Online

Allemeier, David

Graduate Program in Materials Science

2023

Ph. D.

Metal-dielectric photonic crystals are a class of photonic structure which offer fascinating methods to manipulate and investigate the interaction of light and matter. Like other photonic structures, a metal-dielectric photonic crystal consists of a periodic array of materials of dissimilar optical properties. The periodicity of these structures produces optical resonances that allow them to interact strongly with light of compatible wavelength. In a metal-dielectric photonic crystal, the structure is an alternating series of semi-transparent metal layers and transparent dielectrics, where each transparent layer is sandwiched between two partial mirrors. Interference of the trapped light results in the creation of a standing-wave electric field, known as a Fabry-Perot resonance, while the sandwich structure is referred to as an optical microcavity. A metal-dielectric photonic crystal consists of many microcavities stacked on top of one another, such that each cavity is able to interact with its neighbors. Under certain conditions, the cavity resonances undergo hybridization to form a new set of states, just as the states of an atom become hybridized to form molecular orbitals in molecules and energy bands in solids. Hybridization results in a splitting of the Fabry-Perot resonances into a photonic band composed of one state per cavity. The structure of this band depends sensitively on the unit cell geometry of the crystal and the optical properties of the materials, which determines the coupling strength. Breaking the symmetry of the crystal by introducing alternating material thicknesses or composition can introduce a photonic band-gap by doubling the unit cell dimensions, and creates a binary-type MDPC. Higher-order crystals and quasi-crystals can be created by appropriate unit cell design. In this work, we demonstrate the ability to use these effects to produce a novel light-emitting device based on organic light-emitting diode (OLED) technology. We show that OLED microcavities can be stacked to form a metal-dielectric photonic crystal that can be electrically driven to produce light. The electroluminescence spectrum inherits the states of the photonic crystal through enhancement and inhibition of the spontaneous emission rate of the organic molecules through the Purcell effect. We show that this is the result of weak coupling between the excitonic organic emitters and the cavity modes. We discuss the underlying theory that shapes these resonances using the formalism of quasinormal mode theory, coupled-mode theory, and the optical transfer matrix method. Analytical solutions for the resonant states are presented for both simple and binary crystal structures. We further discuss the phenomenon of epsilon-near-zero modes in metal-dielectric photonic crystals, and demonstrate the experimental realization of both binary and simple MDPC OLED devices.