Chiroptical Resolution and Thermal Switching of Chirality in Conjugated Polymer Luminescence via Selective Reflection using a Double‐Layered Cell of Chiral Nematic Liquid Crystal

An optically resolvable and thermally chiral‐switchable device for circularly polarized luminescence (CPL) is first constructed using a light‐emitting conjugated polymer film and a double‐layered chiral nematic liquid crystal (N*‐LC) cell. The double‐layered N*‐LC cell with opposite handedness at each layer is fabricated by adding each of two types of N*‐LCs into each of the cells, and the N*‐LCs consist of nematic LCs and chiral dopants with opposite chirality and different mole concentrations. The selective reflection band due to the N*‐LC is thermally shifted so that the band wavelength is close to the luminescence band of the racemic conjugated polymer, such as disubstituted polyacetylene (diPA), yielding CPL with opposite handedness and high dissymmetry factor values (|g_lum|) of 1.1–1.6 at low and high temperatures. The double‐layered N*‐LC cell bearing the temperature‐controlled selective reflection is useful for generating CPLs from racemic fluorescent materials and for allowing thermal chirality‐switching in CPLs, which present new possibilities for optoelectronic and photochemical applications.     

Hybrid Cholesteric Films with Tailored Polarization Rotation

Incorporation of metal nanoparticles in chiral (Ch) films of cellulose nanocrystals (CNCs) enhances nanoparticle plasmon resonances, due to the coupling of the intrinsic properties of the plasmonic guest and the photonic properties of the Ch‐CNC host. In contrast with previous reports, the properties of the Ch‐CNC host are focused and an effective strategy is developed for tuning the optical polarization rotation of the composite films formed by the CNCs and gold nanoparticles. A twofold enhancement in the polarization rotation power of the composite Ch‐CNC films, with an insignificant change in the incurred optical losses, is achieved by varying the density and dimensions of gold nanoparticles embedded in the Ch‐CNC films. For such films, a new approach is developed to obtain broadband circular dichroism by fabricating films from mixtures of CNC suspensions ultrasonicated for different time intervals. These new findings enable fine‐tuning of the power and spectral range of the polarization rotation and offer a novel strategy for the fabrication of broadband reflectors and polarizers, smart solar windows, and detectors for circularly polarized light.     

Photoinduced Chiral Nematic Organization in an Achiral Glassy Nematic Azopolymer

A liquid crystalline homopolymer that has photoisomerizable methoxyazobenzene groups in the side chain has been synthesized and characterized. Thin films of the nematic glassy phase of this polymer have been processed in order to study the absorption spectra and the vibrational and electronic circular dichroism responses by irradiation with 488 nm circularly polarized light (CPL). Selective reflection of visible light demonstrates that the irradiation of this glassy nematic azopolymer induces a helix as a consequence of the chiral arrangement of the azobenzene units. Moreover, a wedge cell with an aligning layer for planar orientation was filled with the polymer with the aim of investigating the change in the macroscopic optical properties and optical textures of the azopolymer on irradiation with CPL. The transfer of chirality from CPL to azopolymer through chiral conformations is proposed as a model for explaining the supramolecular chirality.     

Biopolymer Templated Glass with a Twist: Controlling the Chirality,Porosity, and Photonic Properties of Silica with Cellulose Nanocrystals

A detailed investigation of the formation and properties of mesoporous silica templated by the chiral nematic liquid crystal phase of cellulose nanocrystals (CNCs) is presented. Under appropriate conditions, CNCs co‐assemble with silica up to loadings of ≈60 wt% to give composite films with periodic chiral nanostructures. The periodicity of these films can be readily controlled to obtain materials that selectively reflect light with wavelengths ranging from ≈400–1400 nm. The co‐assembly of CNCs and silica into ordered chiral nematic structures is demonstrated to occur within a narrow window of pH and is affected by aging: a slow rate of silica condensation appears to be vital for the formation of well‐ordered materials. CNCs can be removed from the composite films by calcination or acid hydrolysis to give high surface area chiral nematic mesoporous silica (CNMS) with tunable pore diameters. The combination of mesoporosity and chiral nematic ordering in CNMS enables it to be used in a unique way for refractometric sensing applications. It is shown that, when using circular dichroism (CD) signals to monitor the chiral photonic properties of CNMS, variations in refractive index can be detected based on changes of both CD signal intensity and peak position with good sensitivity.     

