An orange-reflecting photonic polymer film has been fabricated based on a hydrogen-bonded cholesteric liquid
crystalline (CLC) polymer consisting of non-reactive (R)-(+)-3-methyladipic acid as the chiral dopant. This polymer film
can be patterned easily by evaporating the chiral dopant at specific locations with a hot pen or a laser beam. Removal of
chiral dopant leads to a decrease in the helical pitch at the heat treated areas leading to a change in color from orange to
green revealing a high contrast pattern. The photonic patterns are irreversible and stable at ambient conditions. This
makes such a CLC polymer film interesting as writable photonic paper.
Laser direct-writing is an important technique for the fabrication of complex patterns. There is a continuous need for structures with increasingly small features, i.e., enhanced resolution. Focused radially polarized light is known to exhibit a narrow longitudinal polarization component. Here, a proof-of-concept is shown of enhanced resolution through polarization-selectivity by the selective recording of the longitudinal polarization component in a polarization-selective homeotropic and smectic B photoresist. The full-width-at-half-maximum (FWHM) of the fabricated spots in the polarization-selective resist is up to 56% smaller compared to the FWHM of the same spot in a photoresist that is not polarization-selective, which supports simulations that predict a theoretical maximum reduction of 62%.
A novel responsive lighting system is presented capable of lowering the color temperature of emitted light on dimming. It is based on a single white light emitting LED and a thermo-responsive scattering coating. The coated LED automatically emits light of lower correlated color temperature (CCT) when the power is reduced, while maintaining a chromaticity close to the black body curve. Existing systems all use multiple color LEDs, additional control circuitry and mixing optics. An optical ray tracing model can explain the experimental results.
There has been a huge increase in the global demand of energy over the last few years. One of the main contributors to energy consumption in buildings, cars, greenhouses and indoor spaces is the cooling devices needed to maintain the indoor temperature at comfortable levels. To reduce the energy used by cooling devices, we need improved light control in transparent building elements, such as windows. Infrared (IR) reflectors applied to the windows for rejection of infrared light would be very attractive, especially if they do not affect light in the visible region. A method to selectively and precisely control infrared transmission is via the use of cholesteric liquid crystal (Ch-LC) polymer reflectors. Ch-LCs, also known as chiral-nematic LCs, reflect circularly polarized light as a result of their self-organizing molecular helices. The pitch of the helix in these networks determines the wavelength of reflection. In contrast to existing alternatives, they are characterized by a very sharp cut-off between the transmissive and the reflective state enabling exact tailoring of the heat reflection. In this article we have focused on fabrication of infrared reflectors using Ch-LCs and a computational model was used to predict the energy savings of this IR-reflector in an office building in Abu Dhabi which indicated that 6 % energy savings can be realized.
In this work, we present patterned water-responsive coatings, which alter both their topological and optical properties. The polymer coatings are based on a hydrogen-bonded cholesteric liquid crystalline polymer network. A two-step photopolymerization procedure leads to a patterned coating with repeating liquid crystalline and isotropic areas. The cholesteric liquid crystalline areas reflect green light, whilst the isotropic areas are transparent for visible light. Treatment with alkaline solution results in a hygroscopic polymer salt coating. When placed in demineralized water, the polymer films swells, leading to an enhancement of the surface topography structure in which the liquid crystalline areas swell more. Moreover, the pitch of the helical organization in the cholesteric areas increases due to this swelling leading to a color change from green to red.
We developed a polarization-selective negative photoresist based on a smectic B liquid crystal monomer host functionalized with a dichroic photoinitiator. The smectic phase enables high-order parameter uniaxial alignment of the monomer host molecules. It is shown that the dichroic initiator aligns with the host which provides the polarization selectivity upon UV initiation of the polymerization of the monomer system. The polymerization contrast with respect to its sensitivity for polarized UV light can become infinitely high by the addition of an inhibitor. We tested the new lithographic material for its application in polarization holography. These experiments show that the polymerization contrast can be translated into the formation of well-defined structures but require further optimization.
Defects in liquid crystals have been studied over decades to disclose information and knowledge on the structure of LC
phases. More recently, LC defects have been identified as a tool to implement new physical functions useful in optical
films for polarization conversion or mechanical actuators able to adopt novel exotic shapes. In the present paper we
describe a general methodology to engineer different defect patterns by combining the use of linear photopolymerizable
polymers and liquid crystals.
