Ultrafast and Very Small:

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.

 

Team members:

 

NSF grant 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 structure calculations done by students Yaochun Zhang and Yonas Abraham, together with Professor Natalie Holzwarth, on scintillator materials they have studied experimentally in this project: