Time-resolved observation of fast hotspot dynamics in superconducting nanowires

We present time-resolved measurements of fast hotspots in superconducting nanowires that allow a direct visualization of the hotspot evolution. The measurements were made possible exploiting an innovative parallel nanowire configuration. We measure that the hotspot lifetime is independent of bias current and hotspot width. We observe that the hotspot expansion is slower than its relaxation and that a time delay on the order of 1 ns occurs between the maximum hotspot power dissipation and maximum hotspot expansion rate. The time delay was found to increase with increasing bias current and it decreases slightly for wider hotspots. On the basis of the observed dynamic evolution, we argue that the hotspot expands rapidly when the temperature gradient on the hotspot boundary is low, and that it expands slowly when this gradient is large.