Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H23: Focus Session: Probing and Modifying Materials with Lasers I |
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Sponsoring Units: DMP Chair: Craig Arnold, Princeton University Room: Morial Convention Center 215 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H23.00001: Vibrational energy on surfaces: Ultrafast flash-thermal conductance of molecular monolayers Invited Speaker: Vibrational energy flow through molecules remains a perennial problem in chemical physics. Usually vibrational energy dynamics are viewed through the lens of time-dependent level populations. This is natural because lasers naturally pump and probe vibrational transitions, but it is also useful to think of vibrational energy as being conducted from one location in a molecule to another. We have developed a new technique where energy is driven into a specific part of molecules adsorbed on a metal surface, and ultrafast nonlinear coherent vibrational spectroscopy is used to watch the energy arrive at another part. This technique is the analog of a flash thermal conductance apparatus, except it probes energy flow with angstrom spatial and femtosecond temporal resolution. Specific examples to be presented include energy flow along alkane chains, and energy flow into substituted benzenes. Ref: Z. Wang, J. A. Carter, A. Lagutchev, Y. K. Koh, N.-H. Seong, D. G. Cahill, and D. D. Dlott, Ultrafast flash thermal conductance of molecular chains, Science 317, 787-790 (2007). \newline \newline This material is based upon work supported by the National Science Foundation under award DMR 0504038 and the Air Force Office of Scientific Research under award FA9550-06-1-0235. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H23.00002: Mechanism of Resonant Infrared Laser Ablation of Polystyrene. Stephen Johnson, Richard Haglund, Daniel Bubb, Kannatessen Appavoo Although the ablation of intact polymers by resonant infrared (IR) laser irradiation has been demonstrated, the mechanism has remained mysterious. This is partly because the IR excitation of complex polymer materials is poorly understood, and partly because most of the experiments have been conducted with a tunable infrared free-electron laser (FEL) with an unusual micropulse-macropulse temporal structure. We have fully characterized the resonant and non-resonant IR ablation of polystyrene (PS) at several IR wavelengths. The energy input at each wavelength was ascertained by convoluting the temperature-corrected lineshape functions of individual C-H and C-C bonds with the spectral profile of the picosecond FEL micropulses. Data from ablation rate, ablation depth, time-resolved photoacoustic and photothermal measurements and nanosecond pulsed-laser shadowgraphy were fed into a simple finite-element model of energy deposition and relaxation. The data and model are consistent with a steady-state ablation mechanism, modified by plume shielding late in the microseconds-long FEL macropulse. Thus the mechanism of the resonant IR laser ablation process is apparently connected primarily with the bond-selective absorption leading to relatively shallow absorption depths and a high local density of vibrational excitation. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H23.00003: Resonant Infrared Matrix Assisted Pulsed Laser Deposition of Polymers: Improving the Morphology of As-Deposited Films Daniel Bubb, Michael Papantonakis, Brian Collins, Elijah Brookes, Joshua Wood, Ullas Gurudas Resonant infrared matrix assisted pulsed laser deposition has been used to deposit thin films of PMMA, a widely used industrial polymer. This technique is similar to conventional pulsed laser deposition, except that the polymer to be deposited is dissolved in a solvent and the solution is frozen before ablation in a vacuum chamber. The laser wavelength is absorbed by a vibrational band in the frozen matrix. The polymer lands on the substrate to form a film, while the solvent is pumped away. Our preliminary results show that the surface roughness of the as-deposited films depends strongly on the differential solubility radius, as defined by Hansen solubility parameters of the solvent and the solubility radius of the polymer. Our results will be compared with computational and experimental studies of the same polymer using a KrF (248 nm) laser. The ejection mechanism will be discussed as well as the implications of these results for the deposition of smooth high quality films. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H23.00004: Laser Ablation Electrospray Ionization: A Molecular Probe for Biological Tissues Peter Nemes, Alexis A. Barton, Yue Li, Akos Vertes Interaction of light and matter has long served as the basis of probing and modifying physical and chemical properties of materials. Recent biomedical applications focus on the mid-infrared (mid-IR) region to couple the laser energy into samples through absorption by the native water. For example, mass spectrometry (MS), relying on atmospheric pressure mid-IR matrix-assisted laser desorption ionization, takes advantage of the small amount of ions in the laser plume. In mid-IR laser ablation, owing to the recoil pressure buildup in the sample, most of the material is expelled in the form of neutral molecules, clusters, and particulates. To enhance ion production, we intercept this plume with a cloud of charged droplets to post-ionize them for MS. As a result, laser ablation electrospray ionization (LAESI) can directly probe the molecular makeup of water rich targets with superior ion yield and dramatically extended mass range (up to 66,500 amu). LAESI also enables two and three dimensional imaging of live tissues. Fast imaging of the plume-plume interaction reveals the mechanistic aspects of LAESI. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H23.00005: Deposition of functional nanoparticle thin films by resonant infrared laser ablation. Richard Haglund, Stephen Johnson, Hee K. Park, Kannatessen Appavoo We have deposited thin films containing functional nanoparticles, using tunable infrared light from a picosecond free-electron laser (FEL). Thin films of the green light-emitting molecule Alq$_{3}$ were first deposited by resonant infrared laser ablation at 6.68~$\mu $m, targeting the C=C ring mode of the Alq$_{3}$. TiO$_{2}$ nanoparticles 50-100~nm diameter were then suspended in a water matrix, frozen, and transferred by resonant infrared laser ablation at 2.94~$\mu $m through a shadow mask onto the Alq$_{3}$ film. Photoluminescence was substantially enhanced in the regions of the film covered by the TiO$_{2}$ nanoparticles. In a second experiment, gold nanoparticles with diameters in the range of 50-100~nm were suspended in the conducting polymer and anti-static coating material PEDOT:PSS, which was diluted by mixing with N-methyl pyrrolidinone (NMP). The gold nanoparticle concentration was 8-10{\%} by weight. The mixture was frozen and then ablated by tuning the FEL to 3.47~$\mu $m, the C-H stretch mode of NMP. Optical spectroscopy of the thin film deposited by resonant infrared laser ablation exhibited the surface-plasmon resonance characteristic of the Au nanoparticles. These experiments illustrate the versatility of matrix-assisted resonant infrared laser ablation as a technique for depositing thin films containing functionalized nanoparticles. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H23.00006: Photochemical ablation of Polytetrafluoroethylene (PTFE) under 157-nm irradiation Sharon R. John, Steven C. Langford, J. Thomas Dickinson We report time- and mass-resolved measurements on neutral molecular particles emitted from polytetrafluoroethylene during exposure to 157-nm laser radiation at fluences where relatively rapid etching is observed. By comparing the time-of-flight signals over a range of masses, we conclude that (CF$_{2})_{N}$ fragments for N=1-6 are emitted directly from the surface in substantial quantities. In contrast, the monomer (N=2) is the principal product during irradiation at 248 nm, due to a thermal decomposition mechanism. The time-of-flight signals of all the (CF$_{2})_{N}$ fragments show fast components with kinetic energies of $\sim $0.6 eV, indicating a non-thermal mechanism. These high kinetic energies are consistent with photochemical scission of the polymer backbone, where a part of the excitation energy is delivered to the fragment as kinetic energy. Although clean etching is observed under these conditions, the great majority of the mass removed appears as much larger fragments with a size distribution of 10 nm to 1$\mu $m. The time-of-flight signals also show a slow component. We present a collisional model to explain the slowing down of neutrals molecules created by photochemical scission. Intense electron, positive and negative ion emissions are also observed. Their formation and emission mechanism will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H23.00007: Rational Design of Two-Photon Absorbing Photochromic Materials for Optical Switching and Data Storage I.A. Mikhailov, K.D. Belfield, A.E. Masunov Diarylethenes are able to undergo light-induced transition from the open to closed ring isomer (photocyclization) accompanied by the change in optical properties (photochromism). This ability holds a great promise for photonic applications, including optical data storage and ultrafast optical switching. Photocyclization initiated by absorption of two photons could drastically increase the density of these devices. However, attachment of fluorene substituent to diarylethene to increase two-photon absorbing cross-section led to the loss of photochromic activity. Analysis of the Kohn-Sham orbitals reveals that the relaxation of the lowest excited state of diarylethene fragment leads to photocyclization, while the occupied level of the chromophore substituent generates an excited state below the photoreactive one. To design the molecular switch active in two-photon regime we suggest stabilizing the highest occupied orbital, which can be accomplished by fluorination of the chromophore. We applied time dependent Density Functional Theory to predict potential energy surfaces of excited states and two-photon absorbing profiles. The obtained results are in agreement with the qualitative orbital description. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H23.00008: Toward molecular switches and biochemical detectors employing adaptive femtosecond-scale laser pulses Roland Allen, Petra Sauer The following topics will be discussed: (1) Photoisomerization of azobenzene, with nuclear motion allowing extra electronic transitions for pulse durations > about 50 fs. (2) Photoinduced ring-opening in a model dithienylethene. (3) Response of dipicolinic acid to femtosecond-scale laser pulses, including excited states and nuclear motion. Although real applications (such as molecular switches and biochemical detectors) will involve adaptive techniques -- with femtosecond-scale laser pulses whose durations, photon energies, fluences, shapes, etc. are tailored for specific applications -- as well as larger systems, one needs an understanding of the rich interplay of electronic and nuclear dynamics to guide more empirical approaches. This understanding can be obtained through detailed computational studies of the kind reported here. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H23.00009: Ultrafast carrier-phonon dynamics under intense optical excitation of GaAs Amlan Basak, M. Hase, M. Kitajima, Hrvoje Petek We report the response of n-doped GaAs (n$_{d}$=2x10$^{18}$ cm$^{-3})$ when excited to an e-h pair density n$_{exc}\sim $10$^{19}$-10$^{20}$ cm$^{-3}$ with a 10 fs laser pulse centered at 400 nm. The experiment is performed in reflective electro optic sampling geometry. Coherent LO phonon oscillation is excited through both ultrafast screening of the depletion field and the deformation potential scattering. The time domain signal contains near-instantaneous transient electronic response as well as coupled plasmon-phonon oscillation. The amplitude of reflectivity is sub-linear possibly indicating saturation of the screening effect with excited carrier density. Fourier Transform analysis shows the bare LO phonon and the lower branch of the LO phonon-plasmon coupled modes (L-). With increasing photocarrier density, the LO phonon response is essentially unaffected, while the L- peak red shifts to the TO phonon limit. Time windowed FT analysis reveals complex carrier density dependent spectral evolution. The coupled carrier-phonon dynamics are discussed in the context of deformation potential scattering and high field transport. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H23.00010: Effect of Hot Electron Pressure in Ultrafast Laser Interaction with Metals Zhibin Lin, Leonid Zhigilei Ultrafast laser irradiation can transiently bring a metal into a highly nonequilibrium state in which the electron temperature can reach thousands of Kelvin while the lattice remains cold. Under these conditions the thermal pressure from the hot conduction electrons can play an important role in defining the initial relaxation dynamics of the irradiated target. In this work, a description of the hot electron pressure due to the presence of the excited electrons is incorporated into a continuum-atomic computational model combining the molecular dynamic method with the two temperature model. Computer simulations employing this approach are performed for Al, Au, and Ni metal films and bulk targets. The effect of the hot electron pressure on the generations of acoustic phonons in the laser-irradiated metal film will be discussed and compared with existing experimental data. The relative contributions of the hot electron pressure and thermoelastic stresses due to the lattice heating to the dynamics of the irradiated target and spallation/ablation process are discussed based on the results of the computer simulations. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H23.00011: Coherent LO phonon self-energy renormalization under high photoexcited carrier densities in Si Anca-Monia Constantinescu, Muneaki Hase, Masahiro Kitajima, Hrvoje Petek The study of hot carrier-phonon interaction dynamics is motivated by their influence on optical and electrical properties of semiconductors. Following high-density (10$^{19}$--10$^{20}$ carriers/cm$^{3})$ photoexcitation of Si(001) with 10 fs duration 400 nm laser pulses, the complex self-energy (i.e. frequency and decay rate) of coherent LO phonon (\textbf{\textit{k}}$\approx $0) renormalize due to deformation potential interaction with the photogenerated non-equilibrium plasma. We evaluate the time dependent LO phonon frequency and dephasing time by analyzing the transient electro-optic reflectivity of variously doped Si(100). We measure the coherent LO phonon mode oscillations in the transient reflectivity over a delay time of 6 ps between pump and probe pulses. Varying the pump power from 50 to 5 mW, we observe that the electronic softening of the lattice (i.e. LO phonon frequency change) and the quasi-exponential dephasing time of the phonon depend on the initial photoexcited carrier density. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H23.00012: Mechanistic study of negative ion emission from single crystal alkali halide surfaces due to pulsed UV laser irradiation J. T. Dickinson, Kenichi Kimura, S. C. Langford We report on extensive measurements of negative alkali ion emission from four alkali halides during exposure to 248-nm pulsed excimer laser radiation at fluences well below the threshold for optical breakdown. A detailed study on the emissions from single crystal KCl shows no evidence for negative halide ions, suggesting that negative alkali ions are not formed by electron attachment to thermally emitted neutral particles. Furthermore, the KCl surface charges positively during laser irradiation (due to electron emission from defects), which would hinder direct emission of negative ions from the surface. We present strong evidence for a negative ion formation mechanism involving double electron attachment to singly charged positive alkali ions. Extension of these measurements to single crystal KBr and to other dielectric materials confirm this mechanism. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H23.00013: Observation of Saturable and Reverse Saturable Absoption in Silver Nanodots Ullas Gurudas, Daniel Bubb, Thomas Lippert, Sebastian Heroith Saturable absorption (SA) and reverse saturable absorption (RSA) were observed in Ag nanodots prepared by pulsed laser deposition. The Real [Re $\chi ^{(3)}$] and Imaginary [Im $\chi ^{(3)}$] part of the third order nonlinearity of these films are measured as and respectively, using Z-scan technique. The decrease of absorption under strong optical illumination results in a negative Im $\chi ^{(3)}$ at the photon energy used. At higher input irradiance RSA becomes dominant. The transformation from SA to RSA suggests that another nonlinear process takes place and become dominant. To evaluate the recovery time of these nonlinear processes and get an idea about the underlying mechanism, we conducted a degenerate pump-probe experiment with 25 psec, 532 nm laser pulses. The increased $\chi ^{(3)}$ and fast response time of the Ag nanoparticles can be used for optical pulse compressor, optical switching, laser pulse narrowing and protecting optical sensors from intense laser pulses. [Preview Abstract] |
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