DigiLens Patent | Nanoparticle-based holographic photopolymer materials and related applications

In various embodiments, the method further includes further ashing the exposed holographic material to form an inorganic grating structure made of the nanoparticles.

In still various other embodiments, the method further includes sintering the nanoparticles at a high temperature to remove grain boundaries between the nanoparticles.

In still various other embodiments, the method further includes coating an additional material onto the nanoparticles, where at least a portion of the additional material is positioned between adjacent nanoparticle rich regions.

In still various other embodiments, the method further includes depositing another holographic material on top of the additional material; and exposing the other holographic material with another holographic recording beam to create a top grating.

Further, various embodiments are directed to a waveguide device includes: a waveguide supporting an input grating and a fold grating, where the fold grating comprises alternating nanoparticle rich regions and nanoparticle poor regions, and where the input grating comprises alternating liquid crystal rich regions and liquid crystal poor regions.

In various other embodiments, the liquid crystal poor regions comprise air gaps.

In still various other embodiments, the nanoparticle poor regions comprise air gap regions on top of polymer matrix regions.

In still various other embodiments, the fold grating is an integrated multiplexed grating which functions as both a fold grating and an output grating.

In still various other embodiments, the alternating nanoparticle rich regions and nanoparticle poor regions include nanoparticles comprising a metal.

In still various other embodiments, the nanoparticles includes a metal oxide core.

In still various other embodiments, the metal oxide core comprises ZrO₂, TiO₂, WO₃, ZnO, Co₃O₄, CuO, and/or NiO.

In still various other embodiments, the nanoparticles further include a ligand functionalized derivative of ZrO₂, TiO₂, WO₃, ZnO, Co₃O₄, CuO, and/or NiO which surrounds the metal oxide core.

In still various other embodiments, the metal includes Pt, Au, and/or Ag.

In still various other embodiments, the nanoparticles are diameter less than 15 nm.

In still various other embodiments, the nanoparticles are diameter of about 4 nm to 10 nm.

In still various other embodiments, the alternating nanoparticle rich regions and nanoparticle poor regions include nanoparticles including a piezoelectric material.

In still various other embodiments, the piezoelectric material includes PZT, barium titanate, and/or lithium niobate.

Further, various embodiments are directed to a waveguide device including: a waveguide supporting a grating, wherein the grating comprises: nanoparticle rich regions and nanoparticle poor regions, wherein the nanoparticle poor regions comprise air gap regions on top of polymer matrix regions, wherein the air gap regions along with the nanoparticle rich regions on the same horizontal level make up a surface relief grating, and wherein the polymer matrix regions along with the nanoparticle rich regions on the same horizontal level make up a volume grating.

Further, various embodiments are directed to a waveguide device comprising: a waveguide supporting an inorganic grating, where the grating includes: nanoparticle rich regions, where nanoparticles in the nanoparticle rich regions are sintered at high temperature to remove grain boundaries between the nanoparticles; and air gaps between adjacent nanoparticle rich regions.

Further, various embodiments are directed to a waveguide device including: a waveguide supporting a multi-layered grating produced using the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention.

FIG. 1 conceptually illustrates a single-beam recording process utilizing a master grating in accordance with an embodiment of the invention.

FIGS. 2A and 2B conceptually illustrate HPDLC SBG devices and the switching property of SBGs in accordance with various embodiments of the invention.

FIG. 3 conceptually illustrates a waveguide display implementing integrated gratings in accordance with an embodiment of the invention.

FIG. 4 conceptually illustrates a waveguide with a grating layer having gratings formed from nanoparticle-based photopolymer materials in accordance with an embodiment of the invention.

FIG. 5 is a chart comparing diffraction efficiencies of various material formulations in accordance with various embodiments of the invention.

FIGS. 6A-6C conceptually illustrate general structures of components in the material formulations in accordance with various embodiments of the invention.

FIG. 7 conceptually illustrates a waveguide cell with defined grating areas containing different materials in accordance with an embodiment of the invention.

FIG. 8 is a flow chart conceptually illustrating a process for forming gratings using a holographic master grating in accordance with an embodiment of the invention.

FIGS. 9A-9E illustrates a process of forming a grating using holographic photopolymer materials including nanoparticles in accordance with an embodiment of the invention.

FIGS. 10A-10D illustrates a process forming a grating using holographic photopolymer materials including nanoparticles in accordance with an embodiment of the invention.

FIGS. 11A-11F illustrate a process of forming a grating using holographic photopolymer materials including nanoparticles and an inert liquid in accordance with an embodiment of the invention.

FIG. 12 illustrates a process for producing a grating in accordance with an embodiment of the invention.

FIGS. 13A-13C conceptually illustrate a process for manufacturing multi-layer gratings in accordance with an embodiment of the invention.

FIGS. 14A-14D conceptually illustrates a process for manufacturing multi-layer gratings in accordance with an embodiment of the invention.

FIGS. 15A-15G illustrates a process for manufacturing a multi-layer grating in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

For the purposes of describing embodiments, some well-known features of optical technology known to those skilled in the art of optical design and visual displays have been omitted or simplified in order to not obscure the basic principles of the invention. Unless otherwise stated, the term “on-axis” in relation to a ray or a beam direction refers to propagation parallel to an axis normal to the surfaces of the optical components described in relation to the invention. In the following description the terms light, ray, beam, and direction may be used interchangeably and in association with each other to indicate the direction of propagation of electromagnetic radiation along rectilinear trajectories. The term light and illumination may be used in relation to the visible and infrared bands of the electromagnetic spectrum. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optical design. As used herein, the term grating may encompass a grating comprised of a set of gratings in some embodiments. For illustrative purposes, it is to be understood that the drawings are not drawn to scale unless stated otherwise.

