({}_{2}\) layer on a cold substrate leads to an amorphous layer. The sputtering of YBCO film is performed as the final stage of the fabrication process. In this case, the YBCO film, sputtered on the epitaxially clean surface, becomes superconducting, but on the amorphous CeO\({}_{2}\) layer it becomes insulating. Since the deposited superconducting structures have rather small sizes, this method allows a small number of defects together with high electrophysical properties of the films [17]. The Josephson junctions' length was 50 \(\upmu\)m (along the grain boundary) with 0.3 \(\upmu\)m film thickness. At the temperature \(T=5\) K, the achieved critical current density is 370 kA/cm\({}^{2}\) and the \(I_{c}R_{n}\) product is 1.54 mV. The junctions are rather long, with lengths much larger than the Josephson penetration depth \(\lambda_{f}=0.6\)-0.9 \(\upmu\)m. The oscillator chip is formed with six JJ structures placed inside the modified dipole antennas. Since the generation of each junction can be continuously tuned from 0.05 to 2 THz [21], its oscillation frequency is determined by the antenna band. The numbering of the structures on the substrate is shown in Figure 1a. The scanning electron microscope (SEM) image of a structure is presented in Figure 1b. Here, one can observe the YBCO film, deposited on the epitaxially clean CeO\({}_{2}\) buffer layer on Zr\({}_{1-x}\)Y\({}_{x}\)O\({}_{2}\) substrate (blue) and the walls (burgundy) of a thick amorphous CeO\({}_{2}\) layer as a part of a preliminary mask layer. To develop and study the oscillators based on high-temperature bicrystal Josephson junctions we have used Cold-Electron Bolometer (CEB) array with broadband antennas [23], that were developed in [5, 10, 26]. While these antennas receive signal at multiple harmonics as well [23], they may limit the band of YBCO oscillator antennas, which has been improved in comparison with [23]. The CEB represents double SIN (superconductor-insulator-normal metal) junctions Al/Al\({}_{2}\)O\({}_{3}\)/Al-Fe/Al\({}_{2}\)O\({}_{3}\)/Al with tiny normal metal absorber made of aluminum with 1 nm thin underlayer of Fe to suppress superconductivity, which also allows achieving deep electron cooling [27]. The considered receiving systems are single elements with double-folded slot antennas, connected by coplanar lines, as well as a 2-dimensional array of bow-tie antennas with CEBs. Both types of receiving systems allow achieving rather narrow bandwidths. Besides photon-noise level sensitivity [3, 4, 5] and cosmic rays immunity [28], one more advantage of the CEB is its tiny size of a few microns, which allows depositing bolometers inside the antenna slots or coplanar lines without any additional microwave lines.