Vacuum diffraction grating and manufacturing method

Abstract

Described herein are improvements to diffraction gratings for use in waveguides and methods for fabricating them. Deep surface-relief gratings (SRGs) can offer many advantages over conventional SRGs and Bragg gratings, with a key advantage being higher S diffraction efficiency. In one embodiment, deep SRGs can be implemented as polymer surface-relief gratings or vacuum Bragg gratings (EBGs). EBGs can be formed by first recording a holographic polymer-dispersed liquid crystal (HPDLC) grating. Removing the liquid crystal from the cured grating provides a polymer surface-relief grating. Polymer surface-relief gratings have many applications, including use in waveguide-based displays.

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims priority to U.S. Patent Application No. 62 / 893,715, filed Aug. 29, 2019, the disclosure of which is hereby incorporated by reference in its entirety.

[0002] The present invention generally relates to waveguides and methods for fabricating waveguides, and more specifically, to waveguide displays including diffraction gratings formed from multi - component mixtures from which one material component type has been removed, and methods for fabricating such diffraction gratings.

Background Art

[0003] A waveguide can be referred to as a structure that has the ability to confine and guide waves (i.e., limit the spatial region in which waves can propagate). One subclass includes optical waveguides, which are structures that can guide electromagnetic waves, typically in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using a number of different mechanisms. For example, planar waveguides can be designed to utilize diffraction gratings to diffract incident light and couple it into the waveguide structure, such that the internally - coupled light can continue to propagate within the planar structure via total internal reflection (TIR).

[0004] Waveguide fabrication may involve the use of material systems that enable the recording of holographic optical elements within or on the surface of the waveguide. One class of such materials includes polymer-dispersed liquid crystal (PDLC) mixtures, which are mixtures containing photopolymerizable monomers and liquid crystals. A further subclass of such mixtures includes holographic polymer-dispersed liquid crystal (HPDLC) mixtures. Holographic optical elements, such as volume phase diffraction gratings, can be recorded in such liquid mixtures by irradiating the material with two mutually coherent laser beams. During the recording process, the monomers polymerize, and the mixture undergoes photopolymerization-induced phase separation, forming regions of densely packed liquid crystal (LC) microdroplets interspersed with regions of clear polymer. Alternating regions of liquid crystal richness and liquid crystal depletion form fringes on the grating.

[0005] Waveguide optics, such as those described above, can be considered for a variety of display and sensor applications. In many applications, waveguides containing one or more diffraction grating layers encoding multiple optical functions can be realized using a variety of waveguide architectures and material systems, enabling new innovations in eyepiece displays for augmented reality (AR) and virtual reality (VR), compact head-up displays (HUDs) for aviation and road transport, and sensors for biometric and laser radar (LIDAR) applications. As many of these applications target consumer products, there is a growing demand for efficient and low-cost means of manufacturing holographic waveguides in large quantities. [Overview of the project] [Means for solving the problem]

[0006] Many embodiments relate to polymer diffraction grating structures, their design, manufacturing methods, and materials.

[0007] Various embodiments include, The target is waveguide-based devices including a waveguide that supports a polymer diffraction grating structure for diffracting light propagating through the waveguide by total internal reflection. • Polymer diffraction grating structures are ○Polymer network and, ○Includes voids between adjacent parts of the polymer network.

[0008] In various other embodiments, the polymer diffraction grating structure may further include isotropic material between adjacent portions of the polymer network, the isotropic material having a refractive index higher or lower than that of the polymer network.

[0009] In various other embodiments, the isotropic material may occupy the space at the bottom of the space between adjacent portions of the polymer network, and air may occupy the space from above the upper surface of the isotropic material up to the modulation depth.

[0010] In various other embodiments, the isotropic material may include a birefringent crystalline material.

[0011] In various other embodiments, the birefringent crystalline material may include a liquid crystal material.

[0012] In various other embodiments, the birefringent crystalline material may be a material with a higher refractive index than the polymer.

[0013] Furthermore, in various other embodiments, the polymer diffraction grating structure may have a modulation depth greater than the wavelength of visible light.

[0014] In various other embodiments, the polymer diffraction grating structure may include a modulation depth and a diffraction grating pitch, where the modulation depth is greater than the diffraction grating pitch.

[0015] In various other embodiments, the waveguide may include two substrates, and the polymer diffraction grating structure may be sandwiched between the two substrates or positioned on the outer surface of either substrate.

[0016] In various other embodiments, the Bragg fringe spacing of the polymer network may be from 0.35 μm to 0.8 μm, and the diffraction grating depth of the polymer network may be from 1 μm to 3 μm.

[0017] In various other embodiments, the ratio of the diffraction grating depth to the Bragg fringe spacing of the polymer network may be from 1:1 to 5:1.

[0018] In various other embodiments, the waveguide display may further include an image generation unit, and the polymer diffraction grating structure may include a waveguide diffraction grating.

[0019] In various other embodiments, the waveguide diffraction grating may be configured as a multiple diffraction grating.

[0020] In various other embodiments, the waveguide diffraction grating may be configured to receive light from an image generation unit including a plurality of images.

[0021] In various other embodiments, the waveguide diffraction grating may be configured to externally couple light from the waveguide.

[0022] In various other embodiments, the waveguide diffraction grating may be configured as a beam expander.

[0023] <00000(...)In various other embodiments, the waveguide diffraction grating may be configured to internally couple light including image data generated from an image generation unit.

[0024] In various other embodiments, the waveguide diffraction grating may be further configured to efficiently internally couple S-polarized light.

[0025] In various other embodiments, the diffraction grating may be further configured to internally couple S-polarized light at an efficiency of 70% to 95% at the Bragg angle.

[0026] In various other embodiments, the diffraction grating may be further configured to internally couple P-polarized light with an efficiency of 25% to 50% at the Bragg angle.

[0027] In various other embodiments, the refractive index difference between the polymer network and the voids may be 0.25 to 0.4.

[0028] In various other embodiments, the refractive index difference between the polymer network and the birefringent crystal material may be 0.05 to 0.2.

[0029] In various other embodiments, the polymer diffraction grating structure may include a two-dimensional diffraction grating structure or a three-dimensional diffraction grating structure.

[0030] In various other embodiments, the waveguide display may further include another diffraction grating structure.

[0031] In various other embodiments, the polymer diffraction grating structure may include a diffraction grating for internal coupling, and another diffraction grating structure may include a beam expander or a diffraction grating for external coupling.

[0032] Furthermore, various embodiments · are directed to a waveguide display including a waveguide that supports a polymer diffraction grating structure for diffracting light propagating by total reflection in the waveguide, 〇 The polymer diffraction grating structure ■ includes a polymer network and ■ a birefringent crystal material between adjacent portions of the polymer network, the birefringent crystal material having a higher refractive index than the polymer.

[0033] Furthermore, various embodiments are directed to a method for fabricating a deep surface relief grating (SRG), the method comprising · providing a mixture of a monomer and a liquid crystal, · providing a substrate, · coating a layer of the mixture on the surface of the substrate - By irradiating the layer with a holographic recording beam, a holographic polymer-dispersed liquid crystal diffraction grating having alternating polymer-rich regions and liquid crystal-rich regions is formed. This includes removing at least a portion of the liquid crystal in a liquid crystal-rich region to form a polymer surface relief diffraction grating.

[0034] In various other embodiments, the monomers include acrylates, methacrylates, vinyls, isosinates, thiols, isocyanate-acrylates, and / or thiolines.

[0035] In various other embodiments, the mixture may further include at least one of a photoinitiator, a coinitiator, or additional additives.

[0036] In various other embodiments, the thiol may include a thiol-vinyl-acrylate.

[0037] In various other embodiments, the photoinitiator may include a photosensitive component.

[0038] In various other embodiments, the photosensitive component may include a dye and / or a radical generator.

[0039] In various other embodiments, providing a mixture of monomers and liquid crystals is possible. Mixing a monomer, a liquid crystal, and at least one of a photoinitiator, co-initiator, polyfunctional thiol, or additional additives. • Store the mixture in a place away from light at a temperature of 22°C or below. • Adding additional monomers, • Filter the mixture through a filter of 0.6 μm or less, This may include storing the filtered mixture in a place away from light.

[0040] In various other embodiments, the substrate may include a glass substrate or a plastic substrate.

[0041] In various other embodiments, the substrate may include a transparent substrate.

[0042] In various other embodiments, the method may further include sandwiching the mixture between one substrate and another using one or more spacers to maintain internal dimensions.

[0043] In various other embodiments, the method may further include depositing a non-adhesive release layer onto one surface of another substrate.

[0044] In various other embodiments, the non-stick release layer may include a fluoropolymer.

[0045] In various other embodiments, the method may further include refilling the liquid crystal-rich region with liquid crystal material.

[0046] In various other embodiments, the liquid crystal material may have a different molecular structure from the liquid crystal previously removed.

[0047] In various other embodiments, removing at least a portion of the liquid crystal may include removing substantially all of the liquid crystal in a liquid crystal-rich region.

[0048] In various other embodiments, removing at least a portion of the liquid crystal may further include leaving at least a portion of the liquid crystal in the polymer-rich region.

[0049] In various other embodiments, the method may further include depositing a protective layer on top of a deep SRG.

[0050] In various other embodiments, the protective layer may include an anti-reflective layer.

[0051] In various other embodiments, the protective layer may contain silicates or silicon nitride.

[0052] In various other embodiments, applying a protective layer may include depositing the protective layer on a deep SRG.

[0053] In various other embodiments, the deposition of the protective layer may include chemical vapor deposition.

[0054] In various other embodiments, chemical vapor deposition may be a nanocoating process.

[0055] In various other embodiments, the protective layer may include a parylene coating.

[0056] In various other embodiments, the liquid crystal rich region may include the void remaining after removing at least a portion of the liquid crystal in the liquid crystal rich region.

[0057] In various other embodiments, the method may further include creating a vacuum in the void or filling the void with an inert gas.

[0058] In various other embodiments, removing at least a portion of the liquid crystal may include cleaning the holographic polymer-dispersed liquid crystal diffraction grating with a solvent.

[0059] In various other embodiments, cleaning the holographic polymer-dispersed liquid crystal diffraction grating may include immersing the holographic polymer-dispersed liquid crystal diffraction grating in a solvent.

[0060] In various other embodiments, the solvent may include isopropyl alcohol.

[0061] In various other embodiments, the solvent may be kept at a temperature lower than room temperature while cleaning the holographic polymer-dispersed liquid crystal diffraction grating.

[0062] In various other embodiments, removing at least a portion of the liquid crystal may further include drying the holographic polymer-dispersed liquid crystal diffraction grating with a high-flow air source.

[0063] In various other embodiments, the method may further include curing a holographic polymer-dispersed liquid crystal diffraction grating.

[0064] In various other embodiments, curing the holographic polymer-dispersed liquid crystal diffraction grating may include exposing the holographic polymer-dispersed liquid crystal diffraction grating to low-intensity white light for a period of about one hour.

[0065] In various other embodiments, the polymer surface relief diffraction grating may be configured to internally couple S-polarized light with an efficiency of 70% to 95%.

[0066] In various other embodiments, the polymer surface relief diffraction grating may be further configured to internally couple P-polarized light with an efficiency of 25% to 50%.

[0067] In various other embodiments, the difference in refractive index between the polymer network and the voids may be 0.25 to 0.4.

