Pitch Gradient Research Articles

Photoerasable and photorewritable spatially-tunable laser based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant

This study investigates, for the first time, a photoerasable and photorewritable spatially-tunable laser using a dye-doped cholesteric liquid crystal (DDCLC) with a photoisomerizable chiral dopant (AzoM). UV illumination via a photomask with a transmittance-gradient can create a pitch gradient in the cell such that the lasing wavelength can be spatially tuned over a wide band of 134nm. The pitch gradient is generated by the UV-irradiation-induced gradient of the cis-AzoM concentration and therefore the induced gradient of the cell HTP value, resulting in the spatial tunability of the laser. Furthermore, the laser has advantages of photoerasability and photorewritability. The spatial tunability of the laser can undergo more than 100 cycles of photoerasing and photorewriting processes without decay or damage.

Pitch gradient induced by disklike chiral molecular diffusion in chiral-nematic liquid crystals

Disklike molecules were introduced as chiral dopants to induce a pitch gradient in the polymer stabilized cholesteric liquid crystals films. A sandwich-cell was fabricated with two different substrates, one of which was coated by the disklike molecule layer and another by a parallel-aligned pattern of polyvinyl alcohol. The pitch gradient was formed by molecular diffusion across the film thickness and further stabilized by photo-initiated crosslinking of the polymerizable monomers during the diffusion process. The effect of the deposition amount of the disklike chiral molecules on the diffusion rate and the consequent pitch gradient was studied.

Band-tunable color cone lasing emission based on dye-doped cholesteric liquid crystals with various pitches and a pitch gradient

This study elucidates, for the first time, a novel band-tunable color cone lasing emission (CCLE) based on dye-doped cholesteric liquid crystal (DDCLC) films with various pitches. For several CLC cells with different pitches it was shown experimentally that the lasing band on the CCLE can be tuned among various color regions measured within different angular ranges. Some important features of the tunable CCLE are also identified and discussed. A spatially band-tunable color cone laser, based on a single DDCLC with a gradient pitch, is developed as a real application.

P‐115: Broad Band Selective Reflection from a Single Cholesteric Liquid Crystal Layer with a Helical Pitch Gradient and its Applications

AbstractWe have succeeded in obtaining broadband selective reflection over the entire visible spectrum from a single cholesteric liquid crystal layer with a helical pitch gradient. The broadband cholesteric layer is applied to brightness enhancement films for LCD, to decorative films with metallic reflection, and to IR shielding films. The decorative film and the IR shielding film are immune to radio wave for telecommunication.

Preparation and reflectance properties of new cholesteric liquid crystalline copolymers containing cholesteryl group

AbstractIn this study, a new family of side‐chain cholesteric liquid crystal copolymers derived from cholesterol substituents were synthesized. The chemical structures of the copolymers were validated by FTIR and 1H NMR. A wide‐band reflective polarizer from the above copolymers with stable optical properties was developed. The pitch gradient was formed by molecular diffusion between adjacent liquid crystal layers based on these cholesteric copolymers with different pitch lengths, and the pitch gradient was further stabilized by UV crosslinking of the polymerizable monomers during the diffusion process. The stability of these systems as a function of time was investigated. The polymer network was validated by scanning electron microscopy and this network played an important role in stabilizing the pitch gradient of the system. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Polychromatic liquid crystal laser arrays towards display applications

Band-edge liquid crystal lasers are of interest for a number of applications including laser projection displays. Herein, we demonstrate simultaneous red-green-blue lasing from a single liquid crystal sample by creating a two-dimensional laser array fabricated from dye-doped chiral nematic liquid crystals. By forming a pitch gradient across the cell, and optically pumping the sample using a lenslet array, a polychromatic laser array can be observed consisting simultaneously of red-green-blue colors. Specifically, the two-dimensional polychromatic array could be used to produce a laser-based display, with low speckle and wide color gamut, whereby no complex fabrication procedure is required to generate the individual 'pixels'.

Wide‐band reflective polarizers from cholesteric liquid crystals with stable optical properties

AbstractA wide‐band reflective polarizer from glassy siloxane cyclic side‐chain liquid crystals with stable optical properties was developed. The pitch gradient that formed was first stored by rapid cooling, which, by preventing the crystallization of glassy materials, was a very easy way of keeping the molecular arrangement. Then, the polymer network that formed through UV‐initiated chain crosslinking of the photopolymerizable monomers in the reflective polarizer further stabilized the cholesteric structure stored by the former rapid cooling. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

New micro‐structure designs of a wide band reflective polarizer with a pitch gradient

New microstructure designs for fabricating a wide band reflective polarizer (WBRP) from cholesteric liquid crystals are reported. The pitch difference in the WBRP is formed by orienting a single layer consisting of different glassy siloxane cyclic side chain oligomer powders on a heater. The molecular arrangement obtained is frozen by quenching. The experimental results show that various micro‐areas exhibiting different reflection wavelengths and pitch gradients are formed in the WBRP. A WBRP exhibiting the reflection properties of the original cholesteric liquid crystals is fabricated by a novel experimental process and the experimental results are in accordance with the microstructure designs of the WBRP.

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.

Cholesteric liquid crystals with a helical pitch gradient: Spatial distribution of the concentration of chiral groups by Raman mapping in relation with the optical response and the microstructure

Cholesteric liquid crystals (CLC) selectively reflect light when the helical pitch is of the order of the wavelength of the incident beam propagating along the helix axis. The wavelength bandwidth, related to the optical anisotropy, is typically limited to a few tens of nanometers in the visible part of the spectrum, which is insufficient for applications such as white-or-black polarizer-free reflective displays and smart windows for the control of the solar light and heat. A way to make cholesteric films reflecting in a broad wavelength band consists in associating various cholesteric pitches in the same film. In this work, it is shown how a study by confocal micro Raman spectrometry mapping makes it possible to have access to information accounting for the local organization of CLCs in the case of graded pitch materials. These investigations will be correlated to the optical response and the transverse microstructure of the CLC material as investigated by transmission electron microscopy. An accurate analysis of the vibrational behavior evolution of the C==O can be correlated to the evolution of the populations of the chiral and achiral groups in the case of the interdiffusion of two CLC substances with various stoechiometries. Besides an easy measurement of the Raman spectrum gives the opportunity to quantify the relative ratio of the mesogenic species and thus to go up by a simple way to the pitch of the helical structure.

Evaluation of Optical Properties for Development of Ultrahigh-Resolution Flexible Contact Endoscope

In this study, a new flexible contact endoscope that is similar to a fiber-optic ultrathin endoscope is examined. Using a new optical design with a 0.23 pitch gradient index lens and an ultrathin fiber-optic image guide, we have obtained 228 line pairs/mm ultrahigh-resolution images. The resolution and magnification of the obtained images are measured and analyzed. Also, the relationship between magnification and the working distance from the target to the gradient index (GRIN) lens using GRIN lenses of different pitch lengths is determined. Finally, we have taken an image of red blood corpuscles of 6–9 µm average size.

Theoretical and experimental optical studies of cholesteric liquid crystal films with thermally induced pitch gradients

The reflection properties of cholesteric films with thermally induced pitch gradients are theoretically and experimentally studied. It is shown that the optical behavior of such films corresponds to the averaged contribution of a number of stochastic pitch variation profiles, due to the transversal and longitudinal nonuniformities that develop in the helical structure of such samples. Depending on the annealing time, both narrow-band and broadband behavior can be selectively achieved. The influence of the pitch profile gradient on the broadband reflection performance of cholesteric samples is theoretically analyzed, and a multi-slab structure for achieving optimum efficiency is proposed.

Polarization-dependent filters based on chiral photonic structures with defects

A one-dimensional photonic device to perform optical multiple-wavelength filtering for light of a particular circular polarization state is presented and modelled. It is shown that the transmission spectrum of a stacked structure consisting of two cholesteric films with the same pitch gradient in the helix can be tailored by incorporating the defects of pitch jump and phase jump in the middle of the structure. The introduction of the defects results in the exhibition of multiple-band-pass characteristics inside the photonic band gap when incident circularly polarized light has the same handedness as the cholesterics. Some critical parameters are analysed: the thickness of the double-composite film, co-polarized/cross-polarized transmittance and reflectance, and the twist angle. For light of specified circular polarization, the shift of the filter in transmitted wavelength can be tuned linearly by adjusting the amount of the phase jump. The proposed chiral filter may be useful for multichannel lightwave systems. Some rules for the practical implementation of a filter with polarization sensitivity are given.

Twist defect in chiral photonic structures with spatially varying pitch

The properties of photonic defect modes in a chiral photonic structure were investigated using the finite element method. By stacking two cholesteric liquid crystal (ChLC) films, the defect mode due to the introduction of a twist defect was considered in both cases of chiral structures with constant pitch and spatially varying pitch. Two types of linear pitch gradients for achieving a broadband reflection were analysed, and the number of chiral pitches required for establishing the stop band was simulated. The effect of a finite sample thickness on the energy density distribution of the defect mode and on the required polarization of the incident light to excite the defect mode was studied. In both cases of constant pitch and spatially varying pitch, an unusual crossover behaviour in reflection at the defect resonance wavelength of a single circularly polarized mode appears when the structure thickness increases beyond a specific value. The energy distribution inside the sample also reveals the unusual distribution. Two different resonance wavelengths can be created by a twist defect in the ChLC composite film with linearly varying pitch, while only one resonance wavelength can be created in the identical film with constant pitch.

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

Photonic defect modes of cholesteric liquid crystal with spatially varying pitch

The introduction of photonic defect modes in a cholesteric liquid crystal (ChLC) can be achieved by a global deformation of the helix with a chiral anti-symmetry relative to the middle of the ChLC configuration. The defect modes can be excited only when the circularly polarized incident wave has the same handedness as the ChLC. The tuning of the defect modes can be realized by adjusting the pitch gradient. Whereas, the transmittance of the defect modes is greatly reduced when the structure thickness is large far away from a critical value, and even some defect modes will be completely suppressed. A forbidden band for polarization-selective reflection can be obtained, and the bandwidth is much wider than that in a monotonically varying pitch structure.

Optical diffraction in nonuniform cholesteric liquid crystals: phase-grating mode.

We have worked out the diffraction pattern in the phase-grating mode of a cholesteric liquid crystal (cholesteric) with a pitch gradient. The pitch gradients considered are symmetric and asymmetric with respect to the sample center. For a uniform input beam, the intensity profile of each diffraction order becomes broad. Further, in the symmetric gradient, the profile of each order is irregular, while in the asymmetric gradient it is nearly flat. For a Gaussian input beam, for the symmetrically deformed structure the profile for each order is asymmetric, while for the asymmetrically deformed structure, the profiles just becomes broad. We find that even a 5% nonuniformity in the pitch can drastically alter the diffraction profiles.

Wide-Band Reflective Polarizers from Variable Pitch Cholesteric Liquid Crystal Films

Due to their periodic helical structure, cholesteric liquid crystals (CLC) have a unique ability to selectively reflect visible light. CLC films reflecting a broad wavelength band were made by inducing a pitch gradient in CLC through a diffusion of small molecules and through a thermal diffusion between two CLC oligomer films with different pitch gradients.

Passive Broadband Reflector: Elaboration and Spectral Properties

We present passive broadening obtained by assembling two layers of cholesteric photo-polymerisable monomers having different pitches at the same temperature. A more or less large diffusion occurs between the two layers, which is dependent of various parameters such as degree of cross-linking of the two layers, temperature dependence, time evolution at a given temperature, thickness of the layers… The presented work consider the spectral properties of the sandwich structure and its correlation with the diffusion profile at the interface between the two layers. The band shape of the reflected and transmitted light were studied. Then, the reflected intensity shows a widening of the reflection associated with the existence of a pitch gradient inside the sample, which can be blocked by photo-polymerisation. At short time, the two peaks corresponding to each liquid crystal polymer are still present but the diffusion occurs at the interface. After a longer time, the broadening band still decreases and a single large peak is observed.

How to broaden the light reflection band in cholesteric liquid crystals? A new approach based on polymorphism

Cholesteric liquid crystals (CLCs) may selectively reflect light when the helicoidal pitch is of the order of the wavelength of the incident beam propagating along the helix axis. The reflection bandwidth is dependent on the birefringence and is limited to a few tens of nanometers, which is insufficient for applications such as white-on-black reflective displays. Recent studies have shown that CLC polymer networks with a pitch gradient induce a broadening of the reflection bandwidth over several hundreds of nanometers. Most related processes rely on photocrosslinking reactions with a UV-gradient in a mixture made of chiral and achiral monomers with different UV-reactivities. Here a new experimental route exploiting the polymorphism of the mixture is presented. The basic concept lies in a thermally-induced pitch variation simultaneously carried out with the UV-crosslinking reaction. The optical behaviour is investigated in parallel with the cross-sectional microstructure as observed by transmission electron microscopy.