EP1673646A4 - Matieres de cristaux liquides et dispositifs electro-optiques comportant une gaine qui contient des cristaux liquides - Google Patents

Matieres de cristaux liquides et dispositifs electro-optiques comportant une gaine qui contient des cristaux liquides

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Publication number
EP1673646A4
EP1673646A4 EP04795211A EP04795211A EP1673646A4 EP 1673646 A4 EP1673646 A4 EP 1673646A4 EP 04795211 A EP04795211 A EP 04795211A EP 04795211 A EP04795211 A EP 04795211A EP 1673646 A4 EP1673646 A4 EP 1673646A4
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EP
European Patent Office
Prior art keywords
polymer
optical material
functional optical
functional
refractive index
Prior art date
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Application number
EP04795211A
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German (de)
English (en)
Other versions
EP1673646A2 (fr
Inventor
Steven M Risser
Vincent D Mcginniss
Kevin B Spahr
Richard J Higgins
Richard W Ridgeway
Zhihua Huang
Edward Jiang
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Optimer Photonics Inc
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Optimer Photonics Inc
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Application filed by Optimer Photonics Inc filed Critical Optimer Photonics Inc
Publication of EP1673646A2 publication Critical patent/EP1673646A2/fr
Publication of EP1673646A4 publication Critical patent/EP1673646A4/fr
Withdrawn legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/52—Liquid crystal materials characterised by components which are not liquid crystals, e.g. additives with special physical aspect: solvents, solid particles
    • C09K19/54—Additives having no specific mesophase characterised by their chemical composition
    • C09K19/542—Macromolecular compounds
    • C09K19/544—Macromolecular compounds as dispersing or encapsulating medium around the liquid crystal
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G02B6/1221—Basic optical elements, e.g. light-guiding paths made from organic materials
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/011—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/1326—Liquid crystal optical waveguides or liquid crystal cells specially adapted for gating or modulating between optical waveguides
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1334—Constructional arrangements; Manufacturing methods based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12142—Modulator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12159—Interferometer
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/011—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass
    • G02F1/0115—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass in optical fibres
    • G02F1/0118—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass in optical fibres by controlling the evanescent coupling of light from a fibre into an active, e.g. electro-optic, overlay
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/50—Phase-only modulation

Definitions

  • the present invention generally relates to electrooptic materials and devices and more particularly to a functional polymer-dispersed liquid crystal (PDLC) electrooptic material which may be used as a functional cladding for electrooptic devices.
  • PDLC polymer-dispersed liquid crystal
  • fiber-based optical attenuators See, e.g., K. Takizawa, et al., "Polarization-Independent Optical Fiber Modulator by Use of Polymer-Dispersed Liquid Crystals," Applied Optics, Vol.
  • the PDLC materials of this invention are novel in several aspects.
  • the materials contain a much lower concentration of liquid crystal than is commonly used to cause phase separation into polymer matrix with liquid crystal droplets. This is due to enhanced incompatibility between the liquid crystal and the proprietary low refractive index host polymers.
  • there are additional constraints imposed on the materials as evidenced by the limited function of some liquid crystal/polymer combinations.
  • the use of a low liquid crystal concentration leads to formation of much smaller liquid crystal droplets, leading to greatly reduced light scattering, evidenced by the low transmission loss of devices fabricated with this material as the cladding.
  • the polymer is NOT matched to either refractive index of the liquid crystal, as is the case for scattering-based PDLC materials.
  • the optional inclusion of the chromophore in the system is also unique.
  • the chromophore phase separates in conjunction with the liquid crystal, and acts to enhance the optical anisotropy of the droplets.
  • the choice of chromophore depends on both the polymer and liquid crystal, as the chromophore must preferentially remain in the LC phase, and must align parallel with the liquid crystal director.
  • Mach-Zehnder devices are constructed using these novel liquid crystalline/polymer materials as the functional cladding over silica waveguides.
  • These devices differ from standard Mach-Zehnder devices in that the cladding over the waveguide is the functional material.
  • Application of a DC or low frequency AC voltage to the cladding of one or both arms of the Mach-Zehnder when the temperature is within the range where the droplets exhibit liquid crystalline behavior, leads to orientation of the molecules within the liquid crystal droplets, such that the effective refractive index of the droplet is different along the direction of the electric field.
  • Light propagating through the waveguide interacts with this altered refractive index, giving rise to a phase change in light propagating down one arm of the Mach-Zehnder, allowing for control of the output of the device.
  • one aspect of the present invention is a functional optical material composed of a liquid crystal (LC) evidencing a pair of refractive indices (RI's) and a polymer in which the LC is dispersed.
  • LC liquid crystal
  • RI's refractive indices
  • the refractive index (RI) of said polymer may be outside of the LC RI's by at least about 0.03.
