Braun Group People
Abby Griffith
Graduate Student in Materials Science and Engineering
B.S. in Materials Science and Engineering from North Carolina State University in 2005
There has been much scientific and technological interest in creating holographic optical elements. Such elements are lighter, and more compact than classical optics, yet also have the advantage that they enable multiplexing. Photopolymers have been used as a recording medium for static holograms. However, much effort has been invested to add dynamic functionality to these holograms, thereby enabling new applications such as optical beam stirring and tunable optical filters. One approach to achieve this goal has been to add liquid crystal to the photopolymer, making the holographic structures tunable by an external electric field. These holographic polymer dispersed liquid crystal structures however have limited multifunctionality. I am interested in adding various nanoparticles into the polymer matrix in order to create multifunctional Bragg gratings.
Holographic polymer dispersed liquid crystal (H-PDLC) gratings are produced by first mixing a photoinitiator, a photopolymerizable monomer, and liquid crystal to form a uniform mixture. Nanoparticles that are to be incorporated into the grating are also added. This mixture is then placed at the intersection of interfering laser beams. In the high intensity regions of constructive interference, polymerization of the monomer is initiated, pushing the liquid crystal into the low intensity regions and forming droplets of liquid crystal in the polymer matrix. The nanoparticles are functionalized so that they remain in the polymer regions of the grating.
A typical transmission electron micrograph of the nanoparticle containing H-PDLC:
Due to the periodicity of the refractive index in the grating, a stop-band is formed as can be viewed by measuring the wavelength versus transmission.
The liquid crystal is birefringent, meaning there is an anisotropism of the refractive index (n). Moreover, when the index of refraction of the polymer is equal to that of the liquid crystal, light is transmitted without much scattering. Without the application of an electric field, the directors of the liquid crystal domains are not aligned and the stop-band remains intact. When an electric field is applied, the directors align and the grating becomes transparent in the direction of the applied field, and therefore the stop-band disappears.
*Thanks for the help of John Busbee and Wright Patterson Air Force Base
Professor Paul Braun • Phone: +1.217.244.7293 • Fax: +1.217.333.2736 • Email: pbraun@illinois.edu
Department of Materials Science and Engineering • University of Illinois at Urbana-Champaign