An optical imprinting method for fabricating high-precision sinusoidal cycloidal diffraction waveplates

Abstract

This invention discloses a high-precision optical imprinting method for fabricating sinusoidal cycloidal diffraction waveplates. By using a sinusoidal cycloidal diffraction waveplate with at least two layers of twisted structures as a polarization conversion diffraction waveplate, the precision of optical fabrication is improved, achieving optical imprinting of sinusoidal cycloidal periodic structures on the order of hundreds of nanometers. This method solves the problems of slow fabrication speed, poor resistance to environmental interference, and high cost and difficulty in further scaling up the exposure system, which limit mass production of sinusoidal cycloidal diffraction waveplates based on polarization holographic exposure systems.

Description

Technical Field

[0001] This invention relates to an optical imprinting method for high-precision sinusoidal cycloidal diffraction waveplates. The sinusoidal cycloidal diffraction waveplates include, but are not limited to, diffraction waveplates in which the anisotropic optical axes are locally or entirely distributed in a sinusoidal cycloidal pattern, such as polarization gratings, fork gratings, and polarization lenses. Background Technology

[0002] Compared to traditional mechanical friction alignment techniques, which are prone to damaging and contaminating the alignment layer, contactless optical alignment techniques based on polarized light irradiation can achieve high-quality, high-precision recording of the polarization direction of the light field, thereby realizing the spatial distribution alignment of anisotropic optical axes. For example, the fabrication of sinusoidal cycloidal polarization gratings involves interfering two circularly polarized beams of equal intensity and opposite rotation to generate a sinusoidal polarization direction in space. This technique has attracted widespread attention due to its near 100% single-order diffraction efficiency (C. Oh and MJ Escuti, "Numerical analysis of polarizationgratings using the finite-difference time-domain method," Phys Rev A 76 (2007)). The anisotropic optical axis of a sinusoidal cycloidal diffraction waveplate exhibits a sinusoidal rotation along the coordinate axes. A polarization grating is a typical sinusoidal cycloidal diffraction waveplate, with a rotation of 180° as one period. Macroscopically, countless such periods are repeated in space to form the grating. Polarizing lenses are diffraction waveplates with anisotropic optical axes distributed in two-dimensional space. They have different sinusoidal cycloid periods along the radial direction, called semi-periodic structures, and can therefore be regarded as locally sinusoidal cycloid diffraction waveplates (LSLi, SJShi, J.Kim, and MJEscuti, "Color-selective geometric-phase lenses for focusing and imaging based on liquid crystal polymer films," OpticsExpress 30, 2487-2502 (2022)). The anisotropic optical axes of sinusoidal cycloid diffraction waveplates tend to align in the lowest volume free energy state. As the sinusoidal cycloid period gradually decreases, it becomes difficult for the anisotropic optical axes to maintain a sinusoidal cycloid planar structure. Only when the thickness of the diffraction waveplate does not exceed a critical thickness does it possess a sinusoidal cycloid planar structure. The anisotropic optical axis of a diffraction waveplate with a twist angle is not completely perpendicular to the plane of the glass substrate (JHXiong, R.Chen, and STWu, "Device simulation of liquid crystal polarization gratings," Optics Express 27, 18102-18112 (2019)). Sinusoidal cycloidal diffraction waveplates, due to their high diffraction efficiency, polarization selectivity, simple fabrication, and thinness, have shown great potential advantages in fields such as beam scanning, spectral imaging, augmented reality, and virtual reality.

[0003] Traditional fabrication of sinusoidal cycloidal diffraction waveplates is based on polarization holographic interferometry. This method can produce high-resolution diffraction waveplates, but further scaling up the exposure system increases cost and difficulty, currently limiting its application to laboratory-scale manufacturing and exhibiting poor resistance to environmental interference. Therefore, a faster, lower-cost imprint fabrication method with strong environmental resistance is needed to meet the demands of industrial production.