Holographic materials for use in the formation of waveguides, gratings, and other related applications in accordance with various embodiments of the invention can include a variety of different mixtures and formulations. In conventional holographic waveguide applications, the volume gratings within the waveguides are typically formed using holographic polymer dispersed liquid crystal (HPDLC) materials. Although convenient for many applications, such materials and the gratings formed from such materials have drawbacks that can be critical in certain cases. In addition to typical color non-uniformity and brightness concerns, HPDLC gratings may include the appearance of certain “defects” in the displayed images in some waveguide applications. Such defects can include dimmed patterns, such as but not limited to non-uniformity in the center or corner of the displayed images, and periodic non-uniformities that appear as a series of linear ridges, or striations. These defects may be rooted from the fundamental structure of the HPDLC gratings and can be difficult to address. For example, in typical waveguide applications implementing exit pupil expansion, a cause of striation defects in the displayed images may be related to the anisotropic nature of the HPDLC gratings. With each interaction of a given HPDLC grating, the polarization of the light may be slightly rotated. Given the polarization-sensitive diffractive nature of these gratings, light that continues to propagate within the waveguide and interacts with the grating again may be diffracted in an unexpected manner due to the change in its polarization state. For instance, many HPDLC material systems form gratings that have a high P-polarization response and a low S-polarization response. Unless otherwise stated, polarization response are described with respect to the plane of incidence. Waveguide display systems utilizing these gratings are often configured to utilize P-polarized input light. However, the polarization of the light “rotates” upon interaction with the grating. Light that interacts with the grating a second time, as is the case in grating architectures for implementing exit pupil expansion, may have a different diffraction efficiency profile—e.g., the light may have a lower P component upon the second interaction and may diffract less since the grating may have a high P-polarization response. This process may continue and the changes in the polarization may produce a cyclic diffraction efficiency profile that may manifest as striations and other defects.

The defects described above with regards to HPDLC gratings may often be unacceptable for commercial adoption. Furthermore, these defects may be more conspicuous in waveguide displays utilizing laser optical engines due to the coherence nature of laser illumination. As such, many embodiments of the invention are directed towards material systems capable of forming gratings with higher uniformity and less appearances of the defects described above. In many embodiments, the defects described above are eliminated as the gratings formed from such material systems may not affect the polarization of incident light. In some embodiments, the material system employed may be a nanoparticle-based photopolymer mixture. In some embodiments, the material system employed may be a mixture including at least one type of monomer and at least one type of nanoparticle for forming a photopolymer and nanoparticle material system after holographic exposure. In some embodiments, the starting mixture may further include at least one type of liquid crystal. In some embodiments, the role of the liquid crystal may not be to be removed from the grating after holographic exposure as is the case in EBGs but rather to improve refractive index modulation diffraction efficiency and electrooptical characteristics of the gratings in association with the nanoparticle component.

The mixture can be utilized in a holographic exposure process employing phase separation to form volume gratings. In several embodiments, the material mixture may be formulated to form volume gratings having high modulations of refractive indices. To address some of the drawbacks of gratings formed from HPDLC materials, photopolymer mixtures in accordance with various embodiments of the invention may include materials for forming isotropic gratings—e.g., gratings having no orientational or nematic order or gratings with random or isotropic domains—formed from alternating sections of polymers and nanoparticles. The type of nanoparticle chosen for use in the material system may be critical. For example, the material system can include various components such as but not limited to monomers, dyes, and coinitiators. The type of nanoparticle selected for use with such systems should advantageously have a low reactivity with the components within the systems. In various embodiments, the nanoparticles may have a high refractive index, which can be at least 1.7 in many cases. In several embodiments, the nanoparticles have high transmittance. For example, many types of nanoparticles utilized have transmittance values of at least 95%. In a number of embodiments, the type of nanoparticle selected has low absorption, allowing for implementation of an efficient waveguide display system. Various types of nanoparticles can be utilized. In many embodiments, the nanoparticles include inorganic core structures, such as but not limited to Au, Ag, Zr, Ti, Zn, and Cd cores. Functionalized and non-functionalized nanoparticles can be used as appropriate depending on the application. In some embodiments, the core structures may have surfaces modified with organic ligands. Such nanoparticles, which can also be referred to as nonmetallic nanoparticles, can be utilized to provide low absorption values. In other embodiments, metallic nanoparticles can also be utilized. Grating architectures, HPDLC material systems, nanoparticle-based material systems, methods for forming gratings using such material systems, and related applications are discussed in the sections below in further detail.

Optical Waveguide and Grating Structures

Optical structures recorded in waveguides can include many different types of optical elements, such as but not limited to diffraction gratings. Gratings can be implemented to perform various optical functions, including but not limited to coupling light, directing light, and preventing the transmission of light. The gratings can be surface relief gratings that reside on the outer surface of the waveguide. In other cases, the grating implemented can be a Bragg grating (also referred to as a volume grating), which are structures having a periodic refractive index modulation. Bragg gratings can be fabricated using a variety of different methods. One process includes interferential exposure of holographic photopolymer materials to form periodic structures. Bragg gratings can have high efficiency with little light being diffracted into higher orders. The relative amount of light in the diffracted and zero order can be varied by controlling the refractive index modulation of the grating, a property that can be used to make lossy waveguide gratings for extracting light over a large pupil.

A single-beam recording process utilizing a master grating in accordance with an embodiment of the invention is conceptually illustrated in FIG. 1. As shown, a beam 100 from a single laser source (not shown) may be directed through an master grating 101. Upon interaction with the master grating 101, the beam 100 can diffract as, for example, in the case of the rays interacting with the black shaded region of the master grating 101, or the beam 100 can propagated through the master grating 101 without substantial deviation as a zero-order beam as, for example, in the case of the rays interacting with the cross-hatched region of the master grating 101. The first order diffraction beams 102 and the zero order beams 103 can overlap to create an interference pattern that exposes the optical recording layer 104 of a waveguide cell. In some embodiments, a spacer block 105 may be positioned between the grating 101 and the optical recording layer 104 in order to alter the distance between the two components.

One class of Bragg gratings used in holographic waveguide devices is the Switchable Bragg Grating (SBG). SBGs can be fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between substrates. The substrates can be made of various types of materials, such glass and plastics. In many cases, the substrates are in a parallel configuration. The substrates can also form a wedge shape. One or both substrates can support electrodes, typically transparent tin oxide films, for applying an electric field across the film. The grating structure in an SBG can be recorded in the liquid material (often referred to as the syrup) through photopolymerization-induced phase separation using interferential exposure with a spatially periodic intensity modulation. Factors such as but not limited to control of the irradiation intensity, exposure time, component volume fractions of the materials in the mixture, and exposure temperature can determine the resulting grating morphology and performance. As can readily be appreciated, a wide variety of materials and mixtures can be used depending on the specific requirements of a given application. In many cases, HPDLC material may be used to fabricate SBGs. During the recording process, the monomers polymerize, and the mixture undergoes a phase separation. The LC molecules aggregate to form discrete or coalesced droplets that are periodically distributed in polymer networks on the scale of optical wavelengths. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating, which can produce Bragg diffraction with a strong optical polarization resulting from the orientation ordering of the LC molecules in the droplets.

