Subpicosecond optoelectronic study of superconducting transmission lines

We have studied the propagation of subpicosecond electrical pulses on coplanar superconducting Nb transmission lines. Pulses with 0.6 ps full width at half maximum were generated by photoconductively shorting a
\sim 10 \mu
m region between two charged 1 to 5 μm lines separated by a 2 to 10 μm gap. The propagating pulses were sampled by the delayed shorting of a fast phototconductive switch between a sampling probe and one of the transmission lines at variable distances away from the generation point. Silicon-on-sapphire wafers served as the transmission line substrate, with the 0.5 μm thick Si layer heavily damaged by an oxygen implant to provide the subpicosecond carrier life time for the excitation and probe switches. Measurements and analyses of pulses propagated up to 8 mm distance at temperatures from 2 K to 10 K showed a threshold for strong attenuation and dispersion at a frequency reflecting the onset of pair breaking in the superconducting transmission lines. The results at least qualitatively confirm the superconducting microstrip transmission line calculations of Kautz based on Mattis and Bardeen's formulae for the complex conductivity of superconductors.