Fundamentals of energy cascade during ultrashort laser-material interactions (Invited Paper)

Abstract

During an ultrashort laser pulse, numerous photons are emitted in a very short period of time leading to very high peak power. The photons can excite free electrons in the material to very high temperatures (heating) or strip bound electrons from the atoms (ionization). In ultrashort laser heating there is a time lag between the electron heating and the lattice heating. The two-temperature model has been proposed to calculate the electron temperature and the lattice temperature and the related damage threshold for metals. On the other hand, ablation models based on impact ionization and photoionization have been proposed to predict material removal rates for semiconductors and dielectrics. However, in existing heating or ablation models, some critical thermal and optical properties of the material are assumed to be time, space, and fluence independent or the estimations are limited to temperatures much lower than the Fermi temperature. In this paper, the quantum theories are employed to calculate the free electron heating, free electron relaxation time, and the temporal and spatial dependent thermal and optical material properties. The improved two-temperature model is used to predict damage fluences of gold thin films. The new ablation model based on the Fokker-Planck equation can predict ablation depth and crater shape of semiconductors and dielectrics. The predicted results are in good agreement with experimental data.

Femtosecond Laser Micro/Nano-manufacturing: Theories, Measurements, Methods, and Applications

“…The improvements included (1) calculation of the heat capacity of free electrons from Fermi distribution, (2) determination of free electron relaxation time and electron thermal conductivity by solving the plasma Boltzmann transport equation, and (3) calculation of optical properties such as reflectance and absorption coefficient by using the improved Drude model that considers free electron heating and interband transition. By using the improved TTM, the electron and phonon temperatures of gold films irradiated by femtosecond laser can be calculated theoretically and verified experimentally [23,24]. The experimental results show that the improved TTM yields a more accurate theoretical prediction of the ablation threshold of the femtosecond laser-gold film interaction compared with the conventional model ( Fig.…”

Nonequilibrium Dynamics of Electron Emission from Cold and Hot Graphene under Proton Irradiation

“…To separate the effects of elevated electron and lattice temperatures, we investigate the proton irradiation of a cold lattice with an elevated electronic temperature and an elevated lattice temperature with cold electrons. We use electron and lattice temperatures from previous works based on two temperature models. − …”

Excitation relaxation in the electron subsystem of a metal when it is irradiated with ultrashort laser pulses

“…Graphs of the distribution function of electrons over energy when they are irradiated with a laser pulse having parameters Q a 0.1 J∕cm 2 and t 0 40 fs on the surface, taking into account excitation and relaxation (1), neglecting relaxation(2), and the equilibrium distribution function (3) at time 3 (a), 6 (b), 9 (c), and 11 fs (d).…”

“…Relaxation time versus pulse-penetration depth x for Q a 0.1 J∕cm 2 and t 0 40 (1) and t 0 80 fs(2).…”