Dye-doped Cholesteric Liquid Crystal Research Articles

Enhanced security through dye-doped cholesteric liquid crystal shells for anti-counterfeiting

We explored the application of fluorescent dye doped Cholesteric Liquid Crystal (CLC) shells for anti-counterfeiting measures. These shells exhibit unique optical properties and, when infused with a fluorescent dye, produce a distinct fluorescent color upon exposure to UV light. We developed a microfluidic device using glass capillaries to generate uniform CLC shells. The shells were analyzed using a Polarizing Optical Microscope (POM) to study their defect structures. For anti-counterfeiting, QR code was encoded with CLC shells that were photopolymerized under ultraviolet light for enhanced durability. In daylight, the code reflects red and blue colors, but under UV light, it reveals the fluorescent dye color. This QR code can only be decrypted with UV illumination in both red and blue CLC shells. The color changes are easily visible to the naked eye, making this technique suitable for immediate verification. By utilizing this straightforward technique, security and chromic-based applications can be implemented with significantly increased cost-effectiveness and efficiency, surpassing the results of traditional approaches.

Transmittance spectra expression for dye-doped cholesteric liquid crystal with large optical activity

ABSTRACT Dye-doped cholesteric liquid crystals (DDCLCs) with a homeotropic initial state are more practical due to their lower power consumption and higher security. However, achieving the homeotropic initial state necessitates the use of long-pitch DDCLCs. The conventional formula for determining the planar (P) state transmittance of DDCLCs is inadequate for calculating the P state transmittance of long-pitch DDCLCs with pronounced optical activity. Therefore, in the experimental calculations and fitting, optical activity was introduced to amend the conventional formula and expand the calculation range to include visible wavelengths (400 nm-800 nm). Following numerous validations, a modified formula for calculating the P-state transmittance of DDCLCs was derived, which yields results closer to the actual values when optical activity is significant. This modification is of great significance in the design of long-pitch DDCLCs.

Thermal control band-edge lasing toward near IR from dye-doped cholesteric liquid crystal: Relatively stable operation at high temperatures

The exceptional band-edge lasing from dye-doped cholesteric liquid crystals (DD-CLCs), which can be easily fabricated by blending chiral molecules with nematic liquid crystals, showcases versatile tuning possibilities. In this work, we employed (ISO-(6OBA)2) with high helical twisting power (HTP) as the chiral molecule in the fabrication of DD-CLCs, ensuring superior tuning possibilities and operational stability at high temperatures. By thermally controlling DD-CLCs with a 5 wt% chiral molecule, the band-edge lasing peak exhibited a noticeable red shift with increasing temperature, demonstrating a tuning range of 106 nm as the temperature rose from 20 to 90 °C. In operation at 78 °C, the longest lasing peak of 692.7 nm was achieved from DD-CLCs with 4 wt% ISO-(6OBA)2. Furthermore, the long-term stability of band-edge lasing from DD-CLCs was proven during a three-hour monitoring period at a high operating temperature above 50 °C.

Smart beam via electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystals

Smart beam via electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystals

Electrically Tunable Two-Color Cholesteric Laser.

Two-color lasing emission from an asymmetric structure, consisting of two dye-doped cholesteric liquid crystal (DD-CLC) layers separated by a transparent interlayer, is demonstrated. The DD-CLC mixtures have different reflection bands with long-wavelength band edges located at the green and red wavelengths of the visible spectrum, respectively. For the laser action, the CLC hosts provide the feedback, and the fluorescent laser dyes represent the active medium. When the stacked structure is optically pumped above the threshold, two simultaneous laser lines separated by 123 nm are observed at the long-wavelength band edges of the DD-CLC mixtures. The influence of an electric field on lasing behavior is also analyzed and discussed in terms of the reflection spectrum and laser action. The results show a reversible tuning of the reflection band, accompanied by a modification of the lasing characteristics under the application of an external field. Above a specific threshold voltage, one of the emission lines is suppressed and the other is conserved. With a further increase in the voltage, both laser emissions are entirely inhibited. The investigated structure demonstrates a simple technique to obtain an electrically tunable multi-wavelength laser, which might pave the way for a new generation of organic laser sources.

Polymer-stabilized fast-relaxation dye-doped cholesteric liquid crystals with a wide temperature range by thiol-Michael addition polymerization

The transient planar (TP) state during the relaxation process, from homeotropic (H) to planar (P) state, of dye-doped cholesteric liquid crystals (DDCLCs) can be eliminated by adjusting the elastic constants, reducing the time and haze of the relaxation process. However, the elastic constants are easily influenced by temperature. Therefore, the operating temperature range for fast-relaxation DDCLCs is relatively narrow. This work demonstrates that a polymer helical template can be formed in the P state of DDCLCs by thiol-Michael addition polymerization. It can significantly expand the temperature range from 20 °C to 60 °C for fast-relaxation DDCLCs. More importantly, polymer-stabilized dye-doped cholesteric liquid crystals (PS-DDCLCs) did not show any reduction in the absorption of the dye or discoloration after polymerization. PS-DDCLCs could still achieve tri-state switching and could be used for fast-relaxation smart windows capable of adapting to increasing environmental temperatures. The proposed smart window can be used in vehicles and aircraft to prevent glare and protect privacy.

Electrically tunable thermoresponsive optic switch for smart window application based on dye-doped cholesteric liquid crystal

We demonstrate a cholesteric liquid crystal (CLC) smart window that functions as a reversible thermo-optic switch between a transparent state at a low temperature and a low-transmission state at a higher temperature. This smart window is based on the addition of a thermoresponsive handedness-reversible chiral dopant in a mixture of a typical chiral dopant, a dichroic dye and nematic liquid crystal, yielding a thermosensitized CLC. In such a homeotropically anchored dye-doped CLC, the thickness-to-pitch ratio increases as the temperature rises, causing the device to switch from the homeotropic texture to the fingerprint texture. The absorption characteristic of the dichroic dye enables the homeotropic and fingerprint configurations in the dye-doped CLC to create two distinct transmission levels—the transparent and opaque optically stable states, respectively. Such a switching mechanism entails no additional electric power and, thus, is energy-saving. Moreover, the switching temperature at which the texture transition takes place can be actively increased by increasing voltage for the smart window. This allows the CLC device to be perfectly adaptable to the need of a user and, in turn, applicable to a wider variety of environments.

Energy saving, transparency changing thermochromism in dye-doped cholesteric liquid crystals for smart windows

An electrically switchable smart window is made from a dye-doped Cholesteric liquid crystal (CLC). In this work, the impact of coumarin dye (0.3 and 1 wt%) on the structural and electro-optical properties of CLCs that reflect IR and blue light (Blue) was explored. The results suggest that high transmittance in Visible and NIR useful to preserve interior transmittance as well as heat suited for all seasons. Temperature-dependent microscopic studies confirmed the homogenous solubility of the dye in CLCs. The fabricated smart window was observed to show three switchable modes, comprising planar (P), focal-conic (FC), and homeotropic (H) states, depending on the applied voltages (1 kHz). The structural observations from microscopic studies and the transmission spectra of pure and dye-doped composites with and without the application of voltage are consistent. The visual transmittance of dye doped CLC increased from 84% in the planar state (OFF state) to 18% in the focal conic state (3 V/µm) and 92% in the homeotropic state (5 V/µm) with a short response time (<1ms). Using dye and CLCs, we demonstrated the functionality of the device as smart windows for energy saving, privacy windows as well as for authenticated/unauthenticated QR codes. To investigate the impact of both pure CLC and dye-doped CLC devices on interior temperature management, a solar simulator device was used. The findings demonstrate that the device can be used as a smart window to regulate the internal temperature throughout the year, cutting down on the expense of operating artificial lighting, heating, and cooling systems for buildings. In comparison to pure CLCs, coumarin dye-doped CLCs showed high transmittance in the transparent and homeotropic state and a low transmittance in the scattering state when an electric field is applied and it also exhibited a good stability over repeated switching cycles.

Fluorescent Dye-Doped Brightening Polymer-Stabilized Bistable Cholesteric Liquid Crystal Films.

Brightening polymer-stabilized bistable cholesteric liquid crystal (PSBCLC) films with doped fluorescent dyes were prepared using the polymerization-induced phase separation (PIPS) method. The transmittance performance behavior of these films in both states (focal conic and planar) and absorbance change in multiple dye concentrations were studied using a UV/VIS/NIR spectrophotometer. The change occurring in dye dispersion morphology with different concentrations was obtained by means of the polarizing optical microscope. The maximum fluorescence intensity of different dye-doped PSBCLC films was measured using a fluorescence spectrophotometer. Moreover, the contrast ratios and driving voltages of these films were calculated and recorded to demonstrate film performance. Finally, the optimal concentration of dye-doped PSBCLC films with a high contrast ratio and a relatively low drive voltage was found. This is expected to have great potential applications in cholesteric liquid crystal reflective displays.