The resulting volume phase grating can exhibit very high diffraction efficiency, which can be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets can change, causing the refractive index modulation of the fringes to lower and the hologram diffraction efficiency to drop to low levels. Typically, the electrodes are configured such that the applied electric field may be perpendicular to the substrates. The electrodes may be fabricated from indium tin oxide (ITO) or other transparent conductive oxides (TCO). In some cases, index-matched ITO (IMITO) is used. In the OFF state with no electric field applied, the extraordinary axis of the liquid crystals generally aligns normal to the fringes. The grating thus exhibits high refractive index modulation and high diffraction efficiency for P-polarized light. When an electric field is applied to the HPDLC, the grating switches to the ON state causing the extraordinary axes of the liquid crystal molecules to align parallel to the applied field and hence perpendicular to the substrate. In the ON state, the grating exhibits lower refractive index modulation and lower diffraction efficiency for both S- and P-polarized light. Thus, the grating region no longer diffracts light. Each grating region can be divided into a multiplicity of grating elements such as for example a pixel matrix according to the function of the HPDLC device. Typically, the electrode on one substrate surface is uniform and continuous, while electrodes on the opposing substrate surface are patterned in accordance to the multiplicity of selectively switchable grating elements.

Typically, the SBG elements are switched clear in 100 μs with a longer relaxation time to switch ON. The diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. In many cases, the device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices, magnetic fields can be used to control the LC orientation. In some HPDLC applications, phase separation of the LC material from the polymer can be accomplished to such a degree that no discernible droplet structure results. An SBG can also be used as a passive grating. In this mode, its chief benefit is a uniquely high refractive index modulation. SBGs can be used to provide transmission or reflection gratings for free space applications. SBGs can be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. The substrates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light can be coupled out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition.

FIGS. 2A and 2B conceptually illustrate HPDLC SBG devices 200, 250 and the switching property of SBGs in accordance with an embodiment of the invention. In FIG. 2A, the SBG 200 is in an OFF state. As shown, the LC molecules 201 are aligned substantially normal to the fringe planes. As such, the SBG 200 exhibits high diffraction efficiency, and incident light can easily be diffracted. FIG. 2B illustrates the SBG 250 in an ON position. An applied voltage 251 can orient the optical axis of the LC molecules 252 within the droplets 253 to produce an effective refractive index that matches the polymer's refractive index, essentially creating a transparent cell where incident light is not diffracted. In the illustrative embodiment, an AC voltage source is shown. As can readily be appreciated, various voltage sources can be utilized depending on the specific requirements of a given application.

In some applications, LC can be extracted or evacuated from the SBG to provide an evacuated Bragg grating (EBG). EBGs can be characterized as a surface relief grating (SRG) that has properties very similar to a Bragg grating due to the depth of the

SRG structure (which may be much greater than that practically achievable using surface etching and other conventional processes commonly used to fabricate SRGs). Examples of EBGs are described in U.S. Pat. Pub. No. 2021/0063634 filed on Aug. 28, 2020 and entitled “Evacuating bragg gratings and methods of manufacturing” which is hereby incorporated by reference in its entirety. The LC can be extracted using a variety of different methods, including but not limited to flushing with solvents such as isopropyl alcohol. In many cases, one of the transparent substrates of the SBG may be removed, and the LC is extracted. The removed substrate can be replaced. The SRG can be at least partially backfilled with a material of higher or lower refractive index. Such gratings offer scope for tailoring the efficiency, angular/spectral response, polarization, and other properties to suit various waveguide applications.

Waveguide Cells

A waveguide cell can be defined as a device containing uncured and/or unexposed optical recording material in which optical elements, such as but not limited to gratings, can be recorded. In many embodiments, optical elements can be recorded in the waveguide cell by exposing the optical recording material to certain wavelengths of electromagnetic radiation. A waveguide cell may be constructed such that the optical recording material may be sandwiched between two substrates, creating a three-layer waveguide cell. Depending on the application, waveguide cells can be constructed in a variety of configurations. In some embodiments, the waveguide cell may be constructed by vacuum filling an empty waveguide cell made of two substrates. Other filling methods can also be used. In several embodiments, the waveguide cell may be constructed by depositing the optical recording material onto one substrate and laminating the composite along with a second substrate to form a three-layer laminate. Various deposition techniques, such as but not limited to spin-coating and inkjet printing, can be used. In some embodiments, the waveguide cell may contain more than three layers. In a number of embodiments, the waveguide cell contains different types of layers that can serve various purposes. For example, waveguide cells can include protective cover layers, polarization control layers, and alignment layers.

Substrates of varying materials and shapes can be used in the construction of waveguide cells. In many embodiments, the substrates are plates made of a transparent material, such as but not limited to glass and plastics. Substrates of different shapes, such as but not limited to rectangular and curvilinear shapes, can be used depending on the application. The thicknesses of the substrates can also vary depending on the application. Oftentimes, the shapes of the substrates can determine the overall shape of the waveguide. In a number of embodiments, the waveguide cell contains two substrates that are of the same shape. In other embodiments, the substrates are of different shapes. As can readily be appreciated, the shapes, dimensions, and materials of the substrates can vary and can depend on the specific requirements of a given application.

In many embodiments, beads, or other particles, are dispersed throughout the optical recording material to help control the thickness of the layer of optical recording material and to help prevent the two substrates from collapsing onto one another. In some embodiments, the waveguide cell is constructed with an optical recording layer sandwiched between two planar substrates. Depending on the type of optical recording material used, thickness control can be difficult to achieve due to the viscosity of some optical recording materials and the lack of a bounding perimeter for the optical recording layer. In a number of embodiments, the beads are relatively incompressible solids, which can allow for the construction of waveguide cells with consistent thicknesses. The size of a bead can determine a localized minimum thickness for the area around the individual bead. As such, the dimensions of the beads can be selected to help attain the desired optical recording layer thickness. The beads can be made of any of a variety of materials, including but not limited to glass and plastics. In several embodiments, the material of the beads is selected such that its refractive index does not substantially affect the propagation of light within the waveguide cell.

In some embodiments, the waveguide cell may be constructed such that the two substrates are parallel or substantially parallel. In such embodiments, relatively similar sized beads can be dispersed throughout the optical recording material to help attain a uniform thickness throughout the layer. In other embodiments, the waveguide cell has a tapered profile. A tapered waveguide cell can be constructed by dispersing beads of different sizes across the optical recording material. As discussed above, the size of a bead can determine the local minimum thickness of the optical recording material layer. By dispersing the beads in a pattern of increasing size across the material layer, a tapered layer of optical recording material can be formed when the material is sandwiched between two substrates.

Modulation of Material Composition

High luminance and excellent color fidelity may be beneficial factors in AR waveguide displays. In each case, high uniformity across the FOV can be beneficial. However, the fundamental optics of waveguides can lead to non-uniformities due to gaps or overlaps of beams bouncing down the waveguide. Further non-uniformities may arise from imperfections in the gratings and non-planarity of the waveguide substrates. In SBGs, there can exist a further issue of polarization rotation by birefringent gratings. In applicable cases, one challenge may be in fabrication of fold gratings where there may be millions of light paths resulting from multiple intersections of the beam with the grating fringes. Careful management of grating properties, particularly the refractive index modulation, can be utilized to overcome non-uniformity.

Out of the multitude of possible beam interactions (diffraction or zero order transmission), only a subset contributes to the signal presented at the eye box. By reverse tracing from the eyebox, fold regions contributing to a given field point can be pinpointed. The precise correction to the modulation that may send more into the dark regions of the output illumination can then be calculated. In some embodiments, correction to the modulation may be performed by adding a material of a certain refractive index/composition and coating depth to the spatial resolution cell contributing to the given field point. The type of material deposited can differ from the type of material used in the formation of other spatial resolution cells. Having brought the output illumination uniformity for one color back on target, the procedure can be repeated for other colors. Once the index modulation pattern has been established, the design can be exported to the deposition mechanism, with each target index modulation translating to a unique deposition setting for each spatial resolution cell on the substrate to be coated/deposited. The resolution of the deposition mechanism can depend on the technical limitations of the system utilized. In many embodiments, the spatial pattern can be implemented to 30 micrometers resolution with full repeatability.

Compared with waveguides utilizing surface relief gratings (SRGs), SBG waveguides implementing manufacturing techniques in accordance with various embodiments of the invention can allow for the grating design parameters that impact efficiency and uniformity, such as but not limited to refractive index modulation and grating thickness, to be adjusted dynamically during the deposition process without using a different master. With SRGs where modulation may be controlled by etch depth, such schemes would not be practical as each variation of the grating would entail repeating the complex and expensive tooling process. Additionally, achieving the desired etch depth precision and resist imaging complexity can be very difficult.

Deposition processes in accordance with various embodiments of the invention can provide for the adjustment of grating design parameters by controlling the type of material that is to be deposited. Various embodiments of the invention can be configured to deposit different materials, or different material compositions, in different areas on the substrate. For example, deposition processes can be configured to deposit HPDLC material onto an area of a substrate that is meant to be a grating region and to deposit monomer onto an area of the substrate that is meant to be a non-grating region. In some embodiments, the grating regions can be coated with a mixture of monomers, nanoparticles, and/or LC components. In several embodiments, the deposition process is configured to deposit a layer of optical recording material that varies spatially in component composition, allowing for the modulation of various aspects of the deposited material. The deposition of material with different compositions can be implemented in several different ways. In many embodiments, more than one deposition head can be utilized to deposit different materials and mixtures. Each deposition head can be coupled to a different material/mixture reservoir. Such implementations can be used for a variety of applications. For example, different materials can be deposited for grating and non-grating areas of a waveguide cell. In some embodiments, HPDLC material is deposited onto the grating regions while only monomer is deposited onto the non-grating regions. In several embodiments, the deposition mechanism can be configured to deposit mixtures with different component compositions.

In some embodiments, spraying nozzles can be implemented to deposit multiple types of materials onto a single substrate. In waveguide applications, the spraying nozzles can be used to deposit different materials for grating and non-grating areas of the waveguide. In many embodiments, the spraying mechanism is configured for printing gratings in which at least one of the material composition, birefringence, and/or thickness can be controlled using a deposition apparatus having at least two selectable spray heads. In some embodiments, the manufacturing system provides an apparatus for depositing grating recording material optimized for the control of laser banding. In several embodiments, the manufacturing system provides an apparatus for depositing grating recording material optimized for the control of polarization non-uniformity. In several embodiments, the manufacturing system provides an apparatus for depositing grating recording material optimized for the control of polarization non-uniformity in association with an alignment control layer. In a number of embodiments, the deposition workcell can be configured for the deposition of additional layers such as beam splitting coatings and environmental protection layers. Inkjet print heads can also be implemented to print different materials in different regions of the substrate.

As discussed above, deposition processes can be configured to deposit optical recording material that varies spatially in component composition. Modulation of material composition can be implemented in many different ways. In a number of embodiments, an inkjet print head can be configured to modulate material composition by utilizing the various inkjet nozzles within the print head. By altering the composition on a “dot-by-dot” basis, the layer of optical recording material can be deposited such that it has a varying composition across the planar surface of the layer. Such a system can be implemented using a variety of apparatuses including but not limited to inkjet print heads. Similar to how color systems use a palette of only a few colors to produce a spectrum of millions of discrete color values, such as the CMYK system in printers or the additive RGB system in display applications, inkjet print heads in accordance with various embodiments of the invention can be configured to print optical recording materials with varying compositions using only a few reservoirs of different materials. Different types of inkjet print heads can have different precision levels and can print with different resolutions. In many embodiments, a 300 DPI (“dots per inch”) inkjet print head is utilized. Depending on the precision level, discretization of varying compositions of a given number of materials can be determined across a given area. For example, given two types of materials to be printed and an inkjet print head with a precision level of 300 DPI, there are 90,001 possible discrete values of composition ratios of the two types of materials across a square inch for a given volume of printed material if each dot location can contain either one of the two types of materials. In some embodiments, each dot location can contain either one of the two types of materials or both materials. In several embodiments, more than one inkjet print head is configured to print a layer of optical recording material with a spatially varying composition. Although the printing of dots in a two-material application is essentially a binary system, averaging the printed dots across an area can allow for discretization of a sliding scale of ratios of the two materials to be printed. For example, the amount of discrete levels of possible concentrations/ratios across a unit square is given by how many dot locations can be printed within the unit square. As such, there can be a range of different concentration combinations, ranging from 100% of the first material to 100% of the second material. As can readily be appreciated, the concepts are applicable to real units and can be determined by the precision level of the inkjet print head. Although specific examples of modulating the material composition of the printed layer are discussed, the concept of modulating material composition using inkjet print heads can be expanded to use more than two different material reservoirs and can vary in precision levels, which largely depends on the types of print heads used.

