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 CoherenceWhen 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
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
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,
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 changesFemtosecond
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,
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,
Femtosecond Laser-Induced Heating, Melting, and Phase TransitionsThe
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
CrystalsWe 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 MaterialsElectrons 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
InterfaceSecond-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
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
Selected Previous Research
High Intensity Laser PhysicsUsing 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
FibersThe 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).