Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed Liquid Crystal Films

A Smart Window with Passive Radiative Cooling and Switchable Near-Infrared Light Transmittance via Molecular Engineering

Smart windows with synergetic light modulation have heightened demands for applications in smart cars and novel buildings. However, improving the on-demand energy-saving efficiency is quite challenging due to the difficulty of modulating sunlight with a broad bandwidth in an energy-saving way. Herein, a smart window with switchable near-infrared light transmittance and passive radiative cooling is prepared via a monomer design strategy and photoinduced polymerization. The effects of hydrogen bonds and fluorine groups in acrylate monomers on the electro-optical properties as well as microstructures of polymer-dispersed liquid crystal films have been systematically studied. Some films show a high contrast ratio of 90.4 or a low threshold voltage (Vth) of 2.0 V, which can be roll-to-roll processed in a large area. Besides, the film has a superior indoor temperature regulation ability due to its passive radiative cooling and controllable near-infrared light transmittance properties. Its radiative cooling efficiency is calculated to be 142.69 W/m2 and NIR transmittance could be switched to below 10%. The introduction of a carboxylic monomer and fluorinated monomer into the system endows the film with a highly efficient temperature management capability. The film has great potential for applications in fields such as flexible smart windows, camouflage materials, and so on.

Polymer-Dispersed Liquid Crystal Films on Flexible Substrates with Excellent Bending Resistance and Spacing Stability

Polymer-dispersed liquid crystals (PDLCs) are very attractive due to their electrically switchable properties. However, current PDLC films still have problems such as high driving voltages, low contrast ratio (CR), and poor bending resistance and spacing stability. To solve these problems, a PDLC film with a system of coexisting polymer spacer columns and polymer network was proposed. First, based on the adhesive systems of IBMA and UV6301, the effects of IBMA concentration and LC content on the morphology of the polymer network and the electro-optical properties of PDLC were investigated, respectively. Then, the effects of the process conditions of mask polymerization such as temperature, time, and UV light intensity on the morphology and electro-optical properties of the polymer spacer columns were systematically investigated. It was found that PDLC films with the coexistence system exhibit both excellent electro-optical properties and outstanding bending resistance and spacing stability. Thus, it provides new practical possibilities for the preparation of high-performance PDLC films used in flexible devices.

Smart Diffraction Gratings based on the Shape Memory Effect

The shape memory effect is the capability of a structure or a material that can be deformed into a certain temporary shape under the external stimulus, and the shape will be fixed without the stimulus. The recovery process can be triggered by the same stimulus. The intelligent tunable device based on the shape memory effect has a wide range of applications in many fields. In the optical field, smart diffraction gratings can accomplish in-situ optical diffraction according to requirements, meeting the high demand in the next generation of smart optical systems. However, it is essential to construct high-precision grating structures based on shape memory materials. Here, a smart diffraction grating based on UV-curable shape memory polymer via two-beam interference is reported, with nano-scale precision, excellent deformability and recovery ability and adjustable spectroscopic performance. More importantly, based on the shape memory effect, grating structures that surpass the precision of the fabrication system can be obtained. The smart grating exhibits rapid deformation and recovery upon heating and long-term storage capability, which facilitates them to be applied in optics, electronics and integrated sensing. This article is protected by copyright. All rights reserved.

Polarization-Dependent Gratings Based on Polymer-Dispersed Liquid Crystal Cells with In-Plane Switching Electrodes

A diffraction grating of polymer-dispersed liquid crystal (PDLC) with polarization-selective characteristics is investigated. Electrically controllable gratings are produced using In-Plane Switching (IPS) electrodes. Indium tin oxide (ITO) electrodes with a stripe pattern are used to generate a horizontal electric field parallel to the substrate on a single glass substrate. It is known from the experimental results that the number of diffraction orders can be controlled by applied voltage. Except for the zeroth order, the consistently highest intensity can be obtained for every other order of diffraction, and the polarization direction of the diffraction is perpendicular to the direction of the electrode stripes. The polarization direction of the zeroth order diffraction is parallel to the direction of the electrode stripes. Therefore, it can be used as a filter for light polarization.

