Ultrafast Photoresponse Dynamics of Current-Biased Hg-Ba-Ca-Cu-O Superconducting Microbridges

We report our time-resolved transient photoresponse studies of current-biased Hg-Ba-Ca-Cu-O superconducting microbridges excited by 100-fs-wide and 800-nm-wavelength laser pulses. Our devices consisted of 20-$\mu{\rm m}$-wide and 30-$\mu{\rm m}$-long bridges, incorporated into a 0.1-mm-wide signal line of the coplanar strip transmission line. The structures were patterned in 400-nm-thick precursor films grown on ${\rm LaAlO}_{3}$ substrates by room-temperature magnetron sputtering and, subsequently, exposed to an ex-situ high-temperature mercuration process. The resulting structures consisted of a $c$-axis-oriented, predominantly Hg-1212 compound with the onset superconducting critical temperature $T_{c,{\rm on}}$ around 110 K. At low temperatures (far below $T_{c}$), under low bias current and low optical excitation, the microbridge photoresponse signal was a positive, $\sim$90-ps-wide (measurement- limited) pulse followed by a negative component and was related to the kinetic-inductive response. At high temperatures, or when the bias current was near the bridge critical current, we observed an additional slow resistive response due to the bolometric effect. The observed ultrafast photoresponse dynamics in our Hg-Ba-Ca-Cu-O bridges make this material very promising for optical photodetector and mixer applications.