  • a functional optical material composed of a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein said LC is less than about 5% miscible in said polymer.
  • a further aspect of the present invention is a functional optical material composed of a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein the functional optical material contains not more than about 20 wt-% LC.
  • the functional optical material can be clad to an optical waveguide and can optionally contain a chromophore.
  • a functional optical waveguide is composed of an optical functional material from above having a refractive index, RI P and an optical waveguide clad having a refractive index, RI WG , wherein RI P is at least about 0.3% lower than RI G under operating conditions of said clad optical waveguide.
  • An additional aspect is functional optical material formed from a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein the functional optical material has refractive index of less than 1.46.
  • a further aspect is functional optical material formed from a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein the polymer formed from the reaction product of one or more of (1) at least two pre-polymers having a weight average molecular weight of at least about 1 ,000; or (2) a polymer having self cross- linking functional groups and reactive agent reactive with said self cross-linking functional groups.
  • LC liquid crystal
  • an optical device one or more of fabricated using or clad with a functional optical material of a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein the refractive index (RI) of the polymer is outside of the LC RI's by at least about 0.03, and wherein the optical device is capable of changing the phase of an optical signal without appreciable change in optical scattering of said optical signal.
  • Appreciable change in optical scattering is defined as an optical scattering of less than about 2 db/cm with optical scatterings of less than about 0.5 db/cm demonstrated in the examples set forth herein.
  • Yet another aspect is a photonic band gap composite media formed from a host matrix and an array dispersed in the host matrix, wherein one or more of said host matrix or said array is formed from a liquid crystal (LC) having a refractive index (RI) and a polymer in which the LC is dispersed, wherein the refractive index (RI) of the polymer is outside of the LC RI by at least about 0.03.
  • Another aspect is a method for controlling the electrooptical output of functional optical material formed from a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein the LC is less than about 5% miscible in the polymer.
  • Such process includes adding a plasticizer to the functional optical material, the proportion of plasticizer in the functional optical material controlling the electrical output thereof.
  • a functional optical material formed from a liquid crystal (LC) and a polymer in which the LC is dispersed, wherein one or more of said LC or the polymer containing fluorine groups and the polymer contains at least 5 molar-% polar groups.
  • Fig. 1 depicts sample LC droplet directors for a radial, an axial, and a bipolar droplet, where the straight lines indicate the preferred orientation of the molecules at that point within the droplet;
  • Fig. 2 is a schematic showing orientation of bipolar droplets in the absence of, or under the influence of a field, wherein the black lines on the droplets represent the director orientation;
  • Fig. 3 is a schematic of the Mach-Zehnder device of the present invention;
  • Fig. 4 is a cross-sectional view of the device of Fig.
  • Fig. 5 is a schematic of the sample used in the EO cell from data reported herein was recorded, wherein the light is polarized 45° out of the plane of the paper;
  • Fig. 6 depicts the general requirements for LC/polymer compatibilization, as reported in Example C;
  • Fig. 7 depicts EO response as a function of temperature, as reported in Example J;
  • Fig. 8 depicts EO response as a function of time for the test cell reported in Example M;
  • Fig. 9 depicts refractive index as a function of temperature, as reported in Example O;
  • Fig. 10 depicts the response of a Mach-Zehnder device to sinusoidal voltage, as reported in Example P;
  • FIG. 11 depicts the voltage dependent response of the Mach-Zehnder device to a sinusoidal voltage, as reported in Example P;
  • Fig. 12 depicts performance of a Sea Bass 3 device at 40° C under the influence of an alternative square-wave voltage, as reported in Example Q;
  • Fig. 13 depicts performance of a Sea Bass 3 device at 50° C under the influence of an alternative square-wave voltage, as reported in Example Q;
  • Fig. 14 depicts performance of a Fluorine 4 device at 45° C under the influence of a sinusoidal voltage, as reported in Example R.
  • a liquid-crystal mesophase of interest in the invention is the nematic phase, in which rod-like (e.g., 30 A-long), organic liquid-crystal molecules tend to align along a common direction (so-called director), while possessing no overall translational order.
  • the nematic phase is birefringent, commonly with an extraordinary refractive index (n ⁇ ) along the director, and ordinary refractive index (n 0 ) along the orthogonal short axes.
  • Typical refractive index values are n e about.1.7 and n ⁇ about.1.5, although materials with lower birefringence and lower n 0 are commercially available.
  • Refractive indices of the overall system can be defined as high, intermediate, or low as follows: High RI n > 1.50 Intermediate RI 1.46 > n > 1.50 Low RI n ⁇ 1.46
  • Polymer-dispersed liquid crystals consist of an unordered array of small liquid crystal droplets or regions contained within a host polymer matrix.
  • the dispersion of liquid crystal inside a polymer can be formed in multiple ways. The most common approach is by mixing the liquid-crystal material with a photocurable monomer at an appropriate volume ratio, and then curing the monomer with UV radiation (photopolymerization-induced phase separation or PIPS).
  • PIPS photopolymerization-induced phase separation
  • the cross-linking of the polymer leads to phase separation due to the decreasing miscibility of the liquid crystal in the polymer, trapping the liquid crystal in droplets, the sizes of which are capable of being adjusted by suitable choice of volume fractions and curing intensities.
  • An alternate PIPS approach involves mixing a reactive monomer and the liquid crystal in the appropriate ratios, and thermally initiating the polymerization. As the monomer grows into the polymer, the liquid crystal becomes immiscible in the polymer, leading to phase separation. Again, the choice of volume fractions and polymerization conditions controls the size of the liquid crystal droplets formed in the polymer matrix.
  • a second fabrication approach is thermally induced phase separation, or TIPS, which can be used when the host polymer has a melting temperature below its decomposition temperature. In this method, a homogeneous mixture of liquid crystal and a melted polymer is formed at a temperature above Tg of the polymer.
  • the solution is cooled at a specific rate to induce phase separation due to the decreasing of the miscibility of the liquid crystal in the hardening polymer.
  • Liquid crystal droplets begin to form as the polymer hardens. The droplets continue to grow until the glass transition temperature of the polymer is crossed. Droplet size is most affected by the cooling rate of the polymer melt/liquid crystal solution. Fast cooling rates tend to produce small droplets because there is not sufficient time for large particles to form. Therefore, droplet size and cooling rate are related inversely.
  • a third fabrication approach involves mixing immiscible polymer and liquid crystal in a mutual solvent. As the solvent is evaporated off, the liquid crystal phase separates from the polymer, leading to formation of the liquid crystal droplets (solvation-induced phase separation or SIPS).
  • TFEMA trifluoroethyl methacrylate monomer
  • TFPMA tetrafluoropropyl methacrylate monomer
  • MMA methyl methacrylate monomer
  • HEMA hydroxyethyl methacrylate monomer
  • HFIPA hexafluoroisopropyl acrylate monomer
  • HFBIvlA hexafluorobutyl methacrylate monomer
  • C-PDLC Chromophore-Containinq Polymer-Dispersed Liquid Crystals
  • the separation between the droplets also depends on volume fraction and curing parameters, and generally ranges from separations similar to droplet diameter to cases where droplets are separated by only very narrow polymer walls that have a thickness that is much less than droplet size.
  • the size of the droplets for PIPS and TIPS is controlled by the polymerization conditions, as well as the volume fraction of liquid crystal in the polymer.
  • the droplets may contain only a small fraction of the included LC, with the rest remaining dispersed in the polymer, due to the miscibility of the LC in low molecular weight polymer.
  • the axial configuration of the liquid crystal droplets also occurs when the molecules are oriented perpendicular to the droplet wall, but only when there is weak surface anchoring, or in the presence of an electric field above the threshold magnitude.
  • This configuration creates a line defect that runs around the equator of the spherical droplet, as also seen in Fig. 1.
  • the radial configuration is returned when the field is removed.
  • the bipolar configuration is obtained by tangential anchoring of the liquid crystal molecules. This creates two point defects at the poles of the droplet and is shown further in Fig. 1.
  • the axis connection of the two polar defects rotates to lie along the direction of the applied field.
  • (7) Scattering and PDLC Function Since the nematic liquid crystal phase is uniaxial, the liquid crystal droplets formed inside the polymer matrix also can be uniaxial, except for the radial configuration. For the axial and bipolar droplets shown in Fig. 1 , the droplet will have an extraordinary index along the vertical direction, while the ordinary index is along the horizontal direction.
  • the radial droplet in Fig. 1 is homogeneous, with a droplet refractive index between that of the ordinary and extraordinary indices of the liquid crystal.
  • the PDLC's operate on the principle of electrically controlled light scattering, with the details of the mechanism dependent on the specific droplet director configuration.
  • the droplets in the zero-field state are homogeneous, with refractive index n ⁇ ( ⁇ n ⁇ +2n 0 )/3 ⁇ ); while when the field is applied, the droplet transitions to the axial configuration, with droplet indices approximately equal to n ⁇ and n 0 .
  • the droplets in the zero-field case scatter due to the refractive index mismatch between n 0 and n ; while with the field applied, any light propagating along the direction of the applied field sees an approximately homogeneous medium of index, n 0 , and so is transmitted.
