Functional Bragg reflectors: patterning a block copolymer Presenter: Hilary McWilliams Mentor: Michael Birnkrant Advisor: Christopher Li Dream Program Drexel University, 3141 Chestnut St., Philadelphia PA, 19104

Motivation U.S. Arrests Suspect in Shining of Laser at Aircraft January 5, 2005, Wednesday DAMIEN CAVE A man from Parsippany, N.J., was accused on Tuesday of shining a laser pen into the cockpits of a small passenger jet and a helicopter over the last week, Late Edition - Final, Section B, Page 1, Column 2 Current research: Variable wavelength laser protection Problem: Lasers present a real danger to pilots. Impact and Future work: Producing heads up displays with Inherent laser safety for pilots

Photonic Crystal Photonic crystals are optical materials with periodically changing refractive indices. This periodic change of refractive indices leads to Photonic band gaps that confine light. A Bragg reflector is a specific one Dimensional photonic crystal that has a photonic band gap in the range of the visible spectrum.

Benefits and Applications of Photonic Crystals Small size High integration density Ability to guide light Benefits Highly integrated devices Microanalyzers Applications Optical computers Optical circuitryTelecommunications

Photonic Crystal Fabrication Methods Opal templates assembled by deposition of silica from solution onto a silicon wafer Photonic crystal prepared by a lithographic technique Photonic crystal prepared using Holographic Photopolymerization

Holographic Polymerization Process Holographic writing of prepolymer syrup. 27% PE-b-PEO 70% Photopolymerizable Monomers Prepolymer Syrup containing reactive monomers and block copolymer The standing wave pattern results in local areas of higher polymerization rates; Photopolymerizable Monomers diffuse to those areas while PE-b-PEO diffuses to areas vacated by the reactive monomers. Incident Laser Beam } Standing wave pattern

Results from Holographic Patterning of Polyethylene Glycol PEG produces semi- continuous layers, Left VS. liquid crystals patterning results In droplet formation, Right Holographically Patterned Liquid Crystals Transmission spectra, below Sharp notch around 600nm Grating calculated ~208 nm Transmitted Light Reflected Wavelength λ 0 Λ Christopher Y. Li, Michael J. Birnkrant, Timothy J. Bunning et. al Soft Matter, 2005, in Press SEM shows Semi-continuous layering PEG, the lighter regions, and Photopolymerized Monomers, the dark regions. The grating d-spacings, Λ, ~220nm

Dynamic Behavior: Thermal Switching The results found PEG-based gratings posses a dynamic behavior with respect to thermal stimulation. Thermal switching occurs because PEG melts and Crystallizes Upon heating Reflected wavelength red-shifts Upon cooling Reflected wavelength And original position Christopher Y. Li, Michael J. Birnkrant, Timothy J. Bunning et. al Soft Matter, 2005, in Press

Current Research: Block Copolymers Polyethylene glycol, which is known to form semi continuous layers, is covalently linked to polyethylene, which has limited solubility in the prepolymer syrup to disperse PE evenly in the Bragg Reflector. Tested the compatibility between PE-b-PEG and Norland optical resin Prepared pre-writing syrup Prepared CNT/Norland syrup

Stepping into the Future 1.Inclusion of proteins in BR 2.Holographic patterning CNTs in BR 3.Pattern functionalized CNTs into BR Look at holographically patterned syrups using SEM Evaluate changes in the transmission spectrum as a function of temperature for PE-b-PEO Proteins Carbon nanotubes Functionalized carbon nanotubes