Nonlinear Optical Enhancement from Aggregate and Ensemble Effects in Organic Systems.

Abstract

Organic optoelectronics provide several advantages over traditional Si based systems. These advantages are typically cost, ease of manufacturing, and environmental impact. One area of particular interest in the field of optoelectronics is that of two-photon absorption (TPA), a nonlinear optical process. In recent years it has become apparent that many dyes can exhibit good two-photon properties. However, attempts to generate further enhancement by self-assembly into nanoparticles has been difficult. This is in large part due to a lack of good optical characterization method for single nanoparticles at resolutions appropriate for determining structure-function relationships on a per particle basis. The work presented here will show organic macromolecular and aggregate systems assembled in such a way as to enhance the nonlinear optical properties orders of magnitude above the constituent building blocks. A focus will be placed on the development of tools appropriate for observing multiphoton response in individual nanoparticles on a single particle basis, namely work done to advance Two‐Photon Near Field Optical Microscopy (TPA‐NSOM). This discussion will show advances to both two‐photon absorption NSOM and two‐photon NSOM fluorescence, as well as how these techniques can quantitatively assess per particle and per molecule two-photon enhancement. Additionally, ultrafast excited state behavior of organic nanomaterials will inform us how these enhancements occur. Several systems, including macrocycles which mimic natural light harvesting, liquid crystalline materials, molecular wire arrays and "click" assembled metallated structures will be studied. It will be shown that the self assembly of dimers into macrocycles can enhance the two photon response by two orders of magnitude, dimer assembly into nanoparticles can give a five-fold two-photon enhancement, and that nanorods can be self assembled from marginal two-photon materials into nanoparticles with a fourty-fold two-photon emission enhancement. The viability of TPA-NSOM for determining single particle TPA response will be displayed for several systems, with optical resolutions approaching 10 nm.