Large Area Self‐Assembly of Nematic Liquid‐Crystal‐Functionalized Gold Nanorods

Fascinating nematic‐ and smectic‐like self‐assembled arrays are observed for gold nanorods partially capped with either laterally or terminally attached nematic liquid crystals upon slow evaporation of an organic solvent on TEM grids. These arrays can be manipulated and reoriented by applying an external magnetic field from quasi‐planar to vertical similar to a Fréedericksz transition of common organic nematic liquid crystals. Birefringence and thin film textures of these self‐assembled gold nanorod arrays observed by polarized optical microscopy are strongly reminiscent of common organic nematic liquid crystal textures between crossed polarizers and, additionally, support the formation of ordered liquid crystal‐like anisotropic superstructures. The ordering within these arrays is also confirmed in bulk samples using small angle X‐ray scattering (SAXS).     

Liquid-crystal color light valve

A multilayer color display device employs two kinds of liquid-crystal films, One is a field-effect nematic liquid crystal; the other comprises a mixture of cholesteric and nematic liquid crystals that have the same function as circular polarizers used in place of crossed polarizers. Three colors (red, green, and blue) can be modulated independently by changing three voltages.     

Rigorous optical modeling and optimization of thin‐film photovoltaic cells with textured transparent conductive oxides

Typical thin‐film photovoltaic (PV) cells incorporate a textured transparent conductive oxide to enhance light trapping and efficiently harvest solar energy. Rigorous coherent optical simulations of these devices and a complete characterization of these textured films are a challenging problem because of the several orders of magnitude difference between the wavelengths of interest and the spatial dimension of the sample that needs to be evaluated. In this paper, a practical approach for rigorous and predictive modeling of optical properties of thin‐film PV cells incorporating a vast variety of light‐trapping structures including semi‐coherent textured films and patterned coherent structures is presented. In contrast to the existing semi‐empirical device models, it is demonstrated that the presented methodology can accurately predict the scattering properties of textured fluorine‐doped tin oxide and aluminum‐doped zinc oxide conductive transparent films. It is further shown that the optical response of single‐junction and tandem‐junction PV devices incorporating such films can also be predicted with good accuracy as compared with the measured results. Next, a methodology to identify the sufficient statistical fingerprints of semi‐coherent textured films that are needed to unambiguously predict the light propagation in thin‐film cells is presented. This comprehensive approach then lends itself to identifying the optimal surface morphology needed for strong light trapping. This rigorous approach automatically includes the effects of important loss mechanisms such as the surface plasmon‐enhanced absorption in textured metal surfaces that are otherwise very difficult to account for semi‐coherent approaches based on scalar scattering theory. Copyright © 2011 John Wiley & Sons, Ltd.     

Nano‐cones on micro‐pyramids: modulated surface textures for maximal spectral response and high‐efficiency solar cells

The front‐side reflection represents a significant optical loss in solar cells. One way to minimize this optical loss is to nano‐texture the front surface. Although nano‐textured surfaces have shown a broad‐band anti‐reflective effect, their light scattering and surface passivation properties are found to be generally worse than those of standard micro‐textured surfaces. To overcome these setbacks in crystalline silicon solar cells, advanced texturing and passivation approaches are here presented. In the first approach, we propose a modulated surface texture by superimposing nano‐cones on micro‐pyramidal surface texture. This advanced texture applied at the front side of crystalline silicon wafers completely suppresses the reflection in a broad wavelength range from 300 nm up to 1000 nm and efficiently scatters light up to 1200 nm. In the second approach, we show a method to minimize recombination at nano‐textured surfaces by using defect‐removal etching followed by dry thermal oxidation. These two approaches are applied here in an interdigitated back‐contacted crystalline silicon solar cell and result in decoupling of the interplay between the mechanisms behind short‐circuit current density and open‐circuit voltage. The device exhibits a conversion efficiency equal to 19.8%, record external quantum efficiency (78%) at short wavelengths (300 nm), and electrical performance equal to the performance of the reference interdigitated back‐contacted device based on front‐side micro‐pyramids. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrotunable Non‐reciprocal Laser Emission from a Liquid‐Crystal Photonic Device

A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage.     

Remote‐Controllable Molecular Knob in the Mesomorphic Helical Superstructures

A programed light‐responsive chiral liquid crystal (LC) containing four photochromic azobenzene moieties covalently connected to a central bicyclic chiral core (abbreviated as AZ₄ICD) is newly designed, precisely synthesized, and efficiently applied as a remote‐controllable molecular knob for the optically tunable thin film. First of all, phase evolutions and ordered structures of AZ₄ICD are systematically investigated by a combination of thermal, microscopic, scattering, and simulation techniques. Wide‐angle X‐ray diffractions of oriented AZ₄ICD samples indicate that the AZ₄ICD molecule itself basically forms layer structures: one is a low‐ordered chiral smectic A LC phase (SmA*) with 5.61 nm layer periodicity at high temperatures, and two highly ordered smectic crystal (SmCr₁ and SmCr₂) phases are subsequently formed at lower temperatures with the anticlinically tilted molecular packing structures. The helical superstructures of chiral nematic LC phase (N*) can be spontaneously constructed by doping AZ₄ICD chiral agents into the achiral nematic molecules. Due to the bent conformational geometry of AZ₄ICD, the thermal window of blue LC phase (BP) is expanded by stabilizing the double twisted cylindrical building blocks. Remote‐controllable phase transformations in the mesomorphic helical superstructures are demonstrated by tuning the wavelength of light.     

