Pengfei Ji

Femtosecond laser processing of germanium: an ab initio molecular dynamics study

An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the structural change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by imposing the Nose Hoover thermostat to the electronic subsystem lasting for ~100 fs and continuing with microcanonical ensemble simulation of ~200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find the irradiated germanium distinguishes from the usual germanium crystal by analyzing their melting and dynamic properties.

Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective.

First-Principles Molecular Dynamics Investigation of the Atomic-Scale Energy Transport: From Heat Conduction to Thermal Radiation

Abstract First-principles molecular dynamics simulation based on a plane wave/pseudopotential implementation of density functional theory is adopted to investigate atomic scale energy transport for semiconductors (silicon and germanium). By imposing thermostats to keep constant temperatures of the nanoscale thin layers, the initial thermal non-equilibrium between the neighboring layers is established under the vacuum condition. Models with variable gap distances with an interval of lattice constant increment of the simulated materials are set up and statistical comparisons of temperature evolution curves are made. The equilibration time from non-equilibrium state to thermal equilibrium state of different silicon or/and germanium layers combinations are calculated. The results show significant distinctions of heat transfer under different materials and temperatures combinations. Further discussions on the equilibrium time are made to explain the simulation results. As the first work of the atomic scale energy transport spanning from heat conduction to thermal radiation, the simulation results highlight the promising application of the first-principles molecular dynamics in thermal engineering.

Ab initio determination of effective electron–phonon coupling factor in copper

Abstract The electron temperature T e dependent electron density of states g ( e ) , Fermi–Dirac distribution f ( e ) , and electron–phonon spectral function α 2 F ( Ω ) are computed as prerequisites before achieving effective electron–phonon coupling factor G e – ph . The obtained G e – ph is implemented into a molecular dynamics (MD) and two-temperature model (TTM) coupled simulation of femtosecond laser heating. By monitoring temperature evolutions of electron and lattice subsystems, the result utilizing G e – ph from ab initio calculation shows a faster decrease of T e and increase of T l than those using G e – ph from phenomenological treatment. The approach of calculating G e – ph and its implementation into MD–TTM simulation is applicable to other metals.

Electron-Phonon Coupled Heat Transfer and Thermal Response Induced by Femtosecond Laser Heating of Gold

Ab initio simulation is one of the most effective theoretical tools to study the electrons evolved heat transfer process. Here, we report the use of finite-temperature density functional theory (DFT) to investigate the electron thermal excitation, electron–phonon coupled heat transfer, and the corresponding thermal response induced by energy deposition of femtosecond laser pulse in gold. The calculated results for cases with different scales of electron excitations demonstrate significant electron temperature dependence of electron heat capacity and electron–phonon coupling factor. Bond hardening of laserirradiated gold and structural variation from solid to liquid are observed. The obtained results shed light upon the ultrafast microscopic processes of thermal energy transport from electron subsystem to lattice subsystem and serve for an improved interpretation of femtosecond laser–metal interaction. [DOI: 10.1115/1.4035248]

Effects of Beam Size and Pulse Duration on the Laser Drilling Process

A two-dimensional axisymmetric transient laser drilling model is used to analyze the effects of laser beam diameter and laser pulse duration on the laser drilling process. The model includes conduction and convection heat transfer, melting, solidification and vaporization, as well as material removal resulting from the vaporization and melt ejection. The validated model is applied to study the effects of laser beam size and pulse duration on the geometry of the drilled hole. It is found that the ablation effect decrease with the increasing beam diameter due to the effect of increased vaporization rate, and deeper hole is observed for the larger pulse width due to the higher thermal ablation efficiency.

Multiscale Investigation of Femtosecond Laser Pulses Processing Aluminum in Burst Mode

ABSTRACT Megahertz is the highest femtosecond laser repetition rate that the state-of-the art technology can achieve. In this article, a single femtosecond laser pulse is burst into multiple femtosecond laser pulses to process aluminum. The temporal gap between two consecutive burst pulses is 2 picoseconds, which is much shorter than the temporal gap between two consecutive pulses at the repetition rate of megahertz. By taking the thermophysical scenarios of femtosecond laser induced of electron thermalization, electron heat conduction, electron–phonon-coupled heat transfer and atomic motion into account, a multiscale framework integrating ab initio quantum mechanical calculation, molecular dynamics and two-temperature model are constructed. The effect of femtosecond laser pulse number on the incubation phenomenon is studied. Comparing with the single pulse-processing aluminum film, the femtosecond laser in burst mode leads to smaller thermal stress, which is favorable to reduce the thermal mechanical damage of the material beneath the laser-irradiated surface. Appreciable differences among the simulation results by using electron thermophysical parameters from ab initio quantum mechanical calculation and those from experimental measurement, empirical estimation and calculation are found, indicating the essentials to precisely model the electron thermal response subject to femtosecond laser excitation.

An Ab Initio Molecular Dynamics Simulation of Femtosecond Laser Processing of Germanium

An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the nanostructure change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by imposing the Nose Hoover thermostat to the electron subsystem lasting for ∼100 fs and continuing with microcanonical ensemble simulation of ∼200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find the irradiated germanium distinguishes from the usual germanium crystal by analyzing their melting and dynamic properties.Copyright