Cholesteric Liquid Crystal Cell Research Articles

Cholesteric liquid crystal mixtures for the switchable smart windows and light shutters: Electro-optical and dielectric behaviour

Abstract Cholesteric liquid crystal (CLC) mixtures with variable pitch lengths were prepared for the electrical switchable smart window and light shutter applications by taking varied concentrations (2.6, 3.0, 4.0, and 5.0 wt%) of chiral dopant (R1011) having helical twisting power (HTP) 37.6 μm−1 and nematic E7 liquid crystal (LC). The morphological and electro-optical analysis of these CLC mixtures filled in 10µm thick cells were observed and reported herein, the switching of initial planar (P) and initial focal conic (FC) to homeotropic (H) states as well as vice-versa. The operating voltage of the 2.6 wt% of chiral doped CLC cell was relatively lower among all the sample cells compared with other concentrations. The maximum contrast ratio (CR) was observed for 5.0 wt% chiral doped CLC cell with initial planar state. However, the best CR was observed in 2.6wt% CLC cell with initial FC state. The comparative macroscopic views of the P, FC, and H states in the OFF and ON conditions of applied voltage for prepared CLC cells were observed for transparent and opaque states. Further, UV-visible spectroscopy was performed in the P, FC, and H states under the application of an externally applied field to observe the absorbance and band gap behavior of the fabricated CLC cells. Additionally, chiral concentration-dependent dielectric parameters with variable frequency in initial P and FC states of the fabricated cells were also reported. The temperature-dependent phase behavior of all the prepared cells exhibited a slight decrease in the isotropic phase of LC due to the doping of chiral dopants.

Electro-optically addressable and rewritable transparent liquid crystal bistable waveguide display devices

Soft functional materials are in high demand to develop novel devices with excellent dynamic performances because they respond to specific stimuli with distinct functions. Here, we have examined sodium dodecyl sulfonate (SDS) ions doped into negative cholesteric liquid crystals (CLCs), employing an optically sensitive poly(N-vinyl carbazole) (PVK) film on ITO substrates for demonstrating a bistable light waveguide display device. The coated PVK film has the potential to block direct current (DC) electric fields and transform an insulator into a conductor when exposed to suitable ultraviolet (UV) stimuli, thus achieving pattern-addressing through the utilization of dynamic scattering of liquid crystals (LCs). Hence, the sample exhibits text pattern scattering and remains transparent under patterned UV stimuli. Initially, an external stimulus DC field was applied to the CLC cell to switch the planar textures in the region shining with UV light to focal conic textures that remain prolonged even after stimulus removal. Then,we examine the novel concept of a voltage-free “remote writing display”usingtransparentbistable waveguides displays by edge-lit white LED stimuli. Such advanced transparent displays offer robust stability, manufacturability, and cost-effectiveness for various applications in different environments and industries, such as automotive, e-paper, and augmented reality devices.

Quickly Formed Continuously Tuneable Laser in the Full Visible Spectrum Using Cholesteric Liquid Crystal Cells Based on the Electrothermal Effect (Laser Photonics Rev. 18(7)/2024)

Quickly Formed Continuously Tuneable Laser in the Full Visible Spectrum Using Cholesteric Liquid Crystal Cells Based on the Electrothermal Effect (Laser Photonics Rev. 18(7)/2024)

Bicolor Tuning and Hyper-Reflective Color Switching Based on Two Stacked Cholesteric Liquid Crystal Cells with Asymmetric Electrothermal Optical Responses.

We propose a double-cell cholesteric liquid crystal (CLC) device composed of a left-handed (LH) CLC cell with a pair of sheet electrodes and a right-handed (RH) CLC cell with a tri-electrode configuration characterized by a sheet electrode on the top and an interdigitated electrode on the bottom substrates. Bi-reflected color tuning and hyper-reflective color switching are revealed from this cell stack via the electrothermal control of the central wavelengths of the LH- and RH-bandgaps by voltage-induced pseudo-dielectric heating. The two CLCs are thermally sensitive and exhibit overlapped bandgaps in the field-off state with nearly identical temperature dependence, resulting in a hyper-reflective color at 720 nm at 23.4 °C and 380 nm at 29.8 °C. Upon the application of 4 Vrms at 2 MHz across the stacked device to induce pseudo-dielectric heating, two reflective colors can be resolved due to asymmetrical temperature elevations. Accordingly, the difference in wavelength between the two colors increases with increasing voltage through a series cell connection, while maintaining approximately constant via a parallel connection. This study provides a feasible pathway to developing a multifunctional device with electrothermally tunable bi-reflected and hyper-reflective states based on two conventional cell geometries, which is promising for lasers and color-related display applications.