[0068] In various other embodiments, the difference in refractive index between the polymer network and the liquid crystal material may be 0.05 to 0.2.

[0069] In various other embodiments, the polymer surface relief diffraction grating may include a Bragg fringe spacing of 0.35 μm to 0.8 μm and a diffraction grating depth of 1 μm to 3 μm.

[0070] In various other embodiments, the polymer surface relief diffraction grating may include a Bragg fringe spacing to diffraction grating depth ratio of 1:1 to 5:1.

[0071] In various other embodiments, the liquid crystal content in the mixture of monomer and liquid crystal may be approximately 20% to 50%.

[0072] In various other embodiments, the liquid crystal in the mixture of monomer and liquid crystal may include liquid crystal singles.

[0073] In various other embodiments, the liquid crystal element may contain cyanobiphenyl and / or pentylcyanobiphenyl.

[0074] Furthermore, various embodiments relate to a method for fabricating deep SRGs, and this method is To provide a mixture of monomers and substances, • To provide a substrate, • Coating a layer of the mixture onto the surface of the substrate, - By irradiating layers with a holographic recording beam, a holographic polymer-dispersive diffraction grating having alternating polymer-rich regions and material-rich regions is formed. This may include removing at least a portion of the material in a material-rich region to form a polymer surface relief diffraction grating.

[0075] In various other embodiments, the monomer may be reactive to the holographic recording beam, and the material may be inactive to the holographic recording beam.

[0076] In various other embodiments, the monomers and materials may be a miscible mixture before irradiation with the holographic recording beam and become an immiscible mixture after irradiation with the holographic recording beam.

[0077] In various other embodiments, the material may include liquid crystals.

[0078] In various other embodiments, the material may include a single liquid crystal.

[0079] In various other embodiments, the substance may include solvents, nonreactive monomers, inorganic substances, and / or nanoparticles.

[0080] Furthermore, various embodiments target waveguide displays, • A light-emitting array that emits light in a first wavelength band, • A collimation lens for projecting image-modulated light from the light-emitting array onto the field of view, • Waveguides, ○ An input / output SBG having high diffraction efficiency for S-polarized light in the first wavelength band, and The present invention may include a waveguide supporting an input / output SBG having high diffraction efficiency for P-polarized light in the first wavelength band.

[0081] In various other embodiments, the waveguide may further support an SBG for diffracting S-polarized and P-polarized light in a second wavelength band emitted by the light-emitting array.

[0082] In various other embodiments, the light-emitting array may be an OLED array.

[0083] In various other embodiments, the waveguide may be curved in at least one plane.

[0084] In various other embodiments, the waveguide may be made from plastic.

[0085] In various other embodiments, the light-emitting array may be spatially distorted to pre-compensate for wavefront distortion caused by the curved surface of the waveguide.

[0086] In various other embodiments, the light-emitting array may be formed on a curved or flexible substrate to pre-compensate for wavefront distortion caused by the curved surface of the waveguide.

[0087] In various other embodiments, at least one of the diffraction gratings may be a Bragg diffraction grating recorded on a photopolymer, a Bragg diffraction grating recorded on a liquid crystal and monomer mixture, a deep surface relief diffraction grating, or a hybrid surface relief / Bragg diffraction grating.

[0088] In various other embodiments, the waveguide may support an ophthalmic optical surface.

[0089] In various other embodiments, the light emission may have a pixel array that is patterned using elemental multiplicity, which includes at least one selected from the group of elements that form polygons of the same size, polygons of the same shape, polygons of varying size across the array, polygons of varying shape across the array, Penrose tiles, and non-repeating patterns.

[0090] Furthermore, various embodiments relate to methods for forming images using waveguides, and these methods include To provide a light-emitting array that emits light in a first wavelength band, a collimation lens, and a waveguide that supports an input / output diffraction grating having high diffraction efficiency for S-polarized light in the first wavelength band and supports input / output ratings having high diffraction efficiency for P-polarized light in the first wavelength band. • Collimating the image light emitted by the light emission array using a collimation lens, • Using an S-diffraction input diffraction grating, the image-modulated S-polarized light from the OLED array is coupled to the total internal reflection path of the waveguide. • Using a P-diffraction input diffraction grating, the image-modulated P-polarized light from the OLED array is coupled to the total internal reflection path of the waveguide. • For projection, the S-polarized light from the waveguide is beam-expanded and extracted. This includes beam-expanding and extracting P-polarized light from a waveguide for projection.

[0091] In various other embodiments, the light-emitting array may be an OLED array.

[0092] In various other embodiments, the method may further include the steps of: providing a curved optical surface supported by the waveguide; pre-distorting a pixel pattern on the light-emitting array; forming a pre-distorted wavefront using the collimation lens; reflecting the light of the pre-distorted wavefront on the curved optical surface; and forming a plane wavefront from the pre-distorted wavefront using the refractive power of the curved optical surface.

[0093] In various other embodiments, the curved optical surface may be a shaped optical surface.

[0094] The description is presented as an exemplary embodiment of the present invention and should not be construed as a complete enumeration of the scope of the invention; it will be better understood by referring to the following figures and data graphs. [Brief explanation of the drawing]

[0095] [Figure 1A] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating, in which a mixture of monomers and liquid crystals deposited on a transparent substrate is exposed to a holographic exposure beam, according to embodiments of the present invention. [Figure 1B] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating from an HPDLC Bragg diffraction grating formed on a transparent substrate, according to an embodiment of the present invention. [Figure 1C] This invention conceptually illustrates the steps of a method for producing a surface relief diffraction grating, which involves removing liquid crystal from an HPDLC Bragg diffraction grating to form a polymer surface relief diffraction grating, according to an embodiment of the present invention. [Figure 1D] This conceptually illustrates the steps of a method for coating a surface relief diffraction grating with a protective layer according to embodiments of the present invention. [Figure 2]This flowchart conceptually illustrates a method for forming a polymer surface relief diffraction grating from an HPDLC Bragg diffraction grating formed on a transparent substrate, according to an embodiment of the present invention. [Figure 3] This is an exemplary mounting configuration of a polymer surface relief diffraction grating or a vacuum Bragg diffraction grating. [Figure 4A] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating, in which a mixture of monomers and liquid crystals deposited on a transparent substrate is exposed to a holographic exposure beam, according to embodiments of the present invention. [Figure 4B] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating from an HPDLC Bragg diffraction grating formed on a transparent substrate, according to an embodiment of the present invention. [Figure 4C] This invention conceptually illustrates the steps of a method for producing a surface relief diffraction grating, which involves removing liquid crystal from an HPDLC Bragg diffraction grating to form a polymer surface relief diffraction grating, according to an embodiment of the present invention. [Figure 4D] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating, which involves partially refilling a surface relief diffraction grating with liquid crystal to form a hybrid surface relief Bragg diffraction grating, according to embodiments of the present invention. [Figure 4E] This invention conceptually illustrates the steps of a method for fabricating a surface relief diffraction grating, in which a hybrid surface relief-Bragg diffraction grating is coated with a protective layer, according to embodiments of the present invention. [Figure 5] This flowchart conceptually illustrates a method for forming a hybrid surface relief-Bragg diffraction grating according to embodiments of the present invention. [Figure 6] This graph shows the calculated P-polarized and S-polarized diffraction efficiency versus the angle of incidence for a deep surface relief diffraction grating with a thickness of 1 micrometer, according to an embodiment of the present invention. [Figure 7] This graph shows the calculated P-polarized and S-polarized diffraction efficiency versus the angle of incidence for a deep surface relief diffraction grating with a thickness of 2 micrometers, according to an embodiment of the present invention. [Figure 8] This graph shows the calculated P-polarized and S-polarized diffraction efficiency versus the angle of incidence for a deep surface relief diffraction grating with a thickness of 3 micrometers, according to an embodiment of the present invention. [Figure 9A-9B] Scanning electron microscope images of multiple embodiments containing different thiol concentrations are illustrated. [Figure 10A-10B] This image compares an HPDLC Bragg diffraction grating with a polymer surface relief diffraction grating or a vacuum Bragg diffraction grating. [Figure 11A-11B] These are two plots comparing an HPDLC Bragg diffraction grating with a polymer surface relief diffraction grating or a vacuum Bragg diffraction grating. [Figures 12A-12B] These are two plots of S-diffraction efficiency and P-diffraction efficiency for two exemplary polymer surface relief diffraction gratings with different depths. [Figures 13A-13B] These are two different plots of S-diffraction and P-diffraction efficiencies for various exemplary polymer surface relief diffraction gratings produced at different initial liquid crystal concentrations. [Figures 14A-14B] These are two different plots of S-diffraction and P-diffraction efficiencies for various exemplary polymer surface relief diffraction gratings produced at different initial liquid crystal concentrations. [Figure 15] This conceptually illustrates a waveguide display according to an embodiment of the present invention. [Figure 16] This conceptually illustrates a waveguide display having two waveguide layers separated by air, according to an embodiment of the present invention. [Figure 17] This conceptually illustrates a typical ray path for a waveguide display according to an embodiment of the present invention. [Figure 18] This conceptually illustrates a waveguide display according to an embodiment of the present invention, in which the waveguide supports a curved optical surface. [Figure 19] This conceptually illustrates a waveguide display according to an embodiment of the present invention, in which the waveguide supports the upper and lower curved optical surfaces. [Figure 20]This invention conceptually illustrates a waveguide display in which the waveguide supports a curved optical surface, and the input image is provided pre-distorted using a pixel array to compensate for aberrations introduced by the curved optical surface. [Figure 21] This invention conceptually illustrates a waveguide display in which a waveguide supports a curved optical surface, and the input image is provided using a pixel array supported by a curved substrate, and pre-distorted to compensate for aberrations introduced by the curved optical surface. [Figure 22] This flowchart conceptually illustrates a method for projecting image light for projection using a waveguide including an S-diffraction grating and a P-diffraction grating, according to embodiments of the present invention. [Figure 23] This flowchart conceptually illustrates a method for projecting image light for projection using a waveguide that supports an optically formulated surface and includes an S-diffraction grating and a P-diffraction grating, according to embodiments of the present invention. [Figure 24A] This conceptually illustrates a portion of a pixel pattern having rectangular elements of different sizes and aspect ratios for use in a light-emitting display panel, according to an embodiment of the present invention. [Figure 24B] This conceptually illustrates a portion of a pixel pattern having Penrose tiles for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24C] This conceptually illustrates a portion of a pixel pattern having hexagonal elements for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24D] This conceptually illustrates a portion of a pixel pattern having square elements for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24E] This conceptually illustrates a portion of a pixel pattern having diamond-shaped elements for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24F]This conceptually illustrates a portion of a pixel pattern having isosceles triangular elements for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24G] This conceptually illustrates a portion of a pixel pattern having hexagonal elements with a horizontally biased aspect ratio, for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24H] This conceptually illustrates a portion of a pixel pattern having rectangular elements with a horizontally biased aspect ratio, for use in an emissive display panel according to an embodiment of the present invention. [Figure 24I] This conceptually illustrates a portion of a pixel pattern having diamond-shaped elements with a horizontally biased aspect ratio, for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 24J] This conceptually illustrates a portion of a pixel pattern having a horizontally biased aspect ratio and a triangle, for use in a light-emitting display panel according to an embodiment of the present invention. [Figure 25] This conceptually illustrates a portion of a pixel pattern having diamond-shaped elements, according to embodiments of the present invention, in which different pixels can have different radiation characteristics. [Modes for carrying out the invention]

[0096] To provide diverse functionalities, there is growing interest in the use of various diffraction gratings on waveguides. These include angle-multiplexed diffraction gratings, color-multiplexed diffraction gratings, folded diffraction gratings, double-interaction diffraction gratings, rotated K-vector diffraction gratings, cross-folded diffraction gratings, mosaic diffraction gratings, chirp diffraction gratings, diffraction gratings with spatially varying refractive index modulation, diffraction gratings with spatially varying diffraction grating thickness, diffraction gratings with spatially varying average refractive index, diffraction gratings with spatially varying refractive index modulation tensors, and diffraction gratings with spatially varying average refractive index tensors. In certain examples, diffraction gratings for the diffraction of various polarizations of light (e.g., S-polarized and P-polarized) may be beneficial. Having a diffraction grating that diffracts either S-polarized or P-polarized light would be particularly advantageous. Specific applications of this technique include waveguide-based displays such as augmented reality and virtual reality displays. One example is an input diffraction grating that can be used to input either S-polarized or P-polarized light, or both, into a waveguide. However, in many cases, having a diffraction grating that diffracts either S-polarized or P-polarized light would be advantageous. For example, waveguide displays using non-polarized light sources such as OLED light sources would benefit from having a diffraction grating that can generate both S-polarized and P-polarized light, and therefore diffract both S-polarized and P-polarized light.

