Research on Subpicosecond and Submicrometer
Phenomena in Condensed Matter Physics
Department of Physics, Wake Forest University
We use ultrafast spectroscopy to investigate the
immediate consequences of absorption of a photon by a crystalline
solid. Ultrashort (~130 fs) high-power (~
20 Gw) laser pulses are employed to create an electronic excitation
of the solid and monitor its evolution within the coupled
electron-lattice system. Phenomena of interest include self-trapping of
excitons and of charge carriers in insulators, relaxation of hot
carriers and surface state dynamics in semiconductors, and excitonic
processes of lattice defect formation. Nonlinear optical and
ultrafast phenomena are being studied in inhomogeneous samples and
thin films with 3-dimensional spatial resolution of the order 200 nm
x 700 nm using confocal multiphoton microspectroscopy. This work
involves collaboration with the medical school of Wake Forest
University. Interactions at surfaces are investigated using
ultrahigh vacuum analytical techniques such as fs time-resolved
photoelectron spectroscopy. Research is supported by the National
Science Foundation under Grant No. DMR-9732023.
Ultrafast and 4 pi Confocal Microspectroscopy of Few or Single
Atomic Defects, Migration, and Local Processes in Inhomogeneous
Materials
SELF-TRAPPED ELECTRON AND TRANSIENT DEFECT ABSORPTION IN NIOBATE
AND TUNGSTATE CRYSTALS
R. T. WILLIAMS, K.B. UCER, H. M. YOCHUM, L. G. GRIGORJEVA†, D. K.
MILLERS†, and G. CORRADI*
Department of Physics, Wake Forest University, Winston-Salem, NC
27109, USA; †Institute of Solid State Physics, University of Latvia,
LV-1063, Latvia; Crystal Physics Laboratory, Hungarian Academy of
Sciences, Budapest, Hungary
We have measured transient optical absorption in LiNbO3, KNbO3,
PbWO4, CdWO4, and ZnWO4 following band-gap excitation by 200-fs
laser pulses and by 10-ns electron pulses. A strong infrared
transient absorption band centered near 1 eV was found in a sample
of stoichiometric LiNbO3. We tentatively attribute this infrared
absorption band to intrinsic electron polarons, following observation
and EPR characterization of a similar (persistent) band in Mg-doped,
thermochemically reduced congruent LiNbO3 by Schirmer et al.
So-called blue-light induced infrared absorption in KNbO3 was
investigated by time-resolved spectroscopy as well. In PbWO4 and
CdWO4, broad visible absorption and strong transient infrared
absorption are observed. It is considered whether infrared
absorption in PbWO4 may be partly due to self-trapped electrons
already characterized by Laguta et al and Hofstaetter et al using
EPR.
(Proceedings of the 14th International Conference on Defects in
Insulating Materials (Johannesburg, South Africa, April 2000), to be
published in Radiation Effects and Defects in Solids.)
Wake Forest University's femtosecond time-resolved
photoelectron experiment as shown on the cover of Laser Focus
World.
In the first instants . . . Ultrafast views of radiation effects
R. T. Williams, K. B. Ucer, and J. L. LoPresti* Department of
Physics, Wake Forest University Winston-Salem, NC 27109 USA
In the first instants following high-energy electronic excitation
of a solid, electrons and holes scatter from one another while
establishing the initial partition of excitation among available
states, and scatter from phonons while cooling to a thermalized
population. If significant local lattice relaxation or self-trapping
occur, mobility and recombination time may be strongly altered.
Electron capture and/or defect formation can culminate in luminescence
for scintillation detection or in stored energy that may be the
basis for imaging or dosimetry. A. N. Vasil'ev has presented a
theoretical overview of the early instants in scintillator
excitation at SCINT99. It is now possible to directly observe and
measure some of the consequences of excitation on fs and ps time
scales.
(Proceedings of the 4th Euroconference on Luminescent Detectors and
Transformers of Ionizing Radiation [LUMDETR2000] Riga, Latvia,
August 2000, to be published in Radiation Measurements, Pergamon
Press.)
collaboration with WFU Medical School (MicroMed
Facility) combining subpicosecond laser system (left) and Zeiss 510
NLO confocal microscope (right) for multiphoton microscopy.
MULTIPHOTON EXCITATION MICROSCOPY OF PHOTOLUMINESCENCE
IN GaN EPITAXIAL FILMS
Y. C. Zhang, K. Burak Üçer, and R. T.
Williams Department of Physics,Wake Forest University Winston-Salem,
NC 27109 USA ABSTRACT
Using techniques of confocal scanning microscopy and
multiphoton excitation, the spectra and relative efficiency of
photoluminescence in epitaxial films of GaN have been resolved in 3
spatial dimensions. In both MOVPE- and MBE- grown GaN, the
band-to-band photoluminescence image can be generally described as
dim over most of the surface, with occasional intense spots of 200 nm
to 800 nm size (FWHM). Axial scans suggest nonradiative
recombination (dead-layer) zones at the interfaces.
(Proceedings of the 3rd International Conference on
Excitonic Processes in Condensed Matter [EXCON98], Boston, 1998
Difei Liang and Dr. Burak Ucer with the 4 pi confocal
microscope
4pi CONFOCAL MICROSCOPE FOR MULTIPHOTON OPTICAL
SECTIONING OF GaN FILM LUMINESCENCE
K. B. UCER, DIFEI LIANG, R. T. WILLIAMS Department of
Physics, Wake Forest University Winston-Salem, NC 27109 USA H.
MORKOC Department of Electrical Engineering, Virginia Commonwealth
University PO Box 843072, Richmond, VA 23284 USA
In the 4pi confocal microscope developed by S. W. Hell
et al, laser light coherently illuminates both sides of a thin
sample through a pair of high-NA objectives, effectively producing a
single standing-wave fringe of 2-photon fluorescence excitation with
weak side lobes. Developed initially for biological applications,
the 4pi microscope of Hell et al demonstrated 75 nm axial resolution
with 810 nm light. We have constructed a 4pi confocal multiphoton
microscope for 3d analysis of band-edge/excitonic photoluminescence in
thin films. Excitation is with 130 fs pulses from a Ti:sapphire
laser. Instrumental features and preliminary tests with rhodamine
and GaN and InN films are reported.
(Proceedings of the 4th International Conference on
Excitonic Effects in Condensed Matter [EXCON2000], Osaka, August
2000.)
Electronic Band Structures of the Scheelite Materials – CaMoO4,
CaWO4, PbMoO4, and PbWO4, Y. Zhang, N.
A. W. Holzwarth, and R. T. Williams, Phys. Rev. B 57,
12738 (1998).
Electronic Structure and Optical Properties of Cd MoO4 and CdWO4,
Y. Abraham, N. A. W. Holzwarth, and R. T. Williams, Phys. Rev.
B62, 1733 (2000)
