Cholesteric Liquid Crystal Layer Research Articles

Optical properties of a stack of cholesteric liquid crystal and isotropic medium layers

Some new optical properties of a stack consisting of cholesteric liquid crystal (CLC) and isotropic medium layers are studied. The problem is solved by the modified Ambartsumyan method for the summation of layers. Bragg conditions for the photonic band gaps of the proposed system are presented. It is shown that the choice of proper sublayer parameters can be used to control the band structure of the system. In the general case, the effect of full suppression of absorption, which is observed in a finite homogeneous CLC layer, is not detected in the presence of anisotropic absorption in CLC sublayers. It is shown that this effect can be generated in the system under study if certain conditions are imposed on the isotropic sublayer thickness. Under these conditions, the maximum photonic density of states (PDS) increases significantly at the boundaries of the corresponding band. The influence of a change in the CLC sublayer thickness and the system thickness on PDS is investigated.

Resonant interaction of light with a stack of alternating layers of a cholesteric liquid crystal and an isotropic medium.

Resonance properties of a stack consisting of a cholesteric liquid crystal and isotropic medium layers are investigated. Bragg's conditions for photonic band gaps are presented for the considered system. It is shown that a significant control of the system zone structure is possible in the case of a proper selection of the sublayer parameters. It is also shown that an effect of complete suppression of absorption is not usually observed in the case of anisotropic absorption in cholesteric liquid crystal sublayers. The possibility to achieve this effect in the considered system, imposing certain conditions on the thickness and the refraction index of the isotropic medium sublayers, is shown. Also, it is shown that in such conditions a large increase of the maximum of photonic density of states happens at the appropriate zone borders.

Lasing in cholesteric liquid crystal cells with an isotropic defect layer inside

The photonic density of states (PDS) of eigen polarizations (EPs) in cholesteric liquid crystal (CLC) cells with a defect layer inside are calculated. The dependences for the PDS and light intensity in the defect layer on the parameters characterizing absorption and gain are obtained. We investigated the possibility of connections between the PDS and the density of the light energy accumulated in the system. The influence of the defect layer and CLC layer on the PDS are investigated. It is shown that the PDS is maximum when the defect is in the center of the system. We also showed that the subject system can work as a low threshold laser, a multi-position trigger, filter, etc.

Spectral and polarization structure of field-induced photonic bands in cholesteric liquid crystals.

Transmission of planar layers of cholesteric liquid crystals is studied in pulsed electric fields perpendicular to the helix axis at normal incidence of both linearly polarized and unpolarized light. Spectral and light polarization properties of the primary photonic band and the field-induced bands up to fourth order of Bragg selective reflection are studied in detail. In our experiments we have achieved an electric field strength several times higher than the theoretical values corresponding to the critical field of full helix unwinding. However, the experiments show that despite the high strength of the electric field applied the helix does not unwind, but strongly deforms, keeping its initial spatial period. Strong helix deformation results in distinct spectral band splitting, as well as very high field-induced selective reflectance that can be applied in lasers and other optoelectronic devices. Peculiarities of inducing and splitting the bands are discussed in terms of the scattering coefficient approach. All observed effects are confirmed by numerical simulations. The simulations also show that liquid crystal surface anchoring is not the factor that prevents the helix unwinding. Thus, the currently acknowledged concept of continuous helix unwinding in the electric field should be reconsidered.

Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector.

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Enhanced Reflectance of Cholesteric Liquid Crystal Device with Quantum Dots

Cholesteric liquid crystal (CLC) has a helical structure with either of left-handedness or right-handedness. Due to the helical structure, the selective reflection occurs depending on the chrial pitch. However, the reflectance of the CLC layer is theoretically limited to 50% because only one of right- or left-handed circularly polarized light is reflected. In this paper, we demonstrate high reflectance of CLC layer with quantum dots for the color reflector application.

Absorption and emission of finite layer of chiral photonic crystal

The peculiarities of absorption and emission of the finite layer of cholesteric liquid crystal are investigated. The influence of parameters characterizing the absorption and gain, as well as the layer thickness and the local dielectric anisotropy on the absorption and emission is studied. In particular, it is shown, that the increase of the parameter x′ characterizing the gain results in the increase of emission. However, there are the discrete points in the (λ, x′) plane, where the emission is maximal (λ is the wavelength of incident light). These points determine the critical values of gain at which the low-threshold laser generation takes place. The obtained results can be used to develop the strongly absorbing systems.

High Reflectance of Cholesteric Liquid Crystal Reflector by Double-Layer Structure

Cholesteric liquid crystal (CLC) has a helical structure with either of left-handedness or right-handedness. Due to the helical structure, the selective reflection occurs depending on the chrial pitch. However, the reflectance of the CLC layer is theoretically limited to 50% because only one of right- or left-handed circularly polarized light is reflected. In this paper, we demonstrate high reflectance with double layer structure for the color reflector application.

