Refractive Index Of Liquid Crystal Research Articles

Sensitivity enhanced electric field sensing with etched tilted fiber Bragg grating and a liquid crystal

A temperature-insensitive electric field sensor utilizing HF-etched tilted gratings in a high refractive index solution (E7 liquid crystal) is proposed. The electric field causes a change in the original arrangement of molecules, thereby altering the refractive index of liquid crystals. This study demonstrates that a combination of liquid crystal and HF-etched tilted gratings effectively excites the cladding leakage mode of the tilted grating. Compared to a non-etched tilted fiber Bragg grating, the proposed sensor exhibits higher sensitivity to electric fields. The experimental results show that the electric field sensitivity of a tilted grating with a diameter of 17.7 µm is 0.024 dB/(kV/cm). The influence of external temperature on the electric field sensor of this design is relatively small, only showing a sensitivity of 0.003 dB/°C. This characteristic is very attractive in practical applications such as industry and healthcare.

Study on polymer dispersed liquid crystal films with wide viewing angle

ABSTRACT Polymer Dispersed Liquid Crystal (PDLC) films have been widely used as electronically controlled dimming technology, and solving the problem of narrow angle of PDLC is a cutting-edge challenge for their application. In this paper, a theoretical model of the viewing angle characteristics of PDLC is constructed, and the corresponding samples are experimentally fabricated. It is found that the simulation results of transmittance changing with the viewing angle are in good agreement with the experimental results. Furthermore, the technical approaches to improve PDLC viewing angle characteristics and achieve a wide viewing angle are discussed theoretically. The results show that the PDLC films for wide viewing angle can be obtained by using liquid crystal materials with smaller anisotropy of refractive index and polymer materials with refractive index slightly higher than the ordinary refractive index of liquid crystal.

INFRARED REFRACTIVE INDICES OF HYDROGEN-BONDED LIQUID CRYSTALS FROM THE ENVELOPE METHOD

The refractive index of liquid crystals at infrared wavelengths is to be measured using a novel technique. This method involves constructing the envelopes for the transmitted intensities using the envelope method on Fourier transform infrared spectral data before doing refractive index measurements. The upper and lower envelopes for the envelope approach are made using the transmitted wavenumber intensities. In this case, we consider hydrogenbonded mesogens, specifically p-n-alkyl benzoic acid : p-(p'-Octyloxybenzylidene)-p-cyanoaniline (nBA: OBCA, where n=6, 8). The outcomes of this research demonstrate that the hydrogen-bonded mesogens nBA:OBCA have higher refractive indices than the pure compounds. Refractive index values in the infrared area drop with decreasing wave number and reach saturation. The proposed technique evaluates samples' refractive indices in the infrared region (4000cm-1 - 400cm-1) which is difficult-to-measure. Molecular interactions of hydrogen-bonded liquid crystals are also investigated.

Compact fiber sensor for pH measurement based on the composite effect of hydrogel deformation and LC refractive index variation.

A novel, to the best of our knowledge, type of compact pH fiber sensor combined with a hydrogel based on the whispering gallery mode (WGM) is proposed and integrates a liquid crystal (LC) microdroplet in a capillary in a compact structure as small as 180 µm. In the research, the hydrogel performs both as a supporting frame and a responsive material that causes morphological deformation of the LC microdroplet with pH variation. Moreover, a new phenomenon of pH-induced LC refractive index variation is observed and applied in the pH measurement, so that the acid itself can also lead the LC microdroplet structure transition. Thus, the WGM method is applied to detect the composite effect simultaneously to improve the sensing capability. The sensitivity of the sensor in the pH range from 4.55 to 6.86 reaches 3.19 nm/pH. The response time is short, within 60 s. The simple and compact structure of the sensor reduces the cost and enhances the stability, which is of great potential for biomedical pH measurement.

An Electrically Tunable Terahertz Filter Based on Liquid-Crystal-Filled Slits with Wall Corrugations

We propose a type of hollow planar waveguide with corrugated walls, which can realize electrically tunable terahertz (THz) filtering by filling the slit with liquid crystals. When the THz signals propagate in a planar waveguide with periodic corrugations, the transmission spectrum always exhibits many pass and stop bands. Inserting a section of defects in the middle of the periodic corrugations can excite an extremely narrow transmission peak, which would be a very good THz filter for frequency division. To achieve tunability of this narrow linewidth THz filter, we also fill the slit between the two corrugated walls with a nematic liquid crystal. The effective refractive index of liquid crystals will change with the external electric field, thus tuning the frequency of the narrow peak. The simulated results show that the center frequency of the proposed filter can be tuned linearly in the frequency range of 0.984~1.023 THz by the external electric field. Moreover, the bandwidth of the filter can be adjusted from 3.2 GHz to 0.3 GHz by increasing the number of periods in the waveguide, and a maximum Q value of 2556 can be achieved when the number of periods at both sides of the defect is 12.

Plasmon-induced transparency in a reconfigurable composite valley photonic crystal.

We propose a new kind of reconfigurable topological valley photonic crystal (TVPC), and a novel topological waveguide can be formed by constructing a domain wall between two TVPCs with opposite valley-Chern indices. The topological waveguide mode in the composite TVPC has large group refractive index. A topologically protected coupled waveguide cavity system is then designed by introducing a hexagonal ring cavity at the center of the straight domain wall of a combined TVPC, in which a narrow plasmon induced transparency window rises at 3.8848 GHz with a Q-factor of 1387 and a maximum group refractive index as high as 186. We propose a notch filter with a resonant frequency of 3.8852 GHz and a very high Q-factor of 10224. By changing the refractive index of liquid crystals via an external voltage applied between two parallel metal plates, the filter can be switched between band-pass and band-stop based on the reconfigurable topological interface state.

The experimental average refractive index of liquid crystals and its prediction from the anisotropic indices.

Anisotropic optical properties of liquid crystals (LC), combined with their fluidity, are crucial for the development of new liquid crystal devices and applications. The optical anisotropy implies an existence of various refractive indices that depend on the direction of light oscillation relative to the director vector. For some applications, it is, however, necessary to estimate an average refractive index (nav) for the liquid-crystalline material, which is commonly effected through models of combination of the anisotropic refractive indices. The validity of these models lacks proof as the experimental average refractive index for anisotropic fluids has not been published. This article reports an experimental strategy to measure nav of liquid crystals; the method is based on generating multiple orientations of the liquid crystal molecules through their isotropization in mixtures with isotropic liquids. The refractive index of an isotropic mixture is measured; then the apparent refractive index of the liquid crystal is extracted using validated combination models of refractive indices in mixtures. The method was assessed with two nematic liquid crystals (5CB and MBBA) mixed with several isotropic liquids. The results indicate that the average refractive index of a LC is an extrapolation, in the nematic range of temperatures, of the index variation above the nematic-isotropic transition. A new theoretical model to predict the average refractive indices of liquid crystals was developed. Compared with the traditional formulae, the new model represents a significant improvement for the calculation of nav.

Fringe field-tunable LC refractive index interface for in-plane beam steering applications

We report on the electrically tunable optical structure based on dual-domain nematic liquid crystal (LC) alignment for in-plane beam steering applications. The device operates due to the total internal reflection of an extraordinary beam at the LC refractive index interface that separates homeotropic and planar-aligned nematics. Patterned electrodes were used in order to switch on the refractive index interface in the bulk of a planar-aligned LC layer. An outstanding feature of the proposed device is the function of tuning the spatial position of the LC interface by means of a fringing electric field, which allowed one to implement wide range light beam microscanning, as well as to realize in-plane angular beam steering with a milliradian resolution.

Photoalignment and photopatterning: New liquid crystal technology for displays and photonics

Objectives. Since the end of the 20th century, liquid crystals have taken a leading position as a working material for the display industry. In particular, this is due to the advances in the control of surface orientation in thin layers of liquid crystals, which is necessary for setting the initial orientation of the layer structure in the absence of an electric field. The operation of most liquid crystal displays is based on electro-optical effects, arising from the changes in the initial orientation of the layers when the electric field is turned on, and the relaxation of the orientation structure under the action of surfaces after the electric field is turned off. In this regard, the high quality of surface orientation directly affects the technical characteristics of liquid crystal displays. The traditional technology of rubbing substrates, currently used in the display industry, has several disadvantages associated with the formation of a static charge on the substrates and surface contamination with microparticles. This review discusses an alternative photoalignment technology for liquid crystals on the surface, using materials sensitive to polarization of electromagnetic irradiation. Also, this review describes various applications of photosensitive azo dyes as photo-oriented materials. Results. The alternative photoalignment technology, which employs materials sensitive to electromagnetic polarization, allows to create the orientation of liquid crystals on the surface without mechanical impact and to control the surface anchoring force of a liquid crystal. This provides the benefits of using the photoalignment technology in the display industry and photonics—where the use of the rubbing technology is extremely difficult. The optical image rewriting mechanism is discussed, using electronic paper with photo-inert and photoaligned surfaces as an example. Further, different ways of using the photoalignment technology in liquid crystal photonics devices that control light beams are described. In particular, we consider switches, controllers and polarization rotators, optical attenuators, switchable diffraction gratings, polarization image analyzers, liquid crystal lenses, and ferroelectric liquid crystal displays with increased operation speed. Conclusions. The liquid crystal photoalignment and photopatterning technology is a promising tool for new display and photonics applications. It can be used for light polarization rotation; voltage controllable diffraction; fast switching of the liquid crystal refractive index; alignment of liquid crystals in super-thin photonic holes, curved and 3D surfaces; and many more applications.

Optical effects in liquid crystal cell with photosensitive chalcogenide glass substrate

We report on the observation of dynamic light-induced annular patterns formation in a planar liquid crystal (LC) cell with chalcogenide microfilm As2S3 as orientating layer. The annular patterns appear under the irradiation of the cell by the laser beam (λ = 473 nm) with Gaussian distribution of intensity. The experimental results are explained in terms of the liquid crystal director reorientation in the LC cell after absorption of light by chalcogenide film. The director reorientation results in the LC refractive index change and formation of annular patterns in LC cell. To theoretically model the system under study we first minimized the total free energy functional subject to Gaussian-type modulation of the pretilt angle at chalcogenide covered substrate. Having found the director spatial profile we studied the light beam propagation through the cell with a spatially modulated pretilt angle. We determined the number of the aberration rings depending on the parameters of the laser beam and LC cell. We believe that light-induced change in the LC director anchoring at chalcogenide film can be applied to the development of various optical and electro-optical elements, such as dynamic micro-lenses, light controlled switchers and triggers, etc.

Graphene/liquid crystal hybrid tuning terahertz perfect absorber.

We present, by simulations, a metastructured graphene/liquid crystal hybrid tuning terahertz perfect absorber that consists of graphene disk resonator arrays above a metallic layer separated with liquid crystal substrate. The liquid crystal refractive index and the graphene Fermi level are utilized to implement double-tuning operation to push the spectra scanning limit of the terahertz absorber. Our simulations demonstrate high performance of a near-linear broad tuning region and near-unity absorbance with wide incident angle and polarization independence. The range of the resonant frequency scan is notably enlarged at a spectral ratio of as high as Δf/f=50% while ensuring absorbance beyond 90%. Such graphene/liquid crystal hybrid tuning scheme would be preferable to push the working-band limit of terahertz perfect absorbers.

Reconfigurable thin film filter for compensating AOI deviation effects using LC coupled cavities

An efficient technique to enhance thin-film filters’ angular tolerance is described and numerically demonstrated. The spacer refractive index is controlled using a liquid crystal (LC) in the multi-cavity thin-film filter. The angle of incident (AOI) deviation causes significant degradation in the filter performance. To circumvent the influence of the AOI deviation, a correction effect is introduced through controlling the LC refractive index, while maintaining the normal incident performance of the filter. The proposed LC-based approach has proven effective in increasing the angular tolerance of the thin-film filter. Simulation results indicate an angle-insensitive performance under a 20o incident angle cone deviation. The results illustrate the effectiveness of this approach over the C-band, where the change in the center wavelength shift was reduced from 107.57 nm to 0.38 nm. Moreover, the change in the transmission filter bandwidth was reduced by 70%. The proposed compensation approach offers flexibility in the central wavelength and in the filter bandwidth estimated at the 0.5 dB, 3 dB, and 25 dB passband ranges.

A three-layer egg-configurated Bragg microcavity of polyglycerol coated cholesteric-liquid-crystal encapsulated by hollow-glass-microsphere

ABSTRACTThis letter reports the optical pumped lasing behaviours of a three-layer Bragg resonance cavity consisting of dye-doped cholesteric liquid crystal (DDCLC) microdroplet, polyglycerol-2 and hollow glass microsphere. The function of PG2 is to control the parallel anchoring of the liquid crystal (LC) molecules on the surface of the LC microdroplet. The whispering-gallery mode (WGM), radial Bragg (photonic bandgap, PBG) mode and Bragg WGM (BWGM) are observed in DDCLC microspheres with different helical pitches and LC refractive indices. The formation mechanisms of six types of lasing emission conditions are analysed in detail. The study results present the prospect of controlling the output mode of the laser. Furthermore, such solid shell-based DDCLC microspheres have outstanding potential applications in miniaturised 3D Bragg lasers, sensors, and integrated and tunable optical devices.

Excellent Quality Factor Ultra-Compact Optical Communication Filter on Ring-Shaped Cavity

Abstract In this paper, we propose a high-quality factor (QF) ultra-compact structure for designing all optical filter based on two dimensional (2D) photonic crystals that can operate in third communication window range (1550 nm). The simulation results calculated by using 2D finite difference time domain (FDTD) method and photonic band gap (PBG) region are calculated by plane wave expansion (PWE) method. The average transmission efficiency, average QF and optical full width at half maximum (FWHM) bandwidth more than 95 %, 4300 and 0.4 nm obtained, respectively. The value of QF for the proposed structure is a major QF for ring resonator-based filters. We studied the effect of changing the air background to liquid crystal (LC) surrounding the dielectric rods. Results showed that by increasing LC refractive index (n) peak of wavelengths red shifted. The footprint of proposed filter is 160 ${{\mu}}{{\rm{m}}^{\rm{2}}}$ which make the proposed structure promising candidate for optical integrated circuits.

Research progress of terahertz liquid crystal materials and devices

Liquid crystal (LC) is an excellent tunable functional material which can be controlled by the external stimulus such as electric field, magnetic field and temperature. Terahertz (THz) radiation in a frequency range of 0.1−10.0 THz, has enormous advantages such as a low photon energy, sensitivity to crystal lattice vibration, magnetic spins, hydrogen bonds, intermolecular interaction, and water, and high transparency to non-conducting materials. The THz technology, therefore, has great potential in a diverse range of applications from spectroscopy, security screening to biomedical technology and high-speed wireless communication. But the development of high-performance LC based tunable THz functional devices is still in its infancy stage. The dispersion of LC refractive index induces a comparatively low birefringence in the THz regime. The lack of transparent electrodes makes the electric tuning of LCs difficult to achieve. To achieve certain modulations requires a very thick THz layer, leading to several disadvantages such as high operating voltage, slow response and poor pre-alignment. In this paper, we first present the research progress of large birefringence LCs in THz range. A room-temperature nematic LC NJU-LDn-4 with an average birefringence greater than 0.3 in a frequency range from 0.5 to 2.5 THz is shown in detail. This kind of LC can remarkably reduce the required cell gap, thus reducing the operating voltage and response time. Then we summarize varieties of conventional THz devices based on LC. Many electrodes are used for THz range. Graphene which can be used as a perfect transparent electrode material in THz band is proposed. Not only tunable transmissive but also reflective THz waveplates are introduced. The thickness of the LC layer of the reflective one can be reduced to ~10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. The same tunability as that in the transmissive type just needs half the thickness. We also introduce that LC can generate THz vortex beam based on a photopatterned large birefringence LC. In the area of LC based versatile THz metamaterial devices, the adjacent units of a metasurface layer, such as a fishnet or grating, are usually connected to each other which may cause low-quality (<i>Q</i>) factor and polarization sensitivity, which is undesirable. We emphasize a graphene-assisted high-efficiency tunable THz metamaterial absorber. Few-layer porous graphene is integrated onto the surface of a metasurface layer to provide a uniform static electric field to efficiently control the LC, thereby enabling flexible metamaterial designs. The THz far-field and near-field with large modulation and fast response are realized. A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and the large-birefringence liquid crystals is also demonstrated to be able to generate broadband THz radiation and control the polarization of THz waves perfectly as well as LC based THz reflectarray. Last but not least, a temperature-supersensitive cholesteric LC used for THz detection is shown. It can not only measure the beam profiles but also detect the power values of THz waves generated from a nonlinear crystal pumped by a table-top laser. Quantitative visualization based on not only the thermochromic but also the thermal diffusion effect, can be used conveniently and effectively at room temperature. In this review, we summarize the latest progress of liquid crystal materials and components in THz and discuss the possible prospects of the combination of liquid crystal technology and THz technology. We envision that LCs will play a unique role in THz sources, THz functional devices and THz detectors.

A novel approach for analyzing refractive Indices and birefringence of nematic liquid crystals

In this paper, we have proposed new technique for determining the ordinary and extra ordinary refractive indices and birefringence of liquid crystals EPAP Valerate, HOBA and NOBA through image analysis method in conjunction with polarizing optical microscopy as a function of temperature by keeping the liquid crystal sample between the polarizers attached with a hot stage and a high-resolution camera. Liquid Crystals employed with image intensities is being used to find out the optical transmittance which is necessary for evaluating the refractive indices by validating the Swanepoel's method. Results obtained in this image analysis method are compared with experimentally determined values of EPAP Valerate, HOBA, NOBA performed with wedge technique and are found in good agreement with an accuracy of ±0.0001. Image analysis method is simple and suitable to determine the refractive indices of liquid crystals.

High-contrast switching and high-efficiency extracting for spontaneous emission based on tunable gap surface plasmon

Controlling spontaneous emission at optical scale lies in the heart of ultracompact quantum photonic devices, such as on-chip single photon sources, nanolasers and nanophotonic detectors. However, achiving a large modulation of fluorescence intensity and guiding the emitted photons into low-loss nanophotonic structures remain rather challenging issue. Here, using the liquid crystal-tuned gap surface plasmon, we theoretically demonstrate both a high-contrast switching of the spontaneous emission and high-efficiency extraction of the photons with a specially-designed tunable surface plasmon nanostructures. Through varying the refractive index of liquid crystal, the local electromagnetic field of the gap surface plasmon can be greatly modulated, thereby leading to the swithching of the spontaneous emission of the emitter placed at the nanoscale gap. By optimizing the material and geometrical parameters, the total decay rate can be changed from 103γ0 to 8750γ0, [γ0 is the spontaneous emission rate in vacuum] with the contrast ratio of 85. Further more, in the design also enables propagation of the emitted photons along the low-loss phase-matched nanofibers with a collection efficiency of more than 40%. The proposal provides a novel mechanism for simultaneously switching and extracting the spontaneous emitted photons in hybrid photonic nanostructures, propelling the implementation in on-chip tunable quantum devices.

Refractive indices of liquid crystal E7 depending on temperature and wavelengths

The dependence of refractive indices of liquid crystal (LC) on temperature is represented by the Haller approximation model, and its dependence on the wavelength is expressed by the extended Cauchy model. We derived the refractive indices expressions of nematic LC E7 depending on temperature and wavelength simultaneously by combining these two models. Based on the obtained expressions, one can acquire the refractive indices of E7 at arbitrary temperature and wavelengths. The birefringence, variation rate of refractive indices, macroscopic order parameter Q, and orientational order parameter ⟨P2⟩ of E7 were then discussed based on the expressions.

Manipulating femtosecond pulse shape using liquid crystals infiltrated one-dimensional graded index photonic crystal waveguides composed of coupled-cavities

In this paper, we investigate the transmission of a 10-femtosecond pulse through an ordinary and graded index coupled-cavity waveguide, using finite-difference time-domain and transfer matrix method. The ordinary structure is composed of dielectric/liquid crystal layers in which four defect layers are placed symmetrically. Next, we introduce a graded structure based on the ordinary system in which dielectric refractive index slightly increases with a constant step value from the beginning to the end of the structure while liquid crystal layers are maintained unchanged. Simulation results reveal that by applying an external static electric field and controlling liquid crystal refractive index in graded structure, it is possible to transmit an ultrashort pulse with negligible distortion and attenuation.

Switching terahertz wave with grating-coupled Kretschmann configuration.

We present a terahertz wave switch utilizing Kretschmann configuration which consists of high-refractive-index prism-liquid crystal-periodically grooved metal grating. The switching mechanism of the terahertz switch is based on spoof surface plasmon polariton (SSPP) excitation in the attenuated total reflection regime by changing the liquid crystal refractive index. The results highlighted the fact that the feasibility to "tune" the attenuated total reflection terahertz wave intensity by using the external applied bias voltage. The extinction ratio of the terahertz switch reaches 31.48dB. The terahertz switch has good control ability and flexibility, and can be used in potential terahertz free space device systems.