[0097] One particular class of diffraction gratings includes surface relief diffraction gratings (SRGs) that can be used to diffract either P-polarized or S-polarized light. Another class of diffraction gratings is surface relief Bragg diffraction gratings (SBGs), which are typically P-polarization selective and can lead to a 50% efficiency loss in non-polarized light sources such as organic light-emitting diodes (OLEDs) and light-emitting diodes (LEDs). Hybridizing a composite of S-polarized and P-polarized diffraction gratings can theoretically provide a twofold improvement over waveguides using only P-polarized diffraction gratings. Therefore, having a highly efficient S-polarized diffraction grating would be advantageous. In many embodiments, S-polarized diffraction gratings can be provided by Bragg diffraction gratings formed on holographic photopolymers. In some embodiments, S-polarized diffraction gratings can be provided by Bragg diffraction gratings formed on holographic polymer-dispersed liquid crystals (HPDLCs) having birefringence modified using an array layer or other process to rearrange liquid crystal (LC) directors. In some embodiments, S-polarized diffraction gratings can be formed using liquid crystals, monomers, and other additives that naturally organize into S-polarized diffraction gratings under phase separation. In some embodiments, these HPDLC diffraction gratings can form deep SRGs with excellent S-polarization diffraction efficiency.

[0098] One class of deep SRGs is a polymer-air SRG or vacuum Bragg grating (EBG) that can exhibit high S diffraction efficiency (up to 99%) and low P diffraction efficiency and can be implemented as an input diffraction grating for waveguides. Such diffraction gratings can be formed by removing liquid crystals from an SBG formed from holographic phase separation of a mixture of liquid crystals and monomers. Deep SRGs formed by such a process typically have a thickness in the range of 1 to 3 micrometers with a Bragg fringe spacing of 0.35 to 0.80 micrometers. In some embodiments, the ratio of the diffraction grating depth to the Bragg fringe spacing may be 1:1 to 5:1. As can be easily recognized, such diffraction gratings can be formed in different dimensions depending on the specific requirements of a given application. Examples of how the thickness of the SRG can result in different diffraction efficiencies are described in relation to Figures 6 to 8.

[0099] In many embodiments, the conditions for deep SRGs are characterized by a high grating depth to fringe spacing ratio. In some embodiments, the condition for forming a deep SRG is that the grating depth is approximately twice the grating period. Modeling such deep SRGs using Kogelnik theory can provide fairly accurate estimates of diffraction efficiency, avoiding the need for more advanced modeling, which typically involves numerical solutions to Maxwell's equations. The grating depths that can be achieved using liquid crystals removable from HPDLC gratings far exceed those possible with conventional nanoimprint lithography methods, which cannot achieve the conditions for deep SRGs (typically providing only a depth of 250–300 nm for a grating period of 350–460 nm). (Pekka Ayras, Pasi Saarikko, Tapani Levola, "Exit pupil expander with a large field of view based on diffractive optics", Journal of the SID 17 / 8, (2009), pp659–664). It should be emphasized here that while deep SRGs that perform S-polarization diffraction are highlighted in this application, deep SRGs can offer a variety of polarization response characteristics depending on the thickness of the diffraction grating formulation, particularly the diffraction grating depth, as will be discussed below. Therefore, deep SRGs can be implemented in a wide variety of different applications.

[0100] The literature supports the equivalence of deep SRGs and Bragg gratings. One reference (Kiyoshi Yokomori, “Dielectric surface-relief gratings with high diffraction efficiency,” Applied Optics; Vol.23; Issue 14; (1984); pp.2303-2310) discloses an investigation of the diffraction properties of dielectric surface-relief gratings by numerically solving Maxwell's equations. It was found that the diffraction efficiency of a grating with a groove depth of approximately twice the grating period is equivalent to that of a volume-phase grating. Yokomori's modeling predicted that dielectric surface-relief gratings recorded on photoresist using phase interference could have a high diffraction efficiency of up to 94% (throughput efficiency of 85%). The equivalence between deep SRGs and Bragg diffraction gratings is also discussed in another paper by Golub (MAGolub, AAFriesem, L. Eisen, “Bragg properties of efficient surface relief gratings in the resonance domain”, Optics Communications; 235; (2004); pp261-267). A further paper by Gerritsen discusses the formation of Bragg-like SRGs in photoresists (Gerritsen HJ, Thornton DK, Bolton SR, “Application of Kogelnik's two-wave theory to deep, slanted, highly efficient, relief transmission gratings”, Applied Optics; Vol.30; Issue 7; (1991); pp 807-814).

[0101] Many embodiments of this disclosure provide methods for creating SRGs, such as deep SRGs, which can offer significant advantages over nanoimprint lithography process particles for slant-type diffraction gratings. Bragg diffraction gratings of any complexity can be created using interference or master-and-contact copy replication. In some embodiments, after removing LCs, the SRG can be backfilled with a material having different properties with respect to LCs. This enables Bragg diffraction gratings with modulation properties not limited by the diffraction grating chemistry required for grating formation.

[0102] In some embodiments, the backfill material does not have to be LC material. In some embodiments, the backfill material may have a refractive index higher than air, which can increase the angular bandwidth of the waveguide. In some embodiments, a deep SRG can be partially backfilled with LC to provide a hybrid SRG / Bragg diffraction grating. Alternatively, in some embodiments, the refilling step can be avoided by removing a small portion of LC from the LC-rich region of HPDLC to provide a hybrid SRG / Bragg diffraction grating. The refilling approach has the advantage that different LCs can be used to form the hybrid diffraction grating. This material can be deposited using an inkjet deposition process.

[0103] In some embodiments, photonic crystals can be formed using the methods described herein. Photonic crystals can be implemented to form a wide variety of diffraction structures, including Bragg gratings. Using a Bragg grating as a diffraction grating, it can provide functions including, but are not limited to, input gratings, output gratings, beam-expanding gratings, and diffraction of two or more primary colors. A photonic crystal can be a three-dimensional diffraction grating structure capable of diffraction capabilities unattainable with basic Bragg gratings. Photonic crystals can include many structures, including all 2D and 3D Bravais gratings. Recording of such structures can benefit from three or more recorded beams.

[0104] In some embodiments, waveguides incorporating photonic crystals can be arranged in a stack of waveguides, each having a diffraction grating formulation for diffracting a unique spectral bandwidth. In many embodiments, photonic crystals formed by liquid crystal extraction provide deep SRGs. In many embodiments, deep SRGs formed using a liquid crystal extraction process can typically have a thickness in the range of 1 to 3 microns with a Bragg fringe spacing of 0.35 to 0.80 microns. In many embodiments, the conditions for a deep SRG are characterized by a high diffraction grating depth to fringe spacing ratio. In some embodiments, the condition for forming a deep SRG is that the diffraction grating depth can be approximately twice the diffraction grating period. It should be emphasized here that while this application is primarily interested in deep SRGs that diffract S-polarization, deep SRGs can provide a variety of polarization response characteristics depending on the thickness of the diffraction grating formulation, particularly the diffraction grating depth, as will be discussed below. Deep SRGs can also be used in conjunction with conventional Bragg diffraction gratings to improve the color, uniformity, and other properties of waveguide displays.

[0105] Deep surface relief gratings (SRGs) have been fabricated in glassy monomer azobenzene materials using laser holographic exposure (O. Sakhno, LMGoldenberg, M. Wegener, J. Stumpe, “Deep surface relief grating in azobenzene-containing materials using a low intensity 532 nm laser”, Optical Materials: X, 1, (2019), 100006, pp3-7). Sakhno's reference also discloses a method in which SRGs can be recorded on holographic photopolymers using two orthogonal linearly polarized laser beams.

[0106] This disclosure provides a method for creating surface relief diffraction gratings that can offer significant advantages to nanoimprint lithography processes, particularly for slant-type diffraction gratings. Bragg diffraction gratings of any complexity can be created using interference or master-and-contact copy replication. In some embodiments, after removing LC, the SRG can be backfilled with a material having different properties to the LC. This enables Bragg diffraction gratings with modulation properties not limited by the diffraction grating chemistry required for grating formation. In some embodiments, the SRG can be partially backfilled with LC to provide a hybrid SRG / Bragg diffraction grating. Alternatively, in some embodiments, the refilling step can be avoided by removing a small portion of LC from the LC-rich region of the HPDLC to provide a hybrid SRG / Bragg diffraction grating. The refilling approach has the advantage that different LC can be used to form the hybrid diffraction grating. This material can be deposited using an inkjet process, as disclosed in a previous application by the inventors. In some embodiments, the refilling material may have a refractive index higher than air, which can increase the diffraction efficiency of the diffraction grating.

[0107] While this disclosure has been made in the context of fabricating deep SRGs, it is recognized that many other diffraction grating structures can be fabricated using the methods described herein. For example, any type of SRG, including SRGs in which the diffraction grating depth is less than the diffraction grating frequency (e.g., Raman-Nass diffraction gratings), can be fabricated in the same manner.

[0108] Figures 1A-1D illustrate apparatuses that may be used in methods for producing deep SRGs or EBGs according to embodiments. Figure 1A conceptually illustrates apparatus 190A that may be used in a step of a method for producing a surface relief diffraction grating, according to embodiments of the present invention, in which a mixture 191 of monomer and liquid crystal deposited on a transparent substrate 192 is exposed to a holographic exposure beam 193, 194. In some examples, the mixture may also include at least one of photoinitiators, coinitiators, polyfunctional thiols, adhesion promoters, surfactants, and / or additional additives. In some embodiments, the monomer may be an isocyanate-acrylate system or a thiolene system. In some embodiments, the liquid crystal may be a complete liquid crystal mixture or a single liquid crystal containing only a portion of a complete liquid crystal mixture. Various examples of single liquid crystals include cyanobiphenyl or pentylcyanobiphenyl, one or both. In some embodiments, the liquid crystal may be replaced with another substance that phase-separates from the monomer during exposure to form polymer-rich and substance-rich regions. Advantageously, the substance and individual liquid crystals can be cost-effective alternatives to the complete liquid crystal mixture, which will be removed in a later step, as described below.

