Showing papers by "Keith A. Nelson published in 1992"
TL;DR: A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals and the results illustrate the advantages of ISRS over conventional Raman Spectroscopy of low-frequency, heavily damped soft modes.
Abstract: The microscopic pathway along which ions or molecules in a crystal move during structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a 'soft' phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO3 and BaTiO3 are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.
124 citations
TL;DR: In this paper, a modification of the optical technique of impulsive stimulated thermal scattering that allows real-time data acquisition is introduced, which is used to characterize the pseudo-Rayleigh acoustic modes of thin polyimide films attached to silicon substrates and its potential use as a means for extracting the elastic constants of thin film coatings is demonstrated.
Abstract: A modification of the optical technique of impulsive stimulated thermal scattering that allows real‐time data acquisition is introduced. The method is used to characterize the pseudo‐Rayleigh acoustic modes of thin (∼1 μm) polyimide films attached to silicon substrates and its potential use as a means for extracting the elastic constants of thin‐film coatings is demonstrated.
106 citations
TL;DR: In this paper, the effects of polariton dispersion and propagation were treated and the results provided guidance of accurate analysis of ISRS data from polar phonons from optic phonons.
Abstract: Femtosecond time-resolved impulsive stimulated Raman scattering (ISRS) experiments on optic phonon–polariton modes are discussd. The effects of polariton dispersion and propagation are treated. The results provide guidance of accurate analysis of ISRS data from polar phonons.
96 citations
TL;DR: In this paper, a method for real-time optical characterization of thin-film acoustic modes is demonstrated, where pseudo-Rayleigh acoustic waves of thin polyimide films are optically excited and monitored in real time as they propagate.
Abstract: A method for real‐time optical characterization of thin‐film acoustic modes is demonstrated. Pseudo‐Rayleigh acoustic waves of thin (∼1 μm) polyimide films attached to silicon substrates are optically excited and monitored in real time as they propagate. The results can be used to determine mechanical properties (elastic moduli) and to guide thin‐film fabrication and curing procedures.
48 citations
TL;DR: In this paper, a diamond anvil cell was used to measure the librational motion of liquid carbon disulfide at a series of pressures > 1 kbar in the first picosecond.
Abstract: This paper discusses how librational motion was measured for liquid carbon disulfide at a series of pressures >1 kbar in a diamond anvil cell during the first picosecond. However, computer simulations never showed clear evidence of librational character. The implications of this are discussed in terms of interaction-induced contributions to the molecular signal and collective reorientation. 44 refs., 8 figs., 3 tabs.
37 citations
TL;DR: In this article, the authors used the acoustic information from Impulsive Stimulated Light Scattering (ISS) experiments on aqueous LiCl solutions to characterize the structural relaxation dynamics at temperatures above, near and below the glass transition temperature Tg.
Abstract: Impulsive stimulated light scattering (ISS) experiments have been conducted on aqueous LiCl solutions to characterize the structural relaxation dynamics at temperatures above, near and below the glass transition temperature Tg. The data provide acoustic velocities, attenuation and thermal diffusion rates over wide ranges of temperature and wave vector. At temperatures in the glass transition region, the dynamics of thermal expansion and subsequent contraction (not due to thermal diffusion) are observed as well. The acoustic information from ISS as well as from earlier Brillouin scattering results is used to determine the frequency‐dependent elastic modulus in the MHz–GHz range. Analysis in terms of an empirical (stretched exponential) relaxation function is carried out and shown to be inadequate. The main analysis is inspired by predictions of mode‐coupling theory concerning susceptibility spectra. Simple scaling for the α‐peak and identical power law dependencies on frequency for both electrical and mechanical moduli, as predicted by theory, were confirmed.
27 citations
TL;DR: In this article, structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time-resolved four-wave mixing technique.
Abstract: Structural and orientational relaxation processes in the glass‐forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time‐resolved four‐wave mixing technique. The α structural relaxation processes which can couple to the longitudinal and shear acoustic modes are analyzed phenomenologically in terms of a distribution of relaxation times fL(τL,T) or fS(τS,T), respectively, in the temperature range of 270–240 K. The two distributions appear to be identical, and undergo marked broadening as the sample is cooled. They are best described by a stretched exponential relaxation function whose exponent decreases with temperature. The molecular orientational relaxation times τor follow an Arrhenius temperature dependence and are longer than the average relaxation times 〈τL〉 and 〈τS〉 at high temperature, but converge as the temperature is lowered. We conclude that the observed orientational relaxation is decoupled from the α structural relaxation process and is a manifestation of ...
