Physics Colloquium

Dr. Wei (David) Qian, Georgia Institute of Technology

"Watching Electronic and Vibrational Motions in Complex
Molecules and Nanostructures by Femtosecond Laser Spectroscopy"

Abstract: The intensive efforts of numerous researchers and
laboratories in shortening laser pulse durations after the
advent of the first (ruby) laser in 1960 led to the generation
of ultrashort pulses down to 100 femtoseconds (fs) from solid
state lasers in 1990 and gave birth to the new scientific field
called femtosecond laser spectroscopy. The extremely high time
resolution and very broad spectrum attained using femtosecond
lasers allows novel investigations of fundamental ultrafast
processes in physics, chemistry, biology, and materials science
initiated by light-matter interaction. In the first part of this
presentation, I will discuss nonadiabatic fast electronic dynamics
through a conical intersection between electronic potential energy
surfaces, which play a vital role in many very important
photophysical, photochemical, and photobiological processes,
such as interfacial electron transfer in some proposed next
generation photovoltaics, intramolecular proton-transfer, the
primary photoisomerization in vision, and DNA photoprotection.

We have used femtosecond polarized spectroscopy to probe the
ultrafast electronic movements at conical intersection in silicon
naphthalocyanine molecules. Taking together the experimental data
and the theory of polarization anisotropy of vibrational quantum
beats developed by us, for the first time we directly measure the
time scales of electronic equilibration via conical intersection,
which is about 200 fs in the silicon naphthalocyanine molecules
with 1 mev stabilization energy. The second part is focused on
ultrafast coherent phonon dynamics in plasmonic nanoparticles.
Plasmonic (silver and gold) nanoparticles are remarkable materials
for nanophotonics and are being used for a variety of applications,
such as nanometer scale optical waveguide, single molecule sensing,
and biomedical imaging because of the surface enhanced electric
field stemming from the their unique localized surface plasmon
resonance. In order to optimize performance of nanodevices using
plasmonic nanoparticles as building blocks, it is necessary to
understand how their electronic and phonon properties depend on
the shape, size, and interparticle distance.

Using femtosecond absorption spectroscopy, we have systematically
studied ultrafast coherent phonon dynamics in monolayer of periodic
metallic nanoparticle arrays fabricated by nanosphere and electron
beam lithography. For the first time, we have uncovered the apparent
effect of interparticle coupling on coherent phonon properties.
Finally, I will simply talk about novel nano-photoacoustic
intracellular drug delivery, which cleverly integrates femtosecond
laser technology, nanotechnology, and biotechnology for great
achievements in medical applications.



Dr. Wei (David) Qian is the Assistant Director and Senior Research
Scientist at the Laser Dynamics Laboratory (LDL) at the Georgia
Institute of Technology. Since joining the LDL he has been
performing cutting-edge research in the broad interdisciplinary
areas between ultrafast laser spectroscopy, nanoscience,
nanotechnology and biotechnology. He is the coauthor of ~50
publications in peer-reviewed journals and conference proceedings
and has 4 patents pending. Dr. Qian received a B.S. degree in Physics
from Anhui University and a Ph.D. degree in Optics from Peking
University, China. His Postdoctoral work was with Professor
David Jonas' group in the Department of Chemistry and Biochemistry
at the University of Colorado, Boulder.
 

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