Fast-relaxation, dye-doped cholesteric liquid-crystal smart window with a perfect planar state

A fast-relaxation dye-doped cholesteric liquid crystal (DDCLC) smart window without any defects is prepared by the doping of a low bend elastic constants liquid crystal dimer 1″,7″-bis-(4-cyanobiphenyl-4′-yl)heptane (CB7CB). The ratio of the bend elastic constant to the twist elastic constant of the liquid crystals (LCs) is adjusted so that it equates to unity. The transient planar (TP) state and the planar (P) state are combined, so the relaxation time from the homeotropic (H) state to the P state is greatly. Moreover, the oily streaks of the P state are removed and the light scattering is lessened. The polarisation-independent DDCLC smart window switches between three states: the haze-free dark state, the high-haze dark state and the transparent state. The smart window can be used in situations where absorbed sunlight reduces room temperature and protects privacy.

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.

A cholesteric liquid crystal smart window with a low operating voltage

A dye-doped cholesteric liquid crystal smart window with a haze-free opaque state is demonstrated. We designed a random stripe pattern in the cholesteric phase, known as the fingerprint (FP) texture, as an initial state (opaque state). In the theoretical calculation of the LC alignment states, although the transmittance of the FP state is slightly higher than that of the twisted state (planar state), it is much lower than homogeneously aligned states. Moreover, we confirmed the proposed LC cell shows the lowest operating voltage (4 V) among guest-host liquid crystal cells because of vertical anchoring at the surfaces.

Electro-optical Performance Investigation of Cholesteric Liquid Crystal Containing Azo Dye: Light Shutter Device Application

In this study, the effect of chiral dopant concentration on electro-optical and dielectric properties of a cholesteric liquid crystal (CLC) composite was investigated, and a dye-doped cholesteric liquid crystal (DDCLC) light shutter was developed. The electro-optical properties of samples were investigated by electro-optical switching method without the use of a polarizer. The dielectric characterization of the composites were also realized using dielectric spectroscopy technique with a wide frequency range. Significant electro-optical and dielectric parameters, such as transition voltages, complex dielectric constant, dielectric anisotropy and light transmission values of composites were found from the experimental data. The obtained results show that the chiral dopant causes changes in electro-optical and dielectric parameters of CLC, and the developed DDCLC light shutter can be used in optoelectronic device applications.

Influence of UV light intensity on dielectric behaviours of pure and dye-doped cholesteric liquid crystals

This study aims to explore the ultraviolet (UV) light effects on dielectric properties of azo dye methyl red (MR)-doped cholesteric liquid crystal (CLC) composite. The CLC composite formed in the study contains 5CB nematic liquid crystal (LC) and S811 chiral additive. To obtain CLC/MR composite, CLC was dispersed with 2% wt/wt MR azo dye. Dielectric measurements of the CLC and CLC/MR samples were performed using dielectric spectroscopy technique in wide frequency range in the absence of UV light and at various UV light intensities (30, 60 and 90 mW/cm2) for 0 and 40 V. It was observed that the dielectric constant increased with UV light at low-frequency values. Dielectric loss data were utilized to extract relaxation frequency which was observed to increase with UV light exposure. Dielectric anisotropy of CLC sample exhibited typical transition from p-type to n-type, but this behaviour disappears for CLC/MR composite above 60 mW/cm2 UV light intensity. Cole–Cole plots were used for extraction of dielectric relaxation data and obtained results showed that CLC sample shows Debye type relaxation only at 0 V whereas CLC/MR composite exhibits non-Debye type relaxation both at 0 V and 40 V. Moreover, conductivity properties of the samples were also investigated, and dc conductivity values were extracted from experimental ac conductivity values. It was found that MR incorporation increased dc conductivity, also it was significantly increased by UV light exposure and thus similar effect was also observed in current–voltage characteristics of the samples. The results show that azo dye MR molecules are suitable for CLCs since they lead to some enhancements in dielectric and electrical properties.

Smart Window with Active-Passive Hybrid Control.

Dimming and scattering control are two of the major features of smart windows, which provide adjustable sunlight intensity and protect the privacy of people in a building. A hybrid photo- and electrical-controllable smart window that exploits salt and photochromic dichroic dye-doped cholesteric liquid crystal was developed. The photochromic dichroic dye causes a change in transmittance from high to low upon exposure to sunlight. When the light source is removed, the smart window returns from colored to colorless. The salt-doped cholesteric liquid crystal can be bi-stably switched from transparent into the scattering state by a low-frequency voltage pulse and switched back to its transparent state by a high-frequency voltage pulse. In its operating mode, an LC smart window can be passively dimmed by sunlight and the haze can be actively controlled by applying an electrical field to it; it therefore exhibits four optical states—transparent, scattering, dark clear, and dark opaque. Each state is stable in the absence of an applied voltage. This smart window can automatically dim when the sunlight gets stronger, and according to user needs, actively adjust the haze to achieve privacy protection.

Fiber micro-tip temperature sensor based on cholesteric liquid crystal.