一种新型光敏小分子用于光控取向的研究

通过在3,5-二羟基苯甲醇分子中的双酚基基团上引入双肉桂酰酯光敏基团的酯化反应,合成了一类新型的分子顶部为羟基极性基团,整个分子构型类似于树枝状的光敏小分子化合物。将此种光敏材料配成一定浓度的溶液,旋涂在玻璃基板上成膜,经过线性偏振紫外光辐照后发生交联反应,制备成光控取向膜。以此种取向膜制成向列相液晶的平行液晶器件,在偏光显微镜下观察,发现取得了均一、稳定的取向效果,并且该取向膜具有良好的热稳定性能。     

温度梯度制备宽波反射液晶薄膜

合成了大螺旋扭曲力的手性化合物4,4′-2(戊基-环己基)苯甲酸异山梨醇酯(ISBB)。对60.0μm厚ISBB/液晶性单体/向列相液晶/光引发剂复合体系的液晶层的上、下表面温度施加不同温度,液晶层中形成了温度梯度。由于高温区域的液晶溶解ISBB的能力强,低温区域的ISBB会向高温区域扩散,从而在液晶薄层中形成了ISBB的浓度梯度与螺距梯度。紫外光聚合复合体系中的光可聚合单体,单体间交联聚合形成网络固定住了液晶层内的螺距梯度,从而制备出反射可见光波段的右旋圆偏振光的宽波反射液晶薄膜。     

A Printable Optical Time‐Temperature Integrator Based on Shape Memory in a Chiral Nematic Polymer Network

An optical and irreversible temperature sensor (e.g., a time‐temperature integrator) is reported based on a mechanically embossed chiral‐nematic polymer network. The polymer consists of a chemical and a physical (hydrogen‐bonded) network and has a reflection band in the visible wavelength range. The sensors are produced by mechanical embossing at elevated temperatures. A relative large compressive deformation (up to 10%) is obtained inducing a shift to shorter wavelength of the reflection band (>30 nm). After embossing, a temperature sensor is obtained that exhibits an irreversible optical response. A permanent color shift to longer wavelengths (red) is observed upon heating of the polymer material to temperatures above the glass transition temperature. It is illustrated that the observed permanent color shift is related to shape memory in the polymer material. The films can be printed on a foil, thus showing that these sensors are potentially interesting as time‐temperature integrators for applications in food and pharmaceutical products.     

CdS Quantum Dots Encapsulated in Chiral Nematic Mesoporous Silica: New Iridescent and Luminescent Materials

Simultaneous integration of light emission and iridescence into a semiconducting photonic material is attractive for the design of new optical devices. Here, a straightforward, one‐pot approach for liquid crystal self‐assembly of semiconductor quantum dots into cellulose nanocrystal‐templated silica is developed. Through a careful balance of the intermolecular interactions between a lyotropic tetraalkoxysilane/cellulose nanocrystal dispersion and water‐soluble polyacrylic acid/mercaptopropionic acid‐stabilized CdS quantum dots, CdS/silica/nanocellulose composites that retain both chiral nematic order of the cellulose nanocrystals and emission of the quantum dots are successfully co‐assembled. Subsequent removal of the cellulose template and organic stabilizers in the composites by controlled calcination generates new freestanding iridescent, luminescent chiral nematic mesoporous silica‐encapsulated CdS films. The pores of these materials are accessible to analytes and, consequently, the CdS quantum dots undergo strong luminescence quenching when exposed to TNT solutions or vapor.     

Hydrogen‐Bonded Liquid Crystals in Confined Spaces—Toward Photonic Hybrid Materials

A series of hybrid materials based on chiral nematic mesoporous organosilica (CNMO) films infiltrated with liquid crystalline hydrogen‐bonded assemblies is prepared and characterized with respect to the mutual manipulation of the photonic properties of the host and the liquid‐crystalline behavior of the guest. Detailed differential scanning calorimetry studies reveal the impact of confinement on the mesomorphic behavior of the liquid crystalline assemblies in the pores of the CNMO films. The photonic properties of the chiral nematic mesoporous host can be controlled by changing the temperature or irradiating the films with UV light. These stimuli‐induced phase transitions are accompanied by changes in the orientational order of the mesogens as revealed by ¹⁹F NMR spectroscopy. The combination of confinement and changes in the molecular orientation in a unique hybrid material based on hydrogen‐bonded liquid crystals and a porous host with a chiral nematic mesostructure is an interesting concept for the design of optical sensors, reflectors, or filters.     