Photonic density of states of cholesteric liquid crystal cells

Using the exact analytical expressions for the reflection and transmission matrices for the finite thickness cholesteric liquid crystal (CLC) layer, we calculated its photonic density of states (PDS) of the eigen polarizations (EPs). We investigated the influence of absorption and gain, as well as the CLC cell thickness and CLC local dielectric anisotropy on PDS. We presented the full picture of distribution of total field in the CLC layer and outside it for two EPs. The possibility of connections between the PDS and the density of the light energy accumulated in the medium was investigated, and it was shown that these characteristics had analogous spectra and, besides, the influences of the problem parameters on these characteristics were also analogous. We showed that there existed a critical value of the parameter characterizing the gain beyond which the lasing mode was quenched and the feedback vanished. We showed that in the presence of gain there existed a critical value of numbers of pitches in the CLC layer beyond which the lasing mode was again quenched and the feedback vanished, too. It is shown that the subject system can work as a low-threshold laser or a multi-position trigger.

On the binary nature of the mechanism of orientational instability in cholesteric liquid crystals in wave fields

The prediction of a theory developed with allowance for the processes of orientational order parameter relaxation in liquid crystals about two independent acoustic mechanisms of orientational instability in the planar layer of a cholesteric liquid crystal (CLC) in a wave field, manifesting itself in the occurrence of a system of 2D domains, is experimentally substantiated for the first time. The distortions of the layer macrostructure for the mesophase of this type was observed in the field of longitudinal waves in a wide frequency range, including frequencies above the relaxation frequency of the orientational order parameter in CLC. The values of the spatial domain period at the effect threshold and the threshold compression amplitudes are determined for CLC layers 10–100 μm thick with a helix pitch of 2–30 μm in the frequency range of 0.3–45 MHz. It is shown that a complete theoretical description of the phenomenon, consistent with the experimental data, can be obtained only with allowance for the action of the convection and the nonlinear relaxation mechanisms of the mesophase layer structure destabilization, which have different natures.

About zone structure of a stack of a cholesteric liquid crystal and isotropic medium layers

The optical properties of a stack of metamaterial-based cholesteric liquid crystal (CLC) layers and isotropic medium layers are investigated. CLCs with two types of chiral nihility are defined. The peculiarities of the reflection spectra of this system are investigated and it is shown that the reflection spectra of the stacks of CLC layers of these two types differ from each other. The influence of: the CLC sublayer thicknesses; incidence angle; local dielectric (magnetic) anisotropy of the CLC layers; refraction indices and thicknesses of the isotropic media layers on the reflection spectra and other optical characteristics of the system is investigated.

Photonic Density of States of a Stack of Cholesteric Liquid Crystal and Isotropic Medium Layers

We calculated the photonic density of states (PDS) of the eigen polarizations in the system composed of a stack of layers of a cholesteric liquid crystal and an isotropic medium. The reflection spectra and PDS peculiarities, as well as the peculiarities of absorption and emittance were investigated. We obtained the dependences of the PDS as a function of parameters characterizing absorption and gain. It was shown that the subject system can be used in lasers for obtaining low threshold lasers with the emittance wavelength tunable in wide ranges. The system also can work as a multi-position trigger.

Zone structure and polarization properties of the stack of a metamaterial-based cholesteric liquid crystal and isotropic medium layers

The optical properties of a stack of metamaterial-based cholesteric liquid crystal (CLC) layers and isotropic medium layers are investigated. The problem is solved by a modification of Ambartsumian’s layer addition method. CLCs with two types of chiral nihility are defined. The peculiarities of the reflection spectra of this system are investigated and it is shown that the reflection spectra of the stacks of CLC layers of these two types differ from each other. Besides, in contrast to the single CLC layer case, these systems have multiple photonic band gaps. There are two types of such gaps: those selective with respect to polarization of the incident light and nonselective ones. It is shown that the system eigenpolarizations mainly coincide with the quasi-orthogonal, quasi-circular polarizations for normally incident light, except the regions of diffraction reflection selective with respect to the polarization of incident light. The influence of the CLC sublayer thick-nesses, the incidence angle, the local dielectric (magnetic) anisotropy of the CLC layers, and the refractive indices and thicknesses of the isotropic media layers on the reflection spectra and other optical characteristics of the system is investigated.

Peculiarities of radiation of a stack of cholesteric liquid crystal and isotropic medium layers

Peculiarities of radiation of a stack of cholesteric liquid crystal and isotropic medium layers are considered. The problem is solved by Ambartsumian’s layer addition modified method. Peculiarities of the reflection spectra and the spectra of photonic density of states are studied. It is shown that the system can be used in laser technique for constructing low-threshold lasers, which are tunable in a wide band of the radiation wavelength.