Thermally responsive polymer-dispersed liquid crystal diffusers fabricated using laser speckle pattern irradiation

This study examined the thermal response of polymer-dispersed liquid crystal (PDLC) diffusers, patterned using a two-lens imaging system. Optical modulation was achieved by modifying the PDLC transmittance using temperature-induced changes to liquid crystal (LC) orientation. PDLCs with controllable scattering properties were obtained by irradiating LC-polymer composites with laser speckle patterns. The variation of the scattering characteristics of the PDLCs with temperature, average speckle size, and LC orientation order was analyzed to determine the most suitable parameters for a diffuser for smart window solar-ray control applications. The findings of these experiments demonstrate that using speckle patterns, a one-time laser exposure process, can provide a simple fabrication method of novel optical devices.

Electrically Switchable, Polarization-Sensitive Encryption Based on Aluminum Nanoaperture Arrays Integrated with Polymer-Dispersed Liquid Crystals

Metasurface-based structural coloration is a promising enabling technology for advanced optical encryption with a high-security level. Herein, we propose a paradigm of electrically switchable, polarization-sensitive optical encryption based on designed metasurfaces integrated with polymer-dispersed liquid crystals. The metasurfaces consist of anisotropic and isotropic aluminum nanoaperture arrays. Optical images can be encrypted by elaborately arranging anisotropic and isotropic nanoapertures based on their polarization-dependent plasmonic resonance characteristics. We demonstrate high-quality encrypted images and QR codes with electrically switchable, polarization-sensitive properties based on PDLC-integrated aluminum nanoaperture arrays. The proposed technique can be applied to many fields including high-security optical encryption, security tags, anticounterfeiting, multichannel imaging, and dynamic displays.

Organic Solvent Sensors Using Polymer-Dispersed Liquid Crystal Films with a Pillar Pattern

An organic solvent sensor of polymer-dispersed liquid crystals (PDLCs) film is fabricated by a combination of tri-functional monomers and LCs. When the patterned PDLC film comes into contact with the organic solvent, the organic solvent will penetrate into the film to induce the orientation of the liquid crystals, which will change from an ordered to a disordered state, which causes the PDLC film to scatter incident light. The experiment used acetone and ethanol as the organic solvents of interest. The results show that the patterned PDLC film has a stronger response to acetone than to ethanol. Based on the difference in the intensity of light scattering and the response time of the patterned PDLC film to different organic solvents, the results can be used to identify and recognize different types of organic solvents.

Orthogonal Reconstruction of Upconversion and Holographic Images for Anticounterfeiting Based on Energy Transfer

Crosstalk-free reconstruction of multiple images within a single element can greatly boost the image capacity and information security. We herein demonstrate a viable approach by integrating upconversion and holographic images into a single holographic polymer nanocomposite. The holographic image is reconstructed through photopolymerization-induced phase separation under a 460 nm laser and identifiable under room light, while the upconversion image recognizable under a 980 nm laser is photopatterned via spatially photobleaching of the dye embedded in the upconversion nanoparticle (UCNP) shell under 365 nm light. To this end, the lanthanide-doped UCNP in the core/shell/shell nanostructure of NaYF₄:20%Yb³⁺,0.5%Tm³⁺@NaYF₄@SiO₂ is designed, and the dye, fluorescein isothiocyanate (FITC), is fixed in the outermost SiO₂ shell via the amine-isothiocyanate reaction and the subsequent sol-gel reaction. Energy transfer from the core of the UCNP to FITC embedded in the shell is critical to boosting the contrast of the upconversion image, which dials the emission color from blue to yellow-green. It is also found that the upconversion image can be brightened by increasing the UCNP content while the holographic image is weakened when the UCNP content is over 15 wt %. This study paves a new way toward advanced anticounterfeiting.

Recent Advances in The Polymer Dispersed Liquid Crystal Composite and Its Applications

Polymer dispersed liquid crystals (PDLCs) have kindled a spark of interest because of their unique characteristic of electrically controlled switching. However, some issues including high operating voltage, low contrast ratio and poor mechanical properties are hindering their practical applications. To overcome these drawbacks, some measures were taken such as molecular structure optimization of the monomers and liquid crystals, modification of PDLC and doping of nanoparticles and dyes. This review aims at detailing the recent advances in the process, preparations and applications of PDLCs over the past six years.

