Reorientation Of Liquid Crystal Molecules Research Articles

Optical CO2 gas sensor based on liquid crystals in a textile grid

Given the increasing concerns about global warming, it is undeniable that measuring and controlling carbon dioxide (CO2) levels, a colorless and odorless greenhouse gas, is of great value. In this respect, liquid crystals (LCs) as an anisotropic material hold promise for fabricating such gas sensors. Here, we report a sensitive optical gas sensor for real-time monitoring of CO2 gas, exploiting a textile grid impregnated with LC and diethanolamine (DEA) as a CO2-sensitive material. The sensing mechanism relies on the reorientation of LC molecules upon the interaction of gas analytes with DEA. By tracing optical texture changes and extracting the corresponding intensities, CO2 gas concentrations ranging from 300 to 10,000 ppm were detected. The sensor exhibits a response time of 12 seconds and a recovery time of 7 seconds at 800 ppm. The sensor is simple and cost-effective.

56.2: Invited Paper: Novel Bistable Liquid Crystal Materials for AR/VR Technology, Vehicle and Flexible Display

The object of study is chiral-nematic and smectic A liquid crystals. The aim of the work is to develop new bistable materials with improved physico-chemical and electro-optical properties for various applications. Chiral liquid-crystalline compounds and compounds characterized by the presence of the smectic A phase over a wide temperature range were synthesized during the studies. The influence of the viscoelastic properties of liquid crystals, the anchoring energy of the alignment material, and the helical twisting power of chiral dopants on the memory effect in chiral-nematic liquid crystals was investigated. In order to clarify the possible mechanism of bistability in highly twisted nematics, we performed a numerical simulation of the reorientation of liquid crystal molecules in an electric field. It was also necessary to find out which parameters of the liquid crystal material have the greatest effect on the bistability. Based on the synthesized smectic compounds, mixtures with different values of optical anisotropy and viscosity were developed. The influence of the compounds structure (bridge fragments, polar groups, alkyl chains) on mesomorphic and basic physico-chemical characteristics was determined. Electrically and electrochemically stable ion dopants were also synthesized with a number of organic cations and organic/inorganic anions. The composition of mixtures has been optimized for long-term operation, especially in the upper operating temperature range.

(Keynote) Van Der Waals Active Metasurfaces and Heterostructures for Phase Modulation and Polarization Conversion

A grand challenge for nanophotonics is the realization of tunable metasurfaces enabling active control of the key constitutive properties of light – amplitude, phase, wavevector and polarization. Active metasurfaces that enable dynamic modulation of reflection amplitude, phase and polarization have been recently explored using several active materials and modulation phenomena, including carrier index in plasmonic ENZ structures, reorientation of liquid crystal molecules, electrooptic effects in quantum well heterostructures and index change in phase change materials. The rapid advances in understanding of exciton resonances in layered van der Waals materials has now stimulated thinking about active metasurfaces that exploit excitonic modulation phenomena to enable ‘van der Waals active metasurfaces’. As one example, I will describe recent advances in electrically reconfigurable polarization conversion across the telecommunication wavelength range in van der Waals layered materials, integrated in a Fabry-Pérot cavity. The large electrical tunability of the excitonic birefringence in tri-layer black phosphorus enables spectrally broadband polarization conversion over nearly half the Poincaré sphere. We observe both linear to circular and cross-polarization conversion with voltage, demonstrating dynamic access to polarization diversity. As a second example, we discuss the observed large gate tunability of the complex refractive index in monolayer MoSe2 by Fermi level modulation near the A and B excitonic resonances for temperatures between 4 K to 150 K. By tuning the charge density, we observe both temperature and carrier dependent epsilon-near-zero response in the permittivity and transition from metallic to dielectric near the A exciton energy, and directly observe active phase modulation in monolayer MoSe2 gated heterostructures, whose voltage dependence is consistent with our complex index measurements. These results have broad implications for the use of monolayer transition metal dichalcogenides in active metasurfaces, and I also will give a general outlook for the wide range of possibilities for active van der Waals metasurfaces.

Electrically Tuning Characteristics of LC Selectively Infiltrated PCF Sagnac Interferometer

A photonic crystal fiber was selectively infiltrated only two of the air holes near the core with liquid crystal. The selective infiltration caused geometric asymmetry and created high birefringence. Because of the geometric asymmetry, the response to varying electric field is concerned with electric field direction. A Sagnac interferometer was set to investigate the electric tuning characteristics of the selectively infiltrated photonic crystal fiber. The effective refractive-indices of the fiber varies with electric field due to reorientation of liquid crystal molecules. The highest sensitivity was 1.12 nm/V (504 nm/kV/mm) with tuning range of 7 nm when electric field direction was parallel to the connecting line of infiltrated holes. The response time of the selectively infiltrated photonic crystal fiber was studied. The device can be utilized as voltage sensors, electro-optical modulators and filters.