Varying the composition of the material printed can be advantageous for several reasons. For example, in many embodiments, varying the composition of the material during deposition can allow for the formation of a waveguide with gratings that have spatially varying diffraction efficiencies across different areas of the gratings. In embodiments utilizing HPDLC mixtures, this can be achieved by modulating the relative concentration of liquid crystals in the HPDLC mixture during the printing process, which creates compositions that can produce gratings with varying diffraction efficiencies when the material is exposed. In several embodiments, a first HPDLC mixture with a certain concentration of liquid crystals and a second HPDLC mixture that is liquid crystal-free are used as the printing palette in an inkjet print head for modulating the diffraction efficiencies of gratings that can be formed in the printed material. In such embodiments, discretization can be determined based on the precision of the inkjet print head. A discrete level can be given by the concentration/ratio of the materials printed across a certain area. In this example, the discrete levels range from no liquid crystal to the maximum concentration of liquid crystals in the first PDLC mixture.

In some embodiments, the HPDLC mixture may include nanoparticles. The LC in the HPDLC mixture can remain within the final grating after holographic exposure as opposed to removed from the grating after holographic exposure as is the case for EBGs. Combining the properties of LCs and nanoparticles can be advantageous in various grating applications. Implementations including LCs and nanoparticles may be found in LC Hiroyuki Yoshida et al “Nanoparticle-Dispersed Liquid Crystals Fabricated by Sputter Doping” Adv. Mater. 2010, 22, 622-626 which is hereby incorporated by reference in its entirety. In some embodiments, nanoparticles can affect LC molecule orientation. The nanoparticles can be dispersed within the LC. Suitable nanoparticles for use with LCs may include metals, insulators, carbon nanotubes, and/or ferroelectric materials. As well as enabling high diffraction efficiency, nanoparticles can be used to control switching times and switching voltages. Capping agents can be used to promote solubility of nanoparticles within the host LC. Nanoparticles can be sputtered onto the host LC which may be applied using vacuum based processes.

The ability to vary the diffraction efficiency across a waveguide can be used for various purposes. A waveguide is typically designed to guide light internally by reflecting the light many times between the two planar surfaces of the waveguide. These multiple reflections can allow for the light path to interact with a grating multiple times. In many embodiments, a layer of material can be printed with varying composition of materials such that the gratings formed have spatially varying diffraction efficiencies to compensate for the loss of light during interactions with the gratings to allow for a uniform output intensity. For example, in some waveguide applications, an output grating is configured to provide exit pupil expansion in one direction while also coupling light out of the waveguide. The output grating can be designed such that when light within the waveguide interact with the grating, only a percentage of the light is refracted out of the waveguide. The remaining portion continues in the same light path, which remains within TIR and continues to be reflected within the waveguide. Upon a second interaction with the same output grating again, another portion of light is refracted out of the waveguide. During each refraction, the amount of light still traveling within the waveguide decreases by the amount refracted out of the waveguide. As such, the portions refracted at each interaction gradually decreases in terms of total intensity. By varying the diffraction efficiency of the grating such that it increases with propagation distance, the decrease in output intensity along each interaction can be compensated, allowing for a uniform output intensity.

Varying the diffraction efficiency can also be used to compensate for other attenuation of light within a waveguide. All objects have a degree of reflection and absorption. Light trapped in TIR within a waveguide are continually reflected between the two surfaces of the waveguide. Depending on the material that makes up the surfaces, portions of light can be absorbed by the material during each interaction. In many cases, this attenuation is small, but can be substantial across a large area where many reflections occur. In many embodiments, a waveguide cell can be printed with varying compositions such that the gratings formed from the optical recording material layer have varying diffraction efficiencies to compensate for the absorption of light from the substrates. Depending on the substrates, certain wavelengths can be more prone to absorption by the substrates. In a multi-layered waveguide design, each layer can be designed to couple in a certain range of wavelengths of light. Accordingly, the light coupled by these individual layers can be absorbed in different amounts by the substrates of the layers. For example, in a number of embodiments, the waveguide is made of a three-layered stack to implement a full color display, where each layer is designed for one of red, green, and blue. In such embodiments, gratings within each of the waveguide layers can be formed to have varying diffraction efficiencies to perform color balance optimization by compensating for color imbalance due to loss of transmission of certain wavelengths of light.

In addition to varying the liquid crystal concentration within the material in order to vary the diffraction efficiency, another technique includes varying the thickness of the waveguide cell. This can be accomplished through the use of spacers. In many embodiments, spacers are dispersed throughout the optical recording material for structural support during the construction of the waveguide cell. In some embodiments, different sizes of spacers are dispersed throughout the optical recording material. The spacers can be dispersed in ascending order of sizes across one direction of the layer of optical recording material. When the waveguide cell is constructed through lamination, the substrates sandwich the optical recording material and, with structural support from the varying sizes of spacers, create a wedge-shaped layer of optical recording material. spacers of varying sizes can be dispersed similar to the modulation process described above. Additionally, modulating spacer sizes can be combined with modulation of material compositions. In several embodiments, reservoirs of HPDLC materials each suspended with spacers of different sizes are used to print a layer of HPDLC material with spacers of varying sizes strategically dispersed to form a wedge-shaped waveguide cell. In a number of embodiments, spacer size modulation is combined with material composition modulation by providing a number of reservoirs equal to the product of the number of different sizes of spacers and the number of different materials used. For example, in one embodiment, the inkjet print head is configured to print varying concentrations of liquid crystal with two different spacer sizes. In such an embodiment, four reservoirs can be prepared: a liquid crystal-free mixture suspension with spacers of a first size, a liquid crystal-free mixture-suspension with spacers of a second size, a liquid crystal-rich mixture-suspension with spacers of a first size, and a liquid crystal-rich mixture-suspension with spacers of a second size. Further discussion regarding material modulation can be found in U.S. application Ser. No. 16/203,071 filed Nov. 18, 2018 entitled “SYSTEMS AND METHODS FOR MANUFACTURING WAVEGUIDE CELLS.” The disclosure of U.S. application Ser. No. 16/203,491 is hereby incorporated by reference in its entirety for all purposes.

Waveguides Incorporating Integrated Gratings

Waveguides in accordance with various embodiments of the invention can include different grating configurations. In many embodiments, the waveguide includes at least one input coupler and at least two integrated gratings. In some embodiments, at least two integrated gratings can be implemented to work in combination to provide beam expansion and beam extraction for light coupled into the waveguide by the input coupler. Multiple integrated gratings can be implemented by overlapping integrated gratings across different grating layers or by multiplexing the integrated gratings. In a number of embodiments, the integrated gratings are partially overlapped or multiplexed. Multiplexed gratings can include the superimposition of at least two gratings having different grating prescriptions within the same volume. Gratings having different grating prescriptions can have different grating vectors (grating K-vector) and grating slant angle with respect to the waveguide's surface. The magnitude of the grating vector of a grating can be defined as the inverse of the grating period while its direction can be defined as the direction orthogonal to the fringes of the grating.

In several embodiments, an integrated grating can be implemented to perform both beam expansion and beam extraction. An integrated grating can be implemented with one or more grating prescriptions. In a number of embodiments, the integrated grating is implemented with at least two grating prescriptions. In further embodiments, the integrated grating is implemented with at least three grating prescriptions. In many embodiments, two grating prescriptions within the integrated grating have similar clock angles. In some embodiments, the two grating prescriptions have different slant angles. An integrated grating in accordance with various embodiments of the invention can be implemented using a variety of types of gratings, such as but not limited to SRGs, SBGs, holographic gratings, and other types of gratings including those described in the sections above. In a number of embodiments, the integrated grating includes two surface relief gratings. In other embodiments, the integrated grating includes two holographically-recorded gratings.

The integrated grating can include at least two grating prescriptions that are implemented in separate least partially overlapped layers or multiplexed into one layer. In further embodiments, the integrated grating includes at least two grating prescriptions that are fully overlapped or multiplexed. In a number of embodiments, the integrated grating includes multiplexed or overlapping gratings that have different sizes and/or shapes—i.e., one grating may be larger than the other, resulting in only partial multiplexing of the larger grating. As can readily be appreciated, various multiplexed and overlapping configurations may be implemented as appropriate depending on the specific requirements of a given application. In some embodiments, a given ray may enter the waveguide along its waveguide path encountering regions containing a grating integrated according to any of the above configurations or no grating at all. Although the discussions below may describe configurations as implementing multiplexed or overlapping gratings, such gratings can be substituted for one another as appropriate depending on the application. In several embodiments, the integrated gratings are implemented by a combination of both multiplexed and overlapping gratings. For example, two or more sets of multiplexed gratings can be overlapped across two or more grating layers.

Integrated gratings in accordance with various embodiments of the invention can be utilized for various purposes including but not limited to implementing full color waveguides and addressing some key problems in conventional waveguide architectures. Other advantages include reduced material and waveguide refractive index requirements and reduced waveguide dimensions resulting from the overlapping and/or multiplexing nature of the integrated gratings. Such configurations can allow for large field-of-view waveguides, which would ordinarily incur unacceptable increases in waveguide form factor and refractive index requirements. In many embodiments, a waveguide is implemented with at least one substrate having a low refractive index. In some embodiments, the waveguide is implemented with a substrate having a refractive index of lower than 1.8. In further embodiments, the waveguide is implemented with a substrate having a refractive index of not more than ˜1.5.

Integrated gratings that can provide beam expansion and beam extraction—i.e., the functions of conventional fold and output gratings—can result in a much smaller grating area, enabling a small form factor and lower fabrication cost. By integrating the functions of beam expansion and extraction, instead of performing them serially as in traditional waveguides, beam expansion and extraction can be accomplished with ˜50% of the grating interactions normally required, cutting down haze in the same proportion in the case of birefringent gratings. A further advantage is that, as a result of the greatly shortened light paths, the number of beam bounces at glass/air interface(s) is reduced, rendering the output image less sensitive to substrate nonuniformities. This can enable higher quality images and the potential to use less expensive, lower specification substrates.

In many embodiments, the grating vectors of the input coupler and integrated gratings are arranged to provide a substantially zero resultant vector. The grating vectors of the input coupler and integrated gratings can be arranged to form a triangular configuration. In several embodiments, the grating vectors can be arranged in an equilateral triangular configuration. In some embodiments, the grating vectors can be arranged in an isosceles triangular configuration where at least two grating vectors have equal magnitudes. In further embodiments, the grating vectors are arranged in an isosceles right triangular configuration. In a number of embodiments, the grating vectors are arranged in a scalene triangular configuration. Another waveguide architecture includes integrated diffractive elements with grating vectors aligned in the same direction for providing horizontal expansion for one set of angles and extraction for a separate set of angles. In several embodiments, one or more of the integrated gratings are asymmetrical in their general shape. In some embodiments, one or more of the integrated gratings has at least one axis of symmetry in their general shape. In a number of embodiments, the gratings are designed to sandwich an electro-active material, enabling switching between clear and diffracting states for certain types of gratings such as but not limited to HPDLC gratings. The gratings can be a surface relief or a holographic type.

