Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-Tc superconducting Y-Ba-Cu-O thin films

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa₂Cu₃O_7-x (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-μm-wide, 7-μm-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO₃ substrates by laser deposition. When varying the biasing conditions between the superconducting and switched states, we observed transients of single-picosecond duration that corresponded to the nonequilibrium kinetic-inductance and the electron-heating response mechanisms, respectively. In both cases, experimental waveforms could be accurately simulated using a nonequilibrium (two-temperature) electron-heating model. From the fits, the YBCO intrinsic temporal parameters associated with the nonequilibrium conditions were extracted. The electron thermalization time was found to be 0.56 ps in the state above the material´s critical temperature (T_c=89 K) and 0.9±0.1 ps in the superconducting state at temperatures ranging from 20 to 80 K. The electron-phonon energy relaxation time was found to be 1.1 ps. The single-picosecond pulse distortion due to propagation on a YBCO coplanar waveguide was also studied. Our results show that a YBCO microbridge can intrinsically operate as a photodetector at rates exceeding 100 Gb/s, making it useful as an optical-to-electrical transducer for optoelectronic interfaces in YBCO digital electronics. Simultaneously, YBCO mixers, based on hot-electron effects, should exhibit an intrinsic bandwidth exceeding 100 GHz