Office: S2 - 259
Dr. Jang's Home Page (In Progress)
Professor Jang specializes in the area of experimental condensed matter physics and nonlinear optics.
Prof. Jang received his Ph.D. in physics from the University of Illinois at Urbana-Champaign in 2005. His thesis work involves modeling of relaxation kinetics of excitonic matter in Cu2O based on time- and space-resolved spectroscopic techniques. He completed his B.S. in physics from Yonsei University, Seoul, South Korea in 1994.Dr. Jang joined our department in the fall of 2010 after he worked at Northwestern University as a postdoctoral fellow. He is an internationally recognized expert in the field of fundamental exciton physics. Recently, he was invited to contribute the book chapter in “Optoelectronics” prepared by InTech and to join the Editorial Board Members of the World Journal of Condensed Matter Physics. He is in charge of setting up the Ultrafast Nonlinear Optics Lab (UNOL), which will pursue multi-department research and training projects.
DESCRIPTION OF CURRENT RESEARCH
Imagine a gas within a solid. How is that possible? That the basic electronic excitation of a semiconductor can be viewed as a nearly free particle in a box takes a leap of imagination. However, the fundamental wave nature of particles leads to this picture. An exciton in a semiconductor is the elementary electronic excitation, composed of an electron and a hole bound by screened Coulomb interaction. As a composite boson, an exciton can be optically created and makes itself visible by radiative recombination. If excitons can be raised to sufficient densities, they may undergo Bose-Einstein condensation (BEC) at low temperatures.
A direct-gap semiconductor Cu2O provides a model system for studying thermodynamics as well as population and relaxation kinetics of excitonic matter owing to its unique crystal and coupled band structures. From the perspective of fundamental exciton physics, renewed interest came with the observation of exciton polaritons. An exciton polariton is a coherent quantum superposition of an exciton and a photon, where the initial coherence with a definite wavevector is primarily determined by the incident laser pulse. As a half-matter/half-light quasiparticle, the exciton polariton exhibits unusual properties such as reduced two-body cross section, scattering by atomic-scale impurities, and resonantly enhanced reflections.
In our group, we study excitonic matter including free excitons, bound excitons, biexcitons (excitonic molecules), and exciton polaritons in various natural-growth and synthetic semiconductors, such as Cu2O, CuCl, ZnO, and CdMnTe, using several excitation methods. We also apply external perturbations, such as a mechanical stress, in order to study various interesting properties of excitonic matter arising from quantum mechanical level mixing. Our ultimate goal is the experimental demonstration of BEC of excitonic matter under optimum conditions.
2) Nonlinear Optics
Crystals lacking inversion symmetry naturally exhibit a second order nonlinearity with electric field characterized by a tensor χ(2). This effect can cause sum frequency generation (ω1+ω2) and difference frequency generation (ω1–ω2) in which the input waves of frequencies ω1 and ω2 combine within the medium. Of special interest is the so-called second harmonic generation (SHG), a frequency doubling process, where a single fundamental wave generates another wave with the twice of the optical frequency. When one satisfies a so-called phase matching condition, the SHG grows at the expense of the fundamental wave.
There are numerous nonlinear materials such as lithium niobate (LiNbO3), potassium titanyl phosphate (KTP), and lithium triborate (LBO), exhibiting a high χ(2) in a visible and near IR range. But they become very inefficient in the mid IR range due to a phase matching problem. Therefore, there is a considerable interest in finding materials that will perform in the IR. In collaboration with Prof. Kanatzidis’s group at Northwestern University, we study SHG responses in the mid IR range arising from some novel chalcogenide materials – KPSe6, RbPSe6, LiAsS2, NaAsS2, KZrPSe6, RbZrPSe6, and so on. Our research on this topic is potentially important for high-density-information transfer in telecommunications and broadband high-speed internets (1.3 – 1.6 mm) and medical applications (2 – 12 mm). We also investigate the third harmonic nonlinearity χ(3) using the so-called Z-scan technique. Using closed and open aperture scans, we can estimate not only χ(3) but also multiphoton absorption coefficients.
SELECTED PUBLICATIONS ( .doc, 32kb)
Last Updated: 1/6/11