Date of Award

January 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Engineering

First Advisor

Young L. Kim

Committee Member 1

Bumsoo Han

Committee Member 2

Vladimir M. Shalaev

Committee Member 3

Jenna Rickus

Abstract

Light in natural and biological media is known as freely diffusing. When light undergoes multiple scattering through inhomogeneous dielectric biomacromolecules, interference is ignored. If scattered waves return to their origin points along the time-reversed paths for constructive interference, outgoing waves can be forbidden, occasionally being on-resonance or off-resonance with Anderson localized modes. However, such a phenomenon in optics requires high-refractive-index contrasts, which intrinsically do not exist in organic molecules (e.g. proteins, lipids, carbohydrates, and nucleic acids). Here we predict and show experimentally that the long-standing perception of diffusive nature of light in biological media is broken by an exquisite distribution of silk protein structures produced from Bombyx mori (silkworm) and layered aragonite structures produced from Haliotis fulgens (abalone). We demonstrate how optical transmission quantities (e.g. light flux in transmission channels) are analyzed satisfying critical values of the Anderson localization transition. We find that the size of modes is smaller than specimen sizes and that the statistics of modes, decomposed from excitation at the gain-loss equilibrium, clearly differentiates silk and nacre from diffusive structures sharing the microscopic morphological similarity. This explains how the dimension, the size, and the distribution of disordered biological nanostructures result in enhanced light-matter interactions, in spite of low refractive index contrasts of constituent materials. As wave localization is universal, the presented results of electromagnetic waves will allow us to extend insight into electronic and mechanical waves in biological systems. Importantly, such optical resonances are extremely sensitive to subtle nanoscale perturbations, and thus can be implemented to biosensing platforms.

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