[0109] Figure 1B conceptually illustrates apparatus 190B that may be used in a step of a method for fabricating a surface relief diffraction grating from an HPDLC Bragg diffraction grating 195 formed on a transparent substrate using a holographic exposure beam, according to an embodiment of the present invention. The holographic exposure beam can convert monomers into polymers in several areas. The holographic exposure beam may include a cross-recording beam and may include alternating bright and dark illuminated regions. The polymerization-driven diffusion process causes diffusion of monomers and LCs in opposite directions, where monomers may undergo gelation to form polymer-rich regions (in the bright regions), and liquid crystals may be trapped in the polymer matrix to form liquid crystal-rich regions (in the dark regions).

[0110] Figure 1C conceptually illustrates apparatus 190C that may be used in a step of a method for producing a deep polymer surface relief diffraction grating 196 or EBG, according to an embodiment of the present invention, in which liquid crystal is removed from the HPDLC Bragg diffraction grating of Figure 1B to form a polymer surface relief diffraction grating. Advantageously, the polymer surface relief diffraction grating 196 may have a large depth with a relatively small diffraction grating period to form a deep SRG. The liquid crystal can be removed by washing with a solvent such as isopropyl alcohol (IPA). The solvent should be strong enough to wash away the liquid crystal but weak enough to preserve the polymer. In some embodiments, the solvent may be cooled below room temperature before washing the diffraction grating. Figure 1D conceptually illustrates apparatus 190D that may be used in a step of a method for producing a polymer surface relief diffraction grating, according to an embodiment of the present invention, in which the polymer surface relief diffraction grating is coated with a protective layer 197.

[0111] Figure 2 conceptually illustrates a method for forming a deep SRG from an HPDLC Bragg diffraction grating formed on a transparent substrate, according to an embodiment. As shown, a method 200 for forming a deep SRG or EBG is provided. Referring to the flow chart, method 200 comprises providing a mixture of at least one monomer and at least one liquid crystal (201). The at least one monomer may include an isocyanate-acrylate monomer or a thiolene. In some embodiments, the at least one liquid crystal may be a complete liquid crystal mixture or a single liquid crystal that may consist of only a portion of the liquid crystal mixture, such as a single component of the liquid crystal mixture. In some embodiments, the at least one liquid crystal may be replaced with a solution that can phase-separate from the monomer during exposure. Criteria for such a solution may include the ability to phase-separate from the monomer during exposure, ease of removal after curing and during cleaning, and ease of handling. Exemplary alternative solutions include solvents, non-reactive monomers, inorganics, and nanoparticles.

[0112] Providing a mixture of monomers and liquid crystals may also involve mixing at least one monomer and liquid crystal with one or more of the following additional additives: initiators such as photoinitiators or coinitiators, polyfunctional thiols, dyes, adhesion promoters, surfactants, and / or other crosslinking agents. This mixture may be settled to allow the coinitiator to catalyze the reaction between the monomer and the thiol. The settling period may be carried out at a low temperature (e.g., 20°C) for approximately 8 hours in a dark space or a space with red light (e.g., infrared). After settling, the additional monomer may be mixed with the monomer. This mixture may then be filtered or strained through a filter having a small pore size (e.g., 0.45 μm pore size). After straining, this mixture may be stored at room temperature in a dark space or a space with red light before coating.

[0113] Next, a transparent substrate can be provided (202). In certain embodiments, the transparent substrate may be a glass substrate or a plastic substrate. A layer of the mixture can be deposited or coated onto the surface of the substrate (203). In some embodiments, the mixture is sandwiched between the transparent substrate and another substrate using glass spacers to maintain internal dimensions. Before the mixture is sandwiched, a non-stick coating may be applied to this other substrate. The non-stick coating may include fluoropolymers such as OPTOOL UD509 (Daikin Chemicals), Dow Corning 2634, Fluoropel (Cytonix), and EC200 (PPG Industries). A holographic recording beam can be shone on the mixed layer (204). The holographic recording beam may be a two-beam interference pattern that can induce phase separation of LC and polymer. In response to the holographic recording beam, the liquid monomer changes into a solid polymer, while the neutral, non-reactive substance (e.g., LC) diffuses during holographic exposure in response to a change in chemical potential driven by polymerization. LC can be one implementation of a neutral, non-reactive material, but other materials may also be used. The material and monomer may form a miscible mixture before holographic exposure and become miscible during holographic exposure.

[0114] After being exposed to a holographic recording beam, the mixture can be cured. The curing process may involve leaving the mixture under low-intensity white light for a period of time until the mixture is completely cured. Low-intensity white light may also induce a photobleaching dye process. Thus, an HPDLC diffraction grating having alternating polymer-rich and liquid crystal-rich regions can be formed (205). In some embodiments, the curing process may be carried out in less than two hours. After curing, one of the substrates can be removed by exposing the HPDLC diffraction grating. Advantageously, a non-stick coating may allow for the removal of the other substrate while leaving the HPDLC diffraction grating intact.

[0115] The HPDLC diffraction grating may contain alternating sections of liquid crystal-rich regions and polymer regions. The liquid crystals in the liquid crystal-rich regions can be removed to form a polymer surface relief diffraction grating or EBG that can be used as a deep SRG (206). The liquid crystals can be removed by gently immersing the diffraction grating in a solvent such as IPA. The IPA may be cooled and kept below room temperature while the diffraction grating is immersed in the IPA. The diffraction grating is then removed from the solvent and dried. In some embodiments, the diffraction grating is dried using a high-flow air source such as compressed air. After the LC is removed from the diffraction grating, a polymer-air surface relief Bragg diffraction grating is formed.

[0116] As shown in Figures 1A-1D, the formed surface relief diffraction grating can be further coated with a protective layer. In some cases, the protective layer may be a scratch-resistant moisture and oxygen barrier. In some cases, the protective layer may be a coating that does not fill the void regions where previously removed LCs were located. The coating may be deposited using a low-temperature process. In some implementations, the protective layer may have anti-reflective (AR) properties. The coating may be silicate or silicon nitride. The coating process may be pre-formed by a plasma-assisted chemical vapor deposition (CVD) process, such as a nanocoating process. The coating may be a parylene coating. The protective layer may be a glass layer. Before the protective layer is implemented, a vacuum or inert gas may be used to fill the gaps where previously removed LCs were located. In some embodiments, the coating process may be integrated with an LC removal process (206). For example, the coating material may be mixed with a solvent used to clean the LCs from the diffraction grating.

[0117] Figure 3 illustrates a schematic cross-sectional view of an exemplary embodiment of a polymer-air surface relief Bragg grating 3000 mounted on a waveguide 3002. The polymer-air surface relief Bragg grating 3000 includes periodic polymer compartments 3004a. Adjacent polymer compartments sandwich air compartments 3004b. Air compartments 3004b are sandwiched by polymer compartments 3004a. Air compartments 3004b and polymer compartments 3004a have different refractive indices. Advantageously, the polymer-air surface relief Bragg grating 3000 can be formed with a high grating depth 3006a to Bragg fringe spacing 3006b ratio, which can form a deep SRG. As previously discussed, deep SRGs can exhibit many beneficial qualities, such as high S diffraction efficiency, which may not be present in typical SRGs.

[0118] In one example, the polymer air surface relief Bragg diffraction grating 3000 may have a Bragg fringe spacing 3006b of 0.35 μm to 0.8 μm and a diffraction grating depth of 1 μm to 3 μm. In some embodiments, the polymer compartments 3004a may contain at least some residual liquid crystal when the liquid crystal is not completely removed during step 206 as described in relation to Figure 2. In some embodiments, the presence of residual LC in the polymer-rich region may increase the refractive index modulation of the final polymer SRG. In some embodiments, the air compartments 3004b may contain some residual liquid crystal when the liquid crystal is not completely removed from these air compartments 3004b during step 206. In some embodiments, by leaving some residual liquid crystal in the air compartments 3004b, a hybrid diffraction grating as described in relation to Figures 4-5 may be formed.

[0119] As discussed above, in many embodiments, the present invention also provides a method for fabricating hybrid surface relief / Bragg diffraction gratings. Figure 4A conceptually illustrates apparatus 210A that may be used in a step of a method for fabricating a hybrid surface relief diffraction grating (hybrid SRG) according to an embodiment of the present invention, in which a mixture of monomer and liquid crystal 211 deposited on a transparent substrate 212 is exposed to holographic exposure beams 213, 214. Figure 4B conceptually illustrates apparatus 210B that may be used in a step of a method for fabricating a hybrid SRG from an HPDLC Bragg diffraction grating 215 formed on a transparent substrate using a holographic exposure beam, according to an embodiment of the present invention. Figure 4C conceptually illustrates apparatus 210C that may be used in a step of a method for fabricating a surface relief diffraction grating, in which liquid crystal is removed from an HPDLC Bragg diffraction grating to form a polymer-air SRG 216, according to an embodiment of the present invention. These polymer-air SRGs 216 or EBGs may be deep SRGs. The steps illustrated in and described in relation to Figures 4A to 4C generally correspond to the steps illustrated in and described in relation to Figures 2A to 2C in the process for forming polymer-air SRGs, and therefore it is recognized that the previous explanation is applicable to Figures 4A to 4C.

[0120] In addition, Figure 4D conceptually illustrates additional steps that may be performed to form a hybrid diffraction grating. Apparatus 210D can be used in a step of a method for producing a surface relief diffraction grating according to embodiments of the present invention, in which the surface relief diffraction grating is refilled at least partially with liquid crystal to form a hybrid SRG217. The refilled liquid crystal may have a different consistency than the previously removed liquid crystal, as shown in Figure 4C. Furthermore, it is recognized that the liquid crystal removed in Figure 3C may be partially removed by an alternative method for forming the hybrid SRG217. In addition, Figure 4E conceptually illustrates apparatus 210E that may be used in a step of a method for producing a surface relief diffraction grating according to embodiments of the present invention, in which the hybrid SRG217 formed in the steps illustrated in Figure 4D is coated with a protective layer 218.

[0121] Figure 5 is a flowchart illustrating an exemplary method for forming a hybrid surface-relief Bragg diffraction grating from an HPDLC Bragg diffraction grating formed on a transparent substrate, according to an embodiment of the present invention. As shown, a method 220 for forming a hybrid surface-relief Bragg diffraction grating is provided. Referring to the flowchart, method 220 comprises providing a mixture of at least one monomer and at least one liquid crystal (221). The at least one monomer may include an isocyanate-acrylate monomer. Providing a mixture of monomer and liquid crystal may also comprise mixing at least one monomer and liquid crystal with one or more of the following: photoinitiators, coinitiators, polyfunctional thiols, and / or additional additives. The mixture may be allowed to settle so that the coinitiator can catalyze the reaction between the monomer and the thiol. The settling period may be carried out at a low temperature (e.g., 20°C) for approximately 8 hours in a dark space or a space with red light (e.g., infrared). After settling, additional monomers may be mixed with the monomer. Next, the mixture can be filtered or strained through a filter having a small pore size (e.g., 0.45 μm pore size). After straining, the mixture can be stored at room temperature in a dark space or a space with red light before coating.

[0122] Next, a transparent substrate can be provided (222). In certain embodiments, the transparent substrate may be a glass substrate or a plastic substrate. A non-stick coating may be applied to the transparent substrate before coating the mixture onto the substrate. A layer of the mixture can be deposited on the surface of the substrate (223). In some embodiments, the mixture is sandwiched between the transparent substrate and another substrate using glass spacers to maintain internal dimensions. The mixed layer can be exposed to a holographic recording beam (224). The holographic recording beam may be a two-beam interference pattern that can cause phase separation of LC and polymer. After exposure to the holographic recording beam, the mixture may be cured. The curing process may involve leaving the mixture under low-intensity white light for a period of time under conditions that allow the mixture to fully cure. Low-intensity white light may also cause a photobleaching dye process. Thus, an HPDLC diffraction grating with alternating polymer-rich and liquid crystal-rich regions can be formed (225). In some embodiments, the curing process may be carried out in less than two hours. After curing, one of the substrates may be removed by exposing the HPDLC diffraction grating.

[0123] HPDLC diffraction gratings may contain alternating compartments of liquid crystal-rich regions and polymer regions. To form a polymer surface relief diffraction grating or EBG, which is a form of deep SRG, the liquid crystals in the liquid crystal-rich regions can be removed (226). The liquid crystals can be removed by gently immersing the diffraction grating in a solvent such as isopropyl alcohol (IPA). While the diffraction grating is immersed in IPA, the IPA may be kept at a lower temperature. The diffraction gratings are those removed from the solvent and dried. In some embodiments, the diffraction gratings are dried using a high-flow air source such as compressed air. After the LCs have been removed from the diffraction grating, a polymer-air surface relief Bragg diffraction grating is formed. Steps 221-226 in Figure 5 largely correspond to the steps described in relation to Figure 2 in forming a polymer-air SRG, and therefore these descriptions are applicable to Figure 5.

[0124] Furthermore, method 220 includes at least partially refilling the cleared liquid crystal rich region with liquid crystal to form a hybrid SRG (227). The refilled liquid crystal may have a different consistency than the previously removed liquid crystal, which was previously removed in step 226. Furthermore, it is recognized that the liquid crystal removed in step 226 may be removed only partially by an alternative method to form a hybrid SRG. Advantageously, the hybrid SRG may offer the ability to tune certain beneficial characteristics of the SRG. One particular characteristic that can be improved by including at least some liquid crystal within the SRG is a reduction in haze properties.

[0125] As shown in Figure 4E, the formed surface relief diffraction grating can be further coated with a protective layer. In some cases, the protective layer may be a scratch-resistant moisture and oxygen barrier. In some cases, the protective layer may be a coating that does not fill the void regions where previously removed LCs were located. The coating may be deposited using a low-temperature process. In some implementations, the protective layer may have anti-reflective (AR) properties. The coating may be a silicate or silicon nitride. The coating process may be pre-formed by a plasma-assisted chemical vapor deposition (CVD) process, such as a plasma-treated nanocoating process. The coating may be a parylene coating. The protective layer may be a glass layer. Before the protective layer is implemented, a vacuum or inert gas may be used to fill the gaps where previously removed LCs were located. In some embodiments, the coating process may be integrated with an LC removal process (226). For example, the coating material may be mixed with a solvent used to clean the LCs from the diffraction grating. In some implementations, the coating material may be a material having a refractive index lower or higher than that of the polymer and can fill the spaces between adjacent polymer portions. The difference in refractive index between the polymer and the coating material can allow the polymer SRG to continue diffracting.

[0126] Figures 1–5 illustrate specific methods and apparatus for forming deep SRGs and hybrid surface relief / Bragg diffraction gratings, but various manufacturing methods can be utilized to implement different steps or modifications of such steps. As is readily apparent, a particular process may depend on the specific requirements of a given application. For example, many embodiments utilize another diffraction grating as a protective layer.

[0127] Hybrid SRG / Bragg diffraction gratings with shallow SRG structures can lead to low SRG diffraction efficiency. The method disclosed herein enables the formation of a more effective SRG structure by optimizing the depth of the liquid crystal in the liquid crystal-rich region so that the SRG has a high depth-to-grid pitch ratio while the Bragg diffraction grating is sufficiently thick for efficient diffraction. In many embodiments, the Bragg diffraction grating component of the hybrid diffraction grating can have a thickness in the range of 1 to 3 micrometers. In some embodiments, the SRG component of the hybrid diffraction grating can have a thickness in the range of 0.25 to 3 micrometers. For an initial HPDLC diffraction grating, the thickness will be equal to the sum of the final SRG and Bragg diffraction grating components. As is readily apparent, the thickness ratio of the two diffraction gratings may depend on the waveguide application. In some embodiments, the combination of SRG and Bragg diffraction grating can be used to fine-tune the angular bandwidth of the diffraction grating structure. In some cases, the SRG can increase the angular bandwidth of the diffraction grating structure.

[0128] In many embodiments, the hybrid SRGs shown in Figures 4A–4E can provide a spatially variable relative SRG / Bragg diffraction grating intensity by varying the refilling depth of the liquid crystal region of the diffraction grating across the entire grating. In some embodiments, the liquid crystal in the liquid crystal-rich diffraction grating region can be completely or partially removed during liquid crystal removal and refilling as defined in steps 206, 226, and 227. In some embodiments, the liquid crystal used to refill or partially refill the liquid crystal-cleared region may have a different chemical composition from the liquid crystal used to form the initial HPDLC diffraction grating. In various embodiments, a first liquid crystal having phase separation properties equivalent to those of the monomer may be specified to provide an HPDLC diffraction grating with optimal modulation and diffraction grating definition, while a second refilled liquid crystal may be specified to provide desired index modulation properties in the final hybrid diffraction grating. In some embodiments, the Bragg portion of the hybrid diffraction grating may be switchable by electrodes deposited on the surfaces of the substrate and cover layer. In many embodiments, the refilled liquid crystal may contain additives that may include, but are not limited to, features that improve switching voltage, switching time, polarization, transparency, and other parameters. A hybrid diffraction grating formed using a refilling process would have the added advantage of the LC forming a continuum (rather than an assembly of LC droplets), thereby reducing haze.

[0129] Deep SRGs, EBGs, and / or hybrid SRGs may be described in the context of S and P diffraction gratings, but these diffraction gratings are applicable to many other types of diffraction gratings. These include, but are not limited to, angle-multiplexed diffraction gratings, color-multiplexed diffraction gratings, folded diffraction gratings, double-interaction diffraction gratings, rotated K-vector diffraction gratings, cross-folded diffraction gratings, mosaic diffraction gratings, chirp diffraction gratings, diffraction gratings with spatially variable refractive index modulation, diffraction gratings with spatially variable diffraction grating thickness, diffraction gratings with spatially variable mean refractive index, diffraction gratings with spatially variable refractive index modulation tensors, and diffraction gratings with spatially variable mean refractive index tensors. Furthermore, deep SRGs, EBGs, and / or hybrid SRGs may be switchable or non-switchable diffraction gratings depending on their specific implementation. Deep SRGs, EBGs, and / or hybrid SRGs may be fabricated on plastic or glass substrates. Also, these diffraction gratings may be fabricated on one substrate and transferred to another.

[0130] A consideration of various implementation methods for deep SRG or EBG. In many embodiments, deep SRGs can provide a means for controlling polarization in a waveguide. SBGs are typically P-polarization selectivity, leading to a 50% efficiency loss in non-polarized light sources such as OLEDs and LEDs. Therefore, combining an S-polarization grating with a P-polarization grating can theoretically provide a twofold improvement over a waveguide using only a P-polarization grating. In some embodiments, the S-polarization grating can be provided by a Bragg grating formed from a conventional holographic photopolymer. In some embodiments, the S-polarization grating can be provided by a Bragg grating formed from HPDLC having birefringence modified using an array layer or other process to rearrange the liquid crystal director. In some embodiments, the S-polarization grating can be formed using liquid crystals, monomers, and other additives that naturally organize into an S-polarization grating under phase separation. In many embodiments, the S-polarization grating can be provided by an SRG. Using the processes described above, a deep SRG exhibiting high S-diffraction efficiency (up to 99%) and low P-diffraction efficiency can be formed by removing the liquid crystal from an SBG formed from holographic phase separation of a mixture of liquid crystals and monomers.

[0131] Deep SRGs can also provide other polarization response characteristics. Several prior art theoretical studies, such as the paper by Moharam (MGet al. “Diffraction characteristics of photoresist surface-relief gratings”, Applied Optics, Vol.23, page 3214, Sep 15, 1984), have pointed to deep surface-relief diffraction gratings having both S sensitivity and P sensitivity, with S being dominant. In some embodiments, deep SRGs illustrate the ability to provide S polarization response. However, deep SRGs can also provide other polarization response characteristics. In many embodiments, deep surface-relief diffraction gratings having both S sensitivity and P sensitivity, with S being dominant, are implemented. In some embodiments, the thickness of the SRG can be adjusted to provide a variety of S and P diffraction characteristics. In some embodiments, the diffraction efficiency may be high with respect to P over a certain spectral and angular bandwidth, and low with respect to S over the same spectral and angular bandwidth. In some embodiments, high efficiency can be provided for both S-polarized and P-polarized light. Theoretical analysis of an air-immersed SRG with a refractive index of 1.6 (hence providing an average diffraction grating refractive index of 1.3) with a period of 0.48 microns and an incident angle of 0 degrees and a diffraction angle of 45 degrees for a wavelength of 0.532 microns is shown in Figures 5-7. Figure 5 is a graph showing the calculated P-polarized and S-polarized diffraction efficiency versus incident angle for a deep relief diffraction grating with a thickness of 1 micrometer, illustrating that high S and P responses can be achieved in this case. Figure 6 is a graph showing the calculated P-polarized and S-polarized diffraction efficiency versus incident angle for a deep surface relief diffraction grating with a thickness of 2 micrometers, illustrating that the S-polarized response is dominant over most of the angular range of the diffraction grating in this case.Figure 7 is a graph showing the calculated P-polarized and S-polarized diffraction efficiency versus incident angle for a thickness of 3 micrometers, illustrating that in this case, the P-polarized response is dominant over a significant portion of the angular range of the diffraction grating.

[0132] In many embodiments, the photonic crystal may be a reflective Bragg diffraction grating or deep SRG formed by an LC extraction process. Deep reflective SRGs made using phase separation followed by LC removal can enable wide angular and spectral bandwidths. In many embodiments, the optical path from the picture generating unit (PGU) to the waveguide can be reduced by replacing the current input SRG with a reflective photonic crystal. In some embodiments, the PGU pupil and the waveguide can be brought into contact. In many embodiments, the reflective deep SRG may be approximately 3 microns thick. The diffraction properties of an LC-extracted Bragg diffraction grating result primarily from the refractive index gap between the polymer and air (rather than from the depth of the diffraction grating, as in the case of a typical SRG).

[0133] Consideration of thiol additives in the initial mixture Figures 9A and 9B illustrate scattering electron microscope (SEM) images of a comparative example of mixtures used to prepare polymer-air SRGs. As previously discussed, the monomers in the initial mixture may be acrylate or thiolene-based. With some monomers, such as acrylate monomers, it has been found that during washing after holographic exposure, the solvent removes not only the liquid crystal material but also the incomplete polymer. It has been found that polyfunctional thiol additives can solve this problem by strengthening the polymer and thus enabling it to be strong enough to withstand solvent washing. Without being limited to any particular theory, thiol additives can improve the mechanical strength of formulations consisting of low-functional acrylate monomers, which tend to form mechanically weak polymers due to reduced crosslinking. Acrylate monomer formulations may be advantageous because they can exhibit lower haze and higher diffraction efficiency. Therefore, the addition of thiols may make acrylate monomer formation a viable option in the preparation of polymer SRGs.

