The Evolution of Laser‐Induced Damage Patterns in Polymer Stabilized Liquid Crystals: Insights From Morphological Characterization and Thermo‐Driven Simulations

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials.

A study was performed to rank the influence of the parameters that can determine the resistance to laser-induced damage of polished synthetic fused silica at 355 nm. These parameters include the type of fused silica, the polishing process, the cleaning method, the sample thickness, and the sample diameter. The study involved both measurements of the statistics of laser-induced damage thresholds (LIDT) and characterization of the damage morphologies. The samples were irradiated at 20 degrees incidence angle with a 6.5-ns-pulse at 355 nm on an automated damage testing (ADT) system. The LIDT was measured on a minimum of 100 sites for each sample to gather sufficient statistical information and the damage morphology was characterized by Nomarski optical microscopy and SEM. The statistical damage curves were very reproducible. The LIDT was most influenced by the polishing process. Surface micro-pits were found on the surface after irradiation with damaging fluences close to the LIDT. Their density varied form less than 0.1 X 10<SUP>-3</SUP> to 26 X 10<SUP>-3</SUP> micrometers <SUP>-2</SUP> for the samples polished with the two different processes. The type of fused silica, the cleaning method, and the sample diameter seemed to have secondary effects on the LIDT. Bulk damage was occasionally noticed for thicker samples in areas with very high surface LIDT. Sample cleaning was found to play a role in reducing the number of low LIDT sites, thereby affecting the statistical behavior of the distribution tail, but did not significantly affect the average damage behavior. Finally, input surface (IS) damage occurred statistically less frequently and at higher fluences than output surface (OS) damage. Further work is discussed to understand if this effect is caused by electric field enhancement on the OS, or fundamental differences in crack propagation behavior between IS and OS.

As a kind of polymer stabilised liquid crystal (PSLC) devices, the scattering-type polymer-stabilised cholesteric liquid crystal (PSCLC) devices can be switched between the transparent state and the scattering state, which has huge application potential in the fields of smart windows, light valves, light modulators and others. The effect of polymer network texture on the electro-optical properties of PSLC. The monomers used in traditional polymer-stabilised cholesteric liquid crystals are usually bi-functional liquid crystal structure monomers. The operating voltage is high, and the hysteresis effect is also large. In this article, the mono-functional polymer monomer is added in reverse-mode polymer stabilised cholesteric liquid crystals (RPSCLC), it is found that when the weight content of the mono-functional monomer is close to that of bi-functional monomer, the large drop of the operating voltage is realised and the contrast ratio effect is increased, and the hysteresis effect can be greatly reduced. Combined with the morphology of the polymer network, the physical mechanism of the influence of mono-functional monomers on the electro-optic properties of PSCLC is revealed. The sample has the simple manufacturing process, uniform size of polymer network, low operating voltage, low hysteresis effect and high contrast ratio. The results will provide the important influence on the research of PSLC.

Recently, we have demonstrated a new approach to fabricate a variable focal lens using polymer-stabilized liquid crystals [1, 2]. The approach is based on curing of a polymer/liquid crystals mixture with a circularly symmetric (e.g. Gaussian) shaped laser beam to induce spatially inhomogeneous polymer network. Applying a uniform voltage to the non-pixilated cell leads to circular-symmetric (lens-like) distribution of refractive index in the cell with plane parallel substrates. In this paper we study and optimize the electro-optical characteristics of such lens by varying the wavelength of the polymerizing laser, temperature regime of the process of polymerization as well as frequency of the lens driving voltage. Obtained results are applied to develop lenses that have no moving components and allow the electro-optical zooming. REFERNCES [1] V.V.Presnyakov, T.V.Galstian, K.E.Asatryan, A.Tork. "Polymer-Stabilized Liquid Crystal for Tunable Microlens Applications", Optics Express, 10, 17, 865-870, 2002. [2] V.V.Presnyakov, T.V.Galstian. "Variable Focal Length Lens Based on Polymer-Stabilized Nematic Liquid Crystals", 19th International Liquid Crystal Conference, Edinburgh, UK, June 30 – July 5, 2002, Book of Abstracts, P754.

