Inkjet method for preparing a helically encoded thickness-tapered polarization volume grating, polarization volume grating and optical waveguide display device

Abstract

The application discloses a spiral coding-based thick gradual change polarization volume grating inkjet preparation method and belongs to the field of polarization volume grating manufacturing. The method divides a to-be-printed region into multiple basic printing units, determines a filling factor according to a target dry film thickness, sets non-activated printing pixels in the Archimedes spiral continuous blank lines in each basic printing unit, and adopts a multi-nozzle piezoelectric drop-on-demand inkjet head to deposit liquid crystal ink on the activated pixels with a fixed droplet volume, so that a thick gradual change polarization volume grating is formed through leveling and ultraviolet curing. The application can form a preset thickness gradient while keeping the continuity of the film layer, and can reduce material accumulation by reducing the filling factor of the junction area between adjacent printing widths, and is suitable for optical waveguide display devices.

Description

Technical Field

[0001] This invention relates to a method for fabricating thickness-gradient polarizing gratings using inkjet printing based on spiral coding, a polarizing grating and an optical waveguide display device, and belongs to the field of polarizing grating technology. Background Technology

[0002] Polarization volume gratings (PVGs) are typically formed by chiral nematic liquid crystals with a periodic spiral orientation structure. They have high diffraction efficiency, large diffraction angle, and circular polarization selectivity, and can be used for waveguide coupling, pupil expansion, and coupling out in augmented reality near-eye displays and head-up displays.

[0003] In a pupil-expanding output coupling grating, the guided light is continuously extracted along the propagation direction, and the remaining guided light power decreases with increasing propagation distance. To make the output light intensity at different positions more uniform, it is necessary to improve the local diffraction efficiency along the propagation direction. When parameters such as grating period, helical pitch, and birefringence are fixed, the thickness of the polarizer grating layer is an important parameter affecting the local diffraction efficiency. Therefore, the required spatial response of the optical waveguide can be converted into a corresponding thickness distribution.

[0004] Spin coating is suitable for obtaining uniform films, but it is difficult to distribute material on demand in-plane according to a digital pattern. Single-nozzle line-by-line deposition can adjust local composition and thickness, but ink changing, cleaning, and serial scanning limit processing efficiency. Multi-nozzle piezoelectric on-demand inkjet printing has the advantages of digitalization, non-contact, and parallel deposition, but low-viscosity liquid crystal inks will undergo droplet spreading, coalescence, leveling, and drying after deposition, which may weaken the preset dose gradient. If only the droplet volume of each pixel is reduced, the lateral flow in the continuous wet film can easily homogenize the thickness; if the droplet volume is maintained and pixels are randomly deleted, large and irregularly shaped uncovered areas are likely to appear in low fill factor areas, causing film discontinuity.

[0005] In addition, when using multiple adjacent printing sheets to manufacture large-area polarizer gratings, material accumulation may form at the edge of each printing sheet; when the edges of adjacent sheets overlap at the junction, a significant thickness bulge will be generated, reducing optical uniformity.

[0006] Therefore, existing technologies still require a polarizer grating fabrication scheme that can simultaneously achieve programmable thickness control, continuous film morphology, high printing throughput, and large-area fabrication across multiple widths. Summary of the Invention

[0007] Objective: This invention aims to solve the problems of weakening the preset in-plane dose gradient due to droplet spreading and wet film leveling when preparing polarizing gratings using multi-nozzle inkjet printing, difficulty in balancing thickness gradient and film continuity, and easy material accumulation at the junction of adjacent printing areas. It also provides an inkjet printing method for preparing polarizing gratings with thickness gradient based on spiral coding, as well as a polarizing grating and an optical waveguide display device.

[0008] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0009] Firstly, a method for fabricating thickness-gradient polarizer gratings using inkjet printing based on spiral coding, specifically including:

[0010] A light alignment layer with a predetermined periodic alignment structure is formed on the substrate.

[0011] The area to be printed is divided into multiple basic printing units along the predetermined thickness variation direction, and the local target fill factor of each basic printing unit is determined according to the target dry film thickness distribution.

[0012] Within a basic printing cell where the local target fill factor is less than 100%, pixels located on the Archimedean spiral path are set as inactive pixels, thereby forming a continuous blank line of the Archimedean spiral. By adjusting at least one of the number of turns of the continuous blank line, the initial radius, and the radial growth coefficient, the area ratio of the inactive pixels is changed, and the remaining active pixels form a connected printing area within the basic printing cell.

[0013] Using a multi-nozzle piezoelectric on-demand inkjet head, liquid crystal ink is deposited onto the positions corresponding to the active pixels in the connected printing area with a fixed droplet volume; the deposited liquid crystal ink is leveled and cured to form a polarizer grating layer with a periodic spiral liquid crystal structure and a dry film thickness corresponding to the local target fill factor on the photo-alignment layer.

[0014] Secondly, a thickness-gradient polarizer grating, prepared by the aforementioned method, comprises a substrate, an optical alignment layer, and a polarizer grating layer arranged sequentially.

[0015] Thirdly, an optical waveguide display device includes an optical waveguide and a thickness-gradient polarizer grating disposed in the decoupling region of the optical waveguide, wherein the dry film thickness of the thickness-gradient polarizer grating increases along the light propagation direction to compensate for the attenuation of light guiding power during continuous decoupling.

[0016] Beneficial Effects: This invention provides a method for preparing a thickness-gradient polarizer grating using inkjet printing based on spiral coding, a polarizer grating, and an optical waveguide display device. This invention relates to polarizer grating manufacturing technology, dividing the printing area into basic printing units. Within each unit, inactive pixels are organized using continuous blank lines with an Archimedean spiral, and the local fill factor is adjusted by changing the number of spiral turns or the rasterization path. A multi-nozzle piezoelectric on-demand inkjet head is used to deposit liquid crystal ink onto the remaining active pixels with a fixed droplet volume. After leveling and UV curing, a periodic spiral liquid crystal layer with a thickness varying according to a preset distribution is formed. This method balances continuous film morphology, programmable thickness gradients, and large-area manufacturing, and can suppress boundary material accumulation by reducing the fill factor at the interface between adjacent printing areas. Compared with the prior art, this invention has at least the following beneficial effects:

[0017] First, under the same target dose gradient, the Archimedes spiral continuous blank line takes into account both gradient preservation and continuous film morphology.

[0018] Second, by using a fixed volume of droplets in conjunction with pixel fill factor adjustment, excessive lateral homogenization is avoided after forming a highly connected wet film with extremely small droplets.

[0019] Third, multi-nozzle single-pass deposition significantly improves pattern writing efficiency.

[0020] Fourth, boundary compensation can reduce the thickness bulge across the span and extend the continuous manufacturing width.

[0021] Fifth, the local thickness distribution can be converted into a verifiable spatial diffraction efficiency distribution, which is suitable for uniform decoupling of optical waveguides. Attached Figure Description

[0022] Figure 1 This is a schematic cross-sectional view of the thickness-gradient polarizer grating fabricated by the method of the present invention.

[0023] Figure 2 This is an observation image of the droplet under the 7th grayscale driving condition of the printhead.

[0024] Figure 3 A schematic diagram of the Archimedes spiral compensation pattern set at the splicing position of the 72 mm format.

[0025] Figure 4 A schematic diagram of the basic unit structure of an Archimedean spiral with a local target fill factor of 43%.

[0026] Figure 5 This is a flowchart of the overall manufacturing process for a polarizer grating (PVG).

[0027] Figure 6 This is a comparison chart of the theoretical and measured film thicknesses for continuous grayscale patterns, random dot patterns, and Archimedes spiral patterns within a length range of 28 mm.

[0028] Figure 7 The optical verification results of the thickness gradient and diffraction efficiency formed by the Archimedes spiral pattern are shown in the figure.