Compact and inexpensive solar concentrators can be designed by using transmission gratings that diffract incident
sunlight into a light guide. To this end a grating should have a small period and maintain a high diffraction
efficiency over a wide range of incident angles. We numerically study the angular dependence of the diffraction
efficiency of surface-relief gratings using Rigorous Coupled-Wave Analysis. It is shown how one can control the
angular acceptance of gratings by tuning the refractive index or the grating topology. Gratings with a high
refractive index maintain a high diffraction efficiency over a wide range of incident angles. By adjusting the
topological symmetry one can design a grating with a high diffraction efficiency over a narrow range of incident
angles, or a grating with a more homogenous distribution of the diffraction efficiency.
Organic polymeric chiral nematic liquid crystalline (cholesteric) wavelength selective mirrors can increase the efficiency
of luminescent solar concentrators (LSCs) when they are illuminated with direct sunlight normal to the device.
However, due to the angular dependence of the reflection band, at larger incidence angles the cholesterics reflect away
some incoming sunlight that could have been absorbed by the luminophore. As a result, the increase in LSC efficiency
after application of a cholesteric reflector drops if the light incident to the device is at angles larger than 30 degrees. The
cholesteric reflectors still have a positive impact on device performance for light incident up to 45-50 degrees but at
larger angles efficiency decreases when a cholesteric reflector is added. This affects the performance of the LSC device
when illuminated with indirect incident light, especially when the incident light has a large contribution of photons above
45 degrees.
We report on the design and fabrication of 'smart surfaces' that exhibit dynamic changes in their surface topology in
response to exposure to light. The principle is based on anisotropic geometric changes of a liquid crystal network upon a
change of the molecular order parameter. The photomechanical property of the coating is induced by incorporating an
azobenzene moiety into the liquid crystal network. The responsive surface topology consists of regions with two
different types of molecular order: planar chiral-nematic areas and homeotropic. Under flood exposure with 365 nm
light the surfaces deform from flat to one with a surface relief. The height of the relief structures is of the order of 1 um
corresponding to strain difference of around 20%. Furthermore, we demonstrate surface reliefs can form either convex
or concave structures upon exposure to UV light corresponding to the decrease or increase molecular order parameter,
respectively, related to the isomeric state of the azobenzene crosslinker. The reversible deformation to the initial flat state
occurs rapidly after removing the light source.
Transmission gratings that combine a large diffraction angle with a high diffraction efficiency and low angular
and wavelength dispersion can be used to concentrate sunlight in a light guide and for lighting applications.
Surface-relief gratings with sub-wavelength grating periods can have these properties. In this paper we study
their diffraction efficiency for general conical angles of incidence. We show the presence of regions in the space
of incident angles where light is efficiently coupled into or out of total internal reflection. It is demonstrated how
this distribution of the diffraction efficiency over angular space can be adjusted by changing the grating geometry.
Finally, these properties are qualitatively verified using holographically produced surface relief gratings.
To improve the optical efficiency and to reduce the number of optical components of LCD backlighting systems, two
types of polarized-light backlights have been made from micro-structured birefringent polymeric layers. One type uses
uniaxially oriented PEN and PET foils that have been structured by diamond-tool machining or by hot-embossing, and
subsequently laminated onto a flat PMMA light guide. The second type uses a liquid crystalline polymeric layer
laminated onto a micro-structured light guide. S-polarized light is preferentially extracted from the light guides. The
efficiency has been measured to be 1.6-1.7 times higher than for a conventional backlight. Costs, thickness and
complexity are decreased since no micro-prismatic brightness enhancement foils or reflective polarizer foils are needed.
Photopolymerization of liquid crystal monomers initiated by means of a dichroic photoinitiator provides an additional
degree of freedom in controlling the morphology and structure of the liquid crystal networks formed. The absorption of
the dichroic photoinitiator, and thereby its initiation rate, depends on its position towards the transversal light beam used
for polymerization as well as its position towards the polarization of the light beam. The photoinitiator adapts the
director profile of the liquid crystal monomer. As a result planar oriented areas aligned orthogonal to the propagation
direction of the light beam polymerize faster than the ones parallel to it. Similarly, planar aligned areas with their
orientation parallel to the electrical field vector of the light polymerize faster than the planar aligned areas oriented
perpendicular to that. Based on this principle complex lithographic structures are built, not only forming structures in the
plane of the polymerizing film but also in the third dimension along its cross-section. Additionally, applying the dichroic
photoinitiator together with the principle of polymerization induced diffusion in monomer blends provides a wealth of
new structures, especially when combined further with complicated, but well-controlled, morphologies such as those of
twisted, splayed and cholesteric liquid crystal monomers.
