We studied the photo-induced dynamics in niobium nitride (NbN) thin films by THz spectroscopy in a strong photoexcitation regime. The pump laser pulse immediately breaks all Cooper pairs into separate quasiparticles. Subsequently, the heat is dissipated out of the films towards the substrate and Cooper pairs start to recover: initially, mutually isolated superconducting islands emerge, which subsequently grow on a 100 ps timescale towards a nearly percolated superconducting network. The superconductivity is restored faster than the superconducting gap recovers its equilibrium value. Most interestingly, experiments suggest that the profile of the density of states of strongly photoexcited films differs during the recovery process from the ones obtained for the thermal equilibrium at any temperature. This phenomenon is controlled by confinement effects within the studied thin films.
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