[0004] In recent years, imprinting technology based on patterned grooves for microlens arrays has been proposed. However, it is difficult to prepare small-sized grooves and the orientation quality is generally poor (Ziqian He, Yun-Han Lee, Ran Chen, Debashis Chanda, and Shin-Tson Wu, "Switchable Pancharatnam–Berry microlens array with nano-imprinted liquid crystal alignment", Opt. Lett. 43, 5062-5065 (2018).). Imprinting techniques using sinusoidal cycloidal polarization gratings with thicknesses satisfying the half-wave condition as master plates can only imprint polarization gratings with large periods (micrometer scale). When the period is below 2 μm, polarization gratings with thicknesses satisfying the half-wave condition no longer have high diffraction efficiency and are difficult to meet the requirements as polarization conversion diffraction waveplates (Sarik R. Nersisyan, Nelson V. Tabiryan, Diane M. Steeves, and Brian R. Kimball, "Characterization of optically imprinted polarizationgratings," Appl. Opt. 48, 4062-4067 (2009).).

[0005] Therefore, we propose a high-precision optical imprinting method for sinusoidal cycloidal diffraction waveplates. Since the anisotropic optical axis can be manipulated in space, and its distribution directly affects optical performance, we propose a novel sinusoidal cycloidal diffraction waveplate with at least two twisted layers as a polarization conversion diffraction waveplate to improve the precision of optical fabrication. This enables the optical imprinting of sinusoidal cycloidal periodic structures on the order of hundreds of nanometers. This optical fabrication method offers advantages such as high speed, strong resistance to environmental interference, and low required optical field energy density, providing a feasible solution for large-scale mass production. Summary of the Invention

[0006] A high-precision optical imprinting method for fabricating a sinusoidal cycloidal diffraction waveplate includes a glass substrate, an optically controlled alignment film, and at least two layers of polarization conversion diffraction waveplates with twisted structures.

[0007] This method can spatially control the polarization state of the light field emitted by at least partially coherent light sources. Specifically, it converts the polarization state of the light field into a sinusoidal cycloid-shaped linear polarization state distribution based on a polarization conversion diffraction waveplate. The period of this distribution is half that of the polarization conversion diffraction waveplate, so a diffraction waveplate with a period half that of the polarization conversion diffraction waveplate can be prepared.

[0008] The absorption spectrum of the photo-aligned film includes the wavelength of the light source. The orientation direction of the photo-aligned material is perpendicular to the major axis of the elliptic polarization state, thus it can record the polarization information of the irradiated polarized light.

[0009] The anisotropic optical axis distribution of the polarization conversion diffraction waveplate with at least two twisted layers is as follows:

[0010]

[0011] Where φ(x,y) is the spatial distribution of the anisotropic optical axis in the xy plane, which can be a local or global sinusoidal cycloidal periodic structure, such as a polarization grating φ(x)=πx / Λ x Λ x The period of the sinusoidal cycloid is [value]; the polarizing lens is [value]. This local periodicity can be expressed as f is the focal length. d1, It refers to the thickness and twist angle of the first diffraction plate, d2. It refers to the thickness and twist angle of the second diffraction plate, where the twist angle is determined by the concentration of the chiral agent, φ. offset The compensation distortion angle for the second diffraction plate is usually equal to...

[0012] The optical imprinting method for high-precision sinusoidal cycloidal diffraction waveplates provided by this invention directly affects the optical performance of the device by changing the thickness and twist angle of at least two layers of diffraction waveplates, i.e., changing the spatial distribution of the anisotropic optical axis. This improves the diffraction efficiency of the sinusoidal cycloidal polarization conversion diffraction waveplate to over 95%, effectively converting the polarization state of the light field emitted from the light source into a sinusoidal cycloidal polarization state distribution. The period of this sinusoidal cycloidal polarization state is half that of the polarization conversion diffraction waveplate. This method enables high-quality, high-precision, stable, and rapid mass production of sinusoidal cycloidal diffraction waveplates.

