Lcd

*Liquid Crystal DevicesDr. Sally E. [email protected]

*Abstract:

Liquid Crystals Displays (LCD) very common, low power, light-weight displays, as well as larger area flat panel displays for monitors and TV applications. Liquid Crystals have a remarkable electro-optic coefficient, a large birefringence is switched with a very low voltage. Newer displays require complex structures with careful control of small features in the liquid crystal. This makes them of interest in other applications besides displays.

This tutorial will cover the physical properties essential for the operation of liquid crystal devices including displays and non-display applications.

*Contents:Structure-property relationships in liquid crystalsPhases of liquid crystalsOrder parameter in liquid crystalsAnisotropy in a liquid: Dielectric, optical and viscoelastic propertiesMolecular structure and influence on the physical propertiesOptical properties of liquid crystalsBirefringencePolarisation of lightControl of polarisationStructure of liquid crystal devices (LCD)AlignmentBasic construction of LCDsOptical properties of display and other devicesTwisted nematic, In-plane switching, Vertically aligned nematicHolograms and Beam steering Micro and nano-structures and liquid crystals

*Structure property relationshipsPhases of liquid crystalsLiquid crystal materials are made of organic molecules.

But to understand the phase behaviour these can be considered as rods.

*Structure property relationshipsPhases of liquid crystalsLiquid crystals are liquids, but have some additional order associated with them, which is crystalline like.The simplest is the nematic phase:- the rods align in a particular direction, but have no positional order.Nematic liquid crystals are milky looking liquids

*Structure property relationshipsPhases of liquid crystalsSmectic phases have the additional order of layers, but they are not precise layers, but density wavesIn addition to layering, there may be some other order, e.g. tilting within the layer.Smectic liquid crystals tend to be wax like substances

*Structure property relationshipsPhases of liquid crystalsOther smectic phases have additional order within the layersThis order may be in the form of hexagonal packingThe phases can be identified by the patterns that form and can be seen using a polarising microscope, or by X-ray scattering.The order between the molecules can also be seen by NMR

Some of the polarising microscope images can be seen at http://reynolds.ph.man.ac.uk/~dierking/gallery/gallery1.html

*Structure property relationshipsPhases of liquid crystals Discotic Liquid crystalsDisc shaped molecules are the basic building blocks, and the order can be in in terms of the orientation (nematic discotics) or in the form of columns.

Nematic discotic

*Structure property relationshipsPhases of liquid crystals Discotic Liquid crystalsDisc shaped molecules are the basic building blocks, and the order can be in in terms of the orientation (nematic discotics) or in the form of columns.

The columns can then pack together to form a two dimensional crystalline array.

The columnar structure could be useful for 1-D conductors and semi-conductors and other properties along the columns.Nematic discoticHexagonal Columnar phase

*Structure property relationshipsPhases of liquid crystals Thermotropic liquid crystals phase forms as a function of temperatureLyotropic liquid crystalsPhase forms as a function of concentration in a solvent

*Lyotropic phases occur for molecules dissolved in a solutionDifferent phases occur with concentrationOften the solvent is water and the molecules have an hydrophilic end and an hydrophobic end (e.g. detergents with polar (hydrophilic) and non-polar (hydrophobic) end groups).The lyotropic liquid crystals form many different phases, as with the thermotropic liquid crystals, but depending on concentration as well as temperatureStructure property relationships Phases of liquid crystals Lyotropic liquid crystals.Hexagonal phase

*Liquid crystal templatesLyotropic liquid crystal structures can be converted to solid structures using the sol-gel process to give silicates with the same structure as the liquid crystal phase.Other methods can be used to form metal nano-particles

*The lamellar phases are found in cell membranesThis allows a liquid environment to exist, so transporting material around, but with a layer which controls the transport of material across the layerAn example of the phase transition is from the lamellar liquid crystal phase to a gel phase, sometimes an undesirable transition.This transition occurs at different temperatures and pressures depending on the environment that the organism lives in and what is required

Structure property relationships Phases of liquid crystals Lyotropic liquid crystals.waterLamellar phasewatergel phase

*Chemical structure of liquid crystal moleculesCyano biphenyl, shown above was the first stable liquid crystal developed at Hull University Chemistry Dept. enabled the LCD industry to develop.Generally the rod shaped molecules can have the following structure:

n=1,2,3X,Y CmH2m+1; CmH2m+1-O; CN etcAromaticAliphaticHetrocyclic

*Chirality is an important property of some of the molecules:A chiral molecule cannot be superimposed on its mirror image. The carbon centre of the molecules below is the chiral centre. The enantiomers are identical except for the way in which they are arranged in space.Solutions or mixtures containing chiral molecules will rotate the plane of polarisation of light travelling through: Optical activity.A racemic mixture has equal amounts of each enantiomer.Synthesis of chiral compounds must be carried out carefully to make sure that a racemic mixture is not obtained.Chemical structure of liquid crystal molecules

*Chiral liquid crystalsThe chiral nematic (Cholesteric) liquid crystal phase is a nematic phase, but the average direction of the molecules rotates through the material.

*The different chemical groups affect the physical properties in many ways, some important effects are as follows

Phase transition temperaturesDielectric propertiesOptical propertiesVisco-elastic propertiesFerroelectric, flexoelectric coefficientsChirality

These physical properties in turn affect the performance of the displays and other devices that contain liquid crystalsChemical structure of liquid crystal molecules

*Order parameterThe order parameter is the degree to which the individual molecules align with the average direction. It is defined in terms of the angle that the molecules make with n, the vector describing the average directionAn important property of this vector is that n = - nThe order parameter (S) is typically S 0.65 for a liquid crystal; for a perfectly ordered crystal S = 1 and for an isotropic liquid S = 0 If the temperature is increased in a thermotropic liquid crystal, the molecules become more disordered and so the order parameter will reduce.n the director

*Anisotropy in a liquidThe order in the liquid allows the material to have different properties in different directions

In a liquid domains will form. Alignment methods will have to be used to obtain a uniform structureScattering of light at the domain boundaries give the bulk a milky appearance

*Elastic propertiesThe molecules in the liquid crystal have a preferred orientation (the director) and as a result if there is a distortion in the structure then there is an elastic energy associated with the distortionThe elastic energy is anisotropic and is described by three elastic constants, k11, k22, k33.Bend, k33Twist, k22

*Dielectric propertiesThe electric permittivity of the liquid crystal is anisotropicThe permittivity is concerned with the polarisability of the material and the response of a material to an electric field.D=eoerE

*Dielectric propertiesThe electric permittivity of the liquid crystal is anisotropicThe permittivity is concerned with the polarisability of the material and the response of a material to an electric field.D=eoerE+-+-+-+-+-+-+-EThe field will induce dipoles in the material, which will create a field inside. P the polarisation.

D = eoE+P = eoeE

P

*Dielectric propertiesThe electric permittivity of the liquid crystal is anisotropicThe permittivity is concerned with the polarisability of the material and the response of a material to an electric field.

+-+-+-+-+-+-+-EIf the field direction changes then the size of the dipoles will be different in an anisotropic material

D = eoE+P = eoeEP

*Measurement of permittivityThe permittivity is measured by making a capacitor filled with liquid crystal and measuring the capacitance.The two values are measured by orienting the liquid crystal in two directionsThe anisotropy in the liquid crystal has values in the range from -10 De 40 in mixtures(where De = e-e) Capacitance meterGuard ring to avoid the effect of fringing fields.Ad

*Permittivity, dielectric constants.Reduced Temperature T/ TNI (TNI is the nematic to isotropic transition temperature)Permittivity or dielectric constant, from capacitance measurements

*Dielectric anisotropy and electric fields in a liquid crystal. When an electric field is applied the energy can be minimised by reorientation of the liquid crystal, because it is a liquid.the stored energy of a parallel plate capacitor is:

So W is minimised by making the dielectric constant as large as possible. Note: this is not the effect of a dipole and does not depend on the polarity (sign) of the field A liquid crystal responds to the average (r.m.s) value of the electric field.

*Dielectric anisotropy and electric fields in a liquid crystal. With positive dielectric anisotropy the director will line up with the electric fieldWith negative dielectric anisotropy the director will line up perpendicular to the electric field

*Influence of chemical structure on permittivityConjugation will increase the polarisabilityDipolar groups will increase the dipoles, either el or et depending on the position in the moleculen=1,2,3X,Y CmH2m+1; CmH2m+1-O; CN etcAromaticAliphaticHetrocyclic

*Permittivity as a function of frequencyAs the frequency of the electric field is increased the permittivity will change.

At optical frequencies the dielectric anisotropy will be positive and can be related to the birefringence as follows:

n2 = e and n2 = e

Dn = n - n, the birefringence

The refractive index of the bulk depends on the polarisability of the moleculesthe order parameteras the temperature is increased the birefringence will reduce.

*Measurement of refractive indicesThe optical refractive indices can be obtained from Abb refractometer measurements.An aligned sample of liquid crystal is put onto a prismThe critical angle, qc, at which total internal reflection occurs is measuredBy changing the polarisation of the light observed both refractive indices can be measured.

qcqc

*Refractive indices of liquid crystalReduced Temperature T/ TNI (TNI is the nematic to isotropic transition temperature)Refractive indices, from Abb refractometer measurements

*Refractive indices of liquid crystalThe refractive index depends on the polarisability of the molecules and also depends on the order parameterPolarisability varies with chemical group, increasing with increased conjugationThe birefringence is always positive, because there is no influence due to purely dipoles.increasing conjugation

*Refractive indices of liquid crystalThe birefringence is critical to the optical properties of the liquid crystal and underlies many of the applications of liquid crystals.By reorienting the liquid crystal the effective birefringence will change and so the optical properties will change

*BirefringenceOptically anisotropic materials have different optical properties depending on the polarisation of the light travelling through the material. This is described by different refractive indices in the material. For a uniaxial material such as liquid crystals there are two values for the refractive index.The refractive indices can be described by an optical indicatrix.Shown in the figure.q

*Optical Indicatrixneno

*Optical Indicatrixnenoq

*nenoqOptical Indicatrix

*nenonxqOptical Indicatrix

*nenonxqThe angle of incidenceof the light may changeOptical Indicatrix

*nenonxqOptical Indicatrix

*nenonxqThe orientation of the optical indicatrix may change this occurs when liquid crystals switchOptical Indicatrix

*Light is a transverse electromagnetic wave, the electric field, the magnetic field and the direction of propagation are all at right angles to each other.

The wave is time varying frequency given by n,speed given by c= nlo in a vacuum in a medium of refractive index n, the wavelength is changed by l=lo/n.Polarised lightA full analysis of polarised light must include both the electric and magnetic components of the light; this is particularly necessary when considering reflected components

In transmissive optical systems the reflected light does not have to be considered in such detail and the light is considered only in terms of the electric components.

*Polarised lightLight can have two orthogonal states or polarisations. The waves can be written as follows: withThe direction of propagation is taken to be along zThese are waves travelling in the z-direction with a relative phase between them of f.

If f = 0 or 2p or an integral multiple then the light is plane polarised

*Circularly polarised lightIf the waves have equal amplitudes E0 and the relative phase is -p/2 + 2mp (where m = 0, 1, 2, ..) then circularly polarised light is obtained. The components are then The intensity of the light is E.E = E02, a constant, but the direction of E is time varying and is rotating with angular frequency of w = 2pf. The light is described as circularly polarised it can be right or left circularly polarised.

*Elliptical polarisationIn a general case E0x E0y and f has any value, the light is elliptically polarised. The electric vector E then rotates as a function of time and the amplitude varies as well. Linear and circular polarisations are special cases.

*Optical phase through a birefringent layerAs the two polarisations pass through the crystal the relative phase between them will change, because the values of k=2pn/l will be different.If a crystal has a thickness of d then one polarisation will have phase relative to z=0 of f=2pnod/l, and the other of f=2pnxd/l.The phase difference is Df=2p(nx-no)d/l

*Viewing angleWhen light is incident from different directions, the effective refractive index can change so the change in polarisation will be different for the different directions of the light.As a result the grey levels are not reproduced accurately and colour distortion occurs, at the extreme reverse contrast can occur.ne, nonx, no

*Viewing angle calculation of reflected light- bright stateTotal Twist, =45n.d/=0.29Polarizer orientation, pol=-15Quarter-wave layer orientation, qwp=-60

Reflected Intensity - Bright

Reflected Intensity - Bright

*Total Twist, =45n.d/=0.29Polarizer orientation, pol=-15Quarter-wave layer orientation, qwp=-60Viewing angle calculation of reflected light - dark state

Reflected Intensity - Dark

Reflected Intensity - Dark

*Viewing angle plot covers polar angles from 0 to 60.Viewing angle calculation of reflected light- contrast ratio

Contrast Ratio (Bright / Dark) contour plot (clipped to ~ 20:1) Max:66

Contrast Ratio (Bright / Dark) area plot

(clipped to ~ 20:1) Max:66

*Nematic continuum theoryThe energy density F is expressed as a function ofThe director nThe elastic contants k11, k22, k33.The pitch, PThe electric field E and dielectric anisotropy De.

*Numerical modelling of the director (n) distribution

*Numerical modelling of the director distribution

*Threshold voltage for switchingThe competition between the elastic and electric energies, results in a threshold voltage for switching

*AlignmentThe liquid crystal director must be fixed somewhere, and the best place is at the surface of a substrateThe director must be fixed so that the elastic forces restore the original structure after the electric field has been removed.The director can be fixed parallel to the surface or perpendicular, or at an angle.Alignment is achieved by rubbing a polymer surface, or by treating the surface with a surfactant.Energetically unfavourable because of the distortion in the director fieldEnergetically favourable the director field is undistortedThe alkyl chains of the surfactant interact with the long chains of the liquid crystal to give alignment perpendicular to the substrate

*Structure of an LCDhttp://www.hylcd.com/images/color.gifThe polarisers are needed to modulate the light intensity, when the liquid crystal changes the polarisation state of the lightThe glass is used to give structure to the display spacers are used to maintain the cell gap (typically 5mm)The ITO (Indium Tin Oxide) is a transparent conductor, to allow a voltage to be applied across the liquid crystal layerAn alignment layer is needed to fix the liquid crystal at the surface, so that the un-switched structure is obtained with the voltage is removed.Colour filters are used to give Red, Green and Blue pixels for colour displays.

*Twisted nematic (TN) LCDFigures taken from http://sharp-world.com/sc/library/lcd_e/s2_4_3e.htmA very common type of display uses an alignment on the two glass substrates at 90 to each other, so that there is a 90 twist in the director in going through the cell. When a voltage is applied the director tilts to align with the electric field in the cell.If the electrodes are patterned into lines, then addressing a row and a column selects a particular pixel, defined by the intersection of the electrodes.

*MultiplexingMultiplexing is implemented by applying a select voltage along a row and the data voltages to the columns, as shown in the diagram below. The voltage levels as a function of time are shown and the rms voltage is also given. The liquid crystal responds to the rms voltage.Normally white display i.e. white in the un-switched state and dark in the switched state.Resultant voltage on Arow voltage data voltagetResultant voltage on Btr.m.s voltage on A

tr.m.s voltage on Btframe time = no. of rows (N) row select time

S+DS-DDDThe rms select and non-select voltages are given below

Row select voltagesSData voltagestABDThe multiplexing limit is:

*Active Matrix Twisted Nematic Displays (AMTN)Direct multiplexing only gives about 30 to 200 lines, with limited grey level and response time.An alternative is needed for computer displays or TV/video displays. A non-linear element or a switch is required at each pixel so that row and column addressing can be used. A suitable non-linear device is the transistor. A display may contain millions of pixels and therefore transistors Only very few can be defective before the display appearance is degraded. Displays are generally very large in comparison to normal silicon transistorsOften the displays are back lit so the display must be transparent.

http://sharp-world.com/sc/library/lcd_e/s2_4_3e.htm

*Thin Film Transistors (TFTs) for LCDsThe material used to make the TFT is usually amorphous Silicon. This is easy to deposit over large areas of glass with its low processing temperatureA storage capacitor is also needed so that the charge can be maintained on the LCD pixel after it has been addressedThe change in dielectric constant in the LC layer can alter the voltage for a fixed charge on the pixel capacitor.http://www.xbitlabs.com/articles/other/display/response-2_3.html

*Materials for TFTsAmorphous Silicon +easy to deposit over large areas and at low temperature - compatible with glass substrates-But it has relatively low mobility-Larger areas are required to overcome the low mobility, so that the pixel can be addressed in the line address time-The transistor must be shielded so that no photoconduction occurs- Large shielding reduces the light throughputLarge transistors mean that the leakage current can be a problemPoly-silicon-Amorphous Silicon can be re-crystallised by local heating to form poly-crystalline Silicon+The higher mobility means that transistors are fast enough to be used for control and signal processing as well as the pixel switches+Inclusion of some electronics onto the glass means fewer connections to the glass, improving performance and simplifying fabrication.+Smaller transistors mean lower leakage currents and so the pixels hold the charge better

*Liquid Crystal on Silicon (LCOS)Although Silicon is not transparent, reflective displays can be made with a Silicon backplane. High resolution can be obtained with 10mm pitch pixels and HDTV resolution (1M+ pixels)Complex processing can be carried out in the backplane.Planarisation of the wafer must be carried out so that the pixels are optically flat.Electrical connections are made from the circuit through the planarisation layer to the metal pixel electrode, which can also act as mirrorThe liquid crystal layer is added above the mirrors, with an ITO ground plane.Reflective operation means that the light travels twice through the liquid crystal layer so it can be made thinner resulting in faster switchingThe very small displays can be used in projector displays and for diffraction

*LC modes for Active Matrix Liquid Crystal Displays (AMTN)Conventionally the 90 twisted nematic liquid crystal is used but:-Twisted nematics have a problem with the viewing angle characteristics and the dark state.Other LC modes to improve the properties are-Vertically Aligned Nematic (VAN)-In-Plane SwitchingCompensation films, which are birefringent films, can be added to the front surface of the device to compensate for the off-axis birefringence variations.

*In-plane switching (IPS) LCDsSwitching is in the plane of the cell.The optical properties are then less angle dependant and it is easier to compensate for the changes off-axis.High anisotropy of the dielectric permittivity is possible because positive materials are used, hence fast switching can be obtainedPatterned electrodes are needed, which require relatively high resolution lithography.

*In-plane switching (IPS) LCDsAlternate voltages are applied to the electrodesThe modelled results show the director distribution around the end of an IPS electrode structure

*Vertically Aligned NematicUsing a negative dielectric anisotropy and alignment perpendicular to the substrates, switching can from a uniform dark (off) state to a switched state which can be average to give a more uniform viewing angle.Slits or other shaped holes in the electrodes provide shaped electric fields so that the tilt direction of the switching is controlled

*Vertically Aligned NematicNumerical modelling of the director pattern and the optical properties show the switching of a VAN cell with a cross shaped hole in the electrode.Four domains result, giving very uniform viewing angle propertiesThe off-axis birefringence of the uniform dark state is easily compensated with a birefringent film with a negative birefringence, giving a very good dark state, hence a good contrast ratio.Materials with a high negative dielectric anisotropy are more difficult to obtain and so switching times can be slower than positive materialsThe birefringence may be lower for these materials and so thicker layers are needed, again making the switching slower

*Cholesteric liquid crystalsEssentially the nematic phase, but with a chiral group which gives a twist to the structure.The twist is often temperature dependant, but only if there is a SmA phase below the nematic and then the pitch increases in going closer the the SmA phase, i.e. as the temperature decreasesThe pitch of the twist can match the wavelength of light, l, in which case selective reflection occurs across a band when l = nP, with n, the refractive index, going from ne to no. (Sometimes called Bragg scattering)Only the circular polarisation matching the twist sense is selectively reflectedIn droplet form, encapsulated into a polymer the cholesteric can be used as a thermometerIn an aligned form it can be used as a circular polariser, or as the reflector for a laser system.

*Liquid crystals for laser systems and photonicsLaser dye molecules can be added to the cholesteric liquid crystal and then optically pumped and the liquid crystal structure forms the laser cavity.Changing the temperature slightly would then tune the wavelength of the laserSome cholesteric systems have been shown to have electrically tuned pitch, which allows electrical tuning of the laser wavelengthThe cavity can be made very small, which has advantages in laser operationImprovements will be to synthesis liquid crystal laser dye materials, which would allow very high concentrations of the dye and to find a method of electrically pumping the laserBlue phases are a three dimensional twisted structure, which is very interesting as a 3D distributed Bragg system.Recent developments using dimers have allowed blue phases to exist over a wide temperature range.These have potential as self assembled photonic band gap structuresA defect formed in the 3D structure, or a line of defects give localised transmission of light in the band gap and control of photon movement through the structure.

*Diffractive displaysConventional LC displays rely on transmission/reflection or absorption of the lightThis is inefficient since light is absorbed rather than redirectedDiffraction offers a way of steering the light into the region where it is requiredRequires coherent illumination (i.e. laser light) and very high resolution structures (sub-wavelength) in the diffracting mediumThe Fourier transform of the image is obtained and this is displayed on a Spatial Light Modulator (SLM) or display!Diffraction from the Fourier transform reconstructs the original imageOften image compression relies on taking the transform, so the processing could be simplified, so reducing the data rates required to the displayOnly LCOS systems provide the resolution requiredBoth binary Ferroelectric liquid crystal and nematic LC can be used.Intensity modulation of the diffraction pattern can be used, but phase modulation is more efficient

*Diffractive Display demonstrated at Cambridge UniversityUsing Binary FLC LCOSUsing analogue nematic LCOS

*Beam steering using LCOSA grating can be used to direct light into different regionsThis can be used in telecom systems to reconfigure light from an array of optical fibre outputs into an array of optical fibre inputsModelling of a subset of a 2D array of LC pixels to produce a programmable phase grating

*Holograms in LC devicesAs well as providing a 2D image by diffraction, at a higher resolution a hologram can be generated, which is a 3D image.The phase front of the light reflected from an object is reconstructed and this then appears like the object itself.This has been demonstrated by Qinetiq, but is not in production

*Stanley, M, Conway, P B, Coomber, S, Jones, C J, Scattergood, D C, Slinger C W, Bannister, B W, Brown, C V, Crossland W A, and Travis A R L, "A novel electro-optic modulator system for the production of dynamic images from giga-pixel computer generated holograms", Practical Holography XIV, Proceedings of SPIE Vol 3956, San Jose, 24 January 2000, pages 13 to 223D holographic CAD Workstation ConceptDemonstration SystemLCOS-based 3D Holographic Displays

*Bistable displaysFor portable displays the energy consumption is criticalBistability allows the display to be addressed and then it can be left in that state. The power can be removed once the display has been addressed, giving very low power operation for displays which do not have rapidly varying information Applications are for e-books, supermarket shelf labels, some mobile phone applications

*Device under development at Hewlett-Packard.

Periodic array of microscopic square posts.

Two stable states of operation, planar and tilted.[1] Kitson S. Geisow A., Bistable Alignment of Nematic Liquid Crystals Around Microscopic Posts, Mol.Cryst. Liq. Cryst., vol.412, pp.153-161, 2004[2] Kitson S. Geisow A., Controllable Alignment of Liquid Crystals Around Microscopic Posts: Stabilisation of Multiple States, Appl. Phys. Lett., vol. 80, no. 19, pp. 3635-3637, May 2002 Post Aligned Bistable Device (PABN)3-D Modelling WindowSides: PeriodicTop: HomeotropicBottom and Post: Planar Degenerate2.6 3.0 m1.2 m0.6 m

*The background colour indicates the tilt of the director, showing the two stable states. Switching is via the flexo-electric effect and involves defect movement.Adaptive meshing is used to concentrate calculation where needed e.g. at the defects.PlanarTiltedThe Two Stable States

*Doping liquid crystalsAdding anisotropic dyes to liquid crystals has been done for a long time for simple displays.The alignment in the liquid crystal aligns the dye molecules which can then be switchedAbsorption occurs only along one direction of the dye and so the light absorption can be switchedOther additives are being considered carbon nano-tubes, very small particles of ferroelectric crystal, biological moleculesThis could be to enhance the anisotropy of the liquid crystal properties or to control the orientation of the dopant Control of the position of defects in the liquid crystal can be used to move particles in the liquid crystal and so could be used to position very small particleshttp://www.lci.kent.edu/polmicpic.htmlA Pair of Point Defects

*Colloidal crystals in liquid crystalTwo-Dimensional Nematic Colloidal Crystals Self-Assembled by Topological DefectsMuevi Igor, karabot Miha, Tkalec Uro, Miha Ravnik, Slobodan umer, Science (vol.313 p.954)

*SummaryLiquid crystals give electrically switchable anisotropic propertiesThe organic molecules can be synthesized to have many different properties and mixture formulation allows greater flexibility and controlOptical anisotropy can be switched so as to modulate the polarisation of light, or the phase of light passing through the layerDisplays are the most common application, but there are many different configurations depending on the application requirementsHigh resolution structures in the liquid crystal allow them to be used as diffraction or holographic displaysFuture applications in photonics may be based on the use of particles or dopants in the liquid crystal

*Switching Between the Stable States via defect formation along post.Intermediate, defect state.The Flexoelectric effect responsible for switching between the stable states.Side viewTop viewPostPostPostPost

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