In this paper, we investigated the effect of porosity on the laser induced damage threshold (LIDT) of sol-gel SiO 2 and ZnO 2 single layer films. Sol-gel SiO 2 films were prepared with the dip-coating method from acid and base catalyzed SiO 2 sols, respectively. Some of the SiO 2 base films were subsequently treated in saturated ammonia gas for 20 hours. ZnO 2 sol-gel and physical vapor deposition (PVD) films were prepared by spin method and electron beam evaporation method, respectively. The films were irradiated by a pulsed Nd:YAG laser to obtain the LIDT of each film. In order to study the damage mechanism of films under laser irradiation, four types of tests were used. Thermal absorption of films was detected via Stanford photo-thermal solutions (SPTS). The porous ratio was derived via refractive index measured by ellipsometer. The surface morphologies of films were imaged by atomic force microscopy (AFM) before laser irradiation. Optical microscopy was used to characterize the defects and impurities of films before laser irradiation and damage morphology after laser irradiation. The experimental results [1–3] showed that porous ratio is an essential factor to decide the LIDT for sol-gel films, which benefits the pressure exerted on the film or substrate by the moving particle to dissipate. The films with lower thermal absorption and higher porous ratio have higher LIDT.

ABSTRACTThin composite films consisting of liquid crystal (LC) domains surrounded by polymer networks, termed as polymer stabilised liquid crystals (PSLCs), were prepared by photo-polymerisation of a pre-polymer dissolved in LC. Four composite films were prepared with different rubbing directions and with and without electric field during photo-polymerisation. Morphological characterisation carried our using a polarising optical microscope (POM) and a scanning electron microscope (SEM) reveal significant changes in LC domain morphology and associated polymer networks with the application of electric field during the fabrication of the films. The electro-optic (EO) properties of PSLC films placed between two crossed polarisers were studied using a He–Ne laser under an action of externally applied electric field. It was found that the PSLC film with twisted alignment and polymerised in the presence of electric field showed better EO properties than other films. Transmittance obtained by EO studies was verified with absorbance studies using a Ultraviolet-Visible spectrophotometer. The dielectric behaviour of PSLC films in the frequency range 20–20 MHz was investigated using a precision impedance analyser. The obtained data were modelled using Debye and Cole–Cole methods to calculate relaxation time and distribution parameter. The relaxation time calculated through the Cole–Cole model is in agreement with response time.

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&amp;SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&amp;SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&amp;ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&amp;SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices.

Many volatile organic compounds (VOCs) are hazardous to human health and their concentrations need to be monitored over a large area. However, most chemical sensors can only be used for point detection. In this paper, we report a method to obtain a two-dimensional (2D) concentration profile of toluene vapor by using polymer-stabilized liquid crystals (PSLCs). After the PSLC sample is exposed to toluene vapor ranging from 9300 to 2800 ppm, the PSLC changes its interference color according to different local toluene concentrations. By using the interference color, we can obtain a 2D concentration profile of toluene inside the PSLC and study the diffusion of toluene inside the PSLC in great detail. We also determine the diffusion coefficient of dissolved toluene inside the PSLC to be 1.01 × 10-6 cm2 s-1.

Polymer-stabilized liquid crystals represent composite materials that consist of polymer networks embedded in a continuous phase of liquid crystals. These materials are recognized for their fast switching times and low operational voltages, making them highly suitable for various optoelectronic applications. Traditionally, ultraviolet light photopolymerization has been employed to fabricate polymer-stabilized liquid crystals. However, this method poses significant challenges, especially with certain liquid crystal formulations that exhibit high birefringence and are unstable under ultraviolet exposure. In turn, blue light photopolymerization offers an attractive alternative, providing enhanced stability and compatibility across a broader range of liquid crystal compositions. In this study, the synthesis and characterization of polymer-stabilized liquid crystals using blue light photopolymerization were investigated, focusing on the effects of light intensity, initiator and monomer concentrations, and curing time on the electro−optical properties of the resulting materials. The findings contribute valuable insights for optimizing device performance and advancing liquid crystal technology.