Non‐Uniform Helix Unwinding of Cholesteric Liquid Crystals in Cells with Interdigitated Electrodes

A microspectrophotometer was used to elucidate the local optical properties of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage. The spectra collected from a spatially selective and micron-sized sampling area allow for new insights into the spectral properties of CLCs in the gaps between patterned electrodes. The microscopic electro-optic response is shown to be highly dependent on the cell thickness and the electrode periodicity. Specifically, the helix unwinding of the CLC superstructure does not always occur uniformly in the sample, as a result of field gradients through the cell thickness: for cells with relatively narrow gaps and electrodes, the redshift occurs initially only in the CLC layers closest to the substrate with the electrodes, leading to broad reflection spectra and different reflection colors depending on which side of the cell is illuminated. Theoretical estimates of the expected shift in the reflection band gap based on the critical field for a given CLC material and the spatial variation of electric field in the cell are found to be in good agreement with the complex behavior observed experimentally. In contrast, in thin cells with wider gaps, the pitch increase affects the whole CLC layer uniformly, because the electric field gradient is small.

The photonic density of states and the light energy density in cholesteric liquid crystal cells

The photonic density of states (PDS) of the eigen polarizations (EPs) in cholesteric liquid crystal (CLC) cells are calculated. The dependences of the PDS on the parameters characterizing absorption, gain and refractive index of the CLC layer surroundings were obtained. We investigated the possibility of connections between the PDS and the density of the light energy accumulated in the medium and showed that these characteristics have analogous spectra and, besides, the influences of the problem parameters on these characteristics also are analogous. It was shown that the decrement of the refractive index of the CLC layer surroundings leads to a sharp increase of the maximum PDS and, consequently, leads to a sharp decrement of the laser excitation threshold. We showed that the subject system can work as a low threshold laser or a multi-position trigger.

Dual structure of cholesteric liquid crystal device for high reflectance

The structure of cholesteric liquid crystal (CLC) undergoes a helical distortion which is left-handed or righthanded. By the right-hand CLC layer, Left-handed polarized light is reflected and vice versa. The color reflected by the selective reflection depends on the chiral pitch and the anisotropy of the refractive index. However, the reflectance of the single CLC layer is theoretically limited to 50% because only one of right- or left-handed circularly polarized light is reflected. In this paper, we demonstrate the enhanced reflectance of a dual-CLC device which can reflect both right- and left-handed circularly polarized light.

Optics of anisotropic metamaterial based structurally chiral photonic crystals

Light transmission through and reflection from a medium layer with dielectric and magnetic helicities are discussed. The axes of the dielectric permittivity tensor, , and the magnetic permeability tensor, , as well as the medium helix axis are all parallel to each other and they are perpendicular to the boundary surfaces. The possibilities of formation of some new types of photonic bandgaps (PBGs) are presented for large anisotropies of the medium—namely, direct and indirect nonselective PBGs (with respect to the incident light polarization, in contrast to the usual direct PBGs, which are selective with respect to the polarization of the incident light). It is shown that a transmission region can arise among the three types of PBGs, in certain conditions, of course. In this paper we generalize the concept of nihility for structurally chiral media, such as cholesteric liquid crystals (CLCs) and we identify two types of CLC nihilities. It is shown that, for certain characteristic parameters of the medium, superluminal light propagation is possible in the transmission band. The influence of the anisotropy of the medium on the reflection spectra is considered and it is shown that one can tune the width, number and frequency range of PBGs of this layer, at essentially large limits, tuning the parameters of the layer. The case of oblique light incidence on the CLC layer is also discussed.

Threshold improvement in uniformly lying helix cholesteric liquid crystal laser using auxiliary π-conjugated polymer active layer

We propose a device structure to lower the lasing threshold of a uniformly lying helix cholesteric liquid crystal (ChLC) laser. We place a π-conjugated polymer active layer beneath the ChLC layer to provide auxiliary gain, and demonstrate an improvement in the lasing threshold by a factor of 2.3. We also perform finite difference time domain calculations coupled with rate equations for a four-level system, and clarify the effect of the additional active layer on both the photonic density of states and the inversion population density. Although the addition of an extra layer lowers the photonic density of states, the gain provided by the auxiliary layer is sufficient to overcome the losses and decrease the lasing threshold. Our concept is useful for obtaining high-performance ChLC lasers.

Experimental investigation of one-sided optical transparency in cholesteric liquid crystal thin film

Influence of boundary conditions on optical properties of a layer of cholesteric liquid crystal (CLC) has been investigated. In particular, a sample was obtained which is transparent for circularly polarized light incident from one side and gives losses in the transmission spectrum (within nearly 100 nm wide wavelength range) if light is incident from the opposite side, i.e. exhibits optical diode behavior. Two selective reflection bands were detected in the reflection spectrum of the sample. Spectral position of both selective reflection bands and transmission spectrum are shown to be CLC temperature-controllable.