Partial Order Colloidal Phases as Photonic Solids

Cornell University, Ithaca NY

Investigators

Abstract

Nontechnical Description: The objective of this research project is to fabricate partial order photonic mesophases made of non-spherical colloidal particles and characterize their optical properties. These novel materials exhibit structural ordering between those of crystals and glasses and hold promise to strongly modify spectral and angular transport of light (similar to crystals) and also to promote bandgaps with high isotropy (similar to glasses). The research helps clarify the mechanisms for the formation of photonic band gaps in various photonic media and seeks novel optical properties such as negative refraction for ultrahigh resolution lenses and low speed of light for enhanced nonlinear optics. The education activities of this project include developing informal science learning activities at the Ithaca Sciencenter; broadening participation of underrepresented minorities and women in science through involving them in research; and contributing to a manual of the best practices for undergraduate research developed by the Institute for Broadening Participation. In particular, graduate student facilitators are trained in science communication to public audiences including children. Technical Description: This project exploits the thermodynamics of shape-anisotropic colloids to capture photonic mesophases, a new class of photonic materials in which order and disorder cohabit, and to study their photonic properties. The research aims to develop an understanding of the types and bounds of order that promote significant photonic properties (i.e., photonic band gaps, negative refraction, slow light) and an ability of fabricating photonic mesophase materials for photonic slabs via low-cost, large-area self-assembly. Specifically, the self-organization of anisotropic colloids and shape-binary mixtures is mapped out under wedge-cell confinement to elucidate the conditions for stabilizing photonic mesophases. Confinement height up to five layers, system density and interparticle interactions are varied and the self-organization is studied using quantitative confocal microscopy imaging with precise spatial and temporal resolution. The spatial distributions of the mode fields, transmission (reflection) spectra and density of states are calculated to elucidate the structure and optical property correlations. The photonic mesophases are characterized using angle-resolved specular reflectance measurements to probe the dispersion relations. The effects of partial order on refraction and slow-light propagation properties are determined from the analysis of equifrequency contours and the calculation of the group velocity at the band gap edges, respectively. Simulations of the refraction and the flat lens imaging of a point source are performed for photonic mesophases that exhibit all-angle negative refraction and effective refractive index of negative one.