Effect of intense laser irradiation on the thermal transport properties of metals

Abstract

Ultrafast laser irradiation of metals can elevate the temperature of electrons to the same order as the Fermi temperature while the lattice remains cold. In this transient and highly nonequilibrium regime, the material properties can undergo drastic modifications, revealing insights into the physical phenomena of the condensed state that are otherwise not evident under equilibrium conditions. However, a clear description of these phenomena, even for elemental metals, remains greatly unexplored partly due to the limitations imposed by the phenomenological treatment of electron-phonon scattering with simplified assumptions for these highly nonequilibrium regimes. In this work, using recent advancements in first-principles calculations, we provide a detailed understanding of hot electron dynamics in a free-electron-like metal (aluminum) and a noble metal (gold) to demonstrate the important role played by the electronic structure in dictating their transport properties at elevated electron temperatures. By performing parameter-free density functional theory calculations of the electron-temperature-dependent heat capacities, electron-phonon coupling, electron mean-free paths, and thermal conductivities, we show that semiempirical and free-electron estimations can lead to erroneous predictions, especially for the case of gold where the excitation of the low-lying $d$ bands can drastically modify the transport properties. We find that the diffusive mean-free paths of electrons in gold can be increased from $\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ at ambient conditions to $\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ at electron temperatures of ${k}_{\mathrm{B}}{T}_{e}=6\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ as a result of drastically reduced electron-phonon scattering. In contrast, we find that the mean-free paths of electrons in aluminum are relatively insensitive to high electron temperature perturbations mainly resulting from the unchanged electron-phonon scattering even at elevated electron temperatures. This ultimately results in a much greater increase in the thermal conductivity of gold at electron temperatures of $\ensuremath{\sim}20000$ K, where it increases by more than two orders of magnitude. However, in aluminum, the thermal conductivity increase at the elevated electron temperatures is relatively not as pronounced (which increases by a factor of $\ensuremath{\sim}70$). Our results shed light on the microscopic understanding of hot electron dynamics in metals and is crucial for a plethora of applications such as in plasmonic devices.