This material is based upon work supported by the National Science Foundation under Grant No. DMR #0423914.
Goal: To investigate the use of multilayer structures for Faraday and surface magneto-optic Kerr effect rotation and, in particular, to investigate the application of surface and band edge enhancements effects for optical modulation and optical isolation.
Magneto-Optical effects such as the Faraday effect and the Surface Magneto Optic Kerr Effect (SMOKE) describe the rotation of the polarization of light in the presence of an applied magnetic field. Unlike other rotatory effects such as optical activity and electro-optic effects, the sense of rotation relative to the magnetic field direction is, for a given structure, independent of the direction of propagation of the light. Thus, the amount of rotation increases with repeated reflection back and forth through the same material. When a Faraday rotator is structured so that the effective net rotation angle during traversal is 45 degrees, the rotator, when combined with simple linear polarizers, enables the elimination of back reflections. The property that the rotation direction is independent of the propagation direction thus effectively allows one to build an optical isolator, a device which only conducts light in one direction. This elimination of potentially deleterious back reflections is very useful in, for example, stabilizing lasers, amplifiers and non-linear optical elements.
Such large angle rotations, however, require either large applied magnetic fields or large interaction lengths, the latter effectively preventing the miniaturization of Faraday rotation utilizing bulk materials. Multilayer interference, on the other hand, involves dramatic increases in interaction lengths for light propagating near the band edge of a multilayer photonic crystal. The use of multilayer materials may enable the miniaturization of optical isolators if materials and structures combining both large rotatory dispersion and large interaction lengths can be fabricated.
The design and production of polymer optical interference filters continues to be the subject of intense research and innovation due to the significant manufacturing advantages of polymers, such as cost, flexibility, and low weight. A fundamental problem for a variety of lighting and optical component applications is the reproduction of desired irregular spectral transmittance and reflectance curves near normal incidence. Applications include specialty filters for the lighting of artwork that are designed to eliminate regions of the visible spectrum that are not necessary for viewing a particular work. The use of extruded multilayer films for specialized spectral films offers unique opportunities as compared to more traditional sputtering, evaporation, or spin coating deposition techniques. Notably, the multilayer extrusion process enables rapid, low-cost, production of large-area free-standing films having a large number of layers. Custom multi-bandgap structures can be created through uneven splits during the layer multiplication process and through the post-processing coherent and incoherent layering of films. The layer multiplying process, however, also imposes unique design challenges requiring different optimization techniques. Though amenable to creating a large number of layers, the layer multiplication process limits the variety of different index materials that can be layered in a single process and is best suited to designs with layer profile symmetries compatible with the layer doubling steps of the multilayer extrusion process.
The work is being done in collaboration with Prof. Carl Dirk of University of Texas, El Paso and includes the design and testing of prototype spectral filters for custom optical filters and specialty lighting applications.
Sample Spectral Filter
Gap microlithography with negative photoresist will be used to pattern dome-shaped resist on the polymer surface prior to dry etching to achieve leyer-terminated plano-convex microlens on the CLiPS polymer materials. The resulting graded index microlens has numerous applications including solar concentrators and micro-GRIN lens arrays which promise super resolution imaging and increased lighting efficiency.
3-D Photonic crystals on Multilayer Polymeric Systems
Fabricate 3-D photonic crystal nanostructures (arrays of holes or pillars) on layered polymers. The planar arrays of holes or pillars provide a 2-D photonic crystal structure while the multilayer films in the polymer system will provide the third dimension in the 3-D structures. Arguably, this third dimension would be small with an expected refractive index difference between the layers of about 0.2 – 0.5, and would only be regarded as complementing the larger air/polymer refractive index difference. However, this contribution could yield a significant increase in the 2-D nature of the photonic crystal on LEDs for enhanced light extraction.