Browsing by Author "Michael J. Escuti, Committee Chair"

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Broadband Polarization Gratings for Efficient Liquid Crystal Display, Beam Steering, Spectropolarimetry, and Fresnel Zone Plate.
(2009-11-30) Oh, Chulwoo; Philip J. Bos, Committee Member; Jan Genzer, Committee Member; David E. Aspnes, Committee Member; David Schurig, Committee Member; Michael J. Escuti, Committee Chair
We introduce achromatic polarization gratings (PGs) as broadband polarizing beam splitters, which exhibit practically ~100% efficient diffraction over a wide range of spectrum. We have experimentally demonstrated high-quality achromatic PGs fabricated using holographic photoalignment techniques for liquid crystal (LC) materials. Non-ideal diffraction behaviors of the PGs have been investigated beyond the paraxial limitations (i.e., small grating periods, oblique incidence, and finite gratings) via extensive numerical analysis based on the finite-difference time-domain method. Design and fabrication of small-period PGs are also discussed to show how to achieve high diffraction efficiency and large diffraction angles at the same time. Three key innovative technologies utilizing the unique diffraction properties of the PGs have been introduced and experimentally demonstrated. The first application for light-efficient LC displays is the polymer-PG display. We have developed a prototype projector based on the polymer-PG display as a viable solution for ultra-bright pico-projector applications. Second, two novel beam steering concepts based on the PG diffraction have been proposed: a non-mechanical, wide-angle beam steering system using stacked PGs and LC waveplates and the Risley grating as a thin-plate version of the Risley prism. The third PG application is in advanced imaging and non-imaging spectropolarimetry. In the last part of this Dissertation, we introduce a polarization-type Fresnel zone plates (P-FZPs), comprising of spatial-variant linear birefringence or concentric PG (CPG) patterns. We have experimentally demonstrated high-quality P-FZPs, which exhibit ideal Fresnel-type lens effects, formed as both LC polymer films and electro-optical LC devices. In summary, we have explored the fundamental diffraction behavior of the polarization gratings and their applications in various optics and photonics technologies. We conclude this Dissertation with our suggestions of a number of potential innovations and advances in technologies that can be enabled by polarization gratings and related technologies.
Finite-Difference Time-Domain Analysis of Periodic Anisotropic Media
(2007-08-07) Oh, Chulwoo; Michael J. Escuti, Committee Chair; Gianluca Lazzi, Committee Member; Keith Weninger, Committee Member
The Finite-Difference Time-Domain (FDTD) method is a numerical technique for solving electromagnetic propagation and scattering problems. The FDTD method has been one of the most popular numerical tools in the computational electromagnetics since Kane Yee proposed his efficient and stable algorithm, often called the Yee algorithm. Recent rapid development in computer technologies in the last two decades allows us to have more power in computation and memory capacity, which overcomes the computationally intensive nature, the main hindrance of wide use of the FDTD method. Still, emerging new applications need modification to the original FDTD algorithm. For the applications of periodic structures such as gratings and photonic crystals, the FDTD method can be much more efficient and accurate by taking the advantage of the periodicity of structures. The simulation space can be dramatically reduced into only one unit cell by enforcing periodic boundary conditions (PBC). An efficient way of implementing PBCs is the split-field update method. The main advantage of the split-field FDTD method is its capability of wideband simulation at oblique incidence. However, the previous works were limited to materials that are either isotropic or that have diagonal tensors. Here we present a modified FDTD algorithm for periodic structures in more general anisotropic media, which incorporates the nondiagonal permittivity tensor. PBCs are implemented by using the split-field technique. Validation of the new FDTD method is done by applying it for problems of different structures and comparing the results from FDTD simulations with other analytical or numerical solutions. We report a rigorous numerical analysis of the Polarization Grating (PG) at the first time. Diffraction properties such as the diffraction efficiency and the polarization selectivity of each diffraction order are analyzed for all kinds of PGs. We discuss the effect of the grating regimes and the finite grating width on the diffraction properties of PGs. In addition, the minimum number of grating periods within a single pixel to get high diffraction effciency is presented. We apply this FDTD method, as a simulation tool, to analyze three different structures of broadband PGs. The optical performance of each structure is evaluated by the results from FDTD simulations in terms of the bandwidth for the maximum diffraction effciency. The achromatic PG using three gratings shows the best bandwidth and the twist-PG using two gratings with the opposite twist sense is found to be most attractive because of its simple structure and fairly large bandwidth. Preliminary experimental studies are also presented. Here we report a rigorous numerical analysis of polarization gratings for the first time. To this end, we develop the modified FDTD algorithm for periodic anisotropic media. We successfully demonstrate the FDTD method for a number of problems with different structures and the results from FDTD simulations show excellent agreement with other analytical or numerical solutions. Finally, we present the extensive analysis of polarization gratings and novel structures of broadband PG using our FDTD simulation tool. A package of the FDTD code written in the standard C⁄C++ language will be available in public as an open source.