  • the amount of light scattered by each droplet depends on the size of the droplet relative to the wavelength of the light and the refractive index mismatch between the droplet and the host polymer. For PDLC materials where the droplet is in the bipolar configuration, the scattering mechanism is slightly different.
  • the bipolar droplets are intrinsically uniaxial, with droplet refractive indices approximately equal to n e and n 0 , but the optical axis of each droplet is randomly oriented, as illustrated in Fig. 2.
  • Application of an external field causes reorientation of the optic axis to lie along the direction of the applied field.
  • the polymer index is approximately n 0
  • when there is no field the light is scattered due to the refractive index mismatch between the polymer and most of the droplets; while when the field is applied, light propagating along the direction of the field will see an approximately homogeneous medium, and will not be scattered.
  • the amount of scattering by each droplet depends on the droplet director, the refractive index mismatch between the droplet and the polymer, and the size of the droplet relative to the wavelength of the light.
  • a Mach-Zehnder interferometric device works by splitting light into two equal beams, altering the relative phase of the two beams, and then re- combining them. The relative phase difference between the beams allow for selection of the output port for the light.
  • An EO polymeric device works by using the electrooptic effect in the polymeric material (LC/polymer blend) to create the phase change. In an electrooptic polymeric material, as an electric field is applied, the refractive indices of the polymeric material change.
  • the change in the effective refractive index of one arm can be accomplished by altering the refractive index of (a) the waveguide material, (b) the cladding, or (c) both the waveguide and cladding.
  • the second case is referred to as a functional cladding, while the third case has both functional cladding and waveguide material.
  • Use of a functional cladding has advantages and disadvantages over use of a functional waveguide material.
  • Use of a functional cladding allows for silica to be the primary waveguide, with its ease of fabrication and low optical loss.
  • Fig. 3 shows one possible Mach-Zehnder configuration used for this device, where directional couplers are used to separate the light along the two arms, and then recombine it.
  • Mach-Zehnder device, 10 is formed from a base silica substrate, 12, that carries two additional silica sheets, 14 and 16.
  • some measurements were taken using a Y-splitting Mach- Zehnder, where a Y-splitter is used to separate the light and a Y-combiner is used to bring the light back together.
  • Fig. 4 shows a cross-section of either device in the active region. The material used in the active region consists of either the PDLC or C-PDLC.
  • the specified quantities of polymer, liquid crystal, and optionally chromophore were dissolved in sufficient dioxane (or other suitable solvent) to achieve a solution containing approximately 1-10 percent solids. This solution then was passed through a 0.2 ⁇ m filter, after which solvent was allowed to evaporate until the solution contained approximately 12% solids. If the material was to be applied to an EO test cell, it then was applied with a dropper to the slide containing the electrodes, until a film of approximately 10-15 ⁇ m was formed. If the material was to be applied to a Mach-Zehnder device, the refractive index of the material was checked using the prism coupling method.
  • the refractive index of the material was within the specified range, a small quantity of the solution then was applied to the waveguide chip using a dropper, until a film approximately 30 ⁇ m thick, or more, was formed over the active region of the waveguide chip.
  • the coated chip then was dried in a 70° C oven for 12-72 hours to remove the solvent, after which it was removed for assembly.
  • the device was assembled by heating the coated chip to approximately 120° C, then placing a silica slide containing the electrodes on top of the material. The temperature then was increased and pressure applied to the electrode slide until the thickness of the polymer layer decreased to approximately 10 ⁇ m. After cooling, the device was removed from the assembly jig and wires attached to the electrodes.
  • EO Test Cell The need to rapidly and cheaply investigate the EO response has led to adoption of an alternate experimental method, similar to that used to measure the electrooptic effect in poled polymer systems.
  • Fig. 5 shows a schematic of the test cell used to determine the magnitude of the EO response for various polymer/LC combinations reported herein, and also for different film PDLC formation methods.
  • an EO cell, 30, is seen to be composed of a silica substrate, 32, fitted with an electrode pair, 36 and 38, with a 20 ⁇ m gap between the electrodes. The EO material, 34, then was applied to cover the two electrodes and the gap between them and baked to remove the solvent.
  • the laser beam was passed throug h the gap between the electrodes, in the direction of arrow 40 perpendicular to polymer 34.
  • the beam polarized 45° to the direction of the gap.
  • the refractive index for light polarized in the plane of the paper will be altered, while that for light polarized normal to the paper will be unaffected, leading to a change in the relative phase of the two polarizations of the light as it traverses the sample.
  • the variation in the transmitted power due to this changing phase difference at the same frequency as the AC voltage can then be directly related to the EO response of the polymer film.
  • a DC bias voltage was applied in addition to the sinusoidal, 200 V peak-to-peak, 1 kHz, AC voltage.
  • polarity -OH, -CN, -COOH, -COO-alkyl, -NCO, and other polar functional groups.
  • the following table looks at all hydrocarbon LC materials and address what type of polymer systems are required in order to obtain acceptable EO performance.
  • TFEMA trifluoroethyl methacrylate monomer
  • TFEA trifluoroethyl acrylate monomer
  • TFPMA tetrafluoropropyl methacrylate monomer
  • TFPA tetrafluoropropyl acrylate monomer
  • MMA methyl methacrylate monomer
  • HEMA hydroxyethyl methacrylate monomer
  • liquid crystal materials of this invention have the following range of compositions: TABLE 4 Ratio of Components of LC
  • fluorinated liquid crystal materials can be made in the following manner:
  • the fluorinated liquid crystal also may be attached to the backbone of a fluoropolymers to realize the following advantages: enhanced optical quality, long-term durability, and control of the phase dispersion, which enhances the EO properties of the total system.
  • the fluorinated liquid crystalline structures also enhance phase dispersion, while maintaining optical clarity, durability, and EO properties.
  • EXAMPLE A A mixture of 0.01 mole (1.95 g) of 4'-hydroxy-4-biphenylcarbonithle and 0.01 mole (4.45 g) of 1 -iodoperfluorohexane were reacted in the presence of potassium hydroxide and ethanol under the same conditions as described in references 1 and 6 from above. The resulting product was:
  • This product was blended (10% by weight) with a fluoropolymer mixture (30 wt-% trifluoroethyl methacrylate, 60 wt-% tetrafluoropropyl methacrylate, 8 wt-% hydroxyethyl methacrylate, and 2 wt-% of an alkylsilane ester of methacrylic acid).
  • This product was then reacted with hydroxyethyl methacrylate in pyridine to create a monomer that was further modified with 4'-hydroxy-4- biphenylcarbonitrile.
  • This monomer was subsequently polymerized with other monomers to form a polymeric liquid crystalline product as described below.
  • 10 wt% LCM with 30 wt-% trifluoroethyl methacrylate, 50 wt-% tetrafluoropropyl methacrylate, 5 wt-% hydroxyethyl methacrylate, and 5 wt-% silyl methacrylate monomers were polymerized in dioxane in accord with typical free radical polymerization processes (Soremon and Campbell, Preparing Methods of Polymer Chemistry, 1961 , Interscience Publishers, New York, New York, pp. 149-215).
  • the resulting product had excellent optical quality, good phase dispersion of the liquid crystalline material and good EO properties. This system also did not degrade (migration of the liquid crystalline material or loss of optical quality) under thermal aging at 80° C for 40 hours.
  • EXAMPLE C This example reports a new approach for enhancing the phase dispersion of liquid crystalline materials, either as a guest in a host polymer, or attached to a polymer (high or low refractive index) backbone.
  • This new procedure modifies either a high or low refractive index polymer with a unique combination of liquid crystal (LC) compatibilizing structures that stabilize LC dispersions while maintaining optical clarity and good NLO properties at very low concentrations of LC materials.
  • Table 5 shows some of the problems associated with prior art LC materials and polymers.
  • Table 6 shows how the modification of polymers and LC materials of this invention results in total systems that have a good balance of optical clarity, stable phase dispersions, and good NLO properties at low concentrations of LC materials.
  • TFPMA tetrafluoropropyl methacrylate monomer
  • HEMA hydroxyethyl methacrylate monomer
  • EXAMPLE E Crosslinked LC-Polvmer System A fluoropolymer containing 50% TFEMA, 40% TFPMA, and 10% of an isocyanate functional methacrylate ester was prepared in a conventional solution
  • TMPTA trimethylolpropane triacrylate
  • n D 1.4740
  • TMPBDA trimethylolpropane benzoate diacrylate
  • n D 1.5110
  • TFEMA trifluoroethyl methacrylate
  • n D 1.3610
  • NVP N-vinylpyrrolidone
  • n D 1.5120 (polar monomer)
  • TFHMA tetrahydrofurfuryl methacrylate
  • n D 1.4580
  • LC 5OCB or 8OCB
  • PS diethoxyacetophenone
  • n D 1.4990 where, n D is refractive index at 589 nm
  • Each of the five 100% reactive liquid systems were applied to EO test cells and irradiated with a 275 watt General Electric Company (GE) sunlamp for 40 minutes under an Ar inert atmosphere. All the systems had EO response values that ranged from 1-6 pm/V 2 .
