Effect and applications of patterned laser illumination on superconducting films

Patterned laser illumination incident upon superconducting films is shown to be capable of creating dynamic superconducting-normal ($S-N$) interfaces with almost any desired pattern. The width of such a dynamic interface for a sharp illumination edge is shown to be the effective quasiparticle diffusion length. In the limit of strong phonon trapping in the film, the effective quasiparticle diffusion length approaches the thermal healing length obtained from the heat diffusion model. However, this is not the appropriate limit in general for thin films immersed in liquid helium. The ability to create dynamic $S-N$ patterns can be used in probing spatial inhomogeneity of superconducting films and tunnel junctions. An Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Pb junction was used to demonstrate the technique. For the first time, the spatial distribution of tunneling probability was obtained through the measurements of local Josephson current. It is shown that this is a much more reliable technique than attempting to infer the tunneling probability from the excess quasiparticle tunneling current due to a local laser or electron beam excitation.