Electro-Steering Tapered Fiber-Optic Device with Liquid Crystal Cladding

The paper presents the results of design, manufacturing, and characterization of a hybrid broad band in-line fiber-optic device. It uses nematic liquid crystal as cladding with electro-steering properties in a biconical optical fiber taper structure. Liquid crystal mixtures denoted as 6CHBT and E7 are designed for electric as well as temperature control of electromagnetic wave propagation in a broad wavelength range. The applied taper with 10±0.5 μm diameters has losses lower than 0.5 dB in whole investigated spectrum range. Three kinds of initial liquid crystal molecules’ orientations (parallel, orthogonal, and twist) in relation to the light beam propagating in a taper were applied. The performance of a tuned cladding was studied at an electric field of the range of 0–190 V and the temperature range from 20°C up to 42°C and 59°C for 6CHBT and E7, respectively. The induced reorientation of liquid crystal molecules was measured at a broad wavelength range (550-1550 nm).

Tapered fiber liquid crystal hybrid broad band device

This paper presents the results of design, manufacturing and characterization of a hybrid broad band filter based on taper technology and liquid crystalline structure using a standard single mode fiber. A liquid crystal mixture, denoted 1550*, was designed for the electrically driven modulation of an electromagnetic wave at visible range. The main reason for using a biconical optical fiber taper as a core is surrounding it by a liquid crystalline medium providing the possibility to control an effective refractive index of fiber cladding. Two kinds of rubbing, orientation parallel and orthogonal to the taper axis, were applied. The performance of a tuned cladding was studied at electric field and temperature domains. The induced reorientation of liquid crystal molecules was measured at a broad wavelength range, between 500 and 1200 nm. All fabricated tunable broad band filter devices exhibit internal losses lower than 0.5 dB.

Tunable transmission of a nematic liquid crystal as defect in a 1D periodic structure of dielectric materials by orientation and re-orientation of liquid crystal molecules.

In this paper, we investigate tunable transmission characteristics of a one-dimensional periodic structure (1DPS), designed with periodic dielectric materials containing a nematic liquid crystal (NLC) as a defect layer, on the basis of orientation and re-orientation of LC molecules. The nonlinear differential equation for the director of the liquid crystal under the light field is solved numerically. The relation between the liquid crystal director and the intensity of the electromagnetic wave (EMW) is derived. Transmittances of the liquid crystal defect layer in the 1DPS are calculated with the variation of the intensity of the incident wave and liquid crystal director tilt angles. By varying the director tilt angle of the liquid crystal molecules as well as the incident angle of the EMW, the shifting of the transmitted defect mode wavelengths is studied. Such study is helpful to understand how orientation and reorientation of the molecules affect the transmittance of the considered periodic structure when the EMW interacts with an embedded liquid crystal as a defect layer in the 1DPS of dielectric materials. Such photonic structure of dielectric materials with the liquid crystal layer as a defect layer can be used to fabricate bistable switches, optical filters, feedback lasers, etc.

Photoluminescence of CdSe/ZnS quantum dots in nematic liquid crystals in electric fields.

We have experimentally investigated the effect of the reorientation of a nematic liquid crystal (LC) in an electric field on the photoluminescence (PL) of CdSe/ZnS semiconductor quantum dots (QDs). To the LC with positive dielectric anisotropy, 1 wt % QDs with a core diameter of 5 nm was added. We compared the change of PL intensity and decay times of QDs in LC cells with initially planar or vertically orientated molecules, i.e., in active or passive LC matrices. The PL intensity of the QDs increases four-fold in the active LC matrix and only 1.6-fold in the passive LC matrix without reorientation of the LC molecules. With increasing electric field strength, the quenching of QDs luminescence occurred in the active LC matrix, while the PL intensity did not change in the passive LC matrix. The change in the decay time with increasing electric field strength was similar to the behavior of the PL intensity. The observed buildup in the QDs luminescence can be associated with the transfer of energy from LC molecules to QDs. In a confocal microscope, we observed the increase of particle size and the redistribution of particles in the active LC matrix with the change of the electric field strength. At the same time, no significant changes occurred in the passive LC matrix. With the reorientation of LC molecules from the planar in vertical position in the LC active matrix, quenching of QD luminescence and an increase of the ion current took place simultaneously. The obtained results are interesting for controlling the PL intensity of semiconductor QDs in liquid crystals by the application of electric fields.

Liquid crystal on subwavelength metal gratings

Optical and electrooptical properties of a system consisting of subwavelength metal gratings and nematic liquid crystal layer are studied. Aluminium gratings that also act as interdigitated electrodes are produced by focused ion beam lithography. It is found that a liquid crystal layer strongly influences both the resonance and light polarization properties characteristic of the gratings. Enhanced transmittance is observed not only for the TM-polarized light in the near infrared spectral range but also for the TE-polarized light in the visible range. Although the electrodes are separated by nanosized slits, and the electric field is strongly localized near the surface, a pronounced electrooptical effect is registered. The effect is explained in terms of local reorientation of liquid crystal molecules at the grating surface and propagation of the orientational deformation from the surface into the bulk of the liquid crystal layer.

Dielectric Permittivity of Polymer Composites with Encapsulated Liquid Crystals in Strong Electric Fields

Creation of flexible display units is one of the most perspective directions in the development of optoelectronics. Nowadays intensive investigations of flexible screens, displays, microdisplay technology, modulators, etc. based on liquid crystals encapsulated in a polymer are carried out in research centers using flexible reference surfaces. Along with positive properties of the composites in comparison with pure liquid crystals (LC) used in traditional liquid crystal displays, composites are characterized by an enhanced level of the control field necessary for reorientation of LC molecules in the polymer matrix and a change in the time of LC response to an electric pulse [1]. Therefore, investigations aimed at the decrease of the control field and the improvement of the dynamic characteristics of LC encapsulated in the polymer are important for their practical application in future technologies. Synthesis of new liquid crystals with improved functional characteristics is a difficult and expensive process. A simpler method of modification of polymer liquid crystal composition is incorporation of small-sized metal or carbon particles into the polymer liquid crystal composition [2, 3]. The particles change the electrophysical properties of the material and determine the control electric fields and the response times. The threshold voltage of the electric Fredericks transition in pure LC is independent of the layer thickness [4] and can change from several tenth of a volt to several volts. In composites this voltage is determined by additional factors, such as the dielectric properties of the polymer, the thickness of the layer, the size of LC capsules, etc. The voltage of LC molecule reorientation in composites can increase by several ten and even hundred times [5, 6]. According to the Fredericks effect, the external electric field influences the direction of the LC director in the threshold manner that, in turn, leads to changes in the dielectric permittivity of the composite and in the electromagnetic wave passage through the composite. The threshold voltage in the LC encapsulated in the polymer can be investigated based on the voltage-current characteristics [4]. However, this method can be used only if the polymeric binding

The influence of charged ions on the electro-optical properties of polymer-dispersed liquid crystal films prepared by ultraviolet-initiated cationic polymerization

Polymer-dispersed liquid crystal (PDLC) films based on epoxy resins were prepared by UV-initiated cationic polymerization using UVI-6974 as UV-initiator. The charged ions arose from photolysis of the UV-initiator remained in the liquid crystal (LC) droplet of the polymer matrix. Optical devices based on PDLC films derive their functional properties from the electric field induced reorientation of LC droplets. These PDLC films doped with ions showed different reorientation of LC molecules and electro-optical properties than those prepared without ionic dopants. When an electric field was applied across these PDLC films, the charged ions in the LC droplet migrated along the direction of the electric field in inducement of the electric field and assembled at the LC droplet/polymer interface. According to the situation mentioned above, the model of charged double layers at the LC droplet/polymer interface was set up in our study. The behavior was attributed to the charged double layers that cancel the field strength within the “bulk” of the liquid crystal in time. By using this model, the electric field and initiator concentration dependent results and the difference of the PDLC with and without ions were investigated and explained.

Enhanced non-volatile and updatable holography using a polymer composite system

Updatable holography is considered as the ultimate technique for true 3D information recording and display. However, there is no practical solution to preserve the required features of both non-volatility and reversibility which conflict with each other when the reading has the same wavelength as the recording. We demonstrate a non-volatile and updatable holographic approach by exploiting new features of molecular transformations in a polymer recording system. In addition, by using a new composite recording film containing photo-reconfigurable liquid-crystal (LC) polymer, the holographic recording is enhanced due to the collective reorientation of LC molecules around the reconfigured polymer chains.

63.3: Fast Switching Surface‐Polymer‐Assisted IPS Liquid Crystal Displays

AbstractWe demonstrate fast switching surface‐polymer‐assisted in‐plane switching (SPA‐IPS) liquid crystal displays (LCDs). The formation of polymer nano islands at the substrate surface as the alignment layers is achieved by liquid crystal (LC) directed polymerization of a small amount of a reactive monomer in homogenously aligned liquid crystal. At appropriate concentration, the phase‐separated nano‐sized polymer islands, with a preferential orientation direction following the rubbing direction, act as internal surface alignment layers and enable the fast switching of field‐induced reorientation of liquid crystal molecules. Furthermore, significant improvement in switch‐off time of a SPA‐IPS will be due to the increased surface anchoring strength by the low‐density surface‐localized polymer islands.

Dielectric properties of liquid crystals in polycapillary matrices

This paper reports on the results of investigations into the dielectric properties of liquid crystals embedded in polycapillary matrices and describes a technique for their measurement. It has been revealed that the chemical structure of the rigid core and the length of mobile alkyl groups of liquid-crystal molecules of the alkylcyanobiphenyl group substantially affect the equilibrium configuration of the liquid-crystal director in capillaries. The reorientation of liquid-crystal molecules embedded in capillaries in the nematic phase under the influence of an external magnetic field has been investigated.

Electrically switchable computer-generated hologram using a liquid crystal cell with a proton beam patterned polymethylmethacrylate substrate

An electrically switchable computer-generated hologram (CGH) was fabricated using a liquid crystal (LC) cell. A polymethylmethacrylate (PMMA) film, which was spin-coated on one glass substrate of the LC cell, was patterned by a focused 2 MeV proton beam with a CGH phase pattern (2 microm resolution). With an applied voltage on the LC cell CGH sample, an index modulation was produced between the regions with and without PMMA because of the reorientation of LC molecules under the external electric field. The maximum diffraction efficiency measured was about 28.7%. The operating voltage was below 15 V(rms).

The effect of phototautomerism on the nonlinearity of the liquid crystals

The nonlinearity of hydroxy-azo dye doped liquid crystals is investigated and the effect of tautomerism is considered. The results indicate that, the nonlinearity of guest–host system can be affected by control of phototautomerism and incident light modulation. In special conditions, the variation of nonlinearity can be stable, so it can generate a stable refractive index pattern because of the optical reorientation of liquid crystal molecules.

Electrical reorientation of liquid crystal molecules inside cylindrical pores for photonic device applications

We present the simulated distribution of the local director of a nematic liquid crystal inside cylindrical macropores under the influence of an electric field. The Frank free energy approach is used to describe the nematic behavior. The influence of both molecular anchoring strength and pore radius is investigated. The results of this analysis are applied for simulation of an electrically tunable microcavity based on porous silicon infiltrated with a liquid crystal. The Bruggeman approximation is used while calculating the effective refractive index of each layer in the porous silicon multilayer structure. The reflectivity spectrum of the latter is simulated using the transfer matrix approach. The electrical tuning range of a microcavity designed for near-infrared waves is found to vary from 10.5 up to 23 nm for weak and strong surface anchoring conditions, respectively.

Carbon Nanoparticles in Nematic Liquid Crystals

Fullerene C60, C70, single-walled and multi-walled carbon nanotubes and graphene sheets are doped to nematic liquid crystal (LC) host in the same percentage. Planar samples of these mixtures are prepared and our measurements constitute an optimization basis for possible applications. Fullerene balls are found to be the best compatible material for optical aims and reorientation of LC molecules, while the carbon nanotubes experience some reorientation possibility in LC media and graphene layers are good barriers to preserve reorientation.

Tunable one‐dimensional photonic crystal structures based on grooved Si infiltrated with liquid crystal E7

AbstractTuning of a photonic band gap in a composite one‐dimensional photonic crystal structure infiltrated with nematic liquid crystal E7 has been evaluated by simulations. It has been found that a sufficient shift of the short wavelength edge, λ edge of a main photonic band gap can be obtained in structures with a pronounced initial alignment of the liquid crystal director in the grooves of a single crystalline Si matrix. The largest effect is predicted for the reorientation of liquid crystal molecules from planar homogeneous alignment along the grooves to homeotropic alignment with respect to the Si walls due to an electro‐optical effect. The shift in photonic band gap due to thermal tuning, resulting from the transition of liquid crystal from the mesophase to the isotropic phase, is less than for electric tuning. Thermo‐tuning has been demonstrated experimentally. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrotunable in-plane one-dimensional photonic structure based on silicon and liquid crystal

The model of the electro-optical effect, due to the reorientation of liquid crystal molecules from a pseudoisotropic to a homeotropic state, in a composite photonic structure with a liquid crystal filler, is elaborated. A composite (110) grooved silicon photonic structure for the middle infrared range was designed and fabricated on a silicon-on-insulator platform. Polarized reflection spectra, demonstrating the electro-optical effect, have been obtained by means of Fourier transform infrared microscopy. The relative shift of the band edge at half intensity in the region of 10μm was found experimentally to be 1.6% compared to 2.2% as predicted by theory.