Quickly Formed Continuously Tuneable Laser in the Full Visible Spectrum Using Cholesteric Liquid Crystal Cells Based on the Electrothermal Effect

AbstractWavelength‐tuneable lasers can play a crucial role across many fields, and their applications continue to expand with advancements. Cholesteric liquid crystal (CLC) devices are developed, enabling good wavelength‐tuneable lasers. In CLC devices, pitch control is required to tune the desired wavelength. However, pitch control takes a long time and is difficult, which limits the speed and range of laser wavelength tuning. Here, by applying a voltage once and forming an electric field gradient along the wedge direction in the CLC wedge cell, we could realize CLC pitch gradient arrays that achieve continuous wavelength tuning over 500 nm in only 10 min through the electrothermal effect while continuously generating laser lines in the full visible spectrum. The laser wavelength could be rapidly tuned over the full visible spectrum by moving within the array in the wedge cell. The study of CLC cells for electric field application in real time is able to observe the dynamic process of pitch change directly. This new laser device and strategy has potential in many fields that require active laser tuning and is expected to lead to innovative changes in the development of future practical laser devices in the optical industry.

Temperature Self-Adaptive and Color-Adjustable Smart Window Based on Templated Cholesteric Liquid Crystals.

Cholesteric liquid crystals (CLCs) exhibit selective reflection due to their self-assembled helical superstructures. Reconfigurable templates can achieve integration functions via inducing processes of molecular assemblies. Here we demonstrate temperature self-adaptive and color-adjustable smart windows using CLCs, which are fabricated via the templating method and exhibit simultaneous reflections in the visible and infrared spectra. Reflection bands formed by the refilled CLC materials can be adjusted reversibly both upon thermal and electrical actuation. In CLC with adjustable reflection in the infrared, the central wavelength of the infrared reflection band can be adjusted from 950 nm to 1305 nm via temperature, and from 1150 nm to 950 nm via electric field. A temperature variation of 10.3 °C within 55 s was induced by the single-layer templated CLC cell, and a comfortable temperature range could be effectively maintained by the CLC cell in a varied environment. In CLC with dynamic color in the visible spectrum, color shifts from 530 nm to 650 nm tuned by temperature and from 530 nm to 440 nm adjusted by electric field were obtained. Temperature-responsive reflection in the infrared spectrum contributes to automatic thermal management, and electric-field-induced band shift in the visible spectrum enables active dynamic color adjustment. The presented templated CLC smart windows show considerable potential in energy conservation and biological clock regulation fields.

Pair interaction of localized topological structures in confined chiral media.

We study pairwise interactions between localized topological structures in chiral magnetic and cholesteric liquid crystal (CLC) systems confined in the planar geometry. Our calculations for magnetics are based on the lattice model that takes into account the bulk and surface anisotropies along with the exchange and the Dzyaloshinskii-Moriya interactions. In CLC cells, these anisotropies describe the energy of interaction with an external magnetic or electric field and the anchoring energy assuming that the magnetic or electric anisotropy is negative and the boundary conditions are homeotropic. We have selected the region of the phase diagram, where various localized solitonlike structures, including skyrmion tubes, torons, and leeches, embedded in the ground state of the z-cone (conical phase) coexist, and carried out numerical analysis of the distance dependencies of the effective intersoliton interaction potentials. For skyrmions and torons, the potentials are found to be attractive in the large separation region. It turned out that for these potentials, the effects of axial asymmetry are negligible. By contrast, it turned out that for the intermediate structures between the skyrmions and torons known as the leeches, the leech-leech potentials generally depend on the orientation of the intersoliton separation vector and their large distance parts may become repulsive at certain directions of the vector. All the potentials have the short distance repulsive parts and the local minima located at the equilibrium separations. It is found that the skyrmion-skyrmion potential has an additional metastable configuration shifted towards the short-distance region.