This Letter proposes and demonstrates a novel, miniature fiber-tip temperature sensor with a tapered hollow capillary tube (HCT) filled with glycerin and dye-doped cholesteric liquid crystal (CLC). The function of glycerin is to provide a surface anchoring force to control the uniform orientation of CLC molecules, so that the CLC in the tapered HCT can be considered as a mirrorless photonic bandgap (PBG) microcavity. An unambiguously identifiable PBG mode single peak appears in the emission spectra of the sensor. The CLC-based fiber-tip temperature sensor has a temperature sensitivity of -9.167nm/∘C, and the figure of merit can reach 67.4∘C-1. This sensor offers key features and advantages, including compactness, unambiguous identifiability, and biocompatibility, which can satisfy requirements of temperature measurement in various temperature sensing application fields and has great potential for biochemical detection at cell level. In addition, the CLC was integrated into the optical fiber terminal, and the PBG mode is excited, collected and transmitted by the multimode fiber coupler, which is reported for the first time, to the best of our knowledge.

Plasmonic random laser from dye-doped cholesteric liquid crystals incorporating silver nanoprisms.

Plasmonic random lasers have been demonstrated in combining dye-doped cholesteric liquid crystals (DD-CLCs) and silver nanoparticles (AgNPs). The DD-CLC laser reveals the lowest threshold and highest slope efficiency through the localized surface plasmon resonance of AgNPs with the best coupling of the emission spectrum of lasing dye and resonance of electron oscillation on the metal surface. Thermal control of the DD-CLC lasers has been achieved to simultaneously shift the long- and short-edge lasing peaks. By the α-stable analysis, the DD-CLC random laser (RL) reveals heavy tail distribution with relatively low α∼1.06 to show the Lévy behavior. Owing to its low spatial coherence, the DD-CLC RL has been demonstrated to produce a speckle-reduced image with a lower contrast of about 0.04.

All-Optical Directional Control of Emission in a Photonic Liquid Crystal Fiber Laser

This study investigates the all-optical directional control of emission in a cholesteric liquid crystal (CLC)-infiltrated photonic crystal fiber (PCF) laser. A laser dye-doped CLC (LD-CLC) and an azobenzene LC-doped CLC (ALC-CLC), which are selectively infiltrated into the hollow core and holey-matrix cladding of the PCF, function as an optically active cavity and an all-optical emission direction-controllable light valve, respectively, in the photonic LC fiber (PLCF) laser. Experimental results show that the PLCF has a low energy threshold (approximately 63 nJ/pulse) and a large circular polarization anisotropy (g ≅ 2) in lasing output. The lasing output from the LD-CLC-filled core of the PLCF can be all-optical controlled to emit omni- or semi-directionally or to off-emit by controlling the isothermal phase transition of the CLC between the scattering focal conic state and the transparent isotropic state by the all-optical-induced reversible photoisomerization of the doped ALCs in the cladding region. The emission direction-controllable PLCF laser has significant potential in various applications, such as bio-image, photo-therapy, wearable devices, displays, sensors, and lighting.

Lasing stability of quasi-CW driven cholesteric liquid crystal lasers

Lasing stability in dye-doped cholesteric liquid crystal (ChLC) lasers largely depends on repetition frequency of the pumping laser, and therefore stabilization of output power under quasi-continuous-wave operation with high repetition frequency greater than 1 kHz is a tough challenge. In this study, a significant improvement of lasing stability in dye-doped ChLC lasers is obtained by lowering dye concentration, increasing cell gap, and moving LC cells with a piezo stage. Considering the balance between lasing stability and slope efficiency, the optimal conditions regarding cell gap and dye concentration in a ChLC laser doped with Pyrromethene 597 are determined to be 50 μm and 0.3 wt%, respectively. Under severe conditions of high repetition frequencies of 1 kHz and 10 kHz, the optimized ChLC laser demonstrates durability retaining 100% and more than 72% against the initial output power for 30 min, respectively.

Lasing emission of dye-doped cholesteric liquid crystal microdroplet wrapped by polyglycerol in hollow glass microsphere

In this Letter, a dye-doped cholesteric liquid crystal (DDCLC)-filled hollow glass microsphere is demonstrated to be a resonator with good temperature response. A diglycerol layer is used to wrap the DDCLCs microdroplet to keep it steady and control its orientation. The whispering gallery mode (WGM) lasing and photonic band gap (PBG) lasing caused by two different mechanisms were obtained under the pump of a pulsed laser, and the temperature response of these two kinds of lasing was studied. For the liquid crystal and chiral material used in this Letter, both the WGM lasing and the PBG lasing have a blue shift in wavelength with increasing temperature.