A smart polymer stabilized liquid crystal (PSLC) thin film with temperature-controllable light transmittance was prepared based on a smectic-A (SmA)–chiral nematic (N*) phase transition, and then the effect of the composition and the preparation condition of the PSLC film on its thermo-optical (T-O) characteristics has been investigated in detail. Within the temperature range of the SmA phase, the PSLC shows a strong opaque state due to the focal conic alignment of liquid crystal (LC) molecules, while the film exhibits a transparent state result from the parallel alignment of N* phase LC molecules at a higher temperature. Importantly, the PSLC films with different temperature of phase transition and contrast ratio can be prepared by changing the composition of photo-polymerizable monomer/LC/chiral dopant. According to the competition between the polymerization of the curable monomers and the diffusion of LC molecules, the ultraviolet (UV) curing surrounding temperature and the intensity of UV irradiation play a critical role in tuning the size of the polymer network meshes, which in turn influence the contrast ratio and the switching speed of the film. Our observations are expected to pave the way for preparing smart PSLC thin films for applications in areas of smart windows, thermo-detectors and other information recording devices.

The observation of photorefractive gratings in new crosslinked polymer-stabilized liquid crystals (PSLCs) is discussed and compared to previous PSLCs. The PSLCs easily incorporate reduced or oxidized molecules that are present in a nematic liquid crystal at a concentration of 2 mol%. The PSLCs that are crosslinked provide improved photo-refractive grating resolution, due to their improved functionality as an immobile electron trap. These materials are capable of functioning well into the Bragg diffraction regime. Photoconductivity experiments that support the photorefractive mechanism and a different charge transport mechanism than neat liquid crystals are also performed.

The director configuration, the local order and the molecular-level reorientational dynamics (microviscosity) of the 5CB liquid crystal (LC) inside a polymer-stabilized LC (PSLC) cell prepared with the 4,4′-bis(6-(acryloyloxy)hexyloxy)biphenyl (BAB6) diacrylic monomer have been studied with the EPR spin probe technique across the nematic and the isotropic phase of the LC. The cells made from glass slides, either clean or coated with an aligning layer, were filled with pure 5CB or with 0.5, 1.0 or 2.0 wt% of BAB6, either in the monomeric state or polymerized. Local orientational order parameter (<P2>) in the non treated cells was always slightly lower than in the coated ones and both were lower than in the bulk LC. Order appeared to be independent of polymerization state or BAB6 concentration up to 1.0 wt%. Reorientational dynamics was essentially bulk-like in all cases, indicating that in our PSLC cells a full molecular mobility is retained. At the concentration of 2.0 wt%, in the coated cell with polymerized BAB6, the monodomain did not reform after a 90° rotation of the cell in the magnetic field, suggesting that the polymer network is, as expected, stabilizing the preexisting director configuration.

Effect of the photoinitiator presence and exposure conditions on laser-induced damage threshold of ORMOSIL (SZ2080)

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) systems are the two primary distinct systems in the field of liquid crystal (LC) technology, and they are differentiated by their unique microstructures. Here, we present a novel coexistent system of polymer-dispersed and polymer-stabilized liquid crystals (PD&SLCs), which forms a homeotropically aligned polymer network (HAPN) within the LC droplets after a microphase separation between the LC and polymer matrix and combines the advantages of both the PDLC and PSLC systems. Then, we prepare a novel thermally light-transmittance-controllable (TLTC) film from the PD&SLC system, where the transmittance can be reversibly changed through thermal control from a transparent to a light-scattering state. The film also combines the advantageous features of flexibility and a potential for large-scale manufacturing, and it shows significant promise in future applications from smart windows to temperature sensors.