Polymer beads dispersed liquid crystal devices (PBLCD) achieved by predesigned radially constructed polymeric particles

AbstractIn this article, we demonstrate a novel design of liquid crystal (LC) displays via predesigned radially constructed polydopamine (PDA) particles. The synthesized PDA particles were modified with dimethyloctadecyl‐3‐(trimethoxysilyl)propyl ammonium chloride (DMOAP) showing radial constructions. Both PDA particles and surface‐modified PDA particles were used to disturb the order of the LC arrangement, showing an opaque appearance. Based on the results, the radially constructed PDA particles efficiently enhanced the performance of cholesteric liquid crystal (CLC) devices. Adding 0.1 wt% DMOAP‐modified PDA particles into the CLC device shows a higher contrast of 82.2%, a lower threshold voltage of 2.4 V and a fast response time of 7.7 ms. Without adding polymer beads, no any display contrast was observed. The results suggest that the synthesized surface‐modified PDA particles disturb the alignment of LCs dispersed in LC cells, leading to a great improvement in the contrast of the CLC devices. In addition, a prepared traditional polymer stabilized CLC (PSCLC) cell shows only 2.8% contrast. The best performance, with a high contrast of 93.8%, a low threshold voltage of 10.4 V and a fast response time of 10.2 ms, was achieved by adding 0.1 wt% modified PDA particles into the CLC device.

Fiber-Optic Temperature Sensor Using Cholesteric Liquid Crystals on the Optical Fiber Ferrules.

Cholesteric liquid crystals (CLCs) can be applied to various physical and chemical sensors because their alignment structures are changed by external stimuli. Here, we propose a CLC device fabricated by vertically forming the helical axis of the CLC between the cross-sections of two optical fiber ferrules. An optical fiber temperature sensor was successfully implemented using the proposed optical fiber ferrule-based CLC device. A wideband wavelength-swept laser with a center wavelength of 1073 nm and scanning range of 220 nm was used as a light source to measure the variations in the reflection spectrum band according to the temperature change in the CLC cell. The wavelength variation of the reflection spectrum band according to the temperature applied to the CLC cell was reversible and changed linearly with a change in the temperature, and the long-wavelength edge variation rate according to the temperature change was −5.0 nm/°C. Additionally, as the temperature applied to the CLC cell increased, the reflection spectrum bandwidth gradually decreased; the reflection spectrum bandwidth varied at a rate of −1.89 nm/°C. The variations in the refractive indices with temperature were calculated from the band wavelengths of the reflection spectrum. The pitch at each temperature was calculated based on the refractive indices and it gradually decreased as the temperature increased.

Temperature Dependent Morphological and Electro-Optical Characteristics of Dye Doped Cholesteric Liquid Crystal

Cholesteric liquid crystals (CLCs) have gain remarkable attention due to its property of stabilizing the planar (P) and focal conic (FC) states. This paper presents the temperature dependent behavior of cholesteric liquid crystal (CLC) mixed with appropriate amount of dye. The pitch of sample corresponds to visible region (~1 µm) was adjusted by adding appropriate concentration of chiral dopant. The phase transition and morphological behavior was recorded using polarized optical microscope (POM) from planar state to isotropic state by heating the sample using temperature-controller. Experimental results indicated that increase in temperature impacts the morphology of CLC and strongly affects its electro-optical (E-O) properties. Further, POM studies showed that at room temperature dye doped CLC cell has initial transparent state (planar texture) however, when driving voltage was high (~27V) with lower frequency of 1 kHz, P state switched to FC state. On further increasing the temperature, minor distortion was observed in P state in comparison to P state of dye doped CLC cell at room temperature, however, at higher (~ 45˚C) temperature, low driving voltage was required to achieve the stable FC state. Further, FC state was switched to stable P state on applying high frequency (50 kHz) voltage. Afterward, above 45˚C temperature, the textures showed the sustained FC state of dye doped CLC cell with no switching to P state on applying high frequency (50 kHz) voltage. Thus, temperature dependent dye doped CLC cell showed the bi-stability feature upto 45˚C temperature, however, above this temperature showed the fixed FC state.

From Single to Multi Mode Lasing: The role of materials revealed in optical simulations

ABSTRACT The comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by an external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multilayered samples.

1.1-µm Band Extended Wide-Bandwidth Wavelength-Swept Laser Based on Polygonal Scanning Wavelength Filter.