[0134] There can be trade-offs between phase separation, diffraction grating formation, and mechanical strength between different formulations. Diffraction grating formation can be benefited from mixtures containing low-functionality monomers, which react more slowly during holographic exposure, resulting in less crosslinking and greater diffusion of unreactive components (e.g., LC). Conversely, mixtures consisting of highly functionality monomers may exhibit better phase separation and polymer mechanical strength due to greater crosslinking, but they may react so rapidly that there is not enough time for unreactive components to diffuse, resulting in lower diffraction efficiency.

[0135] Without being limited to any particular theory, thiol additives can circumvent these limitations by reacting with acrylates or isocyanate-acrylates before holographic exposure to form a loose scaffold. This scaffold can improve the mechanical strength and uniformity of the cured polymer. Thus, this mechanical strength can be tuned through slight adjustments of thiol functionality and concentration without significantly increasing the average functionality of the monomer mixture and interfering with diffraction grating formation.

[0136] Figure 9A illustrates an initial mixture, while Figure 9B illustrates a comparative mixture containing 1.5% by weight of thiols. However, other weight percentages of thiol additives are also considered. For example, the weight percentage of the thiol additive may be 1% to 4% or 1.5% to 3%. In some embodiments, the polyfunctional thiol may be trimethylrollpropanetris (3-mercaptopropionate). Both Figures 9A and 9B include polymer high-density regions 902a / 902b and air regions 904a / 904b. As illustrated, added thiols can create a higher density polymer structure in polymer high-density region 902a in Figure 9B than in polymer high-density region 902b in Figure 9A, which can improve diffraction grating performance. It has been found that the weight percentage of the thiol additive should be balanced to provide stability within the polymer structure to withstand solvent washing, but not to make it rigid so as not to release liquid crystals during solvent washing.

[0137] Comparison of HPDLC diffraction grating performance and polymer-air SRG performance Figures 10A and 10B illustrate images of comparative examples of HPDLC diffraction gratings and polymer SRG or EBG. Figure 10A illustrates the performance of an exemplary HPDLC diffraction grating with the liquid crystals not removed. The diffraction grating in Figure 10A exhibits a nominal or near-0% S diffraction efficiency while containing a P diffraction efficiency of 20–30%. Figure 10B illustrates the performance of an exemplary polymer-air SRG with the liquid crystals removed. The diffraction grating in Figure 10B exhibits a P diffraction efficiency of 18–28% while containing an S diffraction efficiency of 51–77%. Thus, polymer-air SRGs with the liquid crystals removed illustrate relatively high S diffraction efficiency while maintaining comparable P diffraction efficiency. Furthermore, the diffraction grating in Figure 10B contains a P diffraction haze of 0.11–0.15% and an S diffraction haze of 0.12–0.16%.

[0138] Figures 11A and 11B illustrate comparative plots of an HPDLC diffraction grating with the liquid crystal still intact and a polymer SRG or EBG with the liquid crystal removed. Figure 11A illustrates the P-diffraction efficiency and S-diffraction efficiency of an HPDLC diffraction grating with the liquid crystal still intact. The first line 1102a corresponds to the P-diffraction efficiency, and the second line 1104a corresponds to the S-diffraction efficiency. Figure 11B illustrates the P-diffraction efficiency and S-diffraction efficiency of a polymer SRG or EBG with the liquid crystal removed. The first line 1102b corresponds to the P-diffraction efficiency, and the second line 1104b corresponds to the S-diffraction efficiency. As illustrated, the S-diffraction efficiency increases dramatically after the liquid crystal is removed, while the P-diffraction efficiency remains relatively constant.

[0139] In some embodiments, the ratio of S diffraction efficiency to P diffraction efficiency can be adjusted by using different diffraction grating periods, diffraction grating slant angles, and diffraction grating thicknesses.

[0140] Various examples of deep SRG depth Figures 12A and 12B illustrate various comparative examples of P-diffraction and S-diffraction efficiencies in deep SRGs of varying depths. Each of these plots shows diffraction efficiency versus angle. In Figure 12A, the deep SRG has a depth of approximately 1.1 μm. The first line 1102a represents the S-diffraction efficiency, and the second line 1104a represents the P-diffraction efficiency. As illustrated, the peak S-diffraction efficiency is approximately 58%, and the peak P-diffraction efficiency is 23%. It is noted that in this example, the haze for S-diffraction is 0.12%, and the haze for P-diffraction is 0.11%. Such high diffraction efficiency with low haze can make deep SRGs with a depth of approximately 1.1 μm particularly suitable for multiplexed diffraction gratings.

[0141] In Figure 12B, the deep SRG has a depth of approximately 1.8 μm. The first line 1102b represents the S diffraction efficiency, and the second line 1104b represents the P diffraction efficiency. As illustrated, the peak S diffraction efficiency is approximately 92%, and the peak P diffraction efficiency is 63%. It is noted that in this example, the haze for S diffraction is 0.34%, and the haze for P diffraction is 0.40%. Thus, both the S diffraction efficiency and the P diffraction efficiency increase dramatically with increasing diffraction grating depth. It is also noted that the haze appears to increase with increasing diffraction grating depth.

[0142] Various exemplary initial LC concentrations in the mixture Figures 13A and 13B illustrate the results of comparative studies of various EBGs with different initial LC concentrations in the initial mixture. Figure 13A illustrates the S diffraction efficiency versus angle. Figure 13B illustrates the P diffraction efficiency versus angle. In Figure 13A, the first line 1202a corresponds to a 20% initial LC content, the second line 1204a corresponds to a 30% initial LC content, and the third line 1206a corresponds to a 40% initial LC content. In Figure 13B, the first line 1202b corresponds to a 20% initial LC content, the second line 1204b corresponds to a 30% initial LC content, and the third line 1206b corresponds to a 40% initial LC content. Table 1 illustrates a summary of the various results of the comparative studies. [Table 1]

[0143] As illustrated in Figures 13A and 13B and noted in Table 1, both the maximum S diffraction and maximum P diffraction appear to increase with higher initial LC content, while the S diffraction haze and P diffraction haze remain nearly constant.

[0144] Figures 14A and 14B illustrate additional exemplary S-diffraction and P-diffraction efficiencies for various initial LC concentrations. Figure 14A illustrates the S-diffraction efficiency of various exemplary EBGs with various initial LC content. Figure 14B illustrates the P-diffraction efficiency of various exemplary EBGs with various LC content. For both Figures 14A and 14B, from top to bottom, the lines represent 32% LC content, 30% LC content, 28% LC content, 26% LC content, 24% LC content, 22% LC content, and 20% LC content. As illustrated, the S-diffraction and P-diffraction efficiencies are directly related to the amount of LC content (e.g., higher LC content yields higher S-diffraction and P-diffraction efficiencies).

[0145] While not limited to any particular theory, the initial LC content is related to the amount of phase separation between LC and monomer that occurs during the holographic exposure and polymerization processes. Therefore, a higher LC content will increase the amount of LC-rich region removed to create more air regions after washing. The increased air regions create a larger refractive index difference (Δn) between the air regions (formerly liquid crystal-rich regions) and polymer-rich regions, increasing both S-diffraction and P-diffraction efficiencies. In some embodiments, the average refractive index of the polymer SRG can be adjusted by controlling the initial neutral substance (e.g., LC) content, thereby increasing or decreasing the volume of the polymer after removal of the neutral substance. Furthermore, increasing the initial neutral substance content can affect mechanical strength. Therefore, increasing or decreasing mechanical strengthening agents, such as thiol additives, can be used to balance the increase or decrease in mechanical strength.

[0146] Embodiments including an OLED array as an image generator There is growing interest in the use of organic light-emitting diode (OLED) arrays as image generators in waveguide displays. OLEDs offer many advantages in waveguide display applications. As a light-emitting technology, OLEDs do not require a light source. OLEDs can be printed over large areas with excellent cost-effectiveness. Non-rectangular pixel array patterns can be printed on curved or flexible substrates. As discussed below, the ability to pre-distort the pixel array to form a curved focal plane adds new design dimensions that can compensate for distortion of the guided beam wavefront caused by the curved waveguide and prescription lens supported by the waveguide. OLEDs with a resolution of 4Kx4K pixels are currently available, with the prospect of higher resolutions in the short term, and offer a faster route to high-resolution wide-field-of-view AR displays than can be provided by technologies such as silicon-based liquid crystal displays (LCoS) and microelectromechanical systems (MEMS) devices such as digital photoprocessing (DLP) devices. Another significant advantage over LCoS is that OLEDs can switch in microseconds (compared to milliseconds in LC devices).

[0147] OLEDs have certain drawbacks. In their basic form, OLEDs are Lambertian emitters, making efficient light collection far more difficult than with LCoS and DLP microdisplays. The red, green, and blue spectral bandwidths of OLEDs are wider than those of light-emitting diodes (LEDs), presenting further light management problems in holographic waveguides. The most significant drawback of OLEDs is that in waveguides using HPDLC diffraction gratings, such as switchable Bragg diffraction gratings (SBGs), which tend to be P-polarization selective, half of the light available from the OLED is wasted. Therefore, many embodiments of the present invention relate to waveguide displays for use with emission unpolarized image sources that can provide high optical efficiency for unpolarized light, and related methods for manufacturing such waveguide displays.

[0148] For the purpose of illustrating embodiments, some well-known features of optical technology known to those skilled in the art of optical design and visual displays are omitted or simplified so as not to obscure the basic principles of the present invention. Unless otherwise specified, the term “on-axis” in relation to ray or beam direction refers to propagation parallel to an axis perpendicular to the surface of the optical components described in relation to the present invention. In the following description, the terms light, ray, beam, and direction may be used interchangeably and in relation to one another to indicate the direction of propagation of electromagnetic radiation along a linear trajectory. The terms light and irradiation may be used in relation to the visible and infrared bands of the electromagnetic spectrum. Parts of the following description are presented using specialized terminology commonly adopted by those skilled in the art of optical design. As used herein, the term diffraction grating may, in some embodiments, encompass a diffraction grating composed of a set of diffraction gratings. For illustrative purposes, it should be understood that the drawings are not drawn to scale unless otherwise specified.

[0149] Referring here to the drawings, various embodiments of the present invention illustrate methods and apparatus for providing a waveguide display using an luminescent input image panel. Figure 15 conceptually illustrates a waveguide display according to an embodiment of the present invention. As shown, the apparatus 100 includes a waveguide 101 supporting an input diffraction grating 102 and an output diffraction grating 103 having high diffraction efficiency for P-polarized light in a first wavelength band, and an input diffraction grating 104 and an output diffraction grating 105 having high diffraction efficiency for S-polarized light in a first wavelength band.

[0150] The apparatus 100 further includes an OLED microdisplay 106 that emits unpolarized light having an emission spectral bandwidth, including a first wavelength band and a collimation lens 107 for projecting light from the OLED microdisplay 106 into a field of view. In exemplary embodiments, the S-diffraction gratings and P-diffraction gratings 102-105 can be layered without requiring voids. In other embodiments, the diffraction grating layers can be separated by voids or transparent layers. The S-diffraction gratings and P-diffraction gratings 102-105 may be the deep SRG or EBG described above.