  • GE General Electric Company
  • EXAMPLE G Polymer Host System A porous sol-gel system was prepared by crosslinking of Si(OCH 3 ) 4 (hydrolysis reactions described in Sol-Gel Techniques for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes, Lisa C. Klein ed., Nukes Publications, Park Ridge, New Jersey, 1988). Novel modifications of these known sol-gel reactions were made using fluorinated silanes (1 H, 1 H, 2H, 2H-perfluorinated triethoxysilanes), either alone or in combination with nano-size silica particles to produce low or medium (n ⁇ 1.5) refractive index gel structures.
  • the porous sol-gel structures were created between coplanar gold electrodes on a quartz slide with a gap of 20 ⁇ m between the electrodes.
  • Conventional or fluorinated liquid crystalline compounds (in solvent) or polymer solutions of the liquid crystalline compounds were absorbed into the porous sol-gel structures, dried, and tested for their ability to respond in a nonlinear-optical behavior under influence of an electric field, using the testing procedure described previously.
  • the NLO results from the sol-gel structures were similar to those polymer liquid crystalline samples that were not absorbed into a sol-gel structure.
  • liquid crystal materials of this invention also can be incorporated into hybrid (organic-inorganic) or nano-composite polyester polymers (these polymers are described by R. van der Linde and S. Frings, in the paper presented at the ef h
  • liquid crystal materials of this invention also can be incorporated into highly branched or dendritic polymers that contain fluorine groups in their backbone structures.
  • a description of how to prepare these branched/dendritic polymers can be found in Polymer Science, and International Review Journal, May 2000, vol. 25, K. Inoue, pp 453-571.
  • EXAMPLE J EO Cell with a PDLC Film (133-46-33)
  • a PDLC film was prepared using the SIPS method, using an OPI low-index terpolymer (Run 46) and 5OCB (4-pentyloxy-4'-biphenylcarbonitrile, purchased from
  • Fig. 7 shows the EO response as a function of temperature.
  • the curve demonstrated the K/(T*-T) behavior expected from a nematic nearing the transition temperature. Note the rapid drop-off of the response as the temperature rises above the critical temperature.
  • These results show the EO response of the PDLC has strong temperature dependence and strong dependence of the bias field. Also, note there is an apparent maximum temperature for operation of the PDLC. Comparison of these results to those from the preceding example show the 5OCB has a response which, when scaled for wt-% of the liquid crystal, is more than 4 times larger than that of 5CB.
  • EXAMPLE L EO Cell with a C-PDLC Film (133-85-9) A C-PDLC film was prepared using the SIPS method. The solution composition was 6.6 mg of 5OCB, 5.2 mg OPI chromophore 1 19-96, and 56.37 mg of polymer KP001 (containing 10% NCO groups), jointly dissolved in dioxane. Drying at 70 ° C led to a crosslinked polymer film, which was resistant to solvents. The material was used to produce EO test cells, which gave the following results.
  • the solution composition was 9.2 mg of 5OCB, 5.3 mg OPI chromophore 119-96, and 79.2 mg of polymer Spike 3, jointly dissolved in dioxane.
  • the material was used to produce EO test cells, which gave the following results.
  • EXAMPLE N Performance of a Mach-Zehnder Device with PDLC cladding (SharkD A Mach-Zehnder device was constructed as is shown in Fig. 3. The PDLC was initially formed using the SIPS method, with 5.9 mg of 5OCB dissolved in 39.5 mg of polymer Run46. After the solvent was removed, the film was heated to greater than 120 ° C to enable attachment of the top electrode cover plate. During this process, the microstructure may have altered, due to a TlPS-like process. The device was tested to determine the voltage required to create a phase shift of ⁇ along one arm of the device. The electrodes were spaced 15 ⁇ m apart, and were several microns above the waveguides.
  • the device was tested at 85° C, where sufficient light would propagate along the waveguides to enable the measurement. By varying the voltage applied to the system, it was determined that approximately 3000 V was required to cause a ⁇ phase change in the device. This large voltage is to be expected, in light of the results from Fig. 8, showing the small EO response at elevated temperatures for this material.
  • a C-PDLC film was prepared using the SIPS method.
  • the solution composition was approximately 11.58% of 5OCB, 5.47% OPI chromophore 119-96, in polymer Spike 3, jointly dissolved in dioxane.
  • the material was used to produce EO test cells and prisms for refractive index measurements, giving the following results.
  • the solution was initially prepared with lower concentration of the liquid crystal and chromophore. An aliquot was removed, applied to a prism, and the dried in a 70° C oven for approximately 1 hour.
  • the refractive index of the LC-containing film was then measured using standard methods (H. Onodera, I. Awai, and J.-l. Ikenoue, "Refractive index measurement of bulk materials: prism coupling method", Applied Optics, Vol. 22, 1194-1197, 1983). By measuring the refractive index at various temperatures, the curve displayed in Fig. 9 was prepared.
  • V peak-to-peak, AC signal and measuring the response of the system at 1 kHz using a lock-in amplifier.
