Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L25: Focus Session: Probing and Modifying Materials with Lasers II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Craig Arnold, Princeton University Room: 327 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L25.00001: Modeling of Laser Material Interactions Invited Speaker: Irradiation of a substrate by laser light initiates the complex chemical and physical process of ablation where large amounts of material are removed. Ablation has been successfully used in techniques such as nanolithography and LASIK surgery, however a fundamental understanding of the process is necessary in order to further optimize and develop applications. To accurately describe the ablation phenomenon, a model must take into account the multitude of events which occur when a laser irradiates a target including electronic excitation, bond cleavage, desorption of small molecules, ongoing chemical reactions, propagation of stress waves, and bulk ejection of material. A coarse grained molecular dynamics (MD) protocol with an embedded Monte Carlo (MC) scheme has been developed which effectively addresses each of these events during the simulation. Using the simulation technique, thermal and chemical excitation channels are separately studied with a model polymethyl methacrylate system. The effects of the irradiation parameters and reaction pathways on the process dynamics are investigated. The mechanism of ablation for thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case where an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. The study provides insight into the influence of thermal and chemical processes in polymethyl methacrylate and facilitates greater understanding of the complex nature of polymer ablation. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L25.00002: Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation Leonid Zhigilei, Elodie Leveugle There are a number of applications that utilize the ability of laser ablation of a volatile matrix to entrain, eject and, if needed, deposit large macromolecules to a substrate with minimum chemical modification. In particular, the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique is used in fabrication of ultra-thin organic films for optoelectronic, biomedical, and chemical sensor applications. In this presentation we report the results of a computational investigation of the mechanisms of molecular ejection in MAPLE. Coarse-grained molecular dynamics simulations are performed for polymer concentrations up to 6 wt.{\%}. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected as parts of polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. An internal release of matrix vapor in the overheated droplets is shown to be capable of pushing polymer molecules to the outskirts of the droplets, forming ``molecular balloons'' in which polymer-rich surface layers enclose the volatile matrix material. The results of the simulations explain some of the complex morphologies observed in polymer films deposited in MAPLE and conventional polymer ablation/deposition experiments. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L25.00003: Mechanism of Resonant Mid-Infrared Laser Ablation of Polystyrene Richard Haglund, Stephen Johnson, Daniel Bubb We investigated the mechanism of resonant-infrared laser ablation of polymers using polystyrene as a model material. The ablation laser was a picosecond mid-infrared free-electron laser tuned to mid-IR laser wavelengths that are resonant with specific vibrational modes of the polystyrene target. Time-resolved plume imaging combined with etch-depth measurements and finite-element calculations indicate that a blowoff model fits the experimentally measured etch depths and plume images, provided one accounts for moderate shielding of the surface by the ablation plume. The finite-element model includes the temperature-dependent absorption coefficient and specific heat that dramatically change the material properties above the glass-transition temperature. Ablation begins after a thin surface layer of the material is superheated to temperatures exceeding 1000 C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area at the target surface. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L25.00004: The influence of thermal confinement and temperature-dependent absorption on resonant infrared ablation of frozen aqueous and alcohol targets Daniel Bubb, Stephen Johnson, Richard Haglund We investigated the mechanism of matrix-assisted resonant infrared laser ablation in frozen aqueous and methanol solutions of polymer, by performing plume shadowgraphy and ablation yield measurements. A picosecond, tunable free-electron laser was used for ablation at two wavelengths, one (2940 nm) that was resonant with the --OH stretch in both water and methanol, and the other (3450 nm) that is resonant with the --CH stretch in methanol. The plume images showed gross similarities, differing only in the time required for the shockwave to appear and in the velocity of the shock front. Typically, 15-25 \textit{$\mu $}s after the ablation laser pulse arrives the primary material ejection commences and lasts for hundreds of \textit{$\mu $}s. In all three cases, the ablation plume appears to consist entirely of vapor with no droplets or solid particles. The ablation yield is either linear or quadratic in fluence. This dependence can be understood if we consider thermal diffusion in the targets and the temperature dependence of the absorption coefficient. . [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L25.00005: VUV excimer laser-materials interactions with fluorocarbon polymers Tom Dickinson, Sharon George, Steve Langford Particle emission from transparent wide bandgap materials at laser fluences below the threshold for optical breakdown can provide important insight on interactions at the higher fluences employed for surface modification, machining, and laser ablation deposition. We present recent studies of ion and neutral molecule emission from polytetrafluorethylene [(C$_{2}$F$_{4})_{N}$---PTFE---Teflon{\textregistered}] and polyvinylidene fluoride [(CH$_{2}$CF$_{2})_{N}$---PVDF] during nanosecond pulsed 157-nm excimer laser irradiation. The chemical and electrical properties of these materials play important roles in many technologies. In PTFE, the primary mechanism for material removal involves bond scission along the backbone of the polymer. In PVDF, HF emission is accompanied by carbonization of the irradiated region. High-energy positive and negative ions are observed from both materials under 157-nm irradiation. We describe critical measurements that reveal the physics and chemistry underlying these processes. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L25.00006: Depth Profiling of Polymer Composites by Ultrafast Laser Ablation Christopher Young, Clive Clayton, Jon Longtin Past work has shown femtosecond laser ablation to be an athermal process at low fluences in polymer systems. The ablation rate in this low fluence regime is very low, allowing for micro-scale removal of material. We have taken advantage of this fact to perform shallow depth profiling ablation on carbon fiber reinforced polymer (CFRP) composites. Neat composite and resin samples were studied to establish reference ablation profiles. These profiles and the effects of the heterogeneous distribution of carbon fibers were observed through confocal laser profilometry and optical and scanning electron microscopy. Weathered materials that have been subjected to accelerated tests in artificial sunlight or water conditions were ablated to determine the correlation between exposure and change in ablation characteristics. Preliminary Raman and micro-ATR analysis performed before and after ablation shows no chemical changes indicative of thermal effects. The low-volume-ablation property was utilized in an attempt to expose the sizing-matrix interphase for analysis. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L25.00007: Laser Interactions with Vertically-Aligned Carbon Nanotube Arrays David Geohegan, Chris Rouleau, Alex Puretzky, Jeremy Jackson, Norbert Thonnard, Ilia Ivanov, Karren More Femtosecond and nanosecond laser interactions with vertically aligned carbon nanotube arrays (VANTAs) have been studied in vacuum and background gases. As-grown VANTAs were synthesized by chemical vapor deposition onto Fe/Al-coated Si wafers to typical heights of 10-20 microns. The forests of aligned nanotubes grow from catalyst nanoparticles anchored to the substrate and typically have a disordered layer of nanotubes at their top surfaces. Laser interactions in vacuum, inert, and oxidizing atmospheres were compared. The nanotubes were found to form periodic surface structures in response to repeated laser pulses. Patterning of the nanotube arrays with scanning beams was studied. Damage to the nanotubes was studied by Raman spectroscopy and high-resolution TEM. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L25.00008: Temperature-dependent energy-transfer between electrons and phonons in nickel around Curie temperature and its relation to ultra-fast demagnetization Xuan Wang, Shouhua Nie, Junjie Li, Rick Clinite, Jim Cao We report on the use of Ultrafast Electron Diffraction (UED) to study the ultrafast dynamics of nickel induced by fs-laser excitation. Particularly, we have observed a significant increase of electron-phonon coupling time when the pre-set sample base-temperature is reduced across its Curie temperature. This implies a strong quenching of magnetic ordering (ultrafast demagnetization), which serves as an extra energy reservoir other than lattice to relax the electron energy. By modeling the energy transfer among these three systems, we conclude that ultrafast demagnetization happens in a time-scale even shorter than electron-phonon coupling and one temperature for both electron and magnetic ordering, as suggested by several former studies, works very well from energy point of view. Our results also support the former observed ultrafast demagnetization in itinerant ferromagnets, which happens in sub-ps time scale. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L25.00009: Ultrafast Time Resolved Reflection High-Energy Electron Diffraction Study of Laser-Matter Interactions for Silicon Hyuk Park, J.M. Zuo We report a study of silicon surface using ultrafast time-resolved reflection high energy electron diffraction (RHEED) based on the pump-probe approach. The probe beam is an electron pulse generated by a femtosecond laser, accelerated to 30 kV and focused by a magnetic lens. Using this probe, we investigated the pulse laser interaction with silicon by monitoring the electron diffraction pattern recorded in the glancing angle geometry. We observed transient angle-dependent electron beam deflection from silicon surfaces. We show that the electron beam deflection comes the change in surface potential and charge produced by laser-matter interaction. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L25.00010: Ultrafast Photoinduced Structural Dynamics in Graphite Zhibin Lin, Roland Allen In this work, we employ the density-functional-based tight-binding approach of Frauenheim and coworkers in simulations employing semiclassical electron-radiation-ion dynamics (SERID), which treats the coupled dynamics of electrons and ions during the nonadiabatic processes induced by laser irradiation. A series of computer simulations are performed in studies of graphite responding to femtosecond-scale laser pulses of various intensities and durations. It is found that the contraction of the interlayer distance in graphite happens shortly after optical excitation. Transient changes in the interlayer distance are related to changes in the interlayer bonding due to ultrafast changes in the populations of the electronic states. Results of these computational studies are compared with recent pump-probe experimental data on graphite. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L25.00011: Photo-Induced Structural Dynamics of Graphite Studied by Ultrafast Electron Crystallography. Ramani K. Raman, Ryan A. Murdick, Yoshie Murooka, Chong-Yu Ruan The graphite to diamond conversion is believed to involve the rhombohedral phase of graphite as an intermediate state. Using ultrafast electron crystallography, we have observed the formation of transient interlayer \textit{sp}$^{3}$ bonds in graphite beyond a threshold fluence, leading to a non-thermal structural change (Raman, R. K. et al$.$ Phys. Rev. Lett. 101, 077401 (2008)). This transient rebonding towards diamondization is likely driven by a compressive Coulomb stress created by the photoinduced charge separation following the initial E$_{2g}$ phonon excitation that alters the layering symmetry within graphite. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L25.00012: Photo-irradiation effect on charge-ordered states in strongly correlated electron systems Yu Kanamori Photo-induced phenomenon has attracted much attention in the research field of strongly correlated electron systems. Recently, the photo-induced effects have been examined experimentally in a charge-ordered (CO) insulating phase associated with the antiferromagnetic (AF) order in perovskite manganites, and changes in the charge and magnetic properties are observed. It is believed that strong coupling between itinerant electrons and localized spins plays a key role on the phenomena. We investigate photo-induced effects in correlated electron systems where conduction electrons couple with localized spins. In particular, the photo-induced phenomena in a CO insulator associated with AF order are examined. Several transient spectra are calculated numerically in the double exchange model. We find that in the photo-excited states, finite spectral weights in the optical absorption spectra appear inside of the insulating gap, and its intensity increases with increasing time. In the one-particle excitation spectra, the in-gap band appears by the photo-irradiation, and its width becomes broad with time evolution. These electronic-structure changes are correlated with the time evolution of the localized spin correlation. These results indicate that the spin degree of freedom plays an important role on the photo-excited states. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L25.00013: Optical, Mechanical, and Opto-Mechanical Switching of Anchored Dithioazobenzene Bridges Ivan Stich, Robert Turansky, Martin Konopka, Nikos Doltsinis, Dominik Marx The ability of anchored photochromic molecules to perform optically, mechanically, and opto-mechanically driven switching cycles is studied using electronic structure methods. As a model for such nanoscale devices we have simulated molecular switches consisting of single dithioazobenzene bridges between gold tips. Purely optical switching cycles are hindered by mechanical effects due to the tips. However, the possibility to perform both combined opto-mechanical and purely mechanical two- way switching is predicted. The simulations help to elucidate also the role played by mechanical and chemical effects due to anchoring. Possible experimental realization of such devices will also be mentioned. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700