Ultrafast Nonlinear Optical Characterization Group
Shining Light on Symmetry, Structure & Emergent Phenomena
Our group probes the symmetry, atomic scale structure, and a range of optical and electronic properties of materials through Ultrafast Lasers and Xrays. We have two main areas of materials focus: Complex Oxides and Semiconductors. Underlying both of these, is our fundamental explorations in discovering new Symmetries in nature. We are primarily a Materials Characterization Group (optical, electronic, structural, thermal properties), who work closely with theory and synthesis groups in team projects, where students have an opportunity to learn multiple skills.
Complex oxides form one of the largest and most active areas of condensed matter physics and materials research since their structure can incorporate a vast majority of the elements in the periodic table giving rise to properties ranging from dielectrics, ferroelectrics, magnetism, correlated electronic behavior, semiconductors, metals and superconductors. In an Interdisciplinary Research Group (IRG 1) titled "Designing Functionality into Layered Oxides" within the NSF-funded Center for Nanoscale Science at Penn State, we are designing new complex oxides that possess novel or contra-indicative properties within a single material, such as competing and metastable polar states, improper ferroelectrics, correlated metals with a built in polarization (polar metals), optically transparent electrical conductors, and materials with both electric and magnetic polarization (multiferroics). The IRG1, led by Gopalan, encompasses theory, synthesis, and characterization groups from five universities (Penn State, Cornell, Berkeley, Northwestern, UC Irvine). In another team project funded by the Department of Energy (DOE), "Ultrafast Emergent Phenomena in Complex Oxide Heterostructures," we are probing ultrafast phenomena in complex oxides. The idea is to excite materials out of their ground state using photons at different frequencies, and study new emergent properties in the excited state. This project, led by Gopalan, involves 10 people from 7 different institutions (Penn State, Argonne National Labs, Rutgers, Columbia, UCSD, Berkeley, Northwestern), and brings theory, synthesis and experiments together. In both these projects, our group focuses on the characterization using linear and nonlinear optics, synchrotron diffraction experiments, scanning probe microscopy, and high resolution electron microscopy.
Semiconductors define our current day digital reality. In close collaboration with John V. Badding group in Chemistry, we have over the past ten years developed the area of semiconductor optical fibers. Using High-Pressure Chemical Vapor deposition, Badding group invented a way to deposit high purity semiconductors such as Si, Ge, and ZnSe into the nanoscale holes of photonic crystal fibers, thus forming fibers with semiconductor cores and silica cladding. Such fibers can enable the field of all-fiber optoelectronics, where light generation, modulation, and detection can all be performed within an optical fiber without the light exiting the fiber, so called, fly-by information processing. Over the years, we have demonstrated in-fiber p-i-n junctions for light modulation, high speed detection, fiber lasers, and endoscopic imaging in the infrared. Gopalan group has also recently demonstrated long single crystal semiconductors fibers using a combination of HPCVD and laser annealing. The NSF-CNS center at Penn State has an IRG(3) led by Badding on semiconductor meta-lattices, where any 3D porous structure can be infiltrated by semiconductors to create quantum solids that can exhibit both long range electron transport as well as quantum confinement. Gopalan group is exploring thermal properties of such metalattices using ultrafast optical techniques.
Symmetry underlies the laws of nature, and the structure of crystalline materials. The symmetry of a crystal determines the form of all tensor properties of that crystal. We have recently identified a new antisymmetry in nature, called Distortion Reversal Symmetry that describes the symmetry of a path of distortion under any external perturbation. We have also listed all of the 17,803 Double Antisymmetry Groups that are applicable to materials with two possible physical phenomena that involve antisymmetries, for example, a magnetic material undergoing a distortion. These can be downloaded here. In collaboration with Dabo group, we are incorporating these symmetries into density functional theory codes such as Quantum Espresso. We continue to explore new symmetries in nature, and their applications in materials research.