Doctor of Philosophy

dissertationPhotonic integration circuits (PICs) have received overwhelming attention in the past few decades due to various advantages over electronic circuits including absence of Joule effect and huge bandwidth. The most significant problem obstructing their commercial application is the integration density, which is largely determined by a signal wavelength that is in the order of microns. In this dissertation, we are focused on enhancing the integration density of PICs to warrant their practical applications. In general, we believe there are three ways to boost the integration density. The first is to downscale the dimension of individual integrated optical component. As an example, we have experimentally demonstrated an integrated optical diode with footprint 3 Ã- 3 m2, an integrated polarization beamsplitter with footprint 2.4 Ã- 2.4 m2, and a waveguide bend with effective bend radius as small as 0.65 m. All these devices offer the smallest footprint when compared to their alternatives. A second option to increase integration density is to combine the function of multiple devices into a single compact device. To illustrate the point, we have experimentally shown an integrated mode-converting polarization beamsplitter, and a free-space to waveguide coupler and polarization beamsplitter. Two distinct functionalities are offered in one single device without significantly sacrificing the footprint. A third option for enhancing integration density is to decrease the spacing between the individual devices. For this case, we have experimentally demonstrated an integrated cloak for nonresonant (waveguide) and resonant (microring-resonator) devices. Neighboring devices are totally invisible to each other even if they are separated as small as /2 apart. Inverse design algorithm is employed in demonstrating all of our devices. The basic premise is that, via nanofabrication, we can locally engineer the refractive index to achieve unique functionalities that are otherwise impossible. A nonlinear optimization algorithm is used to find the best permittivity distribution and a focused ion beam is used to define the fine nanostructures. Our future work lies in demonstrating active nanophotonic devices with compact footprint and high efficiency. Broadband and efficient silicon modulators, and all-optical and high-efficiency switches are envisioned with our design algorithm

Phonon Modes in Single-Walled Molybdenum Disulphide (MoS2) Nanotubes: Lattice Dynamics Calculation and Molecular Dynamics Simulation

We study the phonon modes in single-walled MoS$_{2}$ nanotubes via the lattice dynamics calculation and molecular dynamics simulation. The phonon spectra for tubes of arbitrary chiralities are calculated from the dynamical matrix constructed by the combination of an empirical potential with the conserved helical quantum numbers $(\kappa, n)$. In particular, we show that the frequency ($\omega$) of the radial breathing mode is inversely proportional to the tube diameter ($d$) as $\omega=665.3/d$ {cm$^{-1}$}. The eigen vectors of the first twenty lowest-frequency phonon modes are illustrated. Based on these eigen vectors, we demonstrate that the radial breathing oscillation is disturbed by phonon modes of three-fold symmetry initially, and the tube is squashed by the modes of two-fold symmetry eventually. Our study provides fundamental knowledge for further investigations of the thermal and mechanical properties of the MoS$_{2}$ nanotubes.Comment: Nanotechnology, publishe

Comment on " a unified scheme for flavored mesons and baryons"

We would comment on the results of the paper "a unified scheme for flavored mesons and baryons" (P.C.Vinodkumar, J.N.Panandya, V.M.Bannur, and S.B.Khadkikar Eur. Phys. J. A4(1999)83), and point out some inconsistencies and mistakes in the work for solving the Dirac equation. In terms of an example for a single particle we investigate the reliability of the perturbative method for computing the Coulomb energy and discuss the contribution to the wavefunction at origin from the Coulomb potential. We conclude that the accuracy of their numerical results needs to be reconsidered.Comment: Latex file, 11page

First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defect

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect (substitution of C by Si atom) is investigated by non-equilibrium Green's function (NEGF) formalism combined with first-principle calculations density-functional theory with local density approximation. An efficient correction to the force constant matrix is presented to solve the conflict between the long-range character of the {\it ab initio} approach and the first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a vacancy defect, the thermal conductance is very sensitive to the position of the vacancy defect. A vacancy defect situated at the center of the nanoribbon generates a saddle-like surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. For the Si defect, the position of the defect plays no role for the value of the thermal conductance, since the defective region is limited within a narrow area around the defect center.Comment: accepted by AP

BaFe2Se2O as an Iron-Based Mott Insulator with Antiferromagnetic Order

A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an orthorombic structure, namely BaFe2Se2O, has been successfully fabricated. Experimental results show that this compound is an insulator and has an antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals the narrowing of Fe 3d bands near the Fermi energy, which leads to the localization of magnetism and the Mott insulating behavior. The large distances between the Fe atoms perhaps are responsible for the characters. Linear response calculation further indicates a strong in-plane AF exchange $J$, this can account for the enhanced magnetic susceptibility (which has a maximum at about 450 K) above the Neel temperature.Comment: submitted to PRL on 2 May 2012, resubmitted to PRB on 31 May 2012, and accepted by PRB on 5 July 201