Tunable electro-optic thin film stack for hyperspectral imaging

Abstract

This dissertation focuses on the development of a compact hyperspectral system for airborne imaging based on an H-PDLC tunable filter as the enabling technology. With a primary focus on the development of an H-PDLC stack, this research aims at improving the electro-optic response of Polymer/liquid crystal thin films in both holographically patterned and non-patterned configurations by addition of carbon based nanoparticles namely multiwalled carbon nanotubes (MWNT). Results indicate a reduction in switching voltage, and an improvement in switching speed while at the same time maintaining, reflection efficiency, transmitted wavefront characteristics and minimal dependence on incident polarization. The reduction in the switching voltage is attributed to a change in the dielectric properties of the medium, hence reducing any electrical depolarization effects in the sample and an improvement in switching time is attributed to the reduction in the liquid crystal (LC) droplet sizes hence an increase in relaxation frequency. An optimal nanoparticle doping concentration is arrived at based on the results.The effects on the kinetics of formation of H-PDLC in the presence of MWNT based on a diffusion model and LC redistribution governed by law of conservation of mass is studied. The addition of MWNT slows down the counter diffusion of LC‘s and also acts as physical barriers for the LC‘s hence preventing coalesce.In order to provide electro-optic access to specific wavelengths in a given spectral band two novel stacking techniques have been developed in order to make a stack of H-PDLC integratable into devices. The first technique focuses on reducing the overall optical path length by reducing the number of substrates required to fabricate a stack therefore reducing the stray Fresnel reflections form the substrate layers. An analysis of improved throughput through this device is presented based on wavefront propagation though the stack using a transfer matrix approach incorporating index inhomogeneties and interfacial roughness. The second technique focuses on eliminating all substrate layers except the base by spin coating H-PDLC and coating it with conducting polymer layers.Finally, based on the improved electro-optic response and reduced stack a HPDLC wavelength filter in conjunction with a compressive imaging system is being developed for hyperspectral imaging where the H-PDLC stacks act as the spectral decomposing element. These stacks allow for two modes of operation in the hyperspectral imager- sequential switching and spectral multiplex. A byproduct of the fast response time of the H-PDLC is the ability to modulate each layer at a different frequency. This provides the ability to encode each wavelength in frequency, which can be used to obtain a spectral multiplex and increase system throughput.%%%%Ph.D., Electrical Engineering – Drexel University, 2011