The measurement of complete polarimetric parameters for a broad spectrum of wavelengths is challenging because of the multi-dimensional nature of the data and the need to chromatically separate the light under test. As a result, current methods for spectropolarimetry and imaging polarimetry are limited because they tend to be complex and/or relatively slow. Here we experimentally demonstrate an approach to measure all four Stokes parameters using three polarization gratings and four simultaneous intensity measurements, with potential to dramatically impact the varied fields of air/space-borne remote sensing, target detection, biomedical imaging/diagnosis, and telecommunications. We have developed reactive mesogen polarization gratings using simple spin-casting and holography techniques, and used them to implement a potentially revolutionary detector capable of simultaneous measurement of full polarization information at many wavelengths with no moving or tunable elements. This polarimeter design not only enables measurements over a likely bandwidth of up to 70% of the center wavelength, it is also capable of measurements at relatively high speed (MHz or more) limited only by the choice of photo-detectors and processing power of the system. The polarization gratings themselves manifest nearly ideal behavior, including diffraction efficiencies of greater than 99%, strong polarization sensitivity of the first diffraction orders, very low incoherent scattering, and suitability for visible and infrared light. Due to its simple and compact design, simultaneous measurement process, and potential for preserving image registration, this spectropolarimeter should prove an attractive alternative to current polarization detection and imaging systems.
The filling is reported of the air holes of an InP-based two-dimensional photonic crystal with solid polymer and with liquid crystal 5CB. The polymer filling is obtained by thermal polymerization of an infiltrated liquid monomer, trimethylolpropane triacrylate. The filling procedure for both the monomer and liquid crystal relies on the capillary action of the liquid inside the ~ 200 nm diameter and < 2.5 μm deep air holes. The solid polymer infiltration result was directly inspected by cross-sectional scanning electron microscopy. It was observed that the holes are fully filled to the bottom. The photonic crystals were optically characterized by transmission measurements around the 1.5 μm wavelength band both before and after infiltration. The observed high-frequency band edge shifts are consistent with close to 100% filling, for both the polymer and the liquid crystal. No differences were observed for filling under vacuum or ambient, indicating that the air diffuses efficiently through the liquid infiltrates, in agreement with estimates based on the capillary pressure rise.
Polymer filling of the air holes of indiumphosphide based two-dimensional photonic crystals is reported. The filling is
performed by infiltration with a liquid monomer and solidification of the infill in situ by thermal polymerization.
Complete hole filling is obtained with infiltration under ambient pressure. This conclusion is based both on cross-sectional
scanning electron microscope inspection of the filled samples as well as on optical transmission
measurements.
Polymeric relief structures are extensively used in display technology due to their ability to redirect light in a controlled way. Photo-embossing is a new photo-lithographic technique to generate surface relief structures using photopolymers. In the present paper we show a combinatorial methodology to explore this technique. We have prepared and evaluated (using automated atomic force microscopy) 2-dimensional libraries of photo-embossed gratings, each library with a gradient in period and a gradient in either exposure energy or development temperature or film thickness or photoinitiator concentration or monomer to binder ratio. We show how this combinatorial approach helps us to better understand the photo-embossing process. In addition, we show that this methodology is an effective tool to identify processing conditions resulting in optimum shape and height of the polymeric relief micro-structures to be used in specific applications.
