Flexible optical waveguide preparation method and flexible optical waveguide

Abstract

The invention provides a flexible optical waveguide preparation method and a flexible optical waveguide, and the method comprises the steps: providing a flexible substrate, and carrying out the preprocessing of the flexible substrate, so as to form an adhesion promotion layer on the surface of the flexible substrate; coating uncured photoresist on the adhesion promoting layer of the flexible substrate, and placing the flexible substrate coated with the photoresist on an objective table of a two-photon polymerization direct writing system; constructing a three-dimensional model of the waveguide structure; performing hierarchical slicing and processing on the three-dimensional model of the waveguide structure by taking a Z-axis direction as a slicing direction to obtain a scanning path file; importing the scanning path file into a two-photon polymerization direct writing system, and carrying out scanning exposure on the photoresist on the flexible substrate so as to form a waveguide structure on the flexible substrate; carrying out gradient developing and drying; the prepared flexible optical waveguide has the advantages that the waveguide structure and the flexible substrate are tightly combined, and the performance can still be stable after multiple times of bending, and the yield and the use reliability of flexible optical waveguide preparation are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of energy conversion technology, specifically relating to a method for fabricating a flexible optical waveguide and the flexible optical waveguide itself. Background Technology

[0002] An optical waveguide is an optical device used to guide and transmit optical signals, and it is widely used in many fields such as data communication, sensing, imaging and quantum information processing.

[0003] Traditional optical waveguides are rigid optical waveguides fabricated on rigid substrates (such as silicon and silicon dioxide) using silicon-based photonics processes or electron beam lithography. However, due to their inherent inflexibility, traditional rigid optical waveguides have significant limitations in emerging applications such as wearable devices, bio-integrated sensors, flexible displays, and conformal optical systems, which require optical devices to have flexible and repeatedly bendable characteristics. In these cases, flexible optical waveguides have emerged.

[0004] Currently, flexible optical waveguides are mainly fabricated using the following methods: First, waveguide structures already fabricated on rigid substrates are transferred to flexible substrates. However, since van der Waals forces are closely related to the contact area, this method is only suitable for transferring large-sized waveguide structures and cannot be used for transferring small-sized waveguide structures. Second, waveguide structures are fabricated on flexible substrates using electron beam lithography. Although this method has nanoscale processing precision, its serial writing method results in extremely low efficiency, making it difficult to fabricate on large-area flexible substrates economically and efficiently. At the same time, the subsequent etching process involves many chemical reagents, which can easily lead to problems such as substrate swelling and reduced interface adhesion, resulting in low yield. Third, waveguide structures are fabricated on flexible substrates using traditional femtosecond laser direct writing technology. However, due to the characteristics of flexible substrates, such as low thermal conductivity, low mechanical strength, and high coefficient of thermal expansion, the available laser processing parameter window is very narrow. Slight carelessness can easily cause heat accumulation effects, leading to problems such as burning, carbonization, or microcracks in the flexible substrate. This method has the disadvantages of high processing requirements and high control difficulty.

[0005] To overcome the shortcomings of the above methods, some scholars have proposed applying two-photon polymerization to the fabrication of optical waveguides based on flexible substrates. This utilizes the two-photon absorption effect to confine the region of laser-matter interaction to a very small area near the focal point, breaking the optical diffraction limit and achieving true three-dimensional fabrication of complex micro / nano structures with maskless, non-contact, and extremely high spatial resolution. In two-photon polymerization, the interaction time between the laser and the material is much shorter than the thermal relaxation time, effectively reducing photothermal effects and lowering the risk of thermal damage to the flexible substrate, thus providing a foundation for the economical, efficient, and stable fabrication of flexible optical waveguides. However, in practical applications, it has been found that flexible substrates typically have extremely low surface energy, resulting in weak interfacial adhesion between the formed waveguide structure and the flexible substrate. This makes it easy for the waveguide structure to detach from the flexible substrate during subsequent development or use. Furthermore, during processing, flexible substrates are prone to expansion, contraction, or warping. This macroscopic dimensional instability is transmitted to the micro / nano-scale waveguide structure, leading to twisting, breakage, and other problems, further affecting the yield of flexible optical waveguides. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for fabricating flexible optical waveguides and a flexible optical waveguide, so as to realize the fabrication of high-precision, high-adhesion waveguide structures on flexible substrates, and improve the yield and reliability of flexible optical waveguide fabrication.

[0007] To achieve the above and other related objectives, the present invention provides a method for fabricating a flexible optical waveguide, comprising the following steps:

[0008] A flexible substrate is provided, and the flexible substrate is pretreated to form an adhesion-promoting layer on the surface of the flexible substrate;

[0009] Uncured photoresist was coated onto the adhesion-promoting layer of a flexible substrate, and the photoresist-coated flexible substrate was placed on the stage of a two-photon polymerization direct writing system.

[0010] Construct a three-dimensional model of the waveguide structure;

[0011] The three-dimensional model of the waveguide structure is sliced ​​and processed in layers with the Z-axis as the slicing direction to obtain the scan path file;

[0012] The scan path file is imported into the two-photon polymerization direct writing system to scan and expose the photoresist on the flexible substrate in order to form a waveguide structure on the flexible substrate.

[0013] Gradient development and drying are performed to remove unexposed photoresist.

[0014] Preferably, the pretreatment of the flexible substrate includes surface activation treatment and silane coupling agent grafting; the surface activation treatment includes oxygen plasma treatment and / or ultraviolet ozone treatment to introduce hydrophilic functional groups such as hydroxyl and carboxyl groups on the surface of the flexible substrate to improve surface energy.

[0015] Preferably, the silane coupling agent used for grafting is γ-methacryloyloxypropyltrimethoxysilane. In this way, the methoxy (-OCH3) group of the silane coupling agent undergoes hydrolytic condensation with the hydroxyl groups on the flexible substrate surface to form a strong Si-O-Si covalent bond; simultaneously, its terminal methacrylate groups extend outward, providing active sites for subsequent photoresist polymerization.

[0016] Preferably, the pretreatment of the flexible substrate further includes ultrasonic cleaning; the ultrasonic cleaning process involves ultrasonically cleaning the flexible substrate in acetone, ethanol, and ultrapure water for 5 minutes each, and then drying it with nitrogen gas. This removes organic pollutants, inorganic dust particles, and other impurities adsorbed during the preparation and storage of the flexible substrate, improving cleanliness and avoiding affecting the adhesