Liquid Crystal Gratings Research Articles

Low Driving Voltage and Analysis of Azobenzene Polymer DopedLiquid Crystal Grating

We mix azobenzene polymer and liquid crystal incertain ratio. Then the mixture is injected into cells. Nonlinearly photoinducedbirefringence takes place when linearly polarized ultraviolet is applied with thepattern photomask covering on the cells, which results in the formation ofazobenzene polymer doped liquid crystal grating. The obtained grating ischaracterized by an optical microscope and a He–Ne laser. The results indicate thatthe samples have clear grating structure, and the diffraction efficiencies can bemodulated by electric field. The sample driving voltage is 0.6 V/μm. It islower than the driving voltage of holographic polymer dispersed liquid crystaltransmission grating and could be matched with the driving integrated circuit.

Polarization-independent liquid crystal grating on azo-dye film fabricated through intensity holography

Diffraction phase gratings have found applications in many fields. In this letter the authors described how intensity holography was used to fabricate liquid crystal diffraction phase gratings on azo-dye films. The diffraction efficiency of this liquid crystal phase grating was determined to be independent of the polarization direction of incident light.

Energy transfer between laser beams due to recording of optical axis gratings in liquid crystals

We have studied two-beam-coupling processes related to the generation of optical axis gratings in liquid crystals (LCs) due to the electromagnetic torque acting on the orientation of LCs in superimposed laser beams with spatially modulated polarization. Such gratings result in efficient coupling between the laser beams controlled by the frequency shift introduced between the beams at infrasound frequencies. Larger than 80% efficiency of energy transfer from a pump to a signal beam is reported at power density levels of the laser beams at 1-10 kW/cm2. The distinction between pump and signal beams is relative, since the sign of the frequency shift rather than the power ratio between the beams determines the direction of energy transfer. The experiments were performed for radiation at a 1.06 μm wavelength; high optical nonlinearity and low absorption make LCs suitable for use in visible and near-IR spectra, particularly in high-power laser systems. We have characterized the main fundamental features of the process of recording dynamic orientation gratings, plotting the gain spectra for several LCs and verifying the good quality of the amplified signal beam even for a strongly distorted pump. The obtained results set the groundwork for a variety of novel opportunities and applications that include combining high-power laser beams and wavelength division multiplexing at an ultranarrow interval of frequencies of the order of 1-100 Hz.

Design and Fabrication of High-Performance Liquid Crystal Gratings

We present basic principles of designing and fabricating high-performance liquid crystal (LC) gratings with binary alignment structures which produce a variety of the diffraction properties. The periodic binary structure in the LC layer was fabricated through a selective irradiation of the UV light using the single-masking process. An alternating homeotropic and hybrid alignment configuration produces the input polarization-dependent diffraction suitable for the polarization-separating devices while an oppositely twisted binary alignment configuration gives the input polarization-insensitive diffraction. It is found that a dye-doped bidirectional alignment configuration can be used for obtaining optically controllable polarization properties.

Electrically and Optically Controllable Liquid Crystal Grating with a Patterned Surface-Command Layer

We propose a liquid crystal (LC) binary grating that can be controlled both electrically and optically in a simple scheme. The optical control scheme is obtained through the surface-induced reorientation of the LC molecules on a patterned surface-command layer by a single pump irradiation. The electrical control of the diffraction efficiency is achieved through the change in the phase retardation by the application of a bias voltage for given optically induced LC director distribution. In a coupled scheme of the optical and electrical control, the diffraction efficiency can be easily optimized.

Polymer-Stabilized Cholesteric Diffraction Gratings: Effects of UV Wavelength on Polymer Morphology and Electrooptic Properties

We compare the performance of polymer-stabilized cholesteric liquid crystal diffraction gratings for two limits of the polymer morphology: a one-dimensional array of polymer walls that extend through the grating thickness (“bulk network”), and a thin layer of patterned polymer fibrils localized at one surface (“surface network”). In each case, the polymer is spatially templated by the liquid crystal orientational order, but the depth of the network is dictated by UV absorbance of the liquid crystal and a choice of wavelength used to initiate photopolymerization. Whereas both polymer morphologies yield robust electrically switchable gratings, the surface-stabilized grating operates at lower field thresholds and shorter transition times. However, the patterning of the surface network significantly reduces the contrast between grating on and off states, a limitation that can be potentially offset by employing isotropic monomers index-matched to the liquid crystal.

A dynamic gain equalizer based on holographic polymer dispersed liquid crystal gratings

The dynamic gain equalizer consisting of gratings made of holographic polymer dispersed liquid crystal is explored and the structure and principle presented. The properties of the holographic polymer dispersed liquid crystal grating are analyzed in light of the rigorous coupled-wave theory. Experimental study is also conducted in which a beam of infrared laser was incident to the grating sample and an alternating current electric field applied. The electro-optical properties of the grating and the influence of the applied field were observed. The results of the experiment agree with that of the theory quite well. The design method of the dynamic gain equalizer with the help of numerical simulation is presented too. The study shows that holographic polymer dispersed liquid crystal gratings have great potential to play a role in fiber optics communication.

Anisotropic photonic gratings formed in photocross-linkable polymer liquid crystals

We demonstrated anisotropic photonic gratings in photocross-linkable polymer liquid crystals irradiated with three mutually coherent beams. Both birefringence and surface relief modulation were induced by reorientation and/or migration of mesogenic molecules. Anisotropy and optical properties of photonic gratings were strongly dependent on the combination of polarization states of the three beams. Atomic force microscopy revealed that topological patterns of gratings formed according to interference patterns of the three beams and diffraction efficiencies of recorded gratings depended on polarization direction of the linearly polarized probe beam. These diffraction properties including efficiency and optical anisotropy could be explained by the theory based on Jones calculus.

Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings

Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities within the liquid-crystal-rich layers of the grating.

Simple method of formation of switchable liquid crystal gratings by introducing periodic photoalignment pattern into liquid crystal cell

Abstract We have developed a method for the generation of grating structures in nematic liquid crystals that is a modification of the technique described in [Ph.J. Bos, J.C. Chen, D.L. Johnson, W.E. Glenn, C.E. Holton, B.E. Smith, US Patent 5 825 448, October 20, 1998]. Exploiting UV photoalignment as an alignment technique and an amplitude mask, we introduced a periodical variation of the alignment direction into one of the liquid crystal cell surfaces, while the other surface was aligned uniformly. This significantly simplified the process of grating structure formation and allowed for the generation of polarization independent and electrically controllable gratings. It was found that the grating with a period of 86 μm made using this method demonstrated diffraction efficiency of about 30% in the first diffraction order for a thin grating configuration, and the diffraction signal could be suppressed by more than 20 dB by applying electric field across the cell. Using the edge gradient method we found that a spatial resolution limit of the method was as low as 1.1 μm. This indicates a potential for the proposed method to create periodical structures with a period of about 1 μm.

Two-Dimensional Anisotropic Gratings Formed in Photocrosslinkable Polymer Liquid Crystals by Multiple Interference

We have investigated two-dimensional anisotropic gratings in photocrosslinkable polymer liquid crystals irradiated with three mutually coherent beams. Both birefringence and surface topological modulation were induced by reorientation and migration of mesogenic molecules. Atomic force microscopy reveals a close-packed hexagonal array of gratings formed according to the interference pattern of the three beams. Diffraction properties including efficiency and optical anisotropy were well explained by the theory based on Jones calculus.

Holographically generated twisted nematic liquid crystal gratings

A reflection holographic method is introduced to fabricate an electro-optically tunable twisted nematic (TN) liquid crystal (LC) grating, forgoing the geometrical drawing. The photoisomerization process occurring on the LC alignment layers of an LC cell in the reflection holographic configuration gives a control over the twist angle, and the grating spacing is determined by the slant angle of reflection holographic configuration. The resulting diffraction grating is in a structure of a reverse TN LC, permitting a polarization-independent diffraction efficiency. The electro-optic tunability of the diffraction efficiency is also demonstrated.

Three-Dimensional Measurement of Surface Profile by Focus Method Using Varifocal Lens

The purpose of this study is to measure no-contact three-dimensional surface profile by a focus method. A liquid varifocal lens is utilized to change a focal length by liquid pressure. Contrast images along the focal length are obtained by 4-step phase shifting technique using a liquid crystal grating. A distance can be determined by the relation between the pressure of varifocal lens and the peak position of contrast by the Gaussian fitting . An accuracy check of this measurement is conducted by moving a motor stage. The experiment values are in good agreement with calibrated values in the measurement range from 0 to 60mm with ±0.3 mm of accuracy. Surface profiles of a screw and a deep hole with 40mm have been measured to demonstrate for this method.

Efficient modeling of 3-D photonic crystals for integrated optical devices

We propose a full-wave numerical model of a three-dimensional (3-D) photonic crystal with the absolute photonic bandgap (PBG) centered at /spl lambda//spl sim/1.6 /spl mu/m. The analyzed structure is widely used in integrated optical circuitry. The electromagnetic analysis is performed by using the finite-element method (FEM) and transmission line matrix-integral equation (TLMIE) method. We analyze the reflection properties and compare theoretical results to experimental data. Due to its exact boundary conditions, TLMIE shows much higher accuracy with respect to FEM in the PBG optical band. As a demonstration, we have realized and analyzed a holographic polymer dispersed liquid crystal grating.

