Ultrafast heat transfer on nanoscale in thin gold films

Heat transfer processes, induced by ultrashort laser pulses in thin gold films, were studied with a time resolution of 50 fs. It is demonstrated that in thin gold films heat is transmitted by means of electron–phonon and phonon–phonon interactions, and dissipated on nanoscale within 800 fs. Measurements show that the electron–phonon relaxation time varies versus the probe wavelength from 1.6 to 0.8 ps for λ=560–630 nm. The applied mathematical model is a result of transforming the two-temperature model to the hyperbolic heat equation, based on assumptions that the electron gas is heated up instantaneously and applying Cattaneo’s law to the phonon subsystem, agrees well with the experimental results. This model allows us to define time of electron–phonon scattering as the ratio of the heat penetration depth to the speed of sound in the bulk material that, in turn, provides an explanation of experimental results that show the dependence of the electron–phonon relaxation time on the wavelength.