Holography with Photochromic Diarylethenes

Photochromic materials are attractive for the development of holograms for different reasons: they show a modulation of the complex refractive index, meaning they are suitable for both amplitude and phase holograms; they are self-developing materials, which do not require any chemical process after the light exposure to obtain the final hologram; the holograms are rewritable, making the system a convenient reconfigurable platform for these types of diffractive elements. In this paper, we will show the features of photochromic materials, in particular diarylethenes in terms of the modulation of a transparency and refractive index, which are mandatory for their use in holography. Moreover, we report on the strategies used to write binary and grayscale holograms and their achieved results. The outcomes are general, and they can be further applied to other classes of photochromic materials in order to optimize the system for achieving high efficiency and high fidelity holograms.

Electrochemical Synthesis of Poly(3,4-ethylenedioxythiophene) Film Having Dot Structures for Diffraction Grating

Poly(3,4-ethylenedioxythiophene) (PEDOT) dot micro/nanostructures are synthesized by electrochemical polymerization in a concentrated hydroxypropyl cellulose (HPC) liquid crystal electrolyte solution. Surface observations by scanning electron microscopy and atomic force microscopy reveal micro/nanostructures having hemisphere-like dots on the surface of the PEDOT film, which causes light diffraction at ultraviolet and visible light wavelengths. The size of the dots depends on the concentration of the HPC electrolyte solution, decreasing with increasing the HPC concentration. Electrochemical oxidation and reduction causes changes in the color of the PEDOT film and the diffracted light. Moreover, Au coating on the surface of the PEDOT film enhances the diffracted light reflection intensity by more than tenfold compared to the noncoated PEDOT film.

Liquid Crystalline Nanocolloids for the Storage of Electro-Optic Responsive Images

Liquid crystalline nanocolloids (LCNCs), which are nanostructured composites comprising nanoparticles (NPs) and a liquid crystal (LC) host, have attracted a great deal of attention because of their promising new fundamental physical behaviors and functional properties. Yet, it still remains a big challenge to pattern LCNCs into mesoscale-ordered structures due to the limited NP loading in the LC host. Here, we demonstrate LCNCs in the nematic phase with a high NP loading (∼42 wt %) by in situ co-functionalizing the NP with alkyl and mesogenic ligands. The LCNCs can be assembled into ordered structures through holographic photopolymerization-induced phase separation, giving rise to holographic polymer-dispersed nematic nanocolloids (HPDNNC). Interestingly, high diffraction efficiency, low light-scattering loss, and unique electric-switchable capability are realized in the HPDNNC. In addition, high-quality switchable and unclonable colored images are reconstructed, promising a host of advanced applications (e.g., anticounterfeiting). Our findings pave a way to advance the fundamental understanding of nanostructured LCs and their practical utility in enabling a new breed of inorganic-organic composite materials.

Holographic Characteristics of Photopolymers Containing Different Mixtures of Nematic Liquid Crystals

A holographic polymer dispersed liquid crystal (HPDLC) is used to record holographic diffraction gratings. Several mixtures of nematic liquid crystals (LC) are used as components of the HPDLC to evaluate their influence in static and dynamic basic properties. The diffraction efficiency obtained in the reconstruction of the holograms is evaluated to compare the influence of the different LC. Additionally, the samples are exposed to a variable electric field and the diffracted light intensity as a function of the applied voltage is measured to evaluate the influence of the LC. The results obtained show significant differences depending on the LC incorporated to the photopolymer.

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

The ability to control light direction with tailored precision via facile means is long-desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next-generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.

Image flickering-free polymer stabilized fringe field switching liquid crystal display

Liquid crystal displays are the leading technology for flat panel displays. Their energy efficiency is low, however, due to the light absorption caused by polarizers and color filters and power consumption by driving circuitries. In displaying static images, their energy efficiency can be improved if a low driving frequency is used. As the driving frequency is decreased, the transmittance of the displays may change with time, a phenomenon known as image flickering. In this research we demonstrated that polymer stabilization can significantly reduce the flickering in fringe field switching (FFS) liquid crystal display. Under the polymer stabilization, the driving voltage remains low and the response time becomes shorter. Through simulation study, we find that the polymer stabilization smooths the spatial variation of the liquid crystal orientation in the display, and thus reduce the flexoelectric effect which is responsible for image flickering. The polymer stabilization can be implemented in the current main stream manufacturing to produce displays that can show static images under low power consumption.