Sameet K. Shriyan

Analysis of effects of oxidized multiwalled carbon nanotubes on electro-optic polymer/liquid crystal thin film gratings.

This work focuses on experimentally demonstrating the modification in diffusion kinetics, formation of holographic polymer dispersed liquid crystal gratings and an improvement in its electro optic response by doping them with multi-walled carbon nanotubes. Results indicate a faster rise and fall times which is attributed to the reduction in size of the liquid crystal droplets formed and a reduction in switching voltage due to change in dielectric properties of the medium as manifested by a rise in capacitance. Real time diffraction efficiency measurements reveal a time delay in the appearance of the diffracted order due to non-participation of the nanotube in the polymerization induced phase separation process. An analysis of this effect is presented based on the Stoke-Einsteins diffusion equation incorporating shape anisotropy of the nanotubes.

Electro-optical effects of oxidized multi walled carbon nanotube doping on holographic polymer dispersed liquid crystal films

In this work we demonstrate an improved electro-optic response and quantify the effect on transmitted wavefront properties of thiolene based reflection mode holographic polymer dispersed liquid crystals (HPDLC) gratings doped with oxidized multi-walled carbon nanotubes (MWNT). Effect of various doping levels on the reflection efficiencies is evaluated and optical spectrometry results indicate a reduction in the reflection efficiency and an anomalous electrooptic behavior at higher doping levels of MWNT especially in gratings with longer pitch where the diffusion length for liquid crystal (LC) is long. Wavefront analysis based on Shack-Hartmann wavefront sensor show an increase in the transmitted RMS wavefront error in a 633nm wavefront after a critical level of MWNT doping. Polarized optical microscopy results indicate that the MWNT do not participate in the photo polymerization induced phase separation hence acting as physical barriers for the counter diffusing LC at high MWNT concentrations. Reduction in overall size of the LC droplets in the LC rich planes, observed using scanning electron microscopy imaging, leads to faster rise and fall times hence quicker relaxation time. Observation of reduced switching voltage is attributed to the modification of dielectric properties of the medium manifested by an increase in capacitance and decrease in resistivity in presence of MWNT.

Electro-optic polymer liquid crystal thin films for hyperspectral imaging

The authors present a study focused on the feasibility of using holographic polymer dispersed liquid crystal wavelength filters for hyperspectral imaging (HSI). For this study, stacks of these filters were fabricated in the visible wavelength range of 600 to 800 nm. These filters were demonstrated to have a number of properties useful for HSI applications, including uniform reflection efficiency of 80% across a 35 mm optical aperture, polarization insensitivity for normal incidence, spectral resolution of 10 nm, and fast switching times on the order of microseconds. In addition, the ability to modulate each filter in the stack at a different frequency allows for spectral multiplexing, thus enabling synchronous detection and demodulation of the image data. Although the filters in their current state show promise for HSI applications, techniques to further improve performance in terms of viewing angle range and transmission throughput are presented. Finally, a system level integration of such a stack into the prototype drive and detection unit is discussed.

Multilayer stacking technique for holographic polymer dispersed liquid crystals

We demonstrate an alternate method of stacking holographic polymer dispersed liquid crystal (HPDLC) reflection gratings on substrates coated with indium tin oxide on both sides allowing independent switching of each grating in the stack. Successive layers of the stack are formed by switching existing layers, while exposing the subsequent layer to an interference pattern. Wavefront analysis based on wavefront propagation through HPDLC with electric field on and off is used to substantiate the improvement in the reflection efficiency of the layers in the stack. Results show an optical path length reduction due to elimination of substrate layers at each grating.

Improved Electro-Optic Response of Polymer Dispersed Liquid Crystals Doped with Oxidized Multiwalled Carbon Nanotubes

This work focuses on improving the electro-optic response of thiolene and acrylate based Polymer Dispersed Liquid Crystals by doping them with oxidized multiwalled carbon nanotubes (MWNT). Results indicate a reduction in switching voltage which is attributed to the enhanced electric field experienced by the Liquid Crystal (LC) droplets trapped in the polymer matrix manifested by a decrease in resistivity and an increase in capacitance of the composite medium. Improved switching speeds is related to the reduction in size of the LC droplets since the MWNT act as physical barriers to the diffusing LCs preventing coalesce, confirmed with scanning electron microscopy imaging. PDLCs are doped with various concentrations of MWNT to determine an optimal doping level. An anomalous electro-optic behavior is noted at higher concentrations.

Modification in diffusion kinetics and electro-optic behavior of holographic thin films with carbon nanotubes

This work focuses on the study of the modification in formation and electro-optic behavior of holographically formed polymer -liquid crystal thin film gratings doped with multiwalled carbon nanotubes. Results indicate a time delay in the evolution of the first diffraction order in the presence of carbon nanotubes when compared to ones with no nanotubes. An analysis is presented based on the modification of the diffusion kinetics in terms of photo induced phase separation. This slow down is attributed to the non-participation of the carbon nanotubes in the phase separation process, and acting as physical barriers to the counter diffusing liquid crystals. The diffusion constant of nanotubes, incorporating its shape anisotropy, is computed in such a photo polymerizable system and compared with those of the participating polymers and liquid crystals. An optimal concentration of carbon nanotube doping is arrived at which helps in improve the switching speed while maintaining diffraction efficiency. Improvement is switching speed is attributed to reduction in size of the liquid crystal droplets. Scanning electron microscopy results indicate a change in morphology of the gratings doped with carbon nanotubes. In specific, smaller droplet size and, beyond the optimal level of nanotube doping, imperfect liquid crystal phase separation with scarcity of liquid crystal droplets across the sample is seen.

Holographic electro-optic thin film stacks for airborne hyperspectral imaging

In this work we demonstrate the feasibility of using a holographically formed thin film electro-optic stack for the development of an airborne hyperspectral imaging system in the visible wavelength range of 600nm to 800nm. Each wavelength filtering element in the stack is formed by photo-induced phase separation of a homogenous mixture of liquid crystals and photopolymers, exhibiting a uniform reflection efficiency of up to 80% across a 35mm optical aperture with non-normalized baseline transmission, polarization insensitivity for normal incidence and a spectral resolution of 10nm. Fast switching time on the order of microseconds and techniques to improve view angle in the individual wavelength filtering elements in the stack are discussed and the improvements are discussed from a morphological standpoint. Two techniques for stacking the thin films have been developed which requires lesser number of substrates hence improving transmission throughput and radiometric efficiency through the stack. An advantage of using such a stack is the ability to modulate each wavelength filtering element at a different frequency to obtain a spectral multiplex, thereby enabling synchronous detection and demodulation of each wavelength with a high update rate for the hyperspectral cube. A system level integration of such a stack into the prototype drive and detection unit is discussed in this work.