In many embodiments, a waveguide supporting at least one input coupler and first and second integrated gratings is implemented. The grating structures can be implemented in single- or multi-layered waveguide designs. In single-layered designs, the integrated gratings can be multiplexed. In embodiments where each integrated grating contains at least two multiplexed gratings, the multiplexed integrated gratings can contain at least four multiplexed gratings. As described above, any individual multiplexed grating can be partially or completely multiplexed with the other gratings. In some embodiments, a multi-layered waveguide is implemented with overlapping integrated gratings. In further embodiments, the integrated gratings are partially overlapped. Each of the integrated gratings can be a separate grating or multiplexed gratings.

In many embodiments, the waveguide architecture is designed to couple the input light into two bifurcated paths using an input coupler. Such configurations can be implemented in various ways. In some embodiments, a multiplexed input grating is implemented to couple input light into two bifurcated paths. In other embodiments, two input gratings are implemented to separately couple input light into two bifurcated paths. The two input gratings can be implemented in the same layer or separately in two layers. In a number of embodiments, two overlapping or partially overlapping input gratings are implemented to couple input light into two bifurcated paths. In many embodiments, the input coupler includes a prism. In further embodiments, the input coupler includes a prism and any of the input grating configuration described above.

In addition to various input coupler architectures, the first and second integrated gratings can be implemented in a variety of configurations. Integrated gratings in accordance with various embodiments of the invention can be incorporated into waveguides to perform the dual function of two-dimensional beam expansion and beam extraction. In several embodiments, the first and second integrated gratings are crossed gratings. As described above, some waveguide architectures include designs in which input light is coupled into two bifurcated paths. In such designs, the two bifurcated paths are each directed towards a different integrated grating. As can readily be appreciated, such configurations can be designed to bifurcate the input light based on various light characteristics, including but not limited to angular and spectral bandwidths. In some embodiments, light can be bifurcated based on polarization states—e.g., input unpolarized light can be bifurcated into S and P polarization paths. In many embodiments, each of the integrated gratings performs either beam expansion in a first direction or beam expansion in a second direction different from the first direction according to the field-of-view portion being propagated through the waveguide. The first and second directions can be orthogonal to one another. In other embodiments, the first and second directions are not orthogonal to one another. Each integrated grating can provide expansion of the light in a first dimension while directing the light towards the other integrated grating, which provides expansion of the light in a second dimension and extraction. For example, many grating architectures in accordance with various embodiments of the invention include an input configuration for bifurcating input light into first and second portions of light. A first integrated grating can be configured to provide beam expansion in a first direction for the first and second portions of light and to provide beam extraction for the second portion of light. Conversely, the second integrated grating can be configured to provide beam expansion in a second direction for the first and second portions of light and to provide beam extraction for the first portion of light.

In a number of embodiments, the first integrated grating includes multiplexed first and second grating prescriptions, and the second integrated grating includes multiplexed third and fourth grating prescriptions. In such embodiments, the first grating prescription can be configured to provide beam expansion in a first direction for the first portion of light and to redirect the expanded light towards the fourth grating prescription. The second grating prescription can be configured to provide beam expansion in the first direction for the second portion of light and to extract the light out of the waveguide. The third grating prescription can be configured to provide beam expansion in a second direction for the second portion of light and to redirect the expanded light towards the second grating prescription. The fourth grating prescription can be configured to provide beam expansion in the second direction for the first portion of light and to extract the light out of the waveguide. As can readily be appreciated, the integrated gratings can be implemented with overlapping grating prescriptions instead of multiplexed grating prescriptions. In many embodiments, the first and second grating prescriptions have the same clock angle but different grating slants. In some embodiments, the third and fourth grating prescriptions have the same clock angle, which is different from the clock angles of the first and second grating prescriptions. In a number of embodiments, the first, second, third, and fourth grating prescriptions all have different clock angles. In several embodiments, the first, second, third, and fourth grating prescriptions all have different grating periods. In a number of embodiments, the first and third grating prescriptions have the same grating period, and the second and fourth grating prescriptions have the same grating period.

FIG. 3 conceptually illustrates a waveguide display implementing integrated gratings in accordance with an embodiment of the invention. As shown, the apparatus 300 includes a waveguide 301 supporting an input grating 302 and a grating structure 303. Each grating can be characterized by a grating vector defining the orientation of the grating fringes in the plane of the waveguide. A grating can also be characterized by a K-vector in 3D space, which in the case of a Bragg grating is defined as the vector normal to the grating fringes. The waveguide reflecting surfaces are parallel to the XY plane of the Cartesian reference frame inset into the drawing. In some embodiments, the X and Y axes can correspond to global horizontal and vertical axes in the reference frame of a user of the display.

In the illustrative embodiment of FIG. 3, the input grating 302 includes a Bragg grating 304. In other embodiments, the input grating 302 is a surface relief grating. The input grating 302 can be implemented to bifurcate input light into two different portions. In further embodiments, the input grating 302 includes two multiplexed gratings having different grating prescriptions. In other embodiments, the input grating 302 includes two overlaid surface relief gratings. The grating structure 303 includes two effective gratings 305, 306 that have different grating vectors. The gratings 305, 306 can be integrated gratings implemented as surface relief gratings or volume gratings. In many embodiments, the gratings 305, 306 are multiplexed in a single layer. In several embodiments, the waveguide 301 provides two effective gratings at all points across the grating structure 303 by overlaying more than two separated gratings in the grating structure. For ease of clarity, the gratings 305, 306 that form the grating structure 303 will be referred to as first and second integrated gratings since their role in the grating structure includes providing beam expansion by changing the direction of the guided beam in the plane of the waveguide and beam extraction. In various embodiments, the integrated gratings 305, 306 perform two-dimensional beam expansion and extraction of light from the waveguide 301. The field-of-view coupled into the waveguide can be partitioned into first and second portions, which can be bifurcated as such by the input grating 302. In many embodiments, the first and second portions correspond to positive and negative angles, vertically or horizontally. In some embodiments, the first and second portions may overlap in angle space. In a number of embodiments, the first portion of the field-of-view is expanded in a first direction by the first integrated grating and, in a parallel operation, expanded in a second direction and extracted by the second integrated grating. When a ray interacts with a grating fringe, some of the light that meets the Bragg condition is diffracted while non-diffracted light proceeds along its TIR path up to the next fringe, continuing the expansion and extraction process. Considering next the second portion of the field-of-view, the role of the gratings is reversed such that the second portion of the field is expanded in the second direction by the second integrated grating and expanded in the first direction and extracted by the first integrated grating.