[0029] Figure 8 This image shows a comparison of the polarizer grating before and after cross-width compensation, along with corresponding film thickness test results. Figure 8 Image (a) shows the uncompensated polarizer grating and the corresponding film thickness test results. Figure 8 (b) shows the actual polarizer grating during compensation and the corresponding film thickness test results. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The embodiments described are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention; without departing from the technical concept of the present invention, the pixel size, droplet volume, material concentration, printing speed, curing conditions and compensation parameters can be adjusted according to the equipment conditions and the target optical response.

[0031] Firstly, a method for fabricating thickness-gradient polarizer gratings using inkjet printing based on spiral coding, such as... Figure 1 As shown, it specifically includes:

[0032] A light alignment layer 2 with a predetermined periodic orientation structure is formed on a substrate 1.

[0033] The area to be printed is divided into multiple basic printing units along the predetermined thickness variation direction, and the local target fill factor of each basic printing unit is determined according to the target dry film thickness distribution.

[0034] Within a basic printing cell where the local target fill factor is less than 100%, pixels located on the Archimedean spiral path are set as inactive pixels, thereby forming a continuous blank line of the Archimedean spiral. By adjusting at least one of the number of turns of the continuous blank line, the initial radius, and the radial growth coefficient, the area ratio of the inactive pixels is changed, and the remaining active pixels form a connected printing area within the basic printing cell.

[0035] Using a multi-nozzle piezoelectric on-demand inkjet head, liquid crystal ink is deposited onto the positions corresponding to the active pixels in the connected printing area with a fixed droplet volume; the deposited liquid crystal ink is leveled and cured to form a polarizer grating layer 3 with a periodic spiral liquid crystal structure and a dry film thickness corresponding to the local target fill factor on the photo-alignment layer.

[0036] Furthermore, each basic printing unit comprises a two-dimensional pixel array determined by the printing resolution, the basic printing unit being approximately 1 mm × 1 mm in size and containing 14 × 14 pixels.

[0037] Furthermore, the Archimedean spiral continuous blank line satisfies ,in, This is the radial distance from a point on the helix to the center of the helix. Let the initial radius be , The radial growth factor. Azimuth angle; radial spacing between adjacent spiral coils , Pi is the mathematical constant of a circle.

[0038] Furthermore, the width of the continuous blank line of the Archimedes spiral is preferably one pixel.

[0039] Furthermore, the local target fill factor is defined as follows: ,in: The number of active pixels within a basic printing cell. This represents the total number of pixels within a basic printing unit. Compared to randomly deleting pixels, continuous spiral blank lines confine the unprinted area to narrow channels with controllable width and spacing, and connect the surrounding printed area, thereby shortening the lateral spreading distance required for droplets to cover the blank area.

[0040] Furthermore, the local target fill factor is continuously or segmentally varied along the predetermined thickness variation direction; preferably, the local target fill factor is linearly varied from 100% to 14% within a length of 28 mm.

[0041] Furthermore, such as Figure 2 As shown, the active pixels in each basic printing unit operate using droplets of different gray levels, where i is the gray level value. For example, gray levels 1 to 7 correspond to droplet sizes and volumes of [missing information]. ,like arrive The amount of liquid crystal ink deposited in the activated pixels of the connected printing area is adjusted by the local target fill factor. When the target dose is equal, the pixel droplet volume... With respect to the volume of the i-th order droplet satisfy .

[0042] Furthermore, in terms of droplet volume At that time, the local surface dose is determined by the local fill factor. Adjustment. The dry film thickness can be predicted according to... Confirmed, among which This represents the ink solids mass fraction. For ink density, Density of the cured film This represents the area of ​​a single pixel. Therefore, the local fill factor can be calculated based on the target thickness to generate a digitally printed pattern.

[0043] Furthermore, the liquid crystal ink comprises reactive mesocrystalline RM257, chiral dopant S5011, photoinitiator 184, and xylene solvent, with a total solid mass fraction of 4 wt.%; the liquid crystal ink density is 0.93 g / cm³, the dynamic viscosity is 0.94 mPa·s, and the surface tension is 28 mN / m.

