How does a photovoltaic cell work?
Professor James Wolfe received his Ph.D. in physics from the University of California, Berkeley, in 1971. His thesis work involved a new technique in high resolution NMR, and optical polarization and maser action of nuclear spins. He remained at Berkeley as an assistant research physicist until 1976, when he joined the Department of Physics at the University of Illinois. He is a renowned expert on the imaging and thermodynamics of excitonic matter in semiconductors. His 1998 book, Phonon Imaging , describes the propagation of phonons and ultrasound in solids. A Fellow in the American Physical Society, Professor Wolfe has headed multi-investigator programs for the Department of Energy and the National Science Foundation. He was awarded the 2004 Frank Isakson Prize for Optical Effects in Solids of the American Physical Society "for contributions to the fundamental understanding of excitonic matter and ballistic phonons in semiconductors, made possible by pioneering development of graphic imaging techniques."
Professor Wolfe has written the book "Elements of Thermal Physics", used in the Department's core curriculum for engineering and science majors. He was honored with the Paul Klemens Award in 2010.
Imagine a crystal of silicon immersed in a bath of liquid helium and illuminated with a beam of laser light. The laser light promotes electrons into the conduction band, leaving holes in the valence band. At low temperatures the Coulomb attraction between an electron and a hole causes them to bind together forming a composite particle known as an "exciton", like a hydrogen atom but much lighter. A gas of these neutral particles diffuse in the crystal and recombine within about a microsecond, giving off a characteristic luminescence. At sufficient densities, excitons can combine into excitonic molecules and even condense into an electron-hole liquid. Much has been written about these forms of "excitonic matter."
Professor Wolfe and co-workers have employed time-resolved imaging of the excitonic photoluminescence to characterize their motion and thermodynamics. Excitons are typically very mobile particles and can be pushed around by applying stress to the crystal or by subjecting them to heat pulses (i.e., phonons). Wolfe's group has developed strain confinement of excitonic matter in semiconductor crystals and discovered striking effects of a phonon wind on clouds of electron-hole droplets.
The Fermi statistics of electron-hole liquid in silicon and germanium has been well established, but Bose-Einstein condensation of excitons remains a long-sought goal.
For short-lived excitons in direct-gap semiconductors such as GaAs, Wolfe's group devised picosecond photoluminescence imaging to observe the kinetics of these particles. The excitons in GaAs/AlGaAs quantum wells form biexcitons, and evidence has been obtained for Bose-Einstein statistics of this excitonic gas. In crystals of Cu2O, controlled strain fields have been applied to measure the mobility of excitons and to confine them in potential wells. Excitons in this semiconductor are thought to be prime candidates for observing Bose condensation, however, the present limitation seems to be exciton-exciton anihilation, which is being explored by time-resolved spectroscopy. Recent studies by Wolfe's group indicate that the short exciton lifetime is caused by their combining into biexcitonic molecules, which have extremely short lifetimes.
Laser excitation of a crystal also produces a local source of thermal energy, composed of quantized lattice vibrations (phonons). Wolfe's group invented the method of phonon imaging in order to examine the propagation and scattering of high-frequency phonons in crystals at low temperatures. This technique measures the spatial pattern of heat flux emanating from a point source. Striking patterns of ballistic phonons are observed for many types of crystals due to their inherent elastic anisotropy.
The phonon-imaging technique has been extensively used to examine phonon scattering from electrons, defects and interfaces. Experiments have been performed to probe anisotropies in the superconducting state of conventional superconductors by imaging the transmission of phonons through single crystals. Strong absorption dips are observed in images of ballistic phonon transmission in superconducting Pb which directly measure the free electron (quasiparticle) density. The temperature dependence of quasiparticle density conforms with conventional BCS theory and shows no evidence of a spin density wave ground state.
Other work involves the propagation of ultrasound in the bulk and on the surface of anisotropic solids, which include crystals, composite materials, and fabricated periodic structures known as phononic lattices. Such experiments are not restricted to low temperatures or non-metallic solids.
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