Light propagation and reflection by absorbing cholesteric layers

Multiple-beam optical interferometry of anisotropic soft materials nanoconfined with the surface force apparatus

Soft anisotropic materials that change their macroscopic properties in response to external stimuli such as light, electric field, or pressure are central to several new directions of technology, including optics, micro-mechanics, and bioengineering. Responsiveness is fundamentally connected to the anisotropic ordering of the material's building blocks at the nanometer scale. Here we propose the surface force apparatus as a powerful tool for investigating optically anisotropic materials at the nanometer scale using multiple-beam optical interference, allowing for simultaneous determination of film thickness, alignment, and chiral rotation of the optical axis. We present a method based on 4 × 4 matrices for calculating the exact transmission and reflection coefficients for light incident normal to a planar optical multilayer comprising an arbitrary number of chiral anisotropic layers with the helical axis normal to the layer. The multilayer can also include uniform birefringent media, optical adsorbing (e.g., metals) and isotropic materials. We introduce a technique to analyze and interpret the complex multiple-beam interference patterns arising from such multilayers and demonstrate it for the case of a twisted nematic liquid crystal confined to nanoscale thickness with the surface force apparatus. The analysis opens the prospect of studying the effect of strong confinement on the structure and response of a wide class of anisotropic materials.

Polarization dependence of optically driven polydomain elastomer mechanics

We model how polarized and unpolarized light can cause mechanical response in polydomain nematic and related photoelastomers. The reduction of order by heating and the consequential large strains that are known from nematic elastomers can alternatively be caused by light-induced bending of rodlike dye molecules which then equally reduce the order of their nematic hosts. While there is no mechanical response to heating of polydomain elastomers, mechanical response to light is possible by the selective absorption of light according to how domains are aligned with respect to the polarization direction or with respect to the propagation direction in the case of unpolarized light. We find large contractions or elongations, depending on the nature of polarization. The responses are nonmonotonic with light intensity.

Accurate and efficient calculation of light propagation in one-dimensional inhomogeneous anisotropic media through extrapolation

Berreman's 4 x 4 matrix approach has been generally applied to calculating light propagation in one-dimensional (1-D) inhomogeneous anisotropic media. In numerical calculations the propagator (propagation matrix) of whole 1-D inhomogeneous media is approximated by a stack of N homogeneous slab propagators. We analyze the error of the slab propagator in this slab approximation and show it is correct through the order 1/N(2). By using the extrapolation approach, we eliminate the leading error terms of the product (total propagator) of N homogeneous slab propagators successively. Numerical tests for a cholesteric liquid crystal show that the total propagator constructed through extrapolation is of higher accuracy and efficiency than Berreman's and Abdulhalim's or faster 4 x 4 total propagators.