Ultrafast Investigation Of Electron Dynamics In Metals Under Femtosecond Laser Irradiation

In recent years, due to the multi-layer metal configuration can be wellapplied for satisfaction of thermal, optical, electronic requirements, ultrashortpulse laser heating of multi-layer metal film assembly had attracted greatattention. Generally, the thermal properties for metals are physicallyoriginated from the collision mechanisms for electron-electron andelectron-phonon in the metal targets. For the multi-layer metal film assembly,the thermal properties, such as the electron-phonon coupling strength canactually vary significantly for different layers of the assembly, so the heatingof multi-layer film assembly would take on various characteristics fordifferent padding layer configurations. In this paper, the ultrafast thermaltransfer characteristics in double-layer metal film assemblies irradiated byfemtosecond laser pulse were investigated. For the double-layer metal filmassemblies, the effect of different padding layer configurations on the damagethreshold were well discussed.(1) The development of microscope heat transfer model based on theFourier model was introduced. When the time scale of laser heating metal isless than the nanosecond time scale, the non-equilibrium process will beoccurred between the different carriers. The traditional heat conductionequation considered one carrier cannot descript the heating process in thistime scale. Therefore, the multi-step heat conduction equation was used todescribe the nonequilibrium heating process between carriers.(2) The numerical methods were used in the calculation of the partialdifferential equations. And the numerical methods of the linear differential equations were extended to nonlinear differential equations.(3) The thermal physical parameters of the two-temperature equation weredescripted. A numerical solution of the two-temperature equation had beenperformed up to the femtosecond pulse laser heated metal target. In thecalculated processes, the metal copper as a target, the distributions of electronand lattice temperature were studied. And the effect of laser fluence and laserpulse width on the temperature distribution was discussed.(4) Femtosecond laser heating metal is a nonequilibrium process. This isbased on the electron-lattice coupling mechanism. The two-temperatureequation was extended to calculate the distributions of electron and latticetemperature. The results showed that the temperature distribution will bechanged by the two-layer structure. Take advantage of the two-layer structure,the damage threshold of metal surface can be improved.(5) Take advantage of the femtosecond laser pump-probe technique, theultrafast electron dynamics on the interaction of femtosecond laser and thetwo-layer metal films was studied. At the same time, the theoreticalpredictions were compared with experimental data, which agree well withboth thermal model and transient reflectivity.(6) A numerical solution of the two-temperature equation had beenperformed up to the double-pulse femtosecond laser heated metal target. Weconfirmed the distinctly different results on the single-pulse and double-pulse.The double-pulse laser heated lattice temperature was higher than single-pulse.Through the Boltzmann equation, we estimated the variation of the emissionenhancement. At the same time, this experimental result was qualitatively similar to the theoretical result.