  • Examination of the EO response at the various temperatures shows the EO has a large drift over time at 40° C, while at 45 ° C the response is much more stable. Similar behavior is evident in devices made with this material, as is shown in the next example.
  • EXAMPLE P Performance of a Mach-Zehnder Device with C-PDLC Cladding (Sea Bass 4)
  • the PDLC was initially formed using the SIPS method. After the solvent was removed, the film was heated to greater than 120° C to enable attachment of the top cover plate. During this process, the microstructure may have altered, due to a TlPS-like process.
  • the device was tested to determine the voltage required to create a phase shift of ⁇ along one arm of the device.
  • the electrodes were spaced 15 ⁇ m apart, and were several microns above the waveguides.
  • the device was tested at 40° C, where sufficient light would propagate along the waveguides to enable the measurement.
  • By varying the voltage applied to the system it was determined that approximately 30 V with an 80 V bias was required to cause a ⁇ phase change in the device.
  • the results from modulating the device with a sinusoidal signal are shown in Fig. 10. Note that the total power shows small ripples at both the maximum and minimum, indicating that the device has been slightly overdriven.
  • the device is giving approximately 4 dB extinction, which can be primarily attributed to the power being unbalanced between the two arms of the Mach-Zehnder.
  • the repeatability of the response is shown in Fig. 1 1 , which plots the output power as a function of the applied voltage.
  • the frequency of the square-wave must be large enough that charge buildup does not occur during a half-cycle of the voltage, when a constant voltage is applied.
  • the time to cycle between the positive and negative voltages must be short compared to the orientation relaxation time of the liquid crystal, so no reorientation occurs during the voltage transition.
  • the inclusion of the chromophore in the Sea Bass material adds a component that responds to the direction of the applied field. As this example will demonstrate, this inclusion does not appear to complicate the response of the device nor to prevent simple analysis of the performance.
  • Initial testing of Sea Bass 3 with the alternating square-wave voltage was performed at 40 ° C. Based on the EO results, the maximum voltage applied to the device was 200 V. Shown in Fig.
  • the alternating square-wave was initially driven at a frequency of 1 Hz (left of line 50), and then was driven at a frequency of 100 Hz (right of line 50) and turned on and off manually. Because of the sampling rate, the 100 Hz square wave appears to be a rippled constant voltage.
  • the performance of the device has been greatly enhanced by the use of the alternating square-wave. In particular, the response to voltage is much faster, and shows little drift (arrow 52). However, the device turn-off still shows a much longer response time than is desired (arrow 54). The solution to the turn-off time problem came from closer examination of the
  • a Mach-Zehnder device was constructed as is shown in Fig. 3, using a solution of 13.4% 5OCB and 11.6% HMDI in CP044 (153-087-22). The solution was coated only in the active region of the MZI chip. After the solvent was removed by vacuum drying at room temperature for 4.5 hours, the film was heated to 110 ° C for 2 minutes then 145C for 7 minutes to enable attachment of the top electrode cover plate and allow crosslinking of the polymer. The device was tested to determine the voltage required to create a phase shift of ⁇ along one arm of the device. The electrodes were spaced 15 ⁇ m apart, and were approximately 8 microns above the waveguides.
  • the device was tested at 45° C (Test 16), where sufficient light would propagate along the waveguides to enable the measurement.
  • Test 16 45° C
  • the voltage applied to the system it was determined that approximately 17 V with a 26V DC bias was required to cause a ⁇ phase change in the device.
  • the results from modulating the device with a sinusoidal signal are shown in Fig. 14. Note that the total power shows ripples at both the maximum and minimum voltage, indicating that the device has been slightly overdriven.
  • the device gives more than 24 dB extinction. This level of extinction can only be achieved by having almost perfectly balanced loss along both arms of the MZI. This device verifies that the application of voltage to change the phase along one arm is not altering the optical loss along that arm.
  • the fact that the MZI is able to be overdriven shows that the device is operating through phase change along one arm, not through a loss-based mechanism.
  • TFEMA trifluoroethyl methacrylate
  • TFPMA tetrafluoropropyl methacrylate
  • HEMA hydroxyethyl methacrylate
  • TMPTA trimethylolpropane triacrylate
  • PS photosensitizer DAROCURE 1173
  • EO electrooptic coefficient at 1550 nm, 1 kHz All % are by weight
  • G good mechanical film properties (hard, good adhesion to glass substrates)
  • NG not good mechanical film properties (soft, poor or no adhesion to glass substrate)
  • TFEMA trifluoroethyl methacrylate monomer
  • TFPMA tetrafluoropropyl methacrylate monomer
  • MMA methyl methacrylate monomer
  • HEMA hydroxyethyl methacrylate monomer
  • HFIPA hexafluoroisopropyl acrylate monomer
  • HFBMA hexafluorobutyl methacrylate monomer
  • the chromophore-containing liquid crystal materials of this invention also can be used to provide high-speed modulation of optical signals.
