Walter E. Bron
Professor, Physics & Astronomy
School of Physical Sciences
School of Physical Sciences
PH.D., Columbia University
OTH
OTH
University of California, Irvine
2127 FRH
Mail Code: 4575
Irvine, CA 92697
2127 FRH
Mail Code: 4575
Irvine, CA 92697
Research Interests
Interaction of ultrashort duration laser pulses with semiconductors, normal and superconducting metal films, and biomedical materials
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Academic Distinctions
Appointments
Research Abstract
Synopsis of Previous and Current Research Activity
Non equilibrium electronic carriers and phonon generation, transport, and detection forms the major basis of my present research interests. In earlier work, I have investigated the frequency dependence of the scattering of phonons at impurity ions, the frequency dependence of transport of phonons in the bulk and at solid-interfaces, the evolution in frequency, space, and time of a non-equilibrium distribution of phonons in a solid, and the observation of stimulated phonon emission. The work on phonon spectroscopy has lead to a series of review articles on the subject. The research activity is centered on the lifetime of acoustic and optical phonons, and on the effect of solid surfaces on phonon reflection.
During the past few years new methods of phonon spectroscopy have been developed to resolve the spectral, spatial, and temporal evolution of non equilibrium phonon distributions and their interaction with electronic carriers. A major emphasis of the current research activity involves ultrashort duration dynamics of coherent states of phonons and polaritons. These results, and the experimental methods on which they rely, open a new era of experimental and theoretical work on the basic physics of the transport of electronic carriers and vibrational states in condensed matter.
Five major areas of past and current interest of the author are:
1) The spectral, spatial, and temporal spectroscopy of phonon-phonon interactions in solids. The experiments probe in detail the basic crystal anharmonicities and nonlinear effects. experiments yield the lifetime of a phonon of a given energy. The results have direct bearing on phonon-based thermal energy transport in solids and is, accordingly, relevant to the technological problem of energy efficient structures.
2) The spectral and temporal spectroscopy of phonon transport across solid-solid interfaces. These experiments probe the fundamental barrier which hinders the generation and the transport of phonons at solid discontinuities. Experiments along these lines have already shown that, under certain conditions, the previously assumed frequency independence of the interface transport, is not sustained by experimental results. This area also has important technological implications to the thermal transport out of integrated circuits and other microelectronic structures. These miniaturized entities, which have revolutionized electronic devices, are important not only in that they speed up electronic communication but in that they are highly energy-efficient and minimize the usage of scarce material. The stability and packing densities of such circuits are, however, limited by problems of interface thermal transport.
3) The generation and detection of phonon distributions through the interaction of solids with light. These experiments are being carried out in the author's extensive laser facility that of These experiments are designed to yield spectral and temporal information on the basic direct and nonlinear interactions of monochromatic laser light to produce excited electronic carriers, phonons and polaritons. The present methods determine directly the phonon distribution and its temporal evolution and, therefore, form a direct and detailed test of existing theories. Increasing use of optical communication with laser light, plus the increasing use of conversion of solar energy, make this research area also of some importance to current technological problems.
4) In the past few years a new effort has been made to investigate the temporal response of solids to nonlinear excitations in the pico- and subpico-second time domain. So far we have measured the third order nonlinear (electronic) susceptibility of the GaP, ZnTe and ZnSe and the excitation, and subsequent dephasing, of coherent near zone-center LO phonons and polaritons, in the same material. We have also investigated the effects of an electron-hole plasma on coherent phonon excitation and experiments have been completed on a new optically driven non equilibrium phonon state. A further set of experiments have been completed to determine the effect of a one-component plasma on the dephasing time. The measurements inaugurate a new series of experiments on transient dynamics of carriers and phonons (and their interaction in semiconductors, normal metal and super-conducting thin films). Toward this end a new amplified picosecond and femtosecond laser facility has been constructed; both have characteristics which are suitable for a wide spectrum of experiments in this field.
5) A new series of experiments has been inaugurated on the behavior of solids optically driven into the incipient laser damage, and ultimately, into the laser damage region. These experiments have become possible as a result of our development of dual highly amplified, synchronously pumped tunable laser systems.
6) A series of additional concurrent experiments are well underway on the excitation of electronic carriers and their subsequent return to thermal equilibrium. Experiments have been performed on the ultra short transient dynamics of laser excited gold thin films, superconducting Pb strip lines, and high purity C60, thin films. The interplay between the excited carriers and the subsequent interaction of the carriers with phonons is one of the major results of this phase of the ongoing research.
Most recently we have started preparing for new areas of research which move the research area a number of major steps forward. Specifically, we are preparing to investigate photo induced changes in the index of refraction through the excitation of electrons in metal films which are excited by ultrashort laser pulses. These new investigations have already produced (a) measurements of the electron-phonon interaction parameter in single and polycrystalline gold films, (b) the observation of ballistic electron transport in films up to 400 nm thick, plus (c) a second "interactive" component of the electron transport which involves small angle scattering and eventually appears to evolve in to random walk diffusion.. These results are to be repeated for metallic multtilayer, as e.g. AuTiAu films. Preliminary results reflect the stronger electron-phonon interaction in the Ti layer as compared to that of gold,
The research program outlined briefly above has a further advantage in the type of training it provides to research students. Although the emphasis of the effort is clearly in basic research, it does expose and train students in research techniques which are of direct interest to technology.
Non equilibrium electronic carriers and phonon generation, transport, and detection forms the major basis of my present research interests. In earlier work, I have investigated the frequency dependence of the scattering of phonons at impurity ions, the frequency dependence of transport of phonons in the bulk and at solid-interfaces, the evolution in frequency, space, and time of a non-equilibrium distribution of phonons in a solid, and the observation of stimulated phonon emission. The work on phonon spectroscopy has lead to a series of review articles on the subject. The research activity is centered on the lifetime of acoustic and optical phonons, and on the effect of solid surfaces on phonon reflection.