24 citations
TL;DR: Analysis of mechanical and electrical modulus spectra confirms some results of mode-coupling theory: simple scaling for a peaks and property-independent power laws in time-domain light scattering experiments on ps-μs time scales.
Abstract: Time-domain light scattering experiments on ps-μs time scales have been conducted on a concentrated aqueous LiCl solution. Analysis of mechanical and electrical modulus spectra confirms some results of mode-coupling theory: simple scaling for a peaks and property-independent power laws. This is made without any assumptions about the data which are not derived from theory. A new characteristic time scale, reflecting the short- and intermediate-range ordering dynamics, is also observed
15 citations
TL;DR: The longitudinal acoustic anomaly along the a axis of the crystal [N(CH 3 ) 4 ] 2 ZnCl 4 has been investigated near the normal-incommensurate (N-IC) phase transition temperature by impulsive stimulated scattering, for acoustic frequencies in the range of 350 MHz to 4 GHz.
Abstract: The longitudinal acoustic anomaly along the \ensuremath{\alpha} axis of the crystal [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{ZnCl}}_{4}$ has been investigated near the normal-incommensurate (N-IC) phase transition temperature by impulsive stimulated scattering, for acoustic frequencies in the range of 350 MHz to 4 GHz. Considerable dispersion is seen over this frequency range. The major contirubiton to the anomaly below the N-IC transition is due to coupling to the amplitude mode, whose wave-vector-independent relaxation time \ensuremath{\tau} diverges as the transition temperature ${\mathit{T}}_{\mathit{i}}$ is approached with the temperature dependence given by \ensuremath{\tau}=${\mathrm{\ensuremath{\tau}}}_{0}$${\mathit{T}}_{\mathit{i}}$/(${\mathit{T}}_{\mathit{i}}$-T), with ${\mathrm{\ensuremath{\tau}}}_{0}$\ensuremath{\approxeq}0.6 ps.We find no evidence for the coupling of the acoustic mode to the phase mode. The critical exponents that describe the order-parameter behavior have values consistent with mean-field theory. Critical-exponent values for other universality classes, particularly for the three-dimensional Ising model are found to be inconsistent with the data. The acoustic anomaly above ${\mathit{T}}_{\mathit{i}}$ can be accounted for by the coupling of the longitudinal acoustic mode to energy-density fluctuations associated with the soft mode. A possible explanation for suppression of critical behavior in the acoustic anomaly near the N-IC transition is given.
5 citations
TL;DR: In this paper, the temperature and wave vector dependence of the soft shear acoustic mode and the soft polarization mode of KD2PO4 were examined in the high-temperature phase through impulsive stimulated Brillouin and Raman scattering.
Abstract: The temperature and wave vector dependence of the soft shear acoustic mode and the soft polarization mode of KD2PO4 are examined in the high-temperature phase through impulsive stimulated Brillouin and Raman scattering. With different scattering wave vectors the regimes ωτ 1 are probed where ω is the acoustic frequency and τ is the polarization relaxation time. The results illustrate how the coupled acoustic and polarization response may be observed in the time domain, and indicate advantages for the time-domain approach in the ωτ τ 1 regime. In this regime the softening of the acoustic mode is followed by hardening (increasing of the frequency) as the transition temperature is approached from above. On femtosecond time scales, it is possible to test critically for any inertial component to the polarization response, which would indicate an overdamped oscillator rather than relaxational description of the polar optic phonon mode.
4 citations
TL;DR: In this paper, a purely optical-based method for the excitation and detection of acoustic and thermal disturbances in thin polyimide films is presented, which can be used to detect film delaminations.
Abstract: We demonstrate a new purely optical based method for the excitation and detection of acoustic and thermal disturbances in thin films. This technique is applied to the determination of the viscoelastic properties of unsupported and silicon supported polyimide thin (~1 micron) films. We show how this technique can be used to detect film delaminations and suggest how it may be used to probe film-substrate adhesion quality.
01 Jan 1992
TL;DR: In this paper, the principal limitation to laser mode-selective chemistry seems to be the rapid intramolecular vibrational redistribution (IVR) associated with anharmonic vibrational couplings at high excitation levels.
Abstract: For many years chemical physicists have sought to achieve mode-selective chemistry through laser irradiation. To date, however, attempts at using high-power laser pulses to control molecular reaction pathways have been unsuccessful. The principal limitation to laser mode-selective chemistry seems to be the rapid intramolecular vibrational redistribution (IVR) associated with anharmonic vibrational couplings at high excitation levels1.