  • the chromophores are ordered by applying a large electric field while heating the chromophore/polymer composite to near or slightly above its glass transition temperature, and then cooling the material to lock in the alignment.
  • these devices commonly operate with a constant bias voltage of several to tens of volts per micron to prevent depoling over time.
  • C-PDLC materials can provide similar function.
  • PDLC materials While conventional PDLC materials are capable of modulating light at low frequencies, they have minimal response at the frequencies of interest for optical modulation (100 Hz-40GHz) because the rotation of the molecules does not occur rapidly enough, and the LC molecules typically possess small optical nonlinearities .
  • the chromophore used must incorporate itself into the LC domains, where it aligns with the director.
  • Application of the low voltage needed to orient the director in the droplet will also serve to align the chromophores, providing the anisotropic orientation needed to observe the optical nonlinearity of the chromophore.
  • a low refractive index C-PDLC material as the cladding over silica waveguides, it is possible to modulate the optical signal contained within the waveguides.
  • Photonic band gap composites consist of regularly arranged 3-dimensional arrays of particles or voids that prevent transmission of a specific wavelength(s) of light in specific directions. The wavelengths of the band gap are determined by the size and spacing of the array, and the refractive index mismatch between the two material comprising the array and the host matrix.
  • One or both components of the composite comprise an electrooptical material. By altering the refractive index of the electrooptic material, it is possible to tune the photonic band gap or alter the forbidden propagation direction.
  • EXAMPLE X Crosslinked LC-Polvmer System with Thermal Crosslinking CP044 with 13.25% 5OCB and 5.96% HMDI in dioxane (153-093-25) was applied to an electrooptic test cell, dried into a 20 ⁇ m thick film, covered with a quartz slide, and heated to 170C for approximately 5 minutes. This EO cell was then tested for EO response. EO values of 2.5-6.5 pm/V 2 were observed.
  • EXAMPLE Y Crosslinked LC-Polvmer System with Thermal Crosslinking Polymer CP044 with 13.48% 5OCB was mixed with 20.70% of a polymer which included an isocyanate side group (153-116-33).
  • the material in dioxane was applied to an electrooptic test cell, dried into a 20 ⁇ m thick film, covered with a quartz slide, and heated to 170C for approximately 5 minutes. This EO cell was then tested for EO response. Large EO values of 2.5-6.5 pm ⁇ / 2 were observed.
  • the liquid crystal materials of this invention can also be incorporated onto alternate optical substrates.
  • the examples given previously utilized the PDLC materials as the functional cladding on MZI devices fabricated from silica.
  • the materials of this invention can be utilized equally well as the functional cladding of optical devices made with other materials, such as sol-gel glasses, SiON, or polymers as examples.
  • the primary constraints are those stated previously, that the refractive index of the PDLC clad must be lower than that of the waveguide, and that the PDLC must have droplets small enough to allow good optical transmission while maintaining sufficiently large EO performance to allow for device operation.

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Abstract

Un aspect de l'invention concerne de manière générale une matière optique fonctionnelle se composant d'un cristal liquide qui présente deux indices de réfraction, et d'un polymère dans lequel le cristal liquide est dispersé. L'indice de réfraction du polymère peut s'écarter des indices de réfraction du cristal liquide d'au moins 0,03 environ. Un autre aspect de l'invention concerne une matière optique fonctionnelle se composant d'un cristal liquide et d'un polymère, dans lequel le cristal liquide est dispersé, ce cristal liquide étant miscible dans le polymère à raison de moins de 5 % environ. Un autre aspect de l'invention concerne une matière optique fonctionnelle se composant d'un cristal liquide et d'un polymère, dans lequel le cristal liquide est dispersé, la gaine contenant au plus 20 % en poids environ de cristal liquide. Dans toutes ces formes de réalisation, la matière optique fonctionnelle peut être appliquée comme gaine sur un guide d'ondes optique et contenir éventuellement un chromophore. Dans un autre aspect encore, l'invention concerne un guide d'ondes optique fonctionnel se composant d'un polymère à indice de réfraction RIP, et d'une gaine de guide d'ondes optique à indice de réfraction RIWG, RIP étant inférieur d'au moins 0,3 % environ à RIWG dans les conditions de fonctionnement du guide d'ondes optique gainé.
EP04795211A 2003-10-17 2004-10-14 Matieres de cristaux liquides et dispositifs electro-optiques comportant une gaine qui contient des cristaux liquides Withdrawn EP1673646A4 (fr)

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