Optically tunable/switchable omnidirectionally spherical microlaser based on a dye-doped cholesteric liquid crystal microdroplet with an azo-chiral dopant

Combined electric and photocontrol of selective light reflection at an oblique helicoidal cholesteric liquid crystal doped with azoxybenzene derivative

An oblique helicoidal cholesteric liquid crystal Ch_{OH} represents a unique optical material with a single-harmonic periodic modulation of the refractive index and a pitch that can be tuned by an electric or magnetic field in a broad range from submicrometers to micrometers. In this work, we demonstrate that the oblique helicoidal cholesteric doped with azoxybenzene molecules can be tuned by both the electric field and light irradiation. The tuning mechanism is explained by the kinetics of trans-cis photoisomerization of the azoxybenzene molecules. At a fixed voltage, UV irradiation causes a redshift of the reflection peak by more than 200 nm. The effect is caused by an increase of the bend elastic constant of Ch_{OH} under irradiation. The demonstrated principle has the potential for applications such as smart windows, sensors, tunable lasers, and filters.

Multimode Robust Lasing in a Dye-Doped Polymer Layer Embedded in a Wedge-Shaped Cholesteric

Cholesteric liquid crystals (CLCs) with induced defects are one of the most prominent materials to realize compact, low-threshold and tunable coherent light sources. In this context, the investigation of optical properties of induced defect modes in such CLCs is of great interest. In particular, many studies have been devoted to the spectral control of the defect modes depending on their thickness, optical properties, distribution along the CLC, etc. In this paper, we investigate the lasing possibilities of a dye-doped polymer layer embedded in a wedge-shaped CLC. We show that multimode laser generation is possible due to the observed multiple defect modes in the PBG that enlarges the application range of the system. Furthermore, our simulations based on a Berreman 4 × 4 matrix approach for a wide range of CLC thickness show both periodic and continuous generation of defect modes along particular spectral lines inside the PBG. Such a robust spectral behaviour of induced defect modes is unique, and, to our knowledge, is not observed in similar CLC-based structures.

Control of Large-Area Orderliness of a 2D Supramolecular Chiral Microstructure by a 1D Interference Field

Self-organized periodic micro/nanostructures caused by stimulus-responsive structural deformation often occur in anisotropic self-assembled supramolecular systems (e.g., liquid crystal systems). However, the long-range orderliness of these structures is often beyond control. In this article, we first demonstrate that a large-area disordered two-dimensional (2D) microgrid chiral structure appears in the cholesteric liquid crystal (CLC) reactive mixture because of the photopolymerization-induced Helfrich deformation effect under exposure to the single UV-laser beam. The result is attributed to the impact of an internal longitudinal strain, which is caused by the pitch contraction of the CLC-monomer region through the continuing compression of the thickening CLC polymer layer adhered on the illuminated substrate of the sample during photopolymerization. The experimental results further show that a one-dimensional (1D) UV-laser interference field can be used to effectively control the postformed 2D microgrid structure to arrange in an orderly manner throughout the large exposed area (an order of centimeter). The optimum ability for controlling the orderliness of the microgrid structure can be achieved if the spacing width of the interference field approximates the periodicity of the postformed 2D microgrids. Several factors, such as the pitch of the CLC mixture and the included angle and intensity of the two interfering laser beams, which influence the orderliness and properties of the 2D microgrid structure, are explored in this study. The result of this research opens a new page to improve the applicability of the Helfrich deformation phenomenon and further provides a reference platform for manipulating, modifying, and even tailoring periodic micro/nanostructures in self-organized supramolecular soft-matter systems for application in advanced optics/photonics.

Liquid Crystals in Curved Confined Geometries: Microfluidics Bring New Capabilities for Photonic Applications and Beyond

The quest for interesting properties and phenomena in liquid crystals toward their employment in nondisplay application is an intense and vibrant endeavor. Remarkable progress has recently been achieved with regard to liquid crystals in curved confined geometries, typically represented as enclosed spherical geometries and cylindrical geometries with an infinitely extended axial-symmetrical space. Liquid-crystal emulsion droplets and fibers are intriguing examples from these fields and have attracted considerable attention. It is especially noteworthy that the rapid development of microfluidics brings about new capabilities to generate complex soft microstructures composed of both thermotropic and lyotropic liquid crystals. This review attempts to outline the recent developments related to the liquid crystals in curved confined geometries by focusing on microfluidics-mediated approaches. We highlight a wealth of novel photonic applications and beyond and also offer perspectives on the challenges, opportunities, and new directions for future development in this emerging research area.

Chiral Optical Tamm States at the Interface between a Dye-Doped Cholesteric Liquid Crystal and an Anisotropic Mirror

The resonant splitting of optical Tamm state numerically is demonstrated. The Tamm state is localized at the interface between a resonant chiral medium and a polarization-preserving anisotropic mirror. The chiral medium is considered as a cholesteric liquid crystal doped with resonant dye molecules. The article shows that the splitting occurs when dye resonance frequency coincides with the frequency of the Tamm state. In this case the reflectance, transmittance, and absorptance spectra show two distinct Tamm modes. For both modes, the field localization is at the interface between the media. The external field control of configurable optical and structural parameters paves the way for use in tunable chiral microlaser.

Whispering gallery mode lasing in mesomorphic liquid crystal microdroplets

In recent years, investigation on the non-display applications of liquid crystals has increased considerably. One of the emerging applications is whispering gallery mode (WGM) lasing. Here, we report experimental studies on the morphology and WGM lasing in nematic (N), smectic-A (SmA) and smectic-C (SmC) microdroplets dispersed in a highly transparent and low refractive index perfluopolymer. The mesomorphic microdroplets, obtained by varying the temperature, exhibit radial director configuration. The SmA microdroplets are found to be highly stable and robust against mechanical stress compared to the N and SmC microdroplets. We study lasing properties such as intensity, threshold pump energy and linewidth, and show that overall the SmA microdroplets are superior to the N and SmC microdroplets. The experimental results are discussed based on the orientation of the dye molecules, director fluctuations and tilting at the interface.

Light-activated helical inversion in cholesteric liquid crystal microdroplets

Cholesteric liquid crystal (CLC) droplets exhibit nontrivial topological features, which are controlled by the ratio between the cholesteric pitch and the droplet radius. The radial spherical structure (RSS) is of particular interest, as it reveals an onion-like concentric organization of the cholesteric helices, leading to the expression of spherical Bragg microcavities. Using an overcrowded alkene-based unidirectional molecular motor as a dopant, we show that the topological defect structure in the droplet can be activated by illumination. By using appropriate molecular motor concentrations, light can either break the symmetry of topological defects (as observed for the bent-twisted bipolar structure), or it can induce inversion of handedness in an onion-like organization (in the case of RSS). This latter feature may pave the way toward alternative activation modes of lasers based on cholesteric droplets. By also studying CLC droplets once they have reached full photoconversion at photostationary state (PSS), we highlight that the strong influence of confinement on the droplets structure occurs to the same extent after the helix inversion event. Our results are interpreted in terms of numerical simulations of the droplets' structure, which shed light on the major role played by curvature close to the droplets' center, this latter one becoming dominant when the droplet radius is small.

Tunable liquid-crystal microshell-laser based on whispering-gallery modes and photonic band-gap mode lasing

The lasing behaviors of dye-doped cholesteric liquid crystal (DDCLC) microshells fabricated with silica-glass-microsphere coated DDCLCs were examined. Lasing characteristics were studied in a carrier medium with different refractive indices. The lasing in spherical cholesteric liquid crystals (CLCs) was attributed to two mechanisms, photonic band-gap (PBG) lasing and whispering-gallery modes (WGMs), which can independently exist by varying the chiral agent concentration and pumping energy. It was also found that DDCLC microshells can function as highly sensitive thermal sensors, with a temperature sensitivity of 0.982 nm °C^-1 in PBG modes and 0.156 nm °C^-1 in WGMs.

Optically Controllable Linear-Polarization Rotator Using Chiral-Azobenzene-Doped Liquid Crystals

A linear-polarization rotator based on the optically tunable pitch of chiral-azobenzene-doped liquid crystals (CAdLCs) has been investigated. It is shown that the orientation of linearly polarized (LP) light can be optically tuned using CAdLCs and that the transmitted light possesses a good degree of linear polarization (DoLP). Experimental and simulation (4 × 4 Berreman matrix) results show that the rotation angle is dependent on the pitch as well as the number of turns of the cholesteric LC helix. Some causes to affect the DoLP of the output LP lights during photoisomerization are also discussed. Moreover, a calibration term, β(t), is also introduced to elucidate the behavior of the discontinuous change of the CAdLC pitch in a fixed cell thickness.

Droplet lasers: a review of current progress

It is perhaps surprising that something as fragile as a microscopic droplet could possibly form a laser. In this article we will review some of the underpinning physics as to how this might be possible, and then examine the state of the art in the field. The technology to create and manipulate droplets will be examined, as will the different classes of droplet lasers. We discuss the rapidly developing fields of droplet biolasers, liquid crystal laser droplets and explore how droplet lasers could give rise to new bio and chemical sensing and analysis. The challenges that droplet lasers face in becoming robust devices, either as sensors or as photonic components in the lab on chip devices, is assessed.

Magnetic-field tuning of whispering gallery mode lasing from ferromagnetic nematic liquid crystal microdroplets

We report magnetic field tuning of the structure and Whispering Gallery Mode lasing from ferromagnetic nematic liquid crystal micro-droplets. Microlasers were prepared by dispersing a nematic liquid crystal, containing magnetic nanoparticles and fluorescent dye, in a glycerol-lecithin matrix. The droplets exhibit radial director structure, which shows elastic distortion at a very low external magnetic field. The fluorescent dye doped ferromagnetic nematic droplets show Whispering Gallery Mode lasing, which is tunable by the external magnetic field. The tuning of the WGM lasing modes is linear in magnetic field with a wavelength-shift of the order of 1 nm/100 mT. Depending on the lasing geometry, the WGMs are red- or blue-shifted.

Optical and electrical switching of cholesteric liquid crystals containing azo dye

We propose an optically and electrically switchable cholesteric liquid crystal (ChLC) cell doped with a push–pull azo dye.

Microfluidic fabrication of cholesteric liquid crystal core-shell structures toward magnetically transportable microlasers

We report a magnetically transportable microlaser with cholesteric liquid crystal (CLC) core-shell structure, operating in band-edge mode. The dye doped CLC shells as a water-in-oil-in-water (W/O/W) double emulsion were fabricated by microfluidics. Water-dispersible Fe3O4 magnetic nanoparticles were incorporated in the inner aqueous phase by taking advantage of the immiscibility with the middle CLC oil phase. The influence of temperature and shell thickness on laser properties was discussed in detail. The non-invasive manipulation of microlasers was realized under a magnetic field. The dependence of velocity on the viscosity of the carrying fluid and size of the core-shell structure was theoretically analyzed and experimentally investigated using a prototype electromagnetic platform. We also discussed the design principles for this type of DDCLC core-shell structure. Such magnetically transportable microlasers offer promise in in-channel illumination applications requiring active control inside micro-channels.

Morphological appearances and photo-controllable coloration of dye-doped cholesteric liquid crystal/polymer coaxial microfibers fabricated by coaxial electrospinning technique

This study systematically investigates the morphological appearance of azo-chiral dye-doped cholesteric liquid crystal (DDCLC)/polymer coaxial microfibers obtained through the coaxial electrospinning technique and examines, for the first time, their photocontrollable reflection characteristics. Experimental results show that the quasi-continuous electrospun microfibers can be successfully fabricated at a high polymer concentration of 17.5 wt% and an optimum ratio of 2 for the feeding rates of sheath to core materials at 25 °C and a high humidity of 50% ± 2% in the spinning chamber. Furthermore, the optical controllability of the reflective features for the electrospun fibers is studied in detail by changing the concentration of the azo-chiral dopant in the core material, the UV irradiation intensity, and the core diameter of the fibers. Relevant mechanisms are addressed to explain the optical-control behaviors of the DDCLC coaxial fibers. Considering the results, optically controllable DDCLC coaxial microfibers present potential applications in UV microsensors and wearable smart textiles or swabs.

Creation and manipulation of topological states in chiral nematic microspheres

Topology is a universal concept that is encountered in daily life and is known to determine many static and dynamical properties of matter. Taming and controlling the topology of materials therefore constitutes a contemporary interdisciplinary challenge. Building on the controllable spatial properties of soft matter appears as a relevant strategy to address the challenge, in particular, because it may lead to paradigmatic model systems that allow checking theories experimentally. Here we report experimentally on a wealth of complex free-standing metastable topological architectures at the micron scale, in frustrated chiral nematic droplets. These results support recent works predicting the formation of free-standing knotted and linked disclination structures in confined chiral nematic fluids. We also demonstrate that various kinds of external fields (thermal, electrical and optical) can be used to achieve topological remote control. All this may foster the development of new devices based on topologically structured soft media.

Performance evolution of color cone lasing emissions in dye-doped cholesteric liquid crystals at different fabrication conditions

This work investigates the performance evolution of color cone lasing emissions (CCLEs) based on dye-doped cholesteric liquid crystal (DDCLC) cells at different fabrication conditions. Experimental results show that the energy threshold (E(th)) and relative slope efficiency (η(s)) of the lasing signal emitted at each cone angle (0°-35°) in the CCLE decreases and increases, respectively, when the waiting time in a homogenously rubbed aligned DDCLC cell is increased from 0 hr to 216 hr (9 days). This result occurs because defect lines gradually shrink with the anchoring of the surface alignment when the waiting time is increased. Hence, the scattering loss decreases, and the dwelling time of the fluorescence photons in the resonator increases, which in turn enhances the CCLE performance. With the aligned cell given the pretreatment of a rapid annealing processing (RAP), the waiting time for obtaining an optimum CCLE can markedly be reduced sixfold. The surface alignment of the DDCLC cell also plays a necessary role in generating the CCLE. This work provides an insight into the temporal evolution of the performance for the CCLE laser and offers a method (RAP) of significantly speeding up the formation of a CCLE laser with optimum performance.

Light-directing omnidirectional circularly polarized reflection from liquid-crystal droplets

Constructing and tuning self-organized three-dimensional (3D) superstructures with tailored functionality is crucial in the nanofabrication of smart molecular devices. Herein we fabricate a self-organized, phototunable 3D photonic superstructure from monodisperse droplets of one-dimensional cholesteric liquid crystal (CLC) containing a photosensitive chiral molecular switch with high helical twisting power. The droplets are obtained by a glass capillary microfluidic technique by dispersing into PVA solution that facilitates planar anchoring of the liquid-crystal molecules at the droplet surface, as confirmed by the observation of normal incidence selective circular polarized reflection in all directions from the core of individual droplet. Photoirradiation of the droplets furnishes dynamic reflection colors without thermal relaxation, whose wavelength can be tuned reversibly by variation of the irradiation time. The results provided clear evidence on the phototunable reflection in all directions.

Optical filter with tunable wavelength and bandwidth based on phototunable cholesteric liquid crystals

An optical filter with tunable wavelength and bandwidth is demonstrated using two phototunable cholesteric liquid crystals (CLCs) configured in a reflection mode. In this mode, incident light is first reflected by one Azo-chiral-doped CLC and then by another one. The tuning mechanism, which is based on the pitch modulation of the CLCs that contain an Azo-chiral dopant, can be controlled by two pumping laser beams. The central wavelength can be tuned from 510 to 628 nm, and the bandwidth can be changed from 13 to 79 nm.