[0044] Furthermore, the multi-nozzle piezoelectric on-demand inkjet head has 1024 nozzles, a nozzle resolution of 360 npi, and an effective print width of 72 mm. The driving voltage is 12 V, and the printing platform moves in a single direction at 60 mm / s to perform single-pass deposition on a print area.

[0045] Furthermore, the distance between the printhead and the substrate is approximately 2 mm, the printing environment temperature is approximately 25 ℃ and the relative humidity is approximately 30%; after deposition, it is leveled at room temperature for 60 s, and then cured with ultraviolet light at a wavelength of 365 nm and an exposure dose of 3 J / cm² for 60 s.

[0046] Furthermore, for polarizing gratings with a width greater than the width of a single printing sheet, sheet compensation areas are set on both sides of the boundary line between the first and second printing sheets, and the proportion of active pixels in the compensation area is reduced by using Archimedes spiral continuous blank lines, so that the local target fill factor decreases from the area away from the boundary line to the boundary line, thereby offsetting the material accumulation at the edges of the two printing sheets.

[0047] This compensation method only changes the proportion of digitally activated pixels, without changing the ink formulation, printing conditions, or light orientation structure; Figure 3 An Archimedes spiral compensation pattern is shown, with a 10 mm wide compensation zone on each side of the 72 mm boundary line as the center.

[0048] Furthermore, the width of the area compensation zone on both sides of the boundary line is 10 mm, and the local target fill factor is linearly reduced from 90% outside the area compensation zone to 60% at the boundary line. A continuous polarizer grating is formed on a substrate with a width of 120 mm through the adjacent first and second printing surfaces.

[0049] Secondly, a thickness-gradient polarizer grating, prepared by the aforementioned method, comprises a substrate, an optical alignment layer, and a polarizer grating layer arranged sequentially.

[0050] Furthermore, the dry film thickness of the polarizer grating layer varies from 0.154 μm to 1.22 μm over a length of 28 mm, and the target order diffraction efficiency for unpolarized incident light varies from 2.82% to 38.64%.

[0051] Thirdly, an optical waveguide display device includes an optical waveguide and a thickness-gradient polarizer grating disposed in the decoupling region of the optical waveguide, wherein the dry film thickness of the thickness-gradient polarizer grating increases along the light propagation direction to compensate for the attenuation of light guiding power during continuous decoupling.

[0052] Example 1:

[0053] Determination of target thickness and local dose. For length... Waveguide thickness And the propagation angle The decoupling region, the lateral distance between two adjacent contacts of the decoupling layer , The effective decoupling may occur 8 or 9 times depending on the initial contact location. If the maximum local diffraction efficiency at the end is... The number of effective decoupling operations is The simplified equal-output model then gives a single equal-output... Rigorous coupled-wave analysis shows that a polarizing body grating of about 1 μm thickness can achieve a local efficiency of 69.58% for the chiral circularly polarized component, with corresponding equal outputs of about 11.85% and 10.60% at the 8th and 9th decoupling, respectively. Therefore, about 1 μm was chosen as the target maximum thickness.

[0054] Example 2:

[0055] Spiral pattern generation. For example... Figure 4 As shown, within the basic printing unit, according to Generate an Archimedean spiral and rasterize it into a continuous blank line one pixel wide. Adjust the number of spiral turns and the initial radius. Radial growth coefficient This ensures that the blank lines occupy a set proportion of pixels; the radial spacing between adjacent spiral loops. The example has a fill factor of approximately 43%. Continuous blank lines form narrow, controlled unprinted channels, while surrounding active pixel areas remain connected.

[0056] Example 3:

[0057] Materials and inkjet printing process. The liquid crystal ink was obtained by dissolving reactive mesocrystalline RM257, chiral dopant S5011, and photoinitiator 184 in xylene, with a total solids mass fraction of 4 wt.%; the ink density was... Dynamic viscosity is Surface tension is The printhead uses a Konica Minolta KM-1024i piezoelectric printhead with 1024 nozzles, 360 npi resolution, and 72 mm effective width, driven by 12 V. The print platform moves in a single direction at 60 mm / s, enabling single-page, single-pass deposition.