During the past few years new methods of phonon spectroscopy have been developed to resolve the spectral, spatial, and temporal evolution of non equilibrium phonon distributions and their interaction with electronic carriers. A major emphasis of the current research activity involves ultrashort duration dynamics of coherent states of phonons and polaritons. These results, and the experimental methods on which they rely, open a new era of experimental and theoretical work on the basic physics of the transport of electronic carriers and vibrational states in condensed matter.
Five major areas of past and current interest of the author are:
1) The spectral, spatial, and temporal spectroscopy of phonon-phonon interactions in solids. The experiments probe in detail the basic crystal anharmonicities and nonlinear effects. experiments yield the lifetime of a phonon of a given energy. The results have direct bearing on phonon-based thermal energy transport in solids and is, accordingly, relevant to the technological problem of energy efficient structures.
2) The spectral and temporal spectroscopy of phonon transport across solid-solid interfaces. These experiments probe the fundamental barrier which hinders the generation and the transport of phonons at solid discontinuities. Experiments along these lines have already shown that, under certain conditions, the previously assumed frequency independence of the interface transport, is not sustained by experimental results. This area also has important technological implications to the thermal transport out of integrated circuits and other microelectronic structures. These miniaturized entities, which have revolutionized electronic devices, are important not only in that they speed up electronic communication but in that they are highly energy-efficient and minimize the usage of scarce material. The stability and packing densities of such circuits are, however, limited by problems of interface thermal transport.
3) The generation and detection of phonon distributions through the interaction of solids with light. These experiments are being carried out in the author's extensive laser facility that of These experiments are designed to yield spectral and temporal information on the basic direct and nonlinear interactions of monochromatic laser light to produce excited electronic carriers, phonons and polaritons. The present methods determine directly the phonon distribution and its temporal evolution and, therefore, form a direct and detailed test of existing theories. Increasing use of optical communication with laser light, plus the increasing use of conversion of solar energy, make this research area also of some importance to current technological problems.
4) In the past few years a new effort has been made to investigate the temporal response of solids to nonlinear excitations in the pico- and subpico-second time domain. So far we have measured the third order nonlinear (electronic) susceptibility of the GaP, ZnTe and ZnSe and the excitation, and subsequent dephasing, of coherent near zone-center LO phonons and polaritons, in the same material. We have also investigated the effects of an electron-hole plasma on coherent phonon excitation and experiments have been completed on a new optically driven non equilibrium phonon state. A further set of experiments have been completed to determine the effect of a one-component plasma on the dephasing time. The measurements inaugurate a new series of experiments on transient dynamics of carriers and phonons (and their interaction in semiconductors, normal metal and super-conducting thin films). Toward this end a new amplified picosecond and femtosecond laser facility has been constructed; both have characteristics which are suitable for a wide spectrum of experiments in this field.
5) A new series of experiments has been inaugurated on the behavior of solids optically driven into the incipient laser damage, and ultimately, into the laser damage region. These experiments have become possible as a result of our development of dual highly amplified, synchronously pumped tunable laser systems.
6) A series of additional concurrent experiments are well underway on the excitation of electronic carriers and their subsequent return to thermal equilibrium. Experiments have been performed on the ultra short transient dynamics of laser excited gold thin films, superconducting Pb strip lines, and high purity C60, thin films. The interplay between the excited carriers and the subsequent interaction of the carriers with phonons is one of the major results of this phase of the ongoing research.
Most recently we have started preparing for new areas of research which move the research area a number of major steps forward. Specifically, we are preparing to investigate photo induced changes in the index of refraction through the excitation of electrons in metal films which are excited by ultrashort laser pulses. These new investigations have already produced (a) measurements of the electron-phonon interaction parameter in single and polycrystalline gold films, (b) the observation of ballistic electron transport in films up to 400 nm thick, plus (c) a second "interactive" component of the electron transport which involves small angle scattering and eventually appears to evolve in to random walk diffusion.. These results are to be repeated for metallic multtilayer, as e.g. AuTiAu films. Preliminary results reflect the stronger electron-phonon interaction in the Ti layer as compared to that of gold,
The research program outlined briefly above has a further advantage in the type of training it provides to research students. Although the emphasis of the effort is clearly in basic research, it does expose and train students in research techniques which are of direct interest to technology.
Publications
Imaging Metallic Multilayer Structure through Ultrafast Optically Driven Electron Transport. A. Guerra, III, W.E. Bron, C. Suarez, Applied Physics B68, 405, (1999).
Quasi Electron and Phonon Interactions in the Femtosecond time Domain, Walter E. Bron, Arnold Guerra III and Carlos Suarez. J. Luminescence, 76&77, 518 (1998).
Imaging through quasiparticle transport, W.E. Bron, A. Guerra and C. Suarrez, Opt. Lett. 21, 997, (1996).
Time Resolved Electron Dynamics and Transport Imaging. W.E. Bron. C. Suarez and T. Juhasz. Proceedings of SPIE International Symposium on Optical Science, Engineering and Instrumentation. Vol. 2521, pg 46, (1995).
Dynamics and Transport of Electronic Carriers in Thin Gold Films.C. Suarez, W.E. Bron and T. Juhasz. Physical Review Letters.75,4536 (1995).
Link to this profile
https://faculty.uci.edu/profile/?facultyId=2127
https://faculty.uci.edu/profile/?facultyId=2127
Last updated
04/01/2002
04/01/2002