Photonic Slabs With Anisotropic Colloid And Colloid Inspired Bases
The concept of photonic crystals as 'semiconductors for light' has given rise to strategies for the design and fabrication of periodic dielectric materials with dramatic effects on photon dispersion relations. Photonic crystal theory has revealed numerous reduced symmetry and/or complex basis champion structures that enhance light-matter interactions and promote photonic band gap properties. Realizations of such structures at visible and near infrared wavelengths are often challenging and may be achieved a decade or more after they are envisioned. Colloidal self-assembly routes, in particular, have been desired for visible light control, low cost, ability to span from two- to three- dimensions and potentially large area, parallel processing. Thermodynamic simulation literature in the context of supramolecular chemistry studies indicates non-spherical building blocks hold promise for incorporating a rich diversity of packing arrangements inaccessible with commonly available spherical bases. Investigations in the synthesis and processing science foundations for obtaining order (i.e., mesophases and crystals) along with the optical characterization consistent with the thin film forms are essential to understanding structure-property correlations for photonic solids from colloids. This dissertation demonstrates the application of physical confinement to direct selfassembly. In addition, the photonic band gap characterization of two-dimensional (2D) and quasi-2D structures from mushroom cap and asymmetric dimer shaped colloids was performed using photonic crystal 'slab' configurations. Aqueous suspensions of micronsized mushroom cap-shaped particles were self-organized by gravitational sedimentation in a wedge-shaped confinement cell. The sequence of phases with increased cell height was studied with precise spatial and temporal resolution via fast confocal microscopy. In addition to the phase symmetry transitions found for spheres, rotator (i.e., plastic crystal) and orientation-dependent states were determined. The photonic band gap forming properties of the buckled phase observed in the mushroom-cap particle system under confinement were theoretically modeled. The 2D finite height photonic crystals with dielectric shaping in three dimensions imparted variation to the cross-sectional profile of the slab. The configuration is atypical for lithographic realizations of photonic slabs and also for 2D photonic crystal models assumed to extend infinitely with fixed cross-section along the z-axis. The experimentally tunable variables of shape and degree of buckling were captured using structural parameters. Complete band gaps in the guided modes were determined as a function of shape parameters, lattice distortion, filling fraction, refractive index contrast between low and high dielectric regions, etc. Slab structures were also modeled based on dimers (i.e., two adjacent or overlapping spheres) in centeredrectangular lattices previously realized through convective (i.e., evaporation-assisted) or confinement self-assembly. The degree of lobe fusion and the degree of lobe symmetry shape parameters accessible experimentally were mapped in addition to the structural and materials parameters. Large band gaps for each light polarization as well as polarization independent band gaps were found in a wide range of structures. Additionally, dimer cylinder bases on centered rectangular lattices were modeled to compare properties of realizations consistent with microfabrication using lithography and with colloidal processing for self-assembly of photonic templates. The study suggests that the ideation for structural design in slabs can be enriched by the combination of thermodynamicallyinspired structures with the ease of optimizing slab height (i.e., through lithographic fabrication).
Photonic band gap materials; Photonic crystal; Colloids
Liddell, Chekesha M
Lipson, Michal; Hennig, Richard G.
Materials Science and Engineering
Ph. D., Materials Science and Engineering
Doctor of Philosophy
dissertation or thesis