Nonequilibrium Heating of Electrons, Melting, and Modification of a Nickel Nanofilm by an Ultrashort Terahertz Pulse

The evolution of the optical properties of a 25-nm-thick nickel film on a glass substrate excited by a subpicosecond terahertz pulse with a field strength of 11 MV/cm has been studied by femtosecond interference microscopy with a time resolution of 10^–13 s. The measurements of the complex reflection coefficient and the permittivity in the visible range of the spectrum indicate the non-equilibrium heating of the electron subsystem of nickel to several thousands of degrees, which is accompanied by the induced increase in reflection at the initial time of irradiation and by the subsequent melting after 5–10 ps. Scanning electron and atomic force microscopy studies of the morphology of the modified surface have indicated the local melting of the nanofilm and its delamination from the substrate at this field strength.