[0151] Figure 16 conceptually illustrates a waveguide display according to an embodiment of the present invention, in which the P diffraction grating and S diffraction grating are arranged in separate waveguide layers separated by air. As shown, the apparatus 110 comprises an upper waveguide layer 111 and a lower waveguide layer 112 (supporting diffraction gratings 102, 103 and 104, 105, respectively) separated by an air gap 113. The diffraction gratings 102, 103 and 104, 105 may be the deep SRG and EBG described above.

[0152] Figure 17 conceptually illustrates a typical ray path for a waveguide display according to an embodiment of the present invention. In the embodiment 120 illustrated in Figure 17, the microdisplay 106 is configured to emit unpolarized light 121 in a first wavelength band, which is collimated and projected into the field of view by a collimator lens 107. The S-polarized emission from the microdisplay 106 can be coupled to the total internal reflection path of the waveguide 101 by an S-diffraction input diffraction grating 104 and extracted from the waveguide 101 by an S-diffraction output diffraction grating 105. The P-polarized light from the microdisplay 106 can be internally coupled and extracted in a similar manner using P-diffraction input and output diffraction gratings 102, 103. Provided that the spatial frequencies of the input and output diffraction gratings match, dispersion can be corrected for both S-light and P-light. The input and output diffraction gratings 102, 103 may be the deep SRG or EBG described above.

[0153] Figures 15-17 show a specific waveguide display configuration, but various configurations can be implemented, including modifications to those shown, and the particular implementation may depend on the specific requirements of a given application. Furthermore, such displays can be manufactured using several different methods. For example, in many embodiments, the two diffraction grating layers are formed using an inkjet printing process.

[0154] In many embodiments, the waveguide operates in a monochromatic band. In some embodiments, the waveguide operates in the green band. In some embodiments, waveguide layers operating in different spectral bands, such as red, green, and blue (RGB), can be stacked to provide a three-layer waveguide structure. In further embodiments, the layers are stacked with air gaps between the waveguide layers. In various embodiments, the waveguide layers operate in broader bands, such as blue-green and green-red, to provide a two-layer waveguide solution. In other embodiments, the grating is color-multiplexed to reduce the number of grating layers. Various types of gratings can be implemented. In some embodiments, at least one grating in each layer is a switchable grating.

[0155] The present invention can be applied using a variety of waveguide architectures, including those disclosed in the literature. In many embodiments, the waveguide can incorporate at least one of the following: angle-multiplexed diffraction gratings, color-multiplexed diffraction gratings, folded diffraction gratings, double-interaction diffraction gratings, rotated K-vector diffraction gratings, cross-folded diffraction gratings, mosaic diffraction gratings, chirp diffraction gratings, diffraction gratings with spatially varying refractive index modulation, diffraction gratings with spatially varying diffraction grating thickness, diffraction gratings with spatially varying average refractive index, diffraction gratings with spatially varying refractive index modulation tensors, and diffraction gratings with spatially varying average refractive index tensors. In some embodiments, the waveguide can incorporate at least one of the following: half-wave plates, quarter-wave plates, anti-reflective coatings, beam splitting layers, alignment layers, photochromic back layers for glare reduction, and louver films for glare reduction. In some embodiments, the waveguide can support diffraction gratings that provide separate optical paths for different polarizations. In various embodiments, the waveguide can support diffraction gratings that provide separate optical paths for different spectral bandwidths. In some embodiments, the diffraction gratings for use in the present invention may be HPDLC diffraction gratings, switching diffraction gratings recorded on HPDLC (such as switchable Bragg diffraction gratings), Bragg diffraction gratings recorded on holographic photopolymers, or surface relief diffraction gratings.

[0156] In many embodiments, waveguide displays can provide oblique image fields of view of at least 50°. In further embodiments, waveguide displays can provide oblique image fields of view of at least 70°. In some embodiments, OLED displays can have a brightness greater than 4000 nits and a resolution of 4kx4k pixels. In some embodiments, waveguides can have an optical efficiency greater than 10% so that an OLED display with a brightness of 4000 nits can provide an image brightness greater than 400 nits. Waveguide displays with P diffraction gratings typically have waveguide efficiencies of 5% to 6.2%. Providing S diffraction gratings, as discussed above, can double the waveguide efficiency. In various embodiments, eye boxes greater than 10 mm can be provided with eye relief greater than 25 mm. In many embodiments, the waveguide thickness can be 2.0 to 5.0 mm.

[0157] Figure 18 conceptually illustrates a waveguide display according to an embodiment of the present invention, in which at least one portion of the waveguide optical surface is curved, and the effect of the curved surface portion on the guided beam wavefront. As shown, the apparatus 130 includes a waveguide 131 supporting the curved surface portion 132. In an exemplary embodiment, the waveguide 131 supports input diffraction gratings 102 and output diffraction gratings 103 having high diffraction efficiency for P-polarized light in a first wavelength band, and input diffraction gratings 104 and output diffraction gratings 105 having high diffraction efficiency for S-polarized light in a first wavelength band. A microdisplay 106 displaying a rectangular array of pixels 133 emits unpolarized light 134 in a first wavelength band, which is collimated by a collimator lens 107 and projected into the field of view. P-polarized emission from the microdisplay 106 can be coupled to a total internal reflection path in the waveguide by a P-diffraction input diffraction grating 102 and extracted from the waveguide by a P-diffraction output diffraction grating 103. The presence of any non-planar surface in the waveguide can distort the waterfront of the guided light, causing the output light to exhibit defocus, geometric distortion, and other aberrations when viewed from the eyebox. For example, in Figure 18, light projected from a single pixel by a collimator lens 107 has a plane wavefront 135, which propagates along the waveguide 131 along the TIR path 136 and then forms non-parallel output rays 137-139 that are perpendicular to the curved output wavefront 139A. On the other hand, a perfectly planar waveguide would instead provide parallel beam-expanded light. Figure 19 conceptually illustrates version 140 of the waveguide, in which the waveguide substrate 141 supports two overlapping upper curved surfaces 142 and lower curved surfaces 143.

[0158] Figure 20 conceptually illustrates a waveguide display according to an embodiment of the present invention, in which aberrations introduced by a curved surface portion can be corrected by pre-distorting the pixel pattern of the OLED microdisplay. In the exemplary embodiment, the waveguide apparatus 150 is similar to that illustrated in Figure 18. As shown, the apparatus 150 includes a microdisplay 151 supporting a pre-distorted pixel pattern 152. Unpolarized first-wavelength light 153 emitted by the microdisplay is focused by a lens 107, which substantially collimates the beam entering the waveguide while forming a wavefront 154 that is slightly pre-distorted. After internal coupling and propagation 155 through the waveguide 131, the pre-distorted wavefront is focused by a curved surface 132 to form parallel output rays 156-158 perpendicular to the planar output wavefront 159.

[0159] Figure 21 conceptually illustrates a waveguide display according to an embodiment of the present invention, in which aberrations introduced by a curved surface portion can be corrected by pre-distorting the pixel pattern of an OLED microdisplay formed on the curved surface. The curved microdisplay substrate can help correct focal error, image field curvature, distortion, and other aberrations in relation to the distorted pixel pattern. In the exemplary embodiment, the waveguide apparatus 160 is similar to that illustrated in Figure 18. As shown, the curved substrate microdisplay 161 supports a pre-distorted pixel pattern 164. Unpolarized first-wavelength light 163 emitted by the microdisplay is focused by lens 107 to form a substantially collimated guided beam having a slightly pre-distorted wavefront 164, which, after internal coupling and propagation 165 through waveguide 131, forms parallel output rays 166-168 perpendicular to the planar output wavefront 169.

[0160] Figures 18–21 show specific configurations of waveguides with curved surfaces, but many other different configurations and modifications can be implemented. For example, the techniques and fundamental theories exemplified in such embodiments can also be applied to waveguides supporting ophthalmic optical surfaces. In many embodiments, the prescription waveguide substrate can be custom manufactured using processes similar to those used in the manufacture of ophthalmic eyeglasses, and a standard baseline prescription can be fine-tuned to individual user requirements. In some embodiments, the waveguide diffraction grating can be inkjet printed with a standard baseline prescription. In some embodiments, the OLED display can be custom printed with a pre-distorted pixel pattern. In various embodiments, the OLED display can be printed on a curved backplane substrate. In some embodiments, additional refraction or diffraction pre-compensation elements can be supported by the waveguide. In many embodiments, additional compensation functions can be encoded in at least one of the input and output diffraction gratings. The input and output diffraction gratings may be deep SRGs, EBGs, or hybrid diffraction gratings as described above and can be manufactured in the manner described in relation to Figures 1–5. The input and output diffraction gratings may also have the thickness described in relation to Figures 6–8.

[0161] Figure 22 is a flowchart conceptually illustrating a method for projecting image light for projection using a waveguide including an S-diffraction grating and a P-diffraction grating, according to an embodiment of the present invention. As shown, a method 170 for forming an image is provided. Referring to the flowchart, method 170 includes providing an OLED array emitting light in a first wavelength range, a collimation lens, and a waveguide supporting an input and output diffraction grating having high diffraction efficiency for S-polarized light in the first wavelength band, and an input and output diffraction grating having high diffraction efficiency for P-polarized light in the first wavelength band (171). In some embodiments, the input and output diffraction gratings may be deep SRG, EBG, or hybrid diffraction gratings as previously considered. The image light emitted by the OLED array can be collimated using the collimation lens (172). S-polarized light can be coupled to the total internal reflection path of the waveguide using an S-diffraction input diffraction grating (173). P-polarized light can be coupled to the total internal reflection path of the waveguide using a P-diffraction input diffraction grating (174). S-polarized light can be beam-expanded and extracted from the waveguide for projection (175). P-polarized light can be beam-expanded and extracted from the waveguide for projection (176).

[0162] Figure 23 is a flowchart conceptually illustrating a method for projecting image light for projection using a waveguide that supports an optically formulated surface and includes an S-diffraction grating and a P-diffraction grating, according to an embodiment of the present invention. As shown, a method 180 for forming an image is provided. Referring to the flowchart, method 180 includes providing an OLED array having a pre-distorted pixel pattern emitting light in a first wavelength range, a collimation lens, an input and output diffraction grating having high diffraction efficiency for S-polarized light in the first wavelength band, and a waveguide supporting the input and output diffraction gratings having high diffraction efficiency for P-polarized light in the first wavelength band (181), and further providing a formulated optical surface supported by the waveguide (182). In some embodiments, the input and output diffraction gratings may be deep SRG, EBG, or hybrid diffraction gratings as previously considered. The image light emitted by the OLED array can be collimated using the collimation lens (183). S-polarized light can be coupled to the total internal reflection path of the waveguide using an S-diffraction input diffraction grating (184). P-polarized light can be coupled to the total internal reflection path of the waveguide using a P-diffraction input diffraction grating (185). A pre-distorted wavefront can be reflected by the formulation surface (186). A plane wavefront can be formed from a pre-distorted wavefront using the optical forces of the formulation surface (187). S-polarized light can be beam-expanded and extracted from the waveguide for projection (188). P-polarized light can be beam-expanded and extracted from the waveguide for projection (189).

