Ultrafast heat transfer on nanoscale in thin gold films

Abstract

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. 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.