Transient response of superconducting Pb microbridges irradiated by picosecond laser pulses and its potential applications

We have observed voltage pulses having half-widths of less than 500ps generated by constant-current-biased superconducting Pb variable thickness microbridges driven normal by short (3-5ps) light pulses. This represents a first step in the effort to generate even shorter pulses, which according to our analysis of the Rothwarf-Taylor equations should be possible. The ultimate width should be equal to the phonon pair-breaking time, which for materials such as Nb can be as short as a few picoseconds. In addition to monitoring the voltage pulses directly, we have used a novel adoption of the optical autocorrelation technique having a time resolution limited only by the laser pulse width. It is pointed out that even shorter voltage pulses, and therefore greater potential for device applications, can be achieved by direct injection of quasiparticles.