[0013] The polarization conversion diffraction waveplate preparation process provided by the present invention includes: ultrasonically cleaning a glass substrate and then irradiating it with ozone ultraviolet light to obtain a clean glass substrate; coating a photo-alignment film on the clean glass substrate; performing interference exposure in the central region of the glass substrate coated with the photo-alignment film; spin-coating a liquid crystal polymer doped with a chiral agent onto the exposed glass substrate; curing and polymerization under ultraviolet light in a nitrogen atmosphere; and repeating the spin-coating of the liquid crystal polymer to obtain the polarization conversion diffraction waveplate.

[0014] The optical imprinting principle of the high-precision sinusoidal cycloidal diffraction waveplate provided by this invention is based on the sinusoidal cycloidal periodic spatial modulation of the polarization state of the incident light source by the polarization conversion diffraction waveplate. This can be converted into a sinusoidal cycloidal period that is half the period of the polarization conversion diffraction waveplate. To prevent the influence of Fresnel reflection and to avoid damaging the polarization conversion diffraction waveplate, the prepared polarization conversion diffraction waveplate is placed back-to-back with a glass substrate coated with an optically controlled alignment film. Optical imprinting is performed on the incident ray-polarized light without the need for any additional optical components.

[0015] The polarization conversion diffraction waveplates provided by this invention have diffraction efficiencies exceeding 95%, solving the problems of difficulty in further expanding polarization holographic interferometry exposure systems and high costs. Simultaneously, the optical imprinting fabrication method offers advantages such as strong resistance to environmental interference, fast imprinting speed, low cost, and high precision. This provides a solution for the large-scale mass production of sinusoidal cycloidal diffraction waveplates. Attached Figure Description

[0016] Figure 1 It is a sinusoidal cycloid periodic diffraction waveplate.

[0017] Figure 2 It is a sinusoidal cycloid periodic diffraction waveplate, either locally or as a whole.

[0018] Figure 3 It is a multi-layered twisted structure polarization conversion diffraction waveplate.

[0019] Figure 4 It is the interference of left-handed and right-handed circularly polarized light in a plane that produces sinusoidal cycloid-shaped linearly polarized light.

[0020] Figure 5 This is a schematic diagram of the optical imprinting principle of a polarization conversion diffraction waveplate.

[0021] Figure 6 This is a diffraction efficiency diagram of a polarization conversion diffraction waveplate with a multi-layered twisted structure.

[0022] Figure 7 The polarizer grating waveguide pattern with a period of 500nm was obtained by imprinting. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings:

[0024] like Figure 1 As shown, a sinusoidal cycloid periodic diffraction waveplate has an anisotropic optical axis 101 rotating 180° for one period. In this invention, such a diffraction waveplate with multiple repeated periods is called an integral sinusoidal cycloid periodic diffraction waveplate.

[0025] like Figure 2As shown, local or overall sinusoidal diffraction waveplates include, but are not limited to, polarization grating 102, fork grating 103 and polarization lens 104, and have a sinusoidal cycloid periodic structure in a local area. These types of diffraction waveplates can all be prepared by spatially controlling the polarization state of the light field of the light source through polarization conversion diffraction waveplates.

[0026] like Figure 3 As shown, the multilayer twisted polarization conversion diffraction waveplate proposed in this invention, taking a polarization grating as an example, includes a glass substrate 105, a light-controlled alignment film 106, and multiple layers of anisotropic optical axis diffraction waveplates 107 with twisted angles. The distribution of the anisotropic optical axes is as follows:

[0027]

[0028] By altering the thickness and torsion angle of at least two diffraction waveplates—that is, changing the spatial distribution of the anisotropic optical axis—the optical performance of the device is directly affected. This results in a diffraction efficiency of over 95% for a sinusoidal cycloidal polarization conversion diffraction waveplate, effectively converting the polarization state of the light field emitted from the light source into a sinusoidal cycloidal polarization state distribution. The period of this sinusoidal cycloidal polarization state is half that of the polarization conversion diffraction waveplate. This enables high-quality, high-precision, stable, and rapid mass production of sinusoidal cycloidal diffraction waveplates.