[0163] Consideration of embodiments including various pixel geometric shapes The various devices considered in this disclosure can be applied using light-emitting displays having input pixel arrays of many different geometric shapes, limited only by geometric constraints and practical problems in array implementation. In many embodiments, the pixel array may include pixels that are aperiodic (non-repeating). In such embodiments, asymmetry in the geometric shape and the distribution of pixels can be used to generate uniformity of output illumination from the waveguide. The optimal pixel size and geometric shape can be determined using inverse vector ray tracing from the eyebox to the pixel array through the output and input diffraction gratings (and folded diffraction gratings, if used). A variety of asymmetric pixel patterns can be used in the present invention. For example, Figure 24A conceptually illustrates a portion 230 of a pixel pattern containing rectangular elements 230A-230F of different sizes and aspect ratios for use in a light-emitting display panel according to an embodiment of the present invention. In some embodiments, the pixel array may be based on a non-repeating pattern based on a finite set of polygonal base elements. For example, Figure 24B conceptually illustrates a portion 240 of a pixel pattern having Penrose tiles 240A-240J for use in a light-emitting display panel according to an embodiment of the present invention. The tiles may be based on the principle disclosed in U.S. Patent No. 4,133,152 by Penrose, entitled “Set of tiles for covering a surface.” Naturally occurring patterns, such as honeycomb, are also used in many embodiments.

[0164] In many embodiments, pixels can include arrays of the same regular polygon. For example, Figure 24C conceptually illustrates a portion 250 of a pixel pattern having hexagonal elements according to an embodiment of the present invention. Figure 24D conceptually illustrates a portion 260 of a pixel pattern having square elements 250A to 250C according to an embodiment of the present invention. Figure 24E conceptually illustrates a portion 270 of a pixel pattern having diamond-shaped elements 270A to 270D according to an embodiment of the present invention. Figure 24F conceptually illustrates a portion 280 of a pixel pattern having isosceles triangular elements 280A to 280H according to an embodiment of the present invention.

[0165] In many embodiments, pixels have a vertically or horizontally biased aspect ratio. Figure 24G conceptually illustrates a portion 290 of a pixel pattern having hexagonal elements 290A to 290C with a horizontally biased aspect ratio. Figure 24H conceptually illustrates a portion 300 of a pixel pattern having rectangular elements 300A to 300D with a horizontally biased aspect ratio according to an embodiment of the present invention. Figure 24I conceptually illustrates a portion 310 of a pixel pattern having diamond-shaped elements 310A to 310D with a horizontally biased aspect ratio according to an embodiment of the present invention. Figure 24J conceptually illustrates a portion 320 of a pixel pattern having triangular elements 320A to 320H with a horizontally biased aspect ratio according to an embodiment of the present invention.

[0166] In many embodiments, OLEDs can be fabricated using cavity shapes and multilayer structures to shape the spectral emission characteristics of the OLED. In some embodiments, microcavity OLEDs optimized to provide a narrow spectral bandwidth can be used. In some embodiments, the spectral bandwidth may be less than 40 nm. In some embodiments, a spectral bandwidth of 20 nm or less can be provided. In some embodiments, OLEDs can be fabricated from materials that provide electroluminescence in a relatively narrow bandwidth centered near a selected spectral region corresponding to one of three primary colors. Figure 25 conceptually illustrates a pixel pattern in which different pixels may have different emission characteristics. In some embodiments, pixels may have different spectral emission characteristics depending on their position in the pixel array. In some embodiments, pixels may have different angular emission characteristics depending on their position in the pixel array. In some embodiments, pixels may have both spatially varying spectral and angular emission characteristics across the entire pixel array. The pixel pattern may be based on any of the patterns illustrated in Figures 24A to 24J. In many embodiments, pixels of different sizes and geometric shapes can be arranged to provide spatial emission variation to control uniformity in the final image.

[0167] In many embodiments, OLEDs can have cavity structures designed to transform a given light distribution into a customized form. This is typically achieved by secondary optical elements, which can be bulky for wearable display applications. Such designs also suffer from the problem that the final light source is limited to a single permanent operating mode, which can only be overcome by using mechanically tunable optical elements. In some embodiments, OLEDs can enable real-time beam shaping without relying on secondary optical elements and without using any mechanical adjustments. In some embodiments, OLEDs can be continuously tuned between the forward axis and the off-axis principal emission direction while maintaining high quantum efficiency in any setting, as disclosed in the paper by Fries F. et al. ("Real-time beam shaping without additional optical elements", Light Science & Applications, 7(1), 18, (2018)).

[0168] A significant OLED development, "microcavity OLEDs," may offer the possibility of more controlled spectral bandwidth and emission angle in some embodiments. However, microcavity OLEDs are not yet ready for commercial use. In one embodiment (corresponding to a 2-micron diffraction grating with refractive index modulation of 0.1, average refractive index of 1.65, and incidence angle in a 45-degree waveguide), the diffraction efficiency of the SBG is greater than 75% across the entire OLED emission spectrum (between 25% of the peak points). Narrower bandwidth OLEDs using deeper cavity structures would reduce the bandwidth to less than 40 nm.

[0169] Advantageously, the present invention can utilize OLEDs optimized for use with 460nm blue, which provides better blue contrast in daylight AR display applications, as well as better reliability and lifespan, compared to the more commonly used 440nm OLEDs.

[0170] In some embodiments, the light-emitting display may be an OLED full-color silicon backplane microdisplay similar to that developed by Kopin Corporation (Westborough, MA). The Kopin microdisplay offers a 0.99-inch image diagonal and a pixel density of 2490 pixels / inch. This microdisplay utilizes Kopin's patented Pantile® magnifying lens to enable a compact form factor.

[0171] While the present invention has been discussed in terms of embodiments using OLED microdisplays as input image sources, in many other embodiments, the invention can be applied with any other type of light-emitting microdisplay technology. In some embodiments, the light-emitting microdisplay may be a microLED. MicroLEDs benefit from reduced power consumption and can operate efficiently at higher brightness than OLED displays. However, microLEDs are originally monophosphors typically used to convert the color of LEDs, and do not scale well to small sizes, leading to a more complex device architecture that is difficult to scale down for microdisplay applications.

[0172] While polymer diffraction grating structures have been considered in terms of their use in OLED array-based waveguide displays, polymer diffraction grating structures have advantageous synergistic applications with other classes of displays. Examples of these displays include image generators that use non-emissive display technologies such as LCoS and MEMS-based displays. Although LCoS-based displays typically emit polarized light, which can render the polarization-based advantages of polymer diffraction grating structures less applicable, polymer diffraction grating structures can offer advantageous efficiency and cost savings compared to conventional imprinted diffraction gratings. Furthermore, polymer diffraction grating structures may be applicable to a variety of other non-display waveguide-based implementations, such as waveguide sensors and / or waveguide illumination devices.

[0173] The doctrine of equality While the above description encompasses many specific embodiments of the present invention, these should not be interpreted as limitations on the scope of the invention, but rather as examples of one embodiment thereof. Therefore, it should be understood that the invention may be carried out in ways other than those specifically described without departing from the scope and spirit of the invention. Thus, the embodiments of the invention should be considered in all respects as illustrative and not limiting. Accordingly, the scope of the invention should be determined not by the exemplary embodiments, but by the appended claims and their equivalents.

Claims

1. A method for fabricating a surface relief diffraction grating (SRG), To provide a mixture of monomer and liquid crystal, To provide a substrate, Coating the surface of the substrate with a layer of the mixture, A non-adhesive release layer is applied to one surface of another substrate, The mixture is sandwiched between the substrate and the other substrate using one or more spacers to maintain the internal dimensions, By irradiating the aforementioned layer with a holographic recording beam, a holographic polymer-dispersed liquid crystal diffraction grating having alternating polymer-rich regions and liquid crystal-rich regions is formed. Removing the other substrate from the exposed mixture, This includes removing at least a portion of the liquid crystal in the liquid crystal-rich region to form a polymer surface relief diffraction grating. A method wherein, after removing at least a portion of the liquid crystal in the liquid crystal rich region, the liquid crystal rich region contains voids.

2. The method according to claim 1, wherein the monomer comprises acrylate, methacrylate, vinyl, isosinate, thiol, isocyanate-acrylate, and / or thioline.

3. The method according to claim 2, wherein the mixture further comprises at least one of a photoinitiator, a coinitiator, and an additional additive.

4. The method according to claim 2, wherein the thiol comprises a thiol-vinyl-acrylate.

5. The method according to claim 3, wherein the photoinitiator includes a photosensitive component.

6. To provide a mixture of the monomer and liquid crystal, The monomer and the liquid crystal are mixed with at least one of a photoinitiator, a coinitiator, a polyfunctional thiol, and an additional additive. The mixture shall be stored in a place away from light at a temperature of 22°C or lower. Adding additional monomers to the mixture, The mixture is filtered through a filter of 0.6 μm or less, The method according to claim 1, comprising storing the filtered mixture in a place away from light.

7. The method according to claim 1, further comprising refilling the void with a liquid crystal material.

8. The method according to claim 1, wherein removing at least a portion of the liquid crystal includes removing substantially all of the liquid crystal in the liquid crystal rich region.

9. The method according to claim 1, further comprising removing at least a portion of the liquid crystal to leave at least a portion of the liquid crystal in the liquid crystal rich region.

10. The method according to claim 1, wherein removing at least a portion of the liquid crystal comprises cleaning the holographic polymer dispersed liquid crystal diffraction grating with a solvent.

11. The method according to claim 5, wherein the photosensitive component includes a dye.

12. The method according to claim 7, wherein the liquid crystal material has a different molecular structure from the liquid crystal previously removed.

13. The method according to claim 1, wherein the substrate includes a glass substrate or a plastic substrate.

14. The method according to claim 1, wherein the substrate includes a transparent substrate.

15. The method according to claim 1, wherein the non-adhesive release layer comprises a fluoropolymer.

16. The method according to claim 1, further comprising applying a protective layer on the surface relief diffraction grating (SRG).

17. The method according to claim 16, wherein the protective layer includes an anti-reflective layer.

18. The method according to claim 16, wherein the protective layer comprises a silicate or silicon nitride.

19. The method according to claim 16, wherein the protective layer includes a parylene coating.

20. The method according to claim 1, further comprising creating a vacuum in the void or filling the void with an inert gas.

21. The method according to claim 1, wherein removing at least a portion of the liquid crystal comprises cleaning the holographic polymer dispersed liquid crystal diffraction grating with a solvent.

22. The method according to claim 21, wherein cleaning the holographic polymer dispersed liquid crystal diffraction grating includes immersing the holographic polymer dispersed liquid crystal diffraction grating in the solvent.

23. The method according to claim 1, further comprising curing the holographic polymer dispersed liquid crystal diffraction grating.

24. The method according to claim 23, wherein curing the holographic polymer dispersed liquid crystal diffraction grating includes exposing the holographic polymer dispersed liquid crystal diffraction grating to white light for a period of one hour.

25. The method according to claim 1, wherein the polymer surface relief diffraction grating is in overall contact with the substrate.

26. The method according to claim 25, wherein the polymer surface relief diffraction grating is in direct contact with the substrate.

27. The method according to claim 26, wherein there is no bias layer between the polymer surface relief diffraction grating and the substrate.

28. To provide a mixture of the monomer and the liquid crystal, A mixture containing a monomer and a liquid crystal is mixed with at least one of a photoinitiator, a coinitiator, a polyfunctional thiol, and an additional additive. The mixture shall be stored in a place away from light at a temperature of 22°C or lower. Adding additional monomers to the mixture, The mixture is filtered through a filter of 0.6 μm or less, The method according to claim 1, comprising storing the filtered mixture in a place away from light.

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