Polymer-Stabilized Cholesteric Diffraction Gratings:  Effects of UV Wavelength on Polymer Morphology and Electrooptic Properties

Abstract

We compare the performance of polymer-stabilized cholesteric liquid crystal diffraction gratings for two limits of the polymer morphology: a one-dimensional array of polymer walls that extend through the grating thickness (“bulk network”), and a thin layer of patterned polymer fibrils localized at one surface (“surface network”). In each case, the polymer is spatially templated by the liquid crystal orientational order, but the depth of the network is dictated by UV absorbance of the liquid crystal and a choice of wavelength used to initiate photopolymerization. Whereas both polymer morphologies yield robust electrically switchable gratings, the surface-stabilized grating operates at lower field thresholds and shorter transition times. However, the patterning of the surface network significantly reduces the contrast between grating on and off states, a limitation that can be potentially offset by employing isotropic monomers index-matched to the liquid crystal.

Direction switching and beam steering of cholesteric liquid crystal gratings

“…The half-integer orders (61/2) are caused by a period of 2P near the substrate surfaces associated with the boundary of the substrates. 17,18 The beam spots are a bit broadened. That is due to the facts that the self-assembled stripe directions are slightly uneven, and the grating structure is partially distorted by the electrical field.…”

Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

“…[245] The polymerization of the CLC and monomer mixture was done in two different UV wavelengths, 322 and 365 nm. [246] In the shorter wavelength, there is high absorption and the polymer network forms on the surface while for the longer wavelengths the absorption is slower so the network is distributed in the bulk. The surface grating yields lower threshold voltages and shorter transition times while the bulk grating reduces the contrast between on and off state and both arrangements create robust switchable gratings.…”

Optically and spatially templated polymer architectures formed by photopolymerization of reactive mesogens in periodically deformed liquid crystals

“…Because the LC cell was illuminated by polymerizing UV light from the bottom substrate with patterned electrodes, the intensity gradient of UV light inside the cell may have caused preferential polymerization near the front surface of the cell, followed by frontal polymerization. [25][26][27][28][29]38,39 By increasing the applied voltage, the vertical field in between the signal and top common electrodes strengthened the uniform vertical orientation of LCs with minimum elastic deformation above the signal electrode. At the same time, the increased in-plane field further squeezed the deformed regions toward both the (bottom and top) common electrodes.…”

“…[21][22][23][24] The control of network distribution, perpendicular to the wavefront of reaction-initiating ultraviolet (UV) light, has been demonstrated by manipulating the intensity gradient along the light-propagation direction by using UV-absorbing dye additives or by selecting the appropriate UV wavelength. [25][26][27][28][29] The formation of both spatially and orientationally structured networks, without using external lithographic or holographic implements, has been demonstrated by using pattern-forming states of cholesteric liquid crystals as reaction templates. [30][31][32][33] It has been hypothesized that the spatial templating is driven by the spatially nonuniform elastic deformation of cholesteric pattern-forming states as a reaction medium.…”