Martin E. Garcia

Theory for the Ultrafast Ablation of Graphite Films

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies ( >3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold ( >2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

Laser-induced phonon-phonon interactions in bismuth

Intense femtosecond laser pulses can induce a nonequilibrium state which leads to sudden and dramatic changes in the potential energy surfaces of different solids [1]. This pro perty can be used to excite and manipulate coherent lattice vibrations, as has been recently shown by several experiments [2, 3, 4, 5] and simulations [6]. In the last years, a variety of experiments on laser excitation of coherent phonons has been performed on bismuth [7, 8, 9, 10], which is a particularly interesting solid since its ground-state structure e xhibits a Peierls distortion. From the different studies done so far , a number of fundamental aspects still remain unexplained, like the detection of higher harmonics [9] and the appearance of modes that are forbidden by symmetry in isotropic reflectivi ty measurements [8]. In this letter we perform ab initio calculations which show, for the first time, the existence of a coupling between laserinduced phonon modes of different symmetry. In addition, we demonstrate that the large amplitude atomic vibrations excited by femtosecond laser pulses are affected by the anharmonic part of the potential energy surface, which creates overton es. The structure of Bi can be derived from a simple cubic atomic packing in two steps. First a simple cubic lattice is deformed by elongating it along one of the body diagonals, which is indicated by a thin line in Fig. 1. A Peierls instabil ity

Time-dependent energy absorption changes during ultrafast lattice deformation

The ultrafast time dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We use a microscopic theory to describe laser induced structural changes and their influence on the electronic properties. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy.

Calculation of the partition function using quantum genetic algorithms

We present a new method based on genetic algorithms which permits to determine efficiently the partition function and the excitation spectrum of few-body quantum systems. In our approach, we use a variational formulation for the partition function Z of the system as a functional of its eigenfunctions. Z is obtained by applying the procedure of survival of the fittest, starting from initial random population. During the evolution the best representative converges to a set of eigenfunctions for a given Hamiltonian, while the partition function attains its global extremum (maximum) for a given temperature. We calculate the spectrum and the partition function in the case of few interacting particles in one-dimensional infinite potential well. We investigate formation of the Wigner crystal and study its melting induced by termal and quantum fluctuations.

Theoretical description of the ultrafast ablation of diamond and graphite: dependence of thresholds on pulse duration

A theoretical description of the ultrafast ablation of diamond and graphite is presented. Laser induced lattice deformations and melting are described with the help of molecular dynamics simulations on time dependent potential energy surfaces derived from a microscopic electronic Hamiltonian. Thermalization effects are explicitly taken into account. We calculate the ablation thresholds as a function of the pulse duration for femtosecond pulses. For both materials we obtain smoothly increasing thresholds for increasing duration. The damage and ablation mechanisms are discussed.

Modelling ultrafast laser ablation

This review is devoted to the study of ultrafast laser ablation of solids and liquids. The ablation of condensed matter under exposure to subpicosecond laser pulses has a number of peculiar properties which distinguish this process from ablation induced by nanosecond and longer laser pulses. The process of ultrafast ablation includes light absorption by electrons in the skin layer, energy transfer from the skin layer to target interior by nonlinear electronic heat conduction, relaxation of the electron and ion temperatures, ultrafast melting, hydrodynamic expansion of heated matter accompanied by the formation of metastable states and subsequent formation of breaks in condensed matter. In case of ultrashort laser excitation, these processes are temporally separated and can thus be studied separately. As for energy absorption, we consider peculiarities of the case of metal irradiation in contrast to dielectrics and semiconductors. We discuss the energy dissipation processes of electronic thermal wave and lattice heating. Different types of phase transitions after ultrashort laser pulse irradiation as melting, vaporization or transitions to warm dense matter are discussed. Also nonthermal phase transitions, directly caused by the electronic excitation before considerable lattice heating, are considered. The final material removal occurs from the physical point of view as expansion of heated matter; here we discuss approaches of hydrodynamics, as well as molecular dynamic simulations directly following the atomic movements. Hybrid approaches tracing the dynamics of excited electrons, energy dissipation and structural dynamics in a combined simulation are reviewed as well.

Electric Field-Driven Disruption of a Native β-Sheet Protein Conformation and Generation of a Helix-Structure

We demonstrate that an external constant electric field is able to modify the secondary structure of a protein and induce a transition from a beta-sheet into a helix-like conformation. This dramatic change is driven by a global rearrangement of the dipole moments at the amide planes. We also predict electric-field-induced modifications of the intermediate states of the protein.

Fractional Diffusion in Silicon

Microscopic processes leading to ultrafast laser-induced melting of silicon are investigated by large-scale ab initio molecular dynamics simulations. Before becoming a liquid, the atoms are shown to be fractionally diffusive, which is a property that has so far been observed in crowded fluids consisting of large molecules. Here, it is found to occur in an elemental semiconductor.

All-optical control and visualization of ultrafast two-dimensional atomic motions in a single crystal of bismuth

In a bulk solid, optical control of atomic motion provides a better understanding of its physical properties and functionalities. Such studies would benefit from active control and visualization of atomic motions in arbitrary directions, yet, so far, mostly only one-dimensional control has been shown. Here we demonstrate a novel method to optically control and visualize two-dimensional atomic motions in a bulk solid. We use a femtosecond laser pulse to coherently superpose two orthogonal atomic motions in crystalline bismuth. The relative amplitudes of those two motions are manipulated by modulating the intensity profile of the laser pulse, and these controlled motions are quantitatively visualized by density functional theory calculations. Our control-visualization scheme is based on the simple, robust and universal concept that in any physical system, two-dimensional particle motion is decomposed into two orthogonal one-dimensional motions, and thus it is applicable to a variety of condensed matter systems.

Nonthermal fragmentation of C60

Abstract A theoretical study of the subpicosecond fragmentation of C60 clusters in response to ultrafast laser pulses is presented. We simulate the laser excitation and the consequent nonequilibrium relaxation dynamics of the electronic and nuclear degrees of freedom. The first stages of the nonequilibrium dynamics are dominated by a breathing mode followed by the cold ejection of single C atoms, in contrast to the dimer emission which characterizes the thermal relaxation. We also determine the nonequilibrium damage thresholds as a function of the pulse duration.