[0029] like Figure 4 The diagram shows a polarization holographic interference system: two circularly polarized beams with opposite rotations interfere 108, forming linearly polarized light 101 with a sinusoidal cycloid shape along space on the surface of the optically controlled alignment film 106. The anisotropic optical axes of the spin-coated film align perpendicular to the linear polarization direction. The diffraction waveplates of different periods can be determined by the angle between the two beams. Where λ is the exposure wavelength and θ is half the angle between the two beams.

[0030] like Figure 5 The diagram illustrates the optical imprinting principle of the sinusoidal cycloidal diffraction waveplate proposed in this invention. Based on the polarization conversion diffraction waveplate, the polarization state 109 of the incident light source is spatially controlled in a sinusoidal cycloidal manner, which can be converted into a sinusoidal cycloidal period that is half the period of the polarization conversion diffraction waveplate. To prevent the influence of Fresnel reflection and to avoid damaging the polarization conversion diffraction waveplate, the prepared polarization conversion diffraction waveplate is placed back-to-back with a glass substrate coated with an optical alignment film. Optical imprinting is performed on the incident ray-polarized light without requiring any additional optical components.

[0031] like Figure 6The figure shows the diffraction efficiency of a polarization conversion diffraction waveplate based on a multi-layered twisted structure under different periods. It can be seen that the simulated efficiency (solid line) for periods of 1µm–2µm and 0.8µm–1µm is above 99%, and the experimental efficiency (stars) reaches above 95%. This makes it a suitable polarization conversion diffraction waveplate for imprinting sinusoidal cycloidal diffraction waveplates of 500nm–1µm and 400nm–500nm. The optical imprinting method has strong resistance to environmental interference, fast fabrication speed, and does not require the construction of an interference optical path.

[0032] like Figure 7 Figure 110 shows an optical waveguide diagram of a polarizing holographic grating with a period of 500 nm, embossed based on a polarization conversion diffraction waveplate. This demonstrates that the polarizing holographic grating embossed according to this invention possesses optical waveguide properties and can serve as a master template for the large-scale mass production of subwavelength periodic diffraction waveplates, providing a new fabrication scheme for the industrial production of polarizing holographic gratings, a core component of AR and VR devices.

[0033] The foregoing provides a detailed description of the embodiments of the present invention. However, the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations of these embodiments within the principles and technical concept of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for optically preparing a diffraction waveplate with an anisotropic optical axis orientation of a spatial sinusoidal cycloid periodic distribution, the method comprising: 1) Determine the light source: The light source used must be at least partially coherent and linearly polarized; 2) Determine the light-controlled orientation material: The wavelength of the light source is within the absorption spectrum of the light-controlled orientation material, and the orientation of the light-controlled orientation material is modulated by the polarization state of the light source; 3) Determine the polarization conversion diffraction plate: The polarization conversion diffraction plate spatially and periodically modulates the polarization state of the light source. This spatial period can be local or global, and it is twice the spatial period of the diffraction plate to be fabricated. The polarization conversion diffraction plate comprises at least two layers of diffraction plates with or without a twist angle, and the diffraction efficiency of the polarization conversion diffraction plate for the light source is higher than 95%. 4) Determine the exposure area: The light field emitted by the light source passes through the polarization conversion diffraction plate and exposes the area with the light-controlled orientation material. The polarization state of the exposure area is controlled by the spatial control of the polarization state of the light source by the polarization conversion diffraction plate.

2. According to the method of claim 1, the spatial period of the optically prepared diffraction waveplate is half of the polarization state modulation period of the light source after it is incident on the polarization conversion diffraction waveplate, and the minimum spatial period modulation is several hundred nanometers.

3. The method according to claim 1, wherein the polarization conversion diffraction waveplate, which is spatially periodically distributed with anisotropic optical axes, comprises at least two layers of diffraction waveplates having an optical axis twist angle or no twist angle, wherein the twist axis is completely perpendicular or not completely perpendicular to the substrate.

4. The method according to claim 1, wherein at least one layer of the polarization conversion diffraction plate has a portion of anisotropic optical axes that are periodically distributed in a sinusoidal cycloid pattern.

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