In many embodiments, the integrated gratings 305, 306 in the grating structure 303 can be asymmetrically disposed. In some embodiments, the integrated gratings 305, 306 have grating vectors of different magnitudes. In several embodiments, the input grating 302 can have a grating vector offset from the Y-axis. In a number of embodiments, it is desirable that the vector combination of the grating vectors of the input grating 302 and the integrated gratings 305, 306 in the grating structures 303 gives a resultant vector of substantially zero magnitude. As described above, the grating vectors can be arranged in an equilateral, isosceles, or scalene triangular configuration. Depending on the application, certain configurations may be more desirable.

In many embodiments, at least one grating parameter selected from the group of grating vector direction, K-vector direction, grating refractive index modulation, and grating spatial frequency can vary spatially across at least one grating implemented in the waveguide for the purposes of optimizing angular bandwidth, waveguide efficiency, and output uniformity to increase the angular response and/or efficiency. In some embodiments, at least one of the gratings implemented in the waveguide can employ rolled K-vectors—i.e., spatially varying K-vectors. In several embodiments, the spatial frequencies of the grating(s) are matched to overcome color dispersion.

The apparatus 300 of FIG. 3 further includes an input image generator. In the illustrative embodiment, the input image generator includes a laser scanning projector 307 that provides a scanned beam 307A over a field-of-view that is coupled into total internal reflection paths (TIR paths) (308A, 308B, for example) in the waveguide by the input grating 302 and is directed towards the integrated gratings 305, 306 to be expanded and extracted (as shown by rays 309A, 309B, for example). In some embodiments, the laser projector 307 is configured to inject a scanned beam into the waveguide. In several embodiments, the laser projector 307 can have a scan pattern modified to compensate for optical distortions in the waveguide. In a number of embodiments, the laser scanning pattern and/or grating prescriptions in the input grating 302 and grating structure 303 can be modified to overcome illumination banding. In various embodiments, the laser scanning projector 307 can be replaced by an input image generator based on a reflective or transmissive microdisplay illuminated by a laser or an LED. In many embodiments, the input image can be provided by an emissive display. A laser projector can offer the advantages of improved color gamut, higher brightness, wider field-of-view, high resolution, and a very compact form factor. In some embodiments, the apparatus 300 can further include a despeckler. In further embodiments, the despeckler can be implemented as a waveguide device. Although FIG. 3 shows a specific waveguide application implementing integrated gratings, such structures and grating architectures can be utilized for various applications. In a number of embodiments, a waveguide having integrated gratings can be implemented in a single grating layer for a full color application. In many embodiments, more than one grating layer implementing integrated gratings are implemented. Such configurations can be implemented to provide wider angular or spectral bandwidth operation. In some embodiments, a multi-layered waveguide is implemented to provide a full color application. In several embodiments, a multi-layered waveguide is implemented to provide a wider field-of-view. In many embodiments, a full color waveguide having at least a ˜50° diagonal field-of-view is implemented using integrated gratings. In some embodiments, a full color waveguide having at least a ˜100° diagonal field-of-view is implemented using integrated gratings. Further discussion regarding integrated gratings can be found in U.S. application Ser. No. 16/794,071 filed Feb. 18, 2020 entitled “Methods and Apparatuses for Providing a Holographic Waveguide Display Using Integrated Gratings.” The disclosure of U.S. application Ser. No. 16/794,071 is hereby incorporated by reference in its entirety for all purposes.

HPDLC Optical Recording Material Systems

HPDLC mixtures may include LC, monomers, photosensitive dyes, and coinitiators. The mixture (often referred to as syrup) frequently may include a surfactant. For the purposes of describing the invention, a surfactant is defined as any chemical agent that lowers the surface tension of the total liquid mixture. The use of surfactants in PDLC mixtures is known and dates back to the earliest investigations of PDLCs. For example, a paper by R. L Sutherland et al., SPIE Vol. 2689, 158-169, 1996, the disclosure of which is incorporated herein by reference, describes a PDLC mixture including a monomer, photoinitiator, coinitiator, chain extender, and LCs to which a surfactant can be added. Surfactants are also mentioned in a paper by Natarajan et al, Journal of Nonlinear Optical Physics and Materials, Vol. 5 No. I 89-98, 1996, the disclosure of which is incorporated herein by reference. Furthermore, U.S. Pat. No. 7,018,563 by Sutherland; et al., discusses polymer-dispersed liquid crystal material for forming a polymer-dispersed liquid crystal optical element having: at least one acrylic acid monomer; at least one type of liquid crystal material; a photoinitiator dye; a coinitiator; and a surfactant. The disclosure of U.S. Pat. No. 7,018,563 is hereby incorporated by reference in its entirety.

The patent and scientific literature contains many examples of material systems and processes that can be used to fabricate SBGs, including investigations into formulating such material systems for achieving high diffraction efficiency, fast response time, low drive voltage, and so forth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. both describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. Examples of recipes can also be found in papers dating back to the early 1990s. Many of these materials use acrylate monomers, including:

R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the disclosure of which is incorporated herein by reference, describes the use of acrylate polymers and surfactants. Specifically, the recipe comprises a crosslinking multifunctional acrylate monomer; a chain extender N-vinyl pyrrolidinone, LC E7, photo-initiator rose Bengal, and coinitiator N-phenyl glycine. Surfactant octanoic acid was added in certain variants. Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure of which is incorporated herein by reference, describes a UV curable HPDLC for reflective display applications including a multi-functional acrylate monomer, LC, a photoinitiator, a coinitiators, and a chain terminator. Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999, the disclosure of which is incorporated herein by reference, discloses HPDLC recipes including acrylates. Karasawa et al., Japanese Journal of Applied Physics, Vol. 36, 6388-6392, 1997, the disclosure of which is incorporated herein by reference, describes acrylates of various functional orders. T. J. Bunning et al., Polymer Science: Part B: Polymer Physics, Vol. 35, 2825-2833, 1997, the disclosure of which is incorporated herein by reference, also describes multifunctional acrylate monomers. G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6). 425-430, 1996, the disclosure of which is incorporated herein by reference, describes a PDLC mixture including a penta-acrylate monomer, LC, chain extender, coinitiators, and photoinitiator.