TL;DR: In this paper, the nuclear and electronic responses in highly nonlinear optical glasses were investigated using femtosecond polarization discrimination four wave mixing experiments, and the results showed that the nuclear response was positively correlated with the electronic response.
Abstract: Nuclear and electronic responses in highly nonlinear optical glasses are investigated using femtosecond polarization discrimination four wave mixing experiments.
TL;DR: In this article, structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time-resolved four-wave mixing technique.
Abstract: Structural and orientational relaxation processes in the glass‐forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time‐resolved four‐wave mixing technique. The α structural relaxation processes which can couple to the longitudinal and shear acoustic modes are analyzed phenomenologically in terms of a distribution of relaxation times fL(τL,T) or fS(τS,T), respectively, in the temperature range of 270–240 K. The two distributions appear to be identical, and undergo marked broadening as the sample is cooled. They are best described by a stretched exponential relaxation function whose exponent decreases with temperature. The molecular orientational relaxation times τor follow an Arrhenius temperature dependence and are longer than the average relaxation times 〈τL〉 and 〈τS〉 at high temperature, but converge as the temperature is lowered. We conclude that the observed orientational relaxation is decoupled from the α structural relaxation process and is a manifestation of ...
TL;DR: In this article, the authors used time resolved impulsive stimulated Raman scattering (ISRS) to characterize the lowest frequency A 1 phonon-polariton mode in lithium tantalate, and the anomalously high and wavevector-dependent damping rates observed in terms of coupling of the polariton to a weakly Raman-active relaxational mode and to two heavily damped vibrational modes.
Abstract: Time resolved impulsive stimulated Raman scattering (ISRS) is used to characterize the lowest frequency A 1 phonon-polariton mode in lithium tantalate. The anomalously high and wavevector-dependent damping rates observed are explained in terms of coupling of the polariton to a weakly Raman-active relaxational mode and to two heavily damped vibrational modes. The dynamics of the relaxational mode are explored further through temperature dependent ISRS studies. Femtosecond optical pulse shaping is used for multiple pulse ISRS at temperatures lower than 120K where single pulse excitation led to photorefractive damage.
01 Jan 1992
TL;DR: The microscopic mechanism for the phase transitions between the paraelectric cubic phase and the tetragonal, orthorhombic, and rhombohedral ferroelectric phases remain in doubt.
Abstract: Despite extensive study of the barium titanate (perovskite) family of ferroelectrics, the dynamics and microscopic mechanism for the phase transitions between the paraelectric cubic phase and the tetragonal, orthorhombic, and rhombohedral ferroelectric phases remain in doubt. From a dynamical point of view, the degree to which collective motion of the order parameter, i.e. the polarization, is vibrational (phonon-like) in character versus relaxational (hopping dynamics) is in dispute due to the conflicting results of recent low-frequency Raman spectroscopy studies [1,2]. In terms of microsopic mechanism, relaxational dynamics are indicatative of an order-disorder transition involving collective hopping of ions between different crystalline arrangements. Vibrational dynamics indicate a displacive transition involving motions of ions within a single free energy region whose minimum position shifts with temperature. The vibrational component of the order parameter is often very heavily damped (sometimes overdamped) and so Raman spectral responses are central peaks which merge with relaxational responses. It is often difficult to discern and analyze quantitatively the vibrational and relaxational dynamics.
TL;DR: In this article, an experimental methodology for recording femtosecond time-resolved observations of irreversible change in solids is described, where the trime-dependent evolution must be observed on a single-shot basis since the sample may be permanently altered after each excitation event.
Abstract: An experimental methodology for recording femtosecond time-resolved observations of irreversible change in solids is described. The central problem posed is that the trime-dependent evolution must be observed on a single-shot (i.e. real-time) basis since the sample may be permanently altered after each excitation event. Preliminary demonstrations of real-time femtosecond spectroscopic observations are presented. In addition, one-shot data acquisition techniques open up the possibility of excitation intensities that greatly exceed optical damage threshholds of most samples. Since only one excitation pulse is used, cumulative damage mechanisms may be circumvented. Even if the sample is damaged in a single shot, in some cases the events of interest may be observed before damage occurs. The use of timed sequences of high-intensity excitation pulses to drive large-amplitude, coherent lattice vibrations is discussed. If successful, such large-amplitude lattice vibrations could assist crystalline chemical reactions or structural phase transitions.