We demonstrated a 1.1-µm band extended wideband wavelength-swept laser (WSL) that combined two semiconductor optical amplifiers (SOAs) based on a polygonal scanning wavelength filter. The center wavelengths of the two SOAs were 1020 nm and 1140 nm, respectively. Two SOAs were connected in parallel in the form of a Mach-Zehnder interferometer. At a scanning speed of 1.8 kHz, the 10-dB bandwidth of the spectral output and the average power were approximately 228 nm and 16.88 mW, respectively. Owing to the nonlinear effect of the SOA, a decrease was observed in the bandwidth according to the scanning speed. Moreover, the intensity of the WSL decreased because the oscillation time was smaller than the buildup time. In addition, a cholesteric liquid crystal (CLC) cell was fabricated as an application of WSL, and the dynamic change of the first-order reflection of the CLC cell in the 1-µm band was observed using the WSL. The pitch jumps of the reflection band occurred according to the electric field applied to the CLC cell, and instantaneous changes were observed.

A Switchable Cholesteric Phase Grating with a Low Operating Voltage

We demonstrate a simple fabrication method of a uniform-lying-helix (ULH) cholesteric liquid crystal (ChLC) cell for phase grating device applications. To utilize a stable ULH state, we set the pitches of ChLCs as half of the cell gap to obtain the fingerprint texture with homeotropic anchoring. With the given grating period, the diffraction efficiency of the ULH cell can be maximized by optimizing the cell gap. We found that the fabricated grating device can provide a large diffraction angle of 10° and a low operating voltage of 3 V with a diffraction efficiency of 30%. We expect potential applications of the device for diffraction optics, optical interconnects, and beam steering devices.

PHOTOTUNABLE SELECTIVE REFLECTION OF CHOLESTERIC LIQUID CRYSTALS

The dependence of the helical pitch of a cholesteric liquid crystal based on a composite photosensitive chiral dopant (cChD) on the intensity of light irradiation was studied. The transmission spectra and the selective reflection spectra of cholesteric liquid crystal cells were measured. The concentration of the cChD additive is calculated, so that the peak of selective reflection and its rearrangement occurs in the visible range of the electromagnetic radiation spectrum (380-780 nm). The possibility of photo-control by shifting the peak of the selective reflection of the cChD additive was studied, when exposed to LEDs with wavelengths of 365 nm and 450 nm, while reducing or increasing the intensity, a change in the spiral pitch was observed. Depending on the light intensity, part of the molecules of the chiral additive containing the azo group underwent isomerization, i.e. molecules in the trans-form passed to the cis-form, which led to a spectral shift of the selective reflection peak. If a small intensity of the 365 nm LED was applied, then some of the molecules were forced to transition from the trans- to the cis-form, and then, when illuminated with a 450 nm LED, from the cisto the trans-form. After reversible rearrangement, the properties of cholesteric liquid crystal changes due to interaction with light, because under the influence of light, the equilibrium ratio of the trans- and cis-isomers of the molecules of the substance changed, which macroscopically changed the torsion force of the chiral additive. Using two LEDs of 365 nm and 450 nm with different emission spectra, a reversible control of the cChD selective reflection peak in the visible range was obtained. The maximum displacement occurred at approximately 145 nm.

The Field-Induced Stop-Bands and Lasing Modes in CLC Layers with Deformed Lying Helix

Waveguide optical properties of a cholesteric liquid crystal (CLC) layer with a deformed lying helix (DLH) have been studied by numerical simulations using the finite difference time domain method. The DLH structure, when the helix’s axis is oriented in plane of a CLC layer, is induced by an electric field in a virtual CLC cell with periodic (planar/homeotropic) boundary conditions at one of the alignment surfaces. This in-plane helical structure is stable only in a permanently applied electric field providing the helix deformation. In this work the polarized light reflectance spectra have been studied at different electric fields and light impingement into a waveguide formed by the DLH layer. It is found that for light propagating along the helix axis the reflectance spectrum has multiple stop-bands centred at wavelengths , which is different from set of bands located at , and characteristic of CLC spectra for the Grandjean-plane textures subjected to distortion by an electric or magnetic field perpendicular to the helix axis, where j is a natural number, p is the helix pitch and is the average refractive index. Each of the higher order (j > 1) bands consists of three polarization-dependent sub-bands. In the case of an amplifying CLC DLH layer, depending on an extent of the helix deformation, the lasing modes can be excited at different edges of the sub-bands. While at the strongest deformation the lasing is preferable at the edges of the central sub-band; a lower extent of deformation makes favourable conditions for the lasing at edges of the two other sub-bands.