Fully Chiral Light Emission from CsPbX3 Perovskite Nanocrystals Enabled by Cholesteric Superstructure Stacks

Halide perovskites have received tremendous attention due to their fantastic optical and electrical properties. Here, circularly polarized light emission is successfully demonstrated using a simple configuration consisting of inorganic perovskite nanocrystals embedded within a predefined handedness cholesteric superstructure stack. The helical structured cholesteric liquid crystal film acts as a selective filter to transform the unpolarized light emission from perovskite nanocrystals into circularly polarized luminescence. The transformation is accompanied by an extraordinary dissymmetry factor (|g_lum|) up to 1.6, well‐defined handedness, high photoluminescence quantum yield, and full‐color availability. Furthermore, the circularly polarized luminescence is angular dependent and can easily be modulated by shifting the overlap of the reflection band and the emission band. The proposed method is more straightforward and powerful than the previous approaches, offering new opportunities in optoelectronic and photonic devices.     

A low light natural image statistical model for joint contrast enhancement and denoising

We study the problem of joint low light image contrast enhancement and denoising using a statistical approach. The low light natural image in the band pass domain is modeled by statistically relating a Gaussian scale mixture model for the pristine image, to the low light image, through a detail loss coefficient and Gaussian noise. The detail loss coefficient is statistically described using a posterior distribution with respect to its estimate based on a prior contrast enhancement algorithm. We then design our low light enhancement and denoising (LLEAD) method by computing the minimum mean squared error estimate of the pristine image band pass coefficients. We create the Indian Institute of Science low light image dataset of well-lit and low light image pairs to learn the model parameters and evaluate our enhancement method. We show through extensive experiments on multiple datasets that our method helps better enhance the contrast while simultaneously controlling the noise when compared to other state of the art joint contrast enhancement and denoising methods.     

Wide-View and Broadband Circular Polarizers for Transflective Liquid Crystal Displays

A simple wide-view and broadband circular polarizer comprising of a linear polarizer and two uniaxial films is proposed to enhance the viewing angle of transflective liquid crystal displays (LCDs). For the transmissive mode, over the entire 90deg viewing cone, the normalized light leakage from two stacked circular polarizers is suppressed to below 1.5times10^-2, and contrast ratio over 10:1 is obtained using a normally black vertically aligned transflective LCD. At the same time, this configuration warrants a broadband operation and reasonably good viewing angle (10:1 contrast ratio is over 40 at all directions) for the reflective mode. The physical mechanisms for achieving broadband operation and wide viewing angle are discussed.     

Optimum polarizer combinations for twisted nematic displays

We present a simple phenomenological theory that makes it possible to compute the brightness and contrast ratio of reflective twisted nematic displays from the optical characteristics of their polarizers. The theory satisfactorily explains the large change in contrast ratio that is observed when the positions of the polarizers are interchanged in a reflective display assembled with dissimilar polarizer types. Brightness and contrast measurements are reported for reflective displays assembled with the four possible combinations of Polaroid HN-42 and HN-55 sheet-type polarizers, and the results are in good agreement with theory. Optical characteristics of polarizers are suggested which should markedly improve display viewability over what is currently obtainable. We find that a small increase in contrast ratio is possible if the front polarizer of a display is made stronger than the rear polarizer instead of employing identical polarizers.     

液晶微胶囊制备宽波反射凝胶

手征向列相(N*)液晶能够选择性反射入射光,但其反射波宽一般小于150nm。利用负介电各向异性的向列相液晶SLC10V513-200与手性化合物R1011、CB15配制出5种N*液晶,其反射波长能够覆盖整个可见光波段。使用异佛尔酮二异氰酸酯(IPDI)界面聚合法制备出平均粒径为8.0μm的5种N*液晶聚脲微胶囊之后,将相同质量的5种微胶囊混入OP-10与IPDI中制备出微胶囊的凝胶。对80.0μm厚微胶囊凝胶薄膜施加交流电场,使微胶囊中的液晶处于平面织构状态,紫外固化微胶囊中的液晶性单体,固定微胶囊中N*液晶的平面织构,从而制备出可以反射可见光波段的微胶囊凝胶薄膜。