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June 27, 2003 Polarized Light Drives Anisotropic Expansion in Chalcogenide GlassesG. Chen1, H. Jain1, M. Vlcek2, S. Khalid3, D.A. Drabold4, and S.R. Elliott5 Common oxide glasses are transparent solids known for their inertness and stability. By contrast, their chalcogenide analogs (i.e. sulfides, selenides, and tellurides) exhibit unexpected sensitivity to bandgap light, leading to several novel photoinduced effects. Particularly fascinating are the anisotropic effects created in their random structure by the polarized light. The in situ EXAFS experiments at NSLS provide the first atomistic evidence of the origin of such structural changes where the local structure around Se in an arsenic selenide glass film expands anisotropically, depending on the polarization of the laser beam.
To obtain atomistic insight into the light-induced structural changes that cause the newly discovered vector effects in chalcogenide glasses, it is necessary to have an experimental technique that is capable of probing the element-specific structure of amorphous materials in different directions as defined by the polarized laser beam. The extended x-ray absorption fine structure (EXAFS) analysis, which is conducted with a synchrotron light source, satisfies these requirements. Synchrotron light sources provide not only high-intensity but also polarized X-rays, which are essential for detecting anisotropic structures.
Figure 2 shows an example of the light-induced structural changes for the two polarization directions of the laser beam. There is an expansion of the nearest-neighbor distance around the Se atoms - its magnitude depends on the direction of light polarization with larger expansion being observed when EL // EX than when EL ^ EX. Thus a vector structural change is observed directly for the first time for any amorphous material. The authors believe that the anisotropic local expansion around Se atoms occurs as the bandgap laser light excites and reorients the Se 4p lone pairs, and helps form anisotropic As-Se bonds. Their experiments provide an atomistic view of photo-induced vector phenomena in chalcogenide glasses, and guide the discovery of new materials with enhanced light-induced effects for photonic applications. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
The exposure of solids to light leads to a number of important chemical
reactions and physical phenomena in nature as well as in modern
technology, e.g. photography, xerography, holography,
photopolymerization, photosynthesis of plants, etc. In general,
light interacts with a solid through its electronic states, which
depend upon the constituent atoms and structure of the material.
Light-induced change in the atomic structure involving displacement
of atoms is unusual, except in special cases of disordered
materials, including polymers and glasses. Such changes, when they
occur, are expected to depend on the intensity and wavelength (λ) of
light. However, recently light-induced mass transport, photocrystallization, and an opto-mechanical effect have been
reported on glassy chalcogenide films, which imply a change in the atomic structure that
depends also on the polarization of light. Atomistic, chemical
specific experimental evidence about such changes has been missing.
These so-called vector effects (because they depend on the direction
of the electric field vector of light) are particularly intriguing
because the starting structure of the unilluminated films is
amorphous and isotropic. 
In the present work, the Lehigh University researchers, in
collaboration with colleagues from Ohio, Pardubice (Czech Republic),
and Cambridge (UK) universities and BNL, have used EXAFS analysis
with polarized X-rays (in which the electrical field vector EX //
the ground plane) to probe the changes in the structure around Se
and As atoms, and track the laser-polarization dependence of these
changes in arsenic selenide glasses. The experiments were performed
at NSLS beamline X18B; the schematic for in situ observations is
shown in Figure 1. A helium-neon laser beam (λ=633 nm) intercepts
the X-ray beam on the sample. A half wave plate is used to change
the polarization direction of the laser beam (identified by its
electric field vector, EL) so that EL // EX or EL ^ EX. Two
different spots of the same sample were separately illuminated by
two orthogonal, linearly polarized laser beams, and the structural
changes were probed in situ by the X-ray beam.