Electro-Thermal Formation of Uniform Lying Helix Alignment in a Cholesteric Liquid Crystal Cell

We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field approach to the formation of uniform lying helix (ULH) texture in a cholesteric liquid crystal (CLC) cell. The technique entails a hybrid voltage pulse at frequencies f1 and, subsequently, f2, which are higher and lower than the onset frequency for the induction of dielectric heating, respectively. When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 Vrms at f1 = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 Vrms and f1 = 55 kHz, our results indicate that switching of the voltage frequency from f1 to f2 enables the succeeding formation of well-aligned ULH during either the isotropic-to-CLC phase transition at f2 = 1 kHz or by the electrohydrodynamic effect at f2 = 30 Hz. For practical use, the aligning technique proposed for the first time in this study is more applicable than existing alternatives in that the obtained ULH is adoptable to CLCs with positive dielectric anisotropy in a simple cell geometry where complicated surface pretreatment is not required. Moreover, it is electrically switchable to other CLC textures such as Grandjean planar and focal conic states without the need of a temperature controller for the phase transition, the use of ion-rich LC materials, or mechanical shearing for textural transition.

Continuously Tunable Optical Notch Filter With Functions of a Mirror and a Beam Splitter

The objective of this study was to develop continuously tunable optical notch filters by combining four left- and right-handed circular cholesteric liquid crystal (CLC) cells without subsidiary optical components. By rotating each filter, the photonic band position could be continuously blue shifted over 100-nm spectral range. Using four notch filter sets with four different chiral dopant concentrations, spectral range from 440 to 870 nm could be covered. Since each CLC cell has only one chiral dopant concentration, it could be stable for a long time. Each filter is independent of polarization in the spectral range. Filter performance was highly enhanced by introducing an anti-reflection layer on the filter device. There was no light leakage inside the photonic band. Outside the band, transmittance was about 70%-100%. In addition, these filters had stable operation under extremely high laser intensity (~124 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of CW 532-nm diode laser and ~4.43 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of Nd: YAG pulse laser operation for 2 h) without showing damage. Such filters also have functions as a mirror and a beam splitter. Depending on CLC materials, this simple and easy strategy could be used to prepare filters for applications in VIS and NIR spectral range devices.

Continuously Tunable and Bandwidth Variable Optical Notch/Band-Pass Filters Over 500 nm Spectral Range Using Cholesteric Liquid Crystals

We report continuously tunable and bandwidth variable optical notch and band-pass filters created by combining four left- and right-handed circular cholesteric liquid crystal cells without extra optical components. Filter performance was greatly improved by introducing an anti-reflection layer on the filter device. The filter comprised cholesteric liquid crystal wedge cells with continuous pitch gradient. The band wavelength position was spatially tuned from 470 nm to 1000 nm. The notch filters are polarization-independent in the spectral ranges. The band pass filters can be designed to be polarization-independent or polarization-dependent via cell alignment and the bandwidth can be reversibly controlled from the original bandwidth (60~18 nm). These tunable filter strategies could make a paved road to wavelength tunable spectroscopic instruments applications in the VIS and near infrared response spectral range.

Electro-thermally tunable reflective colors in a self-organized cholesteric helical superstructure

We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (λc) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium–tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state, which is subject to vertically applied voltage, should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, λc of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance–capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of λc in the entire visible spectrum is as low as 12 Vrms in a 5 μm thick cell at a frequency of 2 MHz. The proposed CLC cell, exhibiting a broad electrically tunable spectral range from the near-infrared to ultraviolet, holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors.

In situobservation of dynamic pitch jumps of in-planar cholesteric liquid crystal layers based on wavelength-swept laser.

We report in situ observation of dynamic pitch jumps in cholesteric liquid crystal (CLC) layers that depend on the applied electric field. A high-speed and wide bandwidth wavelength-swept laser is used as an optical broadband source to measure the dynamic pitch jumps. We could not observe the dynamic pitch jump in the quasi-static pitch variation. Instead, we carry out two driving methods, a normal driving and an overdriving method, in order to measure the dynamic pitch jump in the CLC cell. For the case of normal driving, it has been confirmed that the reflection band from the measurement region is discontinuously shifted by movement of the defect wall. The reflection band was compressed and recovered before the band moved, but the dynamic pitch jump of the helix could not be observed. For the case of overdriving, however, it was possible to observe the unwinding of the helix during the dynamic pitch jump. The entire dynamic pitch jump process in the CLC cell could be observed by measuring the transmission spectra from the CLC cell by varying the applied electric field. We confirm that the entire reaction time with the overdriving method was about 800 ms, which was shorter than with the normal driving method. This study contributes to the development of fast in-plane switching research and the development of new CLC devices.