P-167: FDTD and Elastic Continuum Analysis of the Liquid Crystal Polarization Grating

liquid crystal polarization grating (LCPG) has advantages of polarization independence of the zero diffraction order, nearly 100% diffraction efficiency in the first orders, and higher resolution capability over previously reported binary LC gratings. Here we analyze the LCPG, an electrically controlled, polarization-independent light modulator using the finite- difference time-domain (FDTD) method and the elastic continuum theory. The optical performance is studied and critical electrical parameters for a LCPG cell are presented. 1. Introduction of liquid crystal (LC) light modulators operating on unpolarized light have been proposed in an effort to overcome the substantial losses in conventional LC displays that require polarized light. Most prominently, Bos and coworkers suggested several potentially efficient and polarization-independent diffraction gratings using liquid crystals (1-4). These approaches, however, face crucial limitations for real applications due to their binary nature: fabrication difficulty and the easy appearance of defects. They therefore tend to have inherently large periods that result in many diffraction orders with small diffraction angles. A newly demonstrated liquid crystal polarization grating (LCPG) (5) is a switchable diffractive optical element with a continuously varying, periodic, anisotropic index profile. As shown in Fig. 1(a), this can be embodied as a nematic director that follows (6): n(x) = ˆ cos(x / ) + ˆ y sin(x /) + ˆ (0) (1)

Polarization-Independent Liquid-Crystal Grating with Microscale Alignment Pattern

A polarization-independent liquid crystal (LC) grating with a microscale LC molecular alignment pattern is proposed. The microscale LC alignment pattern is achieved using a microrubbing technique. The optical properties of the proposed LC grating are theoretically discussed in detail using Jones matrix method. Optimum condition for obtaining maximum +1st or -1st diffraction efficiencies is derived. The diffraction efficiency of the LC grating is measured and the diffraction efficiency characteristics are compared with the theoretical result. Furthermore, the dependence of the polarization direction of the incident light on the diffraction efficiency is discussed.

Polarization-Selective Liquid-Crystal Device and Its Application to Recordable Integrated Optical Unit

A polarization-selective multiple-beam liquid crystal grating (MBLCG) was designed. It can be applied to an integrated optical unit (IOU) for a recordable optical pickup head in small form factor with the functions of recording data using a single beam and retrieving data using multiple beams, respectively. This device is constructed using the technologies of liquid crystal light modulator for switching diffraction effects of a single transmissive beam and multiple diffracted beams. The proposed IOU composes of a laser diode, a photodetector, a MBLCG and a holographic optical element (HOE). The optical axes of the MBLCG and HOE are mutually perpendicular so that the round-trip light efficiency of the IOU can be maximized. Experimental results show that the MBLCG is capable of generating seven diffracted beams with equal light intensities and polarization-selective corresponding to an incident beam of the ordinary or extraordinary state.

Enhancement of third-harmonic generation in a polymer-dispersed liquid-crystal grating

We report the observation of significant enhancement of one-step third-harmonic generation in a one-dimensional photonic crystal pumped by a near-infrared laser beam tuned to the low-frequency edge of the first photonic band gap. The third-harmonic phase matching can be controlled by changing the angle of incidence of the fundamental radiation, allowing tunability of the third-harmonic wavelength. The observed phenomenon was modeled theoretically using the transfer-matrix method. The enhancement is attributed to the combined action of phase-matching between the pump and harmonic waves and pump-field localization within the photonic crystal.

Visible and near-infrared characterization and modeling of nanosized holographic-polymer-dispersed liquid crystal gratings

We have studied the electro-optical and angular behavior of holographic-polymer-dispersed liquid crystal gratings at different wavelengths, in the visible and in the near-infrared range. As usual in these kinds of materials, a strong polarization dependent behavior was observed. Our samples showed very high diffraction efficiency for p-polarized radiation at 1.55 microm, which is very interesting for many possible applications in the telecom field. However, we also observed a very unusual behavior for visible p-polarized light and we try to suggest some explanation for that. By analyzing the angular dependence of the diffraction efficiency, we could measure the components of the permittivity modulation tensor and infer important information about the main parameters involved in the grating structure: the degree of phase separation and the anisotropy in the liquid crystal droplet distribution. In our opinion, this simple and nondestructive methodology can be very useful for studying these kinds of materials and getting information on their morphology, in view of optimizing their properties. Finally, we discuss the role of the refractive index optical dispersion in order to describe the behavior of these materials at different wavelengths. These remarks are especially important when properties in the infrared range are extrapolated from measurements in the visible.

Optical Diffraction Properties of Polymer Dispersed Liquid Crystals Switched by Interdigitated Electrodes

ABSTRACT We investigated optical properties of a polymer dispersed liquid crystal (PDLC) film in-plane switched by a periodic pattern of interdigitated electrodes. Such a system acts as a thin optical phase grating which diffraction efficiency strongly depends on polarization state of the incident light. The intensity and dynamic response of various diffraction orders were analyzed as a function of the applied voltage amplitude. Our results demonstrate that by using a PDLC very effective phase gratings with response times below 10 ms can be achieved at the expense of slightly higher driving fields than needed for the similar bulk nematic liquid crystal gratings.