Polymer helices with submicron dimensions have been fabricated from a variety of isotropic and liquid crystalline polymers with storage moduli ranging from 38MPa to 1.9GPa (measured at 1Hz, room temperature). These helices are made using a double templating process, in which a thin film comprised of independent helical structures deposited using glancing angle deposition (GLAD) acts as the master. In our process the 'positive' structure of the master is copied into a polymer 'negative', which then itself acts as a template for the final film of polymer helices. Liquid crystalline polymers are of particular interest for use in MEMS because highly ordered liquid crystalline polymers can be actuated by exposing them to a stimulus (such as heat) that causes a decrease in order, leading to a reversible, macroscopic change in shape. The phase behavior, optical properties, and mechanical properties of planar aligned monoacrylate liquid crystalline polymers with varying crosslinker content are investigated, in order to determine the composition that will yield the largest deformations upon heating. We find that films with the lowest crosslinker content investigated (2.5%) undergo the largest reduction in birefringence as they are heated, corresponding to a loss in order. However, we also observe that the films with the highest crosslinker content investigated (10%) undergo the largest physical deformation upon heating. SEM images illustrating the deformation of liquid crystalline polymer helices as they are heated are also presented.
We present polymeric MEMS materials which reversibly respond to either thermal or UV stimuli by moving between nearly flat (r ~ infinity) and tightly curled states (r ~ 5mm) with variations in the radiation environment or temperature. The molecular orientation gradient of a liquid crystal network controls the primary bending axes, while controlled order parameter variations are responsible for the degree of deformation. In the case of thermal activation, these order changes are dominated by thermal motion, while UV-switchable defects bring about reduced network order in the case of UV actuation. We report fabrication and operation of the actuators and supplementary data regarding alignment configurations for controllable deformations, the phase behaviour of the liquid crystal constituents, thermal expansions, and absorption of the UV dyes are included. We find that splayed molecular configurations are preferred over twisted modes due to their single deformation axis, and that the optimum concentration of active molecules for UV-driven actuation is on the order of 7-8wt.%.
Liquid crystal networks change their dimensions when the degree of order is altered. Upon decreasing order, e.g. as a result of temperature increase, the linear dimension decreases in the direction along the director and increases orthogonal to that. When the director changes as a function of position, the local dimensional changes cause stresses that effect in deformation of the sample. In the case of thin films with a twisted molecular orientation over their cross-section a change in the order parameter results in a double, saddle-like, bending of the film as the linear expansion is different for both in-plane axes. For geometric reasons this bending is uncontrolled and irregular. When the linear expansion is chosen to be different along one in-plane axis, but is kept the same for the other axis, the deformation becomes orderly and controlled. Therefore, films of liquid crystal networks with a splayed molecular alignment over their cross-section provide a well-controlled bending deformation as a function of a changing order parameter. In a liquid crystal network the order parameter can be modulated by temperature. The direction- and order parameter dependent linear expansion than comes on top of the volume expansion as caused by induced thermal molecular motions and decreased secondary molecular forces. Besides by temperature the order parameter can also be modulated by light in the presence of photo-sensitive moieties in the liquid crystal network. The deformation behavior is anticipated to be of relevance for polymer based MEMS technology.
In this study the μ-rubbing technique is used to micro-pattern polyimide alignment layers using a metallic sphere at different loads. Optical and atomic force microscopy shows that the width of the patterns ranges from 12-40 μm and depth ranges from 2-14 nm. Our primary finding is that μ-rubbing induces planar alignment in polyimides. We performed μ-rubbing on pre-rubbed polyimide perpendicular to the rubbing direction. It is found that μ-rubbing erases the alignment properties of the pre-rubbing procedure. Liquid crystal cells were constructed using the pre-rubbed polyimide substrates with μ-rubbed patterns on one side and a homogeneously rubbed polyimide layer on the other side. Therefore the pre-rubbed polyimide layers are crossed and consequently a twisted nematic alignment is observed outside the micro-patterns. Within the micro-patterns, the directions of the pre-rubbing and μ-rubbing are parallel, a planar alignment observed. In another configuration, liquid crystal cells were made with pre-rubbed polyimide substrates having micro-patterns on both sides to give grid pattern with planar and twisted nematic configuration. Studies were extended to determine the pretilt of the micro patterned area.
New designs are presented of backlight systems for transmissive and transflective LCD’s based on stretched PET films with a well-defined micro-structure, which emit highly collimated or diffuse and linearly polarized light with a high efficiency. Moreover, edge-lit waveguide systems are discussed equipped slanted phase gratings which combine a range of desirable features such as a high transparency in direct view, a direct emission of light at normal angles to the plane of the waveguide and a purely unidirectional out-coupling of light towards the LCD-side. Moreover, these illumination systems emit colored, linearly polarized light which should contribute significantly to the energy efficiency of transmissive, transflective and reflective LCD displays.
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