[0058] like Figure 5 As shown, the glass substrate was cleaned and subjected to a 30-second plasma treatment, followed by the coating of an SD1 photoalignment layer. A periodic alignment pattern was then written using polarized light exposure. The distance between the printhead and the substrate was approximately 2 mm, and the printing environment temperature was approximately 25 °C with a relative humidity of approximately 30%. After deposition, the substrate was kept at room temperature for 60 seconds to allow the ink to level, and then cured by irradiation with an ultraviolet light source at a wavelength of 365 nm and an exposure dose of 3 J / cm² for 60 seconds, forming a polarizing grating layer. The liquid crystal self-assembled into a periodic spiral structure under the constraint of the photoalignment layer. A thickness-gradient polarizing grating was obtained using the above fabrication process. This polarizing grating, from bottom to top, consists of a glass substrate, an alignment layer, and a polarizing grating layer, wherein the dry film thickness of the polarizing grating layer gradually decreases from left to right along the direction shown in the figure.

[0059] In this embodiment, the structural parameter is fixed as the effective refractive index. Bragg cycle Bragg surface tilt angle Helical pitch In-plane grating period center wavelength The structure satisfies as well as Position-dependent diffraction efficiency can be obtained by simply changing the thickness of the polarizer grating layer.

[0060] Example 4:

[0061] Thickness Gradient and Optical Verification. This embodiment aims to obtain a dry film thickness gradient of approximately 1 μm over a length of 28 mm. Under the same linear target dose gradient, continuous grayscale patterns, random dot patterns, and Archimedean spiral patterns were used for printing. The continuous grayscale pattern adjusted the droplet volume of a single pixel by changing grayscale levels 1 to 7; the random dot patterns and Archimedean spiral patterns both used fixed 7-level grayscale droplets, and the local target fill factor was adjusted by changing the number of active pixels. When the target surface dose is the same, the continuous grayscale pattern shows the position... Droplet volume at the location With a fixed droplet volume satisfy: like Figure 6As shown, although the continuous grayscale pattern forms a continuous film layer, the wet film leveling significantly homogenizes the preset dose gradient, with the thickness changing by only 0.048 μm within 28 mm. The random dot pattern retains a thickness variation of 0.48 μm, but film discontinuities appear at the low fill factor ends. The Archimedes spiral pattern sets inactive pixels as continuous spiral blank lines, allowing the remaining active pixels to form a connected printing area. Its film layer remains continuous over the entire length, with the thickness increasing from 0.154 μm to 1.22 μm, forming a thickness gradient of approximately 1.07 μm. The root mean square errors between the measured thickness and the theoretical thickness of the three patterns are 0.3294 μm, 0.1486 μm, and 0.0191 μm, respectively, indicating that the Archimedes spiral pattern can achieve a thickness gradient consistent with the theoretical distribution while maintaining film layer continuity.

[0062] Thickness was measured using a Chotest CP200 profilometer. Unpolarized light was incident perpendicularly, and the target diffraction order was collected at 57.9°. Eighteen locations were selected along a 28 mm gradient, and the measurement spot was approximated as a uniform circular beam with a diameter of 1 mm; when the beam center was located at x, the theoretical efficiency was calculated according to... Perform area averaging. For example... Figure 7 As shown, the measured diffraction efficiency of the representative sample increases from 2.82% to 38.64% from the thin end to the thick end. The mean absolute error relative to the predictions of rigorous coupled-wave analysis at 18 locations is 0.61 percentage points, and the root mean square error is 0.73 percentage points. The diffraction efficiencies of the three replicate samples at the thin and thick ends are 2.62% ± 0.27% and 37.74% ± 0.79%, respectively, and the mean standard deviation between samples at 18 locations is 0.51 percentage points.

[0063] Example 5:

[0064] Cross-format compensation. A 120 mm × 120 mm polarizer grating is fabricated using a first and a second printing plane, with a boundary line formed between the two planes 72 mm from the starting point of the first plane. A 10 mm cross-format compensation zone is set on each side of the boundary line, causing the fill factor to linearly decrease from 90% outside the compensation zone to 60% towards the boundary line, while the remaining material formulation, printing conditions, and optical orientation structure remain unchanged. Figure 8 As shown in (a) and (b), without compensation, the thickness of the main region is approximately 1.1 μm, and the peak thickness at the interface is approximately 4.7 μm. After the above compensation is applied, the thickness at the interface is reduced to approximately 1.1 μm, and the thickness at the interface is basically the same as the thickness of the main region. The protrusion in the physical image is no longer obvious, and the continuous manufacturing width is extended from 72 mm to 120 mm. The diffraction efficiency variation coefficients at the nine measurement locations are also shown. ,in The standard deviation of the sample is 1. The value is the average; after compensation, the CV decreased from 13.33% to 1.49%.

[0065] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating thickness-gradient polarizing gratings using inkjet printing based on spiral coding, characterized in that: Specifically, it includes: A light alignment layer with a predetermined periodic alignment structure is formed on a substrate; The area to be printed is divided into multiple basic printing units along the predetermined thickness variation direction, and the local target fill factor of each basic printing unit is determined according to the target dry film thickness distribution. Within a basic printing unit where the local target fill factor is less than 100%, pixels located on the Archimedes spiral path are set as inactive pixels, thereby forming a continuous blank line of the Archimedes spiral; by adjusting at least one of the number of turns of the continuous blank line, the initial radius, and the radial growth coefficient, the area ratio of the inactive pixels is changed, and the remaining active pixels form a connected printing area within the basic printing unit. Liquid crystal ink is deposited at the positions corresponding to the activated pixels within the connected printing area with a fixed droplet volume; The deposited liquid crystal ink is leveled and cured to form a polarizer grating layer with a periodic spiral liquid crystal structure and a dry film thickness corresponding to the local target fill factor on the photo-alignment layer.

2. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: The basic printing unit is configured as a two-dimensional pixel array determined by the printing resolution.

3. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: The Archimedean spiral continuous blank line satisfies ,in, This is the radial distance from a point on the helix to the center of the helix. Let the initial radius be , The radial growth factor. Azimuth angle; radial spacing between adjacent spiral coils , Pi is the mathematical constant of a circle.

4. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: The local target fill factor The expression is as follows ； in: The number of active pixels within a basic printing cell. This represents the total number of pixels within a basic printing unit.

5. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: The fixed droplet volume The expression is as follows: ； in, For local target fill factor, Let i be the ink droplet volume corresponding to gray level i.

6. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: The local target fill factor of each basic printing unit is determined based on the target dry film thickness distribution. The relationship between the target dry film thickness and the local target fill factor of the basic printing unit is as follows: ； in, Indicates the target dry film thickness. This represents the ink solids mass fraction. For ink density, Density of the cured film For the area of ​​a single pixel, For local target fill factor, Let i be the ink droplet volume corresponding to gray level i.

7. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: Also includes: For a polarizing grating with a width greater than the width of a single printing sheet, a sheet compensation area is set on both sides of the boundary line between the first and second printing sheets. The proportion of active pixels in the compensation area is reduced by using an Archimedes spiral continuous blank line, so that the local target fill factor decreases from the distance from the boundary line to the boundary line, thereby offsetting the material accumulation at the edges of the two printing sheets.

8. The method for fabricating a thickness-gradient polarizer grating based on spiral coding according to claim 1, characterized in that: Also includes: A multi-nozzle piezoelectric on-demand inkjet head is used to deposit liquid crystal ink at the corresponding positions of activated pixels in the connected printing area with a fixed droplet volume.

9. A thickness-gradient polarizing grating, characterized in that: The substrate, prepared by any one of claims 1-8, comprises a substrate, an optical alignment layer, and a polarizer grating layer arranged sequentially.

10. An optical waveguide display device, characterized in that: The device includes an optical waveguide and a thickness-gradient polarizer grating disposed in the outcoupling region of the optical waveguide, wherein the thickness-gradient polarizer grating is prepared by the fabrication method according to any one of claims 1-8.

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