Preparation method of geometric phase vortex liquid crystal lens

Abstract

The invention discloses a geometric phase vortex liquid crystal lens preparation method and belongs to the technical field of liquid crystal lenses. According to the geometric phase vortex liquid crystal lens, based on the anisotropy characteristic of nematic phase liquid crystals, a light alignment layer is prepared through a light alignment method, a polarization holographic method, spin coating and exposure, then the nematic phase liquid crystals are spin-coated, the light alignment layer induces nematic phase liquid crystal molecule directors to deflect, a specific arrangement structure is formed, and the geometric phase vortex liquid crystal lens is obtained. A light beam passes through the geometric phase vortex liquid crystal lens, precise control over right-handed circularly polarized light is achieved, a vortex light field carrying orbital angular momentum can be generated, and the characteristic of edge enhancement is shown. The method has the advantages of simple preparation process, low cost, single exposure and the like, and meanwhile, the size and the thickness of the device are reduced, so that the device is more suitable for integrated and portable application.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid crystal lenses, and specifically relates to a preparation method of a geometric phase vortex liquid crystal lens. Background Art

[0002] Vortex beams are beams with special optical properties. Their uniqueness lies in that the light intensity distribution is annular and they have orbital angular momentum in the beam propagation direction. Vortex beams have broad application prospects in the fields of optical communication, optical tweezers, microscopy imaging, etc. The generation of traditional complex vortex beams mostly relies on bulky or expensive devices, such as spiral phase plates, which have problems such as large volume and low precision, and are difficult to meet the requirements of modern optical systems for miniaturization and integration.

[0003] As a new type of optical element, although the metasurface vortex focusing lens solves the deficiencies of traditional methods to a certain extent, there are still some problems. For example, the metasurface vortex focusing lens requires high-precision nanofabrication technologies, such as electron beam lithography and atomic layer deposition. Although these technologies can achieve nanofabrication precision at the nanometer level, the processing area is limited and the cost is high. In addition, the efficiency of the metasurface vortex liquid crystal lens is low and there are strict requirements for the polarization state of the incident beam, which limits its application range. As a new type of optical element, the liquid crystal lens uses the birefringence effect of liquid crystals to achieve phase modulation of light, and has the advantages of being thin, adjustable, and having a wide range of application possibilities. The preparation method of the geometric phase vortex liquid crystal lens is simple, efficient, and low-cost, and can achieve miniaturization and integration, providing a new way for the generation and application of vortex beams. Summary of the Invention

[0004] The present invention discloses a preparation method of a geometric phase vortex liquid crystal lens. The geometric phase vortex liquid crystal lens provided by the present invention realizes the modulation of right-handed circularly polarized light and can generate a vortex light field carrying orbital angular momentum, thereby achieving edge enhancement in imaging.

[0005] The present invention provides a preparation method of a geometric phase vortex liquid crystal lens, which is characterized by including the following steps:

[0006] Step 1: Design the phase of the geometric phase vortex liquid crystal lens so that its phase profile is the sum of a hyperbolic phase and a vortex phase with a topological structure, and the charge number is l, where l is an integer. The phase formula is:

[0007]

[0008] where (r, θ) are the polar coordinates on the lens plane, f is the focal length, λ is the wavelength of light, and the liquid crystal horizontal orientation angle is half of this phase function;

[0009] Step 2. Prepare the geometric phase vortex liquid crystal lens by adopting an interference exposure orientation optical path based on the principle of polarization holography. The interference exposure orientation optical path based on the principle of polarization holography includes optical devices such as a laser, a 4f system, a half-wave plate, a polarization beam splitter prism, a quarter-wave plate, a vortex wave plate, a convex lens, and a beam splitter (also used for beam combination); the specific optical path is as follows: The laser used for exposure emits laser light, and then the beam is expanded by the 4f system composed of two convex lenses; the expanded beam passes through a diaphragm to filter the light at the edge of the light spot, and then successively passes through the half-wave plate, and the laser polarization vector is parallel to the polarization beam splitter prism, and then is divided into two optical paths by the polarization beam splitter prism: along the transmission light direction of the polarization beam splitter prism, there are successively a first quarter-wave plate and a first mirror, and along the reflection light direction of the polarization beam splitter prism, there are successively a second quarter-wave plate and a second mirror; the transmitted light beam and the reflected light beam along the polarization beam splitter prism are combined by the beam splitter; further, a vortex wave plate is arranged behind the first mirror, and a convex lens is arranged behind the second mirror. Both optical paths are adjusted to circularly polarized light. The circularly polarized light passes through the vortex wave plate to form a vortex beam carrying a spiral phase distribution and capable of generating a topological charge of ±l, and the beam is circularly polarized light with opposite rotation directions. Finally, it is combined by the beam splitter and interferentially exposed and oriented at the glass substrate with an alignment layer.

[0010] Step 3. Prepare the geometric phase vortex liquid crystal lens; the alignment layer solution includes an alignment layer solution formed by Brilliant Yellow (BY) and dimethylformamide DMF. Spin-coat the alignment layer solution on the glass substrate, and use the interference exposure orientation optical path described in Step 2 to expose and orient the alignment layer. During this process, it is modulated by the vector field, so that the alignment molecules rotate perpendicular to the polarization direction of the vector field, thereby replicating the spatial polarization information to obtain the glass substrate after optical orientation; then spin-coat the liquid crystal solution on the alignment layer and cure it with ultraviolet light. The photoinitiator initiates the cross-linking polymerization reaction between the liquid crystal monomers under ultraviolet light and forms a stable liquid crystal polymer film. The oriented liquid crystal molecules are further fixed by the molecules with a specific distribution corresponding to the alignment layer, and finally the geometric phase vortex liquid crystal lens is obtained. The liquid crystal solution is a liquid crystal reactive mesogenic mixture, including liquid crystal RM257, photoinitiator Irgacure651, and toluene.

[0011] Furthermore, the two beams of light can be adjusted to left-handed circularly polarized light or right-handed circularly polarized light. When the beam is right-handed circularly polarized light, after incident on the vortex wave plate, it forms a vortex beam carrying a spiral phase distribution and capable of generating a left-handed circularly polarized state with a topological charge of l; when the beam is left-handed circularly polarized light, after incident on the vortex wave plate, it forms a vortex beam carrying a spiral phase distribution and capable of generating a right-handed circularly polarized state with -l; finally, it is combined by the beam splitter prism and exposed and oriented at the glass substrate.

[0012] Preferably, in the optical path: the distance between the glass substrate and the convex lens along the reflection optical path of the polarization beam splitter prism is preferably at the 2f position to ensure the same spot size.

[0013] Preferably, an alignment layer is prepared on the surface of the glass substrate to obtain a glass substrate with an alignment layer. The alignment layer solution includes a solution formed by Brilliant Yellow (BY) and DMF. The mass content of the alignment material Brilliant Yellow is 1-2%; the alignment layer is aligned by the interference exposure optical path to obtain an optically aligned glass substrate; the liquid crystal solution is configured, and a liquid crystal reactive mesogenic mixture is prepared, including liquid crystal RM257, photoinitiator Irgacure651, and toluene. The mass ratio of the sum of liquid crystal RM257 and photoinitiator Irgacure651 to toluene is 1:5.1, and the liquid crystal used is nematic liquid crystal.

[0014] Based on the above solution, the coating method used is spin coating. Spin coat at a speed of 800-1000 r / min for 5 s, and then at a speed of 2500-3000 r / min for 25-30 s. Bake at 80° for 3-5 min, and cure with 365 nm ultraviolet light for 7-10 min to form a firm alignment layer.

[0015] Further adjust the charge number l through the vortex wave plate.

[0016] Place the obtained geometric phase vortex liquid crystal lens into the optical path where the light beam is right-handed circularly polarized light. The focused spot can be captured by a CCD, and its spot shape is similar to a doughnut shape.

[0017] The geometric phase vortex liquid crystal lens obtained by the present invention is used in the fields of optical imaging systems and optical information processing. It can compress the imaging and edge enhancement functions into a single-layer lens to achieve the edge enhancement effect.

[0018] The light beam passes through the geometric phase vortex liquid crystal lens, achieving precise control of right-handed circularly polarized light and being able to generate a vortex light field carrying orbital angular momentum, showing the characteristics of edge enhancement. This method has the advantages of simple preparation process, low cost, and single exposure. At the same time, it reduces the volume and thickness of the device, making it more suitable for integrated and portable applications.

[0019] Beneficial effects:

[0020] 1. By adopting the photoalignment technology in combination with the principle of polarization holography, a geometric phase vortex liquid crystal lens is prepared, reducing the volume and thickness of the device and simplifying the optical system.

[0021] 2. The preparation process of the present invention is simple, time-consuming is short, and cost is low. Description of the drawings

[0022] Figure 1 It is a schematic diagram of the phase distribution of the geometric phase vortex liquid crystal lens of the present invention.

[0023] Figure 2 It is the interference exposure orientation optical path of the present invention.

[0024] Figure 3 It is a modulation characteristic diagram of the geometric phase vortex liquid crystal lens of the present invention.

[0025] Figure 4 It is a simulated focused spot diagram of the geometric phase vortex liquid crystal lens of the present invention.

[0026] Figure 5 It is the edge enhancement effect shown by the present invention under a simplified optical path system compared with bright field imaging. Detailed implementation manners

[0027] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0028] Embodiment 1

[0029] This embodiment provides a preparation method of a geometric phase vortex liquid crystal lens. The geometric phase vortex liquid crystal lens prepared by the present invention is a liquid crystal vortex liquid crystal lens with red, green, and blue wavelength bands of 442 nm and a focal length of 15 cm. The specific steps are as follows:

[0030] Step 1: Design the phase of the geometric phase vortex liquid crystal lens. Based on the geometric phase vortex liquid crystal lens, a vortex light with a spiral phase distribution is generated. Its phase profile is the sum of a hyperbolic phase and a vortex phase with a topological structure. The charge number is 1, and its schematic diagram is as shown in the appendix Figure 1 as shown.

[0031] The phase of the vortex liquid crystal lens is the sum of a vortex phase with a topological charge number of 1 and a hyperbolic phase. Its formula can be expressed as:

[0032]

[0033]

[0034] where (r, θ) are the polar coordinates on the lens plane, f is the focal length, and the liquid crystal horizontal orientation angle is half of this phase function.Step 2: Prepare the geometric phase vortex liquid crystal lens using the adopted orientation optical path. The exposure interference optical path includes a 4f system, a half-wave plate, and a polarization beam splitter prism. Along the transmission light direction of the polarization beam splitter prism, it includes a first quarter-wave plate and a first mirror. Along the transmission light direction of the polarization beam splitter prism, it includes a second quarter-wave plate and a second mirror. Combine the transmitted light beam and the reflected light beam. It also includes a vortex wave plate arranged behind the first mirror and a convex lens arranged behind the second mirror. Both optical paths are adjusted to left-handed circularly polarized light. Among them, the left-handed circularly polarized light passes through the vortex wave plate to form a right-handed circularly polarized light vortex beam carrying a helical phase distribution and capable of generating l = 1. Finally, it is combined by the beam splitter prism and exposed to the orientation optical path structure on the glass substrate as shown in the appendix Figure 2 as shown, combined with the appendix Figure 2 to elaborate on the optical path in detail. In the present invention, the orientation laser 201 is used to emit orientation light. The orientation optical path provided by the present invention includes a 4f system 202 composed of two lenses along the optical path direction of the orientation light. The 4f system is used to expand the beam. The orientation optical path provided by the present invention includes a diaphragm 203 along the optical path direction of the orientation light. The diaphragm 203 is used to filter the light at the edge of the light spot to make the beam more uniform during propagation. The orientation optical path provided by the present invention includes a half-wave plate 204 along the optical path direction of the orientation light. The orientation optical path provided by the present invention includes a polarization beam splitter prism 205. The half-wave plate 204 ensures that the laser polarization vector is parallel to the polarization beam splitter prism 205.

[0035] The orientation optical path provided by the present invention includes a first quarter-wave plate 206 and a second mirror 207 along the optical path direction of the first split light beam obtained from the polarization beam splitter prism. Along the optical path direction of the second split light beam obtained from the polarization beam splitter prism, it includes a second quarter-wave plate 208 and a third mirror 209.

[0036] The orientation optical path provided by the present invention includes a beam splitter 210 for combining the first split light beam and the second split light beam. The orientation optical path provided by the present invention also includes a vortex wave plate 211 arranged between the second mirror and the beam splitter and a convex lens 212 arranged between the third mirror and the beam splitter. In the present invention, the functions of the vortex wave plate 211 and the convex lens 212 are to form a specific interference pattern on the alignment layer of the liquid crystal lens. After adding a lens alone, a lens-shaped circular interference pattern appears. After adding a vortex wave plate, a spiral circular interference pattern appears. Such a pattern is as shown in the appendix Figure 1 as shown.

[0037] Step 3: Prepare the solutions required for the geometric phase vortex liquid crystal lens based on the optical path. The alignment layer solution is prepared from Brilliant Yellow (BY) and DMF solution. The masses of the two solutions are 0.006 g and 0.4 g respectively, and the mass ratio is 1:66.7. The liquid crystal reactive mesogenic mixture is prepared from liquid crystal RM257 (0.1395 g), photoinitiator Irgacure 651 (0.0058 g), and toluene (0.75 g). The liquid crystal used is nematic liquid crystal. The BY solution is placed on the cleaned ITO glass substrate, rotated at a speed of 800 r / min for 5 s, spin-coated at a speed of 3000 r / min for 25 s, and baked at 80° for 3 min to form a firm alignment layer. Use the attached Figure 2 Put it into the optical path for exposure treatment. Use light with a wavelength of 442 nm as the alignment light. At this time, the angle between the half-wave plate 204 and the fast axis is 112°. The polarization beam splitter prism 206 is used to equally divide the linearly polarized light passing through the half-wave plate 204 into two linearly polarized lights with horizontal and vertical polarization directions. The angle between the first quarter-wave plate 207 and the fast axis is 62°, which adjusts the light beam to right-handed circularly polarized light, and the angle between the second quarter-wave plate 210 and the fast axis is 86°, which adjusts the light beam to left-handed circularly polarized light. The focal length of the convex lens is selected to be 15 cm. The distance between the convex lens and the glass substrate is at the 2f position to ensure the same spot size. The total irradiation energy is 2 J / cm 2 . After the alignment is completed, spin-coat the liquid crystal reactive mesogenic mixture at 3000 r / min for 30 s in an environment rich in nitrogen. Place the spin-coated liquid crystal on a heating plate at 80°C and heat it for 3 min. Then immediately cure the sample with ultraviolet light of 365 nm for 7 min to form a stable cross-linked liquid crystal film. After testing, it is necessary to repeat the spin-coating 3 times to obtain a geometric phase vortex liquid crystal lens with better diffraction efficiency.

[0038] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well-known to those skilled in the art.

[0039] For easy understanding, first introduce the geometric phase vortex liquid crystal lens. The vortex liquid crystal lens can adjust the polarization state of the incident light to change the focal length of the outgoing light. As Figure 3 shown in (a) below, when right-handed circularly polarized light is incident on the geometric phase vortex liquid crystal lens, the outgoing light is left-handed circularly polarized light with a focal length of +f, and it is a vortex light carrying a topological charge number of l; as Figure 3 shown in (b) below, when left-handed circularly polarized light passes through the geometric phase vortex liquid crystal lens, the outgoing light is a lens with a focal length of -f, and it is also a vortex light carrying a topological charge number of l.

[0040] The present invention provides a geometric phase vortex liquid crystal lens prepared by the preparation method described in the above technical solution. When it is placed in the optical path where the light beam is a right-handed circularly polarized light, its focused spot can be captured by a CCD, and its simulated focused spot image is as shown in the appendix Figure 4 As shown, the shape of the spot is similar to a donut.

[0041] The geometric phase vortex liquid crystal lens prepared in this embodiment can be applied to edge enhancement imaging, and its imaging effect is as shown in the appendix Figure 5 As shown (the edge enhancement imaging effect of the lower part on the upper part), the geometric phase vortex liquid crystal lens achieves the edge enhancement effect. The reason for the poor edge enhancement effect is that the liquid crystal spin coating is uneven or impurities are introduced during the spin coating process, resulting in uneven film thickness distribution, which in turn affects the optical phase modulation.

[0042] The above specific description further details the purpose, technical solution and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a geometric phase vortex liquid crystal lens, characterized in that: The following steps are involved: Step 1: Design the phase of the geometric phase vortex liquid crystal lens so that its phase profile is the sum of the hyperbolic phase with topological structure and the vortex phase. The charge number is l, where l is an integer. The phase formula is: Where (r, θ) is the polar coordinate on the lens plane, f is the focal length, λ is the wavelength of light, and the horizontal orientation angle of the liquid crystal is half of the phase function; Step 2, adopting the interference exposure orientation optical path based on the principle of polarization holography to prepare the geometric phase vortex liquid crystal lens, the interference exposure orientation optical path based on the principle of polarization holography includes a laser, a 4f system, a half-wave plate, a polarization beam splitter prism, a quarter-wave plate, a vortex wave plate, a convex lens and a beam splitter optical device; the optical path specifically includes: the exposure uses a laser to emit laser, and then the light beam is expanded through a 4f system composed of two convex lenses; the expanded light beam passes through an aperture to filter the light at the edge of the light spot, and then passes through the half-wave plate in sequence, the laser polarization vector is parallel to the polarization beam splitter prism, and then passes through the polarization beam splitter prism to be divided into two light paths: along the polarization beam The transmission light direction of the polarization beam splitter prism includes a first quarter wave plate and a first reflector in sequence, and the reflected light direction of the polarization beam splitter prism includes a second quarter wave plate and a second reflector in sequence; the transmitted light beam and the reflected light beam along the polarization beam splitter prism are combined by a beam splitter; further, a vortex wave plate is arranged behind the first reflector, and a convex lens is arranged behind the second reflector, and the two light paths are adjusted to circularly polarized light. The circularly polarized light passes through the vortex wave plate to form a vortex light beam carrying a spiral phase distribution and capable of generating a topological charge of ±1, and the light beam is circularly polarized light of opposite hand direction, and finally combined by a beam splitter, and oriented by interference exposure at a glass substrate with an orientation layer; Step 3, preparing a geometric phase vortex liquid crystal lens; the orientation layer solution includes an orientation layer solution formed by brilliant yellow (BY) and dimethylformamide DMF, the orientation layer solution is spin-coated on a glass substrate, and the orientation layer is exposed and oriented using the interference exposure orientation optical path described in step 2. During this process, the orientation molecules are modulated by a vector field, so that the orientation molecules rotate perpendicular to the polarization direction of the vector field, thereby replicating the spatial polarization information, and obtaining a glass substrate after light orientation; The liquid crystal solution is then spin-coated onto the orientation layer and cured using ultraviolet light. The photoinitiator initiates a cross-linking polymerization reaction between the liquid crystal monomers under ultraviolet light to form a stable liquid crystal polymer film. The oriented liquid crystal molecules are further fixed by the molecules with a specific distribution corresponding to the orientation layer, and finally the geometric phase vortex liquid crystal lens is obtained.

2. The method according to claim 1, characterized in that The two beams of light can be adjusted to left-handed circularly polarized light or right-handed circularly polarized light. When the light beam is right-handed circularly polarized light, after it is incident on the vortex wave plate, a vortex light beam carrying a spiral phase distribution and capable of generating a left-handed circularly polarized state with a topological charge of l is formed; when the light beam is left-handed circularly polarized light, after it is incident on the vortex wave plate, a vortex light beam carrying a spiral phase distribution and capable of generating a right-handed circularly polarized state of -l is formed; finally, the beams are combined by a beam splitter prism and exposed and oriented on a glass substrate.

3. The method according to claim 1, characterized in that In the optical path, the distance between the glass substrate and the convex lens along the reflected optical path of the polarization beam splitter prism is preferably at the 2f position to ensure the same spot size.

4. The method according to claim 1, characterized in that An alignment layer is prepared on the surface of a glass substrate to obtain a glass substrate having an alignment layer, wherein the alignment layer solution comprises a solution formed by brilliant yellow (BY) and DMF, wherein the mass content of the alignment material brilliant yellow is 1-2%; the alignment layer is oriented by adopting the interference exposure optical path to obtain a glass substrate after light orientation; and the liquid crystal solution is prepared to prepare a liquid crystal reactive mesogen mixture comprising liquid crystal RM257, photoinitiator Irgacure651 and toluene, wherein the mass ratio of the total of liquid crystal RM257 and photoinitiator Irgacure651 to toluene is 1: 5.1, the liquid crystal used is nematic liquid crystal.

5. The method according to claim 1, characterized in that The coating method used is spin coating, with a speed of 800-1000 r / min for 5 seconds, a speed of 2500-3000 r / min for 25-30 seconds, baking at 80° for 3-5 minutes, and using 365nm ultraviolet light for curing to form a firm alignment layer, and the curing time is 7-10 minutes.

6. The method according to claim 1, characterized in that The charge number l is adjusted by a vortex wave plate.

7. A geometric phase vortex liquid crystal lens prepared according to the method described in any one of claims 1 to 6.

8. The geometric phase vortex liquid crystal lens prepared by the method described in any one of claims 1 to 6 is placed in the light path of a right-handed circularly polarized light beam, and its focused light spot can be captured by a CCD, and its light spot shape is similar to a donut shape.

9. Application of the geometric phase vortex liquid crystal lens prepared by the method described in any one of claims 1 to 6. The geometric phase vortex liquid crystal lens is used in optical imaging systems and optical information processing fields. The imaging and edge enhancement functions can be compressed into a single-layer lens to achieve an edge enhancement effect.

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