CBET Award Achievements
Notable Accomplishments from CBET Awards

Computer Modeling of Short Pulse Laser Interaction with Metals

Leonid Zhigilei    University of Virginia Main Campus

Background:  Short (pico- and femtosecond) pulse laser irradiation has the ability to bring material into a highly non-equilibrium state and, combined with new optical, x-ray and electron diffraction time-resolved probe techniques, provides a unique opportunity to study the transient atomic dynamics under extreme conditions that can hardly be achieved by any other means.  Atomistic simulations have a potential for serving as a bridge between the experimental observations and structural changes in the irradiated material.  In this project we develop a computational model capable of a realistic description of short pulse laser interactions with metal targets.  The model is applied for investigation of the atomic-level mechanisms of laser melting, plastic deformation, photomechanical damage, and ablation.  The results have implications for a range of current and emerging laser processing and micro/nano-fabrication applications.

Results:  Below are two scientific contributions made in 2007 with the support of this grant.

(1Establishing the kinetic limit of heterogeneous melting in metals.  Based on large-scale atomic-level simulations, the velocity of the melting front is found to be strongly affected by the local drop of the lattice temperature.  This local temperature drop is defined by the kinetic balance between the transfer of thermal energy to the latent heat of melting, the electron heat conduction from the overheated solid, and the electron-phonon coupling.  A surprising and practically important result is that the maximum velocity of the melting front is below 3% of the room temperature speed of sound, more than an order of magnitude lower than typically assumed in interpretation of experimental results on melting under conditions of fast heating.  The results of the simulations have important implications for the analysis of the relative contributions of the heterogeneous and homogeneous melting mechanisms to the kinetics of melting process at strong superheating.  The results are reported in [Physical Review Letters 98, 195701, 2007].

(2Investigation of the electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium.  Computational analysis based on first-principles calculations of the electron density of states predicts large deviations (up to an order of magnitude) from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling.  These thermophysical properties are found to be very sensitive to details of the electronic structure of the material.  The strength of the electron-phonon coupling can either increase (Al, Au, Ag, Cu, W), decrease (Ni, Pt), or exhibit non-monotonic changes (Ti) with increasing electron temperature.  The electron heat capacity can exhibit either positive (Au, Ag, Cu, W) or negative (Ni, Pt) deviations from the linear temperature dependences.  The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in energetic ion bombardment and femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with ion bombardment or short pulse laser irradiation of metal targets.  The results are reported in [Physical Review B 77, in press, 2008].

Leonid Zhigilei Image 1
Figure 1.  Shape of the melting front (left) and distributions of the lattice and electron temperatures (right) in a simulation of an overheated Ni crystal with a melting front propagating from a free surface.  Constant temperature of 1.19Tm is maintained at the depth of 100 nm under the surface.  [Physical Review Letters 98, 195701, 2007]

Leonid Zhigilei Image 2
Figure 2.  Electron-phonon coupling factor of Cu, Ni, and Ti as a function of the electron temperature. Dashed lines show the values of the electron-phonon coupling "constants" based on experimental measurements.  The ranges of the electron temperature variation in the experiments are shown by bold segments.  Data is from [Physical Review B 77, in press, 2008] and is also accessible in tabulated form from this University of Virginia Faculty Website.

Credit for both images:  Leonid V. Zhigilei, Zhibin Lin, and Dmitriy S. Ivanov, University of Virginia

Primary Strategic Outcome Goal:  (1) Discovery
  - Disciplinary/Interdisciplinary Research
  - International Collaborative Research
  - CAREER: Faculty Early Career Program

Secondary Strategic Outcome Goal:  (2) Learning
  - Teacher Education and In-service professional Development
  - Undergraduate Education and Undergraduate Student Research
  - Graduate Education and Graduate Student Research
  - International Research Experiences for Undergraduate and Graduate Students

This project addresses the strategic outcome goals, as described in the NSF Strategic Plan 2006-2011, of:

These research results are notable because they:
      (a) establish the limiting velocity of the melting front propagation in heterogeneous melting and
      (b) reveal a clear connection between the electronic structure and the temperature dependence of the electron heat capacity and the strength of the electron-phonon coupling in metals.

(1) Discovery:  The results obtained with the support of this CAREER grant advance our understanding of the fundamental processes involved in laser-materials interaction, help in interpretation of experimental data on the kinetics of laser melting, damage and ablation, and have implications for practical applications based on short pulse laser processing and micromachining.  International collaborations have been established with research groups from France, Russia, Germany, and Canada.

(2) Learning:  This CAREER grant enabled the PI to establish an interdisciplinary research group that brings together graduate and undergraduate students from Materials Science, Engineering Physics, Physics, Computer Science, Mechanical Engineering, and Biomedical Engineering Programs.  Students participate in national and international conferences, are involved in collaborative research projects, and connect to colleagues from different institutions and countries.  The international collaborations include both short-term and long-term visits of graduate and undergraduate students to our international partners.  The PI served as a Chair of the 5th International Conference on Photo-Excited Processes and Applications (ICPEPA), which took place in Charlottesville, Virginia.  The conference attracted more than 130 researchers from 20 countries and exposed US students and young researchers to international opportunities.  Training opportunities for undergraduate students and a high-school teacher have been facilitated by the support from Research Experience for Undergraduates (REU) and Research Experience for Teachers (RET) supplements to this grant.

This research is transformative. The results are addressing fundamental materials properties and are transformative for the field of theoretical analysis of materials behavior far from equilibrium.  The quantitative changes introduced to the existing theoretical treatment of material behavior under non-equilibrium conditions (overheating or electronically excited states) by the computational findings can exceed an order of magnitude, leading to a major shift in the theoretical interpretation of experimental observations.

This project represents Broadening Participation.  Two out of four graduate students and two out of four undergraduate students supported by this grant belong to the groups underrepresented in Science and Engineering.

Program Officer:
  Patrick Phelan
CBET Program Director, Thermal Transport Processes
NSF Award Number:   0348503
Award Title:   CAREER: Computer Modeling of Short Pulse Laser Interaction with Metals
PI Name:   Leonid Zhigilei
Institution Name:   University of Virginia Main Campus
Program Elements:   1406, 7641, 7316
NSF Investments:   None Applicable
CBET Nugget:   FY 2008

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This Nugget was Updated on 18 December 2008.