Our Research Program revolves around two themes:
1. " Collective Dynamics of Disordered Solids":
Order in solids is characterized by a correlation length. In perfectly ordered solids, this length is infinite and the characteristic lattice dynamics of such solids is the “phonon dynamics”. In disordered solids however, long range order is perturbed and a new correlation length appears, this time finite. To be more specific, in disordered solids such as glasses or crystals with substitution disorder, the new correlation length characterizes what has been called short or intermediate range order, a much studied topic in recent years. As could be expected, this new correlation length is usually accompanied by a new type of collective dynamics, on a similar short or intermediate spatial scale.
We are studying the collective dynamics of both types of disordered solids, glasses and disordered crystals, using a variety of experimental techniques: Raman spectroscopy in the frequency and time-domain, neutron scattering, time-resolved x-ray diffraction, dielectric and polarization spectroscopies.
Our research has focused particularly on “relaxor ferroelectrics”, a family of highly polarizable crystals (with ferroelectric tendencies) characterized by substitution disorder, with randomly substituted ions moving off-center from their high symmetry crystallographic sites. These crystals display a very rich spectrum of new phenomena and, in certain cases, phase transitions. I am particularly interested in the similarities between their dynamics and that of glasses. It is interesting to note that these relaxor ferroelectrics are disordered perovskite oxides, similar to the high Tc superconductors or the giant magneto-resistance manganites.
2. "Nonlinear Optical Phenomena in Optical Waveguides":
As an extension of my interest in disordered solids, and particularly glasses, I developed a research program for the study of nonlinearities in glasses, optical fibers and planar optical waveguides (Stimulated Brillouin, Stimulated Raman, Four-Wave Scattering, Optical control of propagation through carrier generation, optical emission). This program runs on two separate levels, in parallel. On a fundamental level, we are studying these optical nonlinearities in order to characterize them and understand their precise mechanisms in specific situations. On an applied level, we are attempting to demonstrate new optical effects based on these nonlinearities in new fibers or new waveguide structures. For example, we have carried out an extensive study of Stimulated Brillouin Scattering in microstructured fibers, more specifically photonic crystal fibers. We have also demonstrated all-optical control of the propagation of light in photonic crystal waveguides through optical generation of carriers. We have investigated Slow Light induced by SBS in the high gain regime, as in a Brillouin Laser. We recently completed a comprehensive study of inter-core coupling in a multicore photonic crystal fiber. We are presently involved in a study of Raman-assisted Four-Wave, with the goal of mitigating the limitations of both effects and optimizing their combined advantages.
The above optical studies are coupled to a fabrication activity. We are fabricating tellurite (TeO2)-based preform and fibers, both conventional and microstructured. The preforms are made through rotational suction casting or rod-in-tube methods as well as by extrusion.
Experimental Techniques Used:
- Raman and Brillouin Scattering
- Neutron Scattering
- Ultrasonics, Dielectric and Polarization Spectroscopies
- X-ray Diffraction
- Pulsed NMR
- Fiber optics-Stimulated Raman and Brillouin Scattering- Four Wave Mixing
Research Areas of Interest:
- Non-linear Optics in glasses and optical fibers (e.g. stimulated Raman and Brillouin)
- Propagation of light in photonic crystals
- Experimental and theoretical studies of structural and ferroelectric phases transitions in disordered and partially ordered crystals (phonon dynamics and localized vibrations in relaxor ferroelectrics)
- Lattice dynamics in disordered ferroelectrics (soft modes and central peaks)
- Non-linear properties of disordered ferroelectrics
- Low frequency vibrational dynamics and slow relaxation in glasses (relationship between structural and dynamical features on different length and time or frequency scales, e.g. Boson peak)
- Liquid-Glass transition in strong and fragile glasses
Recent Grants and Active Contracts:
- National Science Foundation, 2001-2003, instrumentation grant for fiber optics research.
- National Science Foundation, 2000-2001, in collaboration with Mark Bickhard (Philosophy/Cognitive Science, Lehigh University), "Complex Systems from Physics to Biology".
- Department of Energy, 2000-2004, "Nanoscopic Study of the Polarization-Strain Coupling in Relaxor Ferroelectrics and the Search for New Relaxor Materials for Transducer and Optical Applications".
- Center for Optical Technologies, Lehigh University, 2003-2006, for support of the All-Optical Functionalities Thrust Team (10-12 faculty/graduate students/postdocs participants, not own individual grant).
- National Science Foundation, 2004-2007, "Optical Nonlinearities in Microstructured Fibers for All Optical Network Functionalities".
- National Science Foundation, 2004-2006, "Microstructure Tellurite Fibers Produced by Extrusion for Nonlinear Applications".
- Office of Naval Research, 2005-2009, "Development of Active Infrared Sensing and Teraherts Imaging Systems for Countering Improvised Explosive Devices.
- US Department of Energy, 2006-2009, "Multiscale Dynamics of Relaxor Ferroelectrics".
- Pennsylvania Department of Community & Economic Development, 2007-2009,"The All Optical Network".
- National Science Foundation, 2007-2010, "Glass Science, Processing and Optical Properties of Tellurite Fibers".
- Virginia Tech, 2008-2011, "Materials World Network: Nanoscale".
- Fiber Tek, 2009-2011, "Optical Fiber Fabrication Processes".
- Lehigh University, 2009-2010, "Lehigh Energy Initiative".