Harry W. K. Tom, Professor of Physics, specializes in nonlinear optics and femtosecond time-resolved laser techniques and is particularly interested in surface dynamics, laser-induced surface chemical reactions, laser-induced phase transitions in bulk materials, nonlinear optics of the water/solid interface, and most recently in terahertz spectroscopy. Tom is Co-Director of the Environmental Physics graduate program, an interdisciplinary program in which condensed matter physicists and environmental scientists collaborate in joint research and graduate training. 

Opportunities for Graduate Students

 I am currently recruiting graduate students in the areas of coherent spectroscopy of surface optical phonons and laser-induced  surface chemical reactions.  Interested students already enrolled at UCR may contact me directly. Prospective graduate students should contact me by email and obtain general information and the downloadable application form here.

Current Research Projects

Coherent Optical Spectroscopy of Surface Phonons and Low Frequency Molecule-Substrate Vibrational Modes and Electronic State Coherence

When a femtosecond laser pulse impinges on a surface, it can excite normal mode vibrations of the surface atoms (ie., phonons), much as striking a bell can excite the normal mode vibrations of a bell. The surface atoms will continue to vibrate at the normal mode frequencies and the mode amplitudes will decay by collisional processes that "dephase" the coherent oscillation. We detect these modes by time-resolved second-harmonic generation: a second "probe" pulse irradiates the surface and the second-harmonic generation from this probe is detected as a function of the pump-probe time-delay. In the figure below we show the results of one such experiment. The "ringing" in time can be Fourier-transformed to give the surface phonon frequency spectrum.

Surface vibrational modes are significant in the study of surface dynamics and reactions because they mediate the transfer of vibrational energy between the bulk material and molecules that may stick or chemically react on the surface. The significance of this new all optical surface phonon technique is that it is the only one that can be used at buried interfaces, ie., gas/solid, liquid/solid, solid/solid interface. We are still investigating the potential of this technique and are using it to investigate surface reactions on clean and adsorbate-covered semiconductor and metal surfaces.  We have observed surface phonon at clean and native-oxide covered GaAs (100) and (110).   We have seen bulk LO phonon hole-plasmon coupled oscillations.  We have also observed phonon changes during Ar+-ion sputtering, insitu oxidation of clean GaAs (100)-(4X6) and laser-induced surface disorder of GaAs (110)-(1X1).

We have also developed a new coherent electronic spectroscopy of surfaces using time-resolved SHG.  By measuring both the time-resolved pump-probe cross-correlation and an optical superposition of the SH fields from the probe and autocorrelation, we can deduce the amplitude, frequency, and phase of the induced optical polarization of the surface electronic states and the laser parameters (pulse duration and chirp).

This work is funded by the National Science Foundation under NSF-CHE-9707143.  For more information see:

  • Y.-M. Chang, L. Xu, and H.W.K. Tom, "Coherent phonon spectroscopy of GaAs surfaces using time-resolved second-harmonic generation," in Special Issue entitled "Electron Dynamics in Metals", Chemical Physics 251/1-3, 283-308 (2000).
  • Y.-M. Chang, L. Xu, and H.W.K. Tom, "Observation of coherent local optical phonons localized at buried interfaces using time-resolved second harmonic generation," Phys. Rev. B 59, 12220-12223 (1999).
  • H.W.K. Tom, Y.M. Chang, and H. Kwak, "Coherent phonon and electron spectroscopy on surfaces using time-resolved second-harmonic generation," Applied Physics B 68, 305-313 (1999), preprint in pdf.
  • H. Kwak, K.C. Chou, and H.W.K. Tom, " Measurement of Surface Electronic Dephasing Time using Surface Second Harmonic Generation," Annual Meeting of the American Physical Society, March 1998.
  • K.C. Chou, H. Kwak, and H.W.K. Tom, "LO phonon hole-plasma coupling near the surface of GaAs (110) after femtosecond laser irradiation," Annual Meeting of the American Physical Society, March 1998.
  • Y.-M. Chang, L. Xu, and H.W.K. Tom, "Observation of Coherent Surface Optical Phonon Oscillations by Time-Resolved Surface Second-Harmonic Generation," Phys. Rev. Lett. 78, 4649 (1997).
  • H.W.K. Tom, C.M. Mate, X.D. Zhu, J.E. Crowell, T.F. Heinz, Y.R. Shen and G.A. Somorjai, "Surface Studies by Optical Second-Harmonic Generation: The Adsorption of O2, CO and Sodium on the Rh(111) Surface," Phys. Rev. Lett. 52, 348 (1984).
  • G.C. Cho, W. Kutt, and H. Kurz, Subpicosecond time-resolved coherent-phonon oscillations in GaAs, Phys. Rev. Lett, 65, 764 (1990).

    Femtosecond surface chemical reactions and physical changes

    Femtosecond laser pulses deposit energy into the electronic states of a system on a time scale short compared to electron-phonon relaxation.  Femtosecond laser desorption of molecules from surfaces is enhanced by many orders of magnitude over nanosecond laser desorption due to novel energy transfer associated with the electronic excitation.  The substrate temperature can be several 1000K while the atomic motion is still cold.  Under these conditions multiple electronic excitations must also be considered.  For CO desorbed from Cu(111), the yield from a single pulse can be as high as 100%.   We are exploring mechanisms of energy transfer to molecules from metals by exploring differences in the isotopic yield of CO desorbed from Cu crystals in UHV.  We have also studied laser-induced damage of the reconstructed (1X1) surface of GaAs(110).  In both cases, reactions occur due to electronic rather than purely thermal driving mechanisms.  The laser intensities are far below the damage threshold for the bulk material.  Because we are able to measure the depletion field through DC-field induced SHG, we can also obtain information about the carrier dynamics including surface hole recombination which turns out to be much slower than the so called surface recombination velocity.   This work has been partially supported by the National Science Foundation under NSF-CHE-9707143 and Lawrence Livermore National Laboratory under MI-98-013.  For more information see:

  • H. Kwak, K.C. Chou, J. Guo, and H. W. K. Tom, "Femtosecond laser-induced disorder of the (1X1)-relaxed GaAs (110) surface," Phys. Rev. Lett. 83, 3745-3748 (1999).
  • J. Guo, H. Kwak, and H.W.K. Tom, "The desorption yield dependence on wavelength of femtosecond laser from CO/Cu(111)," Annual Meeting of the American Physical Society, March 1999.
  • K.C. Chou and H.W.K. Tom, "Study of carrier dynamics near GaAs surface using time-resolved second-harmonic generation," Annual Meeting of the American Physical Society, March 1999.
  • H.W.K. Tom and J.A. Prybyla, "Femtosecond Probing of Chemical Reaction Dynamics at Surfaces," Ch. 20 in Laser Spectroscopy and Photochemistry on Metal Surfaces, Hai-Lung Dai and Wilson Ho, eds., Advanced Series in Physical Chemistry, Vol. 5 (World Scientific, Singapore 1995) pp. 827-896.
  • W.S. Fann, R. Storz, H.W.K. Tom, and J. Bokor, "Direct Measurement of Nonequilibrium Electron-Energy Distribution in Subpicosecond Laser-Heated Gold Films," Phys. Rev. Lett. 68, 2834 (1992).
  • W.S. Fann, R. Storz, H.W.K. Tom, and J. Bokor, "Electron Thermalization in Gold," Phys. Rev. B46, 13592 (1992).
  • J.A. Prybyla, H.W.K. Tom, G.D. Aumiller, "Femtosecond Time-Resolved Surface Reactions: Desorption of CO from Cu(111) in <325 fsec," Phys. Rev. Lett. 68, 503 (1992).

    Femtosecond Laser-Induced Heating, Melting, and Phase Transitions

    The absorption of light in semiconductors promotes electronic states that are generally bonding in character to states that are generally anti-bonding.  Thus the absorption of light generally weakens the interatomic bonds in a material.  During a femtosecond laser pulse, one can promote 5-10% of the electron population all before the lattice acquires much energy.  Under those conditions, the bandgap can collapse and the material will disorder spontaneously--within 100 fs.  We are exploring mechanisms of energy transfer, bandgap collapse, and electronically induced phase transitions in the regime near and just above damage.  We hope to electronically bias these systems close to static high pressure phase transitions by conducting these experiments in diamond anvil cells.  These studies are conducted in collaboration with investigators at Lawrence Livermore National Laboratory and is currently funded under LLNL-MI-099-008.  For more information see:

  • H.W.K. Tom, G.D. Aumiller, and C.H. Brito-Cruz, Time-Resolved Study of Laser-Induced Disorder at Si Surfaces, Phys. Rev. Lett. 60, 1438 (1988).

    Time-Resolved Imaging Studies of Laser-Induced Damage of Optical Crystals

    We are able to obtain images of bulk structural changes on a nanosecond timescale after an optical crystal is initially irradiated by a laser pulse.   The damage nucleation site absorbs energy and the energy propagates away from the damage site in a shockwave or in dislocation loops.  In some cases what we would consider permanent damage does not occur until many 10's of nanoseconds after the laser pulse.  Fundamental studies of shockwave propagation, structural phase transitions, and fracture have in general not been conducted in spherical geometry with point like excitation sources.  We are thus developing new tools for understanding and predicting structural damage in this geometry.  These results are relevant not only to optical damage but also earthquake propagation and structural fatigue of materials.  This project is performed in collaboration with several investigators at Lawrence Livermore National Laboratory and is currently funded under LLNL-B346502.  For more information see:

  • H. Jiang, J. McNary, H. W. K. Tom, M. Yan, H. B. Radousky, and S. G. Demos, "Nanosecond time-resolved multiprobe imaging of laser damage in transparent solids," Appl. Phys. Lett. 81, 3149 (2002).

     
     
     

    Terahertz Spectroscopy of Correlated Electron Materials

    Electrons and holes are created when ultrashort laser pulses are absorbed in a semiconductor. Their rapid creation and subsequent motion provide a rapidly varying polarization that radiates in the terahertz frequency range. Spectroscopy based on this terahertz source has favorable signal to noise and frequency range compared to spectroscopy based on globar sources or discrete frequency devices. This femtosecond laser-based source is also more convenient than free electron lasers. Ward Beyermann (UCR) and I are building a femtosecond laser-based terahertz spectrometer that will be used to study correlated electron materials at low temperatures. Correlated electron materials are expected to show interesting spectral features in the terahertz range because their critical temperatures are such that kBTc are comparable to hf where f is in the terahertz range. This work is funded by the National Science Foundation under NSF-DMR-9704032.  For more information on correlated electron materials see the webpages for Ward Beyermann and " Douglas MacLaughlin.  For references on ultrashort laser-based terahertz spectroscopy see:

  • M.C. Nuss, et al., "Dynamic Conductivity and Coherence Peak in YBa2Cu3O7 Superconductors," Phys. Rev. Lett. 66, 3305 (1991).
  • M. Van Exter, Ch. Fattinger, and D.R. Grischowsky, "Terahertz Time-Domain Spectroscopy of Water Vapor," Opt. Lett. 14, 1128 (1989).
  • Q. Wu and X.-C. Zhang, "Ultrafast Electro-Optic Field Sensors," Appl. Phys. Lett. 68, 1604 (1996).

    Environmental Physics: Nonlinear Optical Studies of the Water/Solid Interface

    Second-Harmonic Generation and Visible-Infrared Sum-Frequency Generation are two nonlinear optical probes that can provide information about molecular adsorbates at interfaces of environmental importance (such as the water/solid, air/solid, or air/liquid interface). We are currently studying the orientation of water within the double charge layer at  water/silica interface on planar and colloidal particle samples.  We also study the adsorption of simple molecules at the surface.  Our fundamental contribution is the ability to measure the field in the double charge layer in an insitu  non-destructive manner.  The solid surface charge is effected self-consistently with the adsorbates and pH in the double-charge layer.  This work is interesting from the fundamental point of view because chemistry at the water/solid interface is not well understood and certainly not as well studied as chemistry at the vacuum/solid interface. This work is relevant to environment science because current models for pesticide degradation and transport in the environment require microscopic reaction rates. This interdisciplinary research is performed in collaboration with Michael Anderson in the Soil and Environmental Sciences Department. The graduate student who is performing this research will receive both a M.Sc. in Soil Physics and a Ph.D. in Physics. We are currently seeking more graduate students to do related research. Students who are interested in this kind of interdisciplinary research may read more about the Environmental Physics graduate program.  Currently 6 graduate students are supported under a National Science Foundation Graduate Research Traineeship program in Environmental Science, NSF-DGE-9554506.  For more information about nonlinear optics applied to water interfaces, see:

  • C.S.C. Yang and H.W.K. Tom, "Adsorption of Organic Molecules at Planar and Colloidal Quartz/Water Interfaces," Annual Meeting of the American Physical Society, March 1999.
  • X.D. Xiao, V. Vogel and Y.R. Shen, "Probing the proton excess at interfaces by second harmonic generation," Chem. Phys. Lett. 163, 555 (1989).
  • A. Castro, K. Bhattacharyya, and K.B. Eisenthal, "Energetics of adsorption of neutral and charged molecules at the air/water interface by second harmonic generation: hydrophobic and solvation effects," J. of Chem. Phys. 95, 1310 (1991).
  • Q. Du, R. Superfine, E. Freyz, and Y.R. Shen, "Vibrational spectroscopy of water at the vapor/water interface," Phys. Rev. Lett. 15, 2313 (1993).
  • R.A. Bradley, R. Georgiadis, S.D. Kevan, and G.L. Richmond, 1993. "Nonlinear optical spectroscopy of the Ag(111) surface in an electrolyte and in vacuum," J. Chem. Phys. 99, 5535 (1993).
  • X. Zhao, S. Ong and K.B. Eisenthal, "Polarization of water molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface," Chem. Phys. Lett. 202, 513 (1993).
  • X. Zhao, S. Ong, H. Wang, and K.B. Eisenthal, "New method for determination of surface pKa using second harmonic generation," Chem. Phys. Lett. 214, 203 (1993).

    Selected Previous Research

    High Intensity Laser Physics

    Using extremely short and energetic laser pulses, it is possible to generate optical fields of 1014 to 1018 W/cm2 on target. At these intensities, most atoms ionize and the electric field in atomic units becomes close to 1. In contrast to traditional laser-atom physics which treats the electric field as a perturbation to the atom--in high intensity laser physics the atom is a small perturbation to the electric field. Novel effects can be observed in atomic ionization, as well as in the excitation of high density plasmas that can generate radiation in the extreme ultraviolet and soft-x-ray. We would like to make a table top short-pulse soft-x-ray source for time-resolved UV and soft-x-ray spectroscopy. For more information see:

  • O.R. Wood II, W.T. Silfvast, H.W.K. Tom, et al., "Pulse Duration on XUV Emission from Femtosecond and Picosecond Laser-Produced Ta Plasmas," Appl. Phys. Lett. 53, 654 (1988).
  • U. Mohideen, M.H. Sher, H.W.K. Tom, et al., "High Intensity Above-Threshold Ionization of He," Phys. Rev. Lett. 71, 509 (1993).
  • M.H Sher, U. Mohideen, H.W.K. Tom, et al., "Picosecond Soft X-ray Pulse-Length Measurement by Pump-Probe Absorption Spectroscopy," Optics Lett. 18, 646 (1993).

    Second-Harmonic Generation in Optical Fibers, Induced-Gratings in Optical Fibers

    The interference between a fundamental frequency optical wave and its second-harmonic can induce excitations that can create permanent defects in glass fibers that modulate spatially along the length of the fiber at the phase-matching period. This grating of defects can generate phase-matched second-harmonic when the prepared fiber is irradiated by only the fundamental. In fact, because a small amount of second-harmonic can be generated even in centrosymmetric glass without phase-matching, this process is self-seeding. That is, if you irradiate a fiber with intense fundamental, a second-harmonic generating grating will "write" itself. A number of exciting variations of this basic phenomena have been explored, such as stretching the fiber to create a phase-matched second-harmonic generator at a different laser frequency, intentionally poling a material using an external electric field to form a phase-matched crystal, or using this same idea to pole materials other than glass. This phenomena is related to the writing of "Hill gratings" in fibers which are formed by the interference of two counter-propagating optical waves of the same frequency. For more information see:

  • R.H. Stolen and H.W.K. Tom, "Self-Orgnaized Phase-Matched Harmonic Generation in Optical Fibers," Optics Lett. 12, 585 (1987).
  • H.W.K. Tom, R.H. Stolen, G.D. Aumiller, and W. Pleibel, "Preparation of Long Coherence-Length Second-Harmonic Generating Optical Fibers Using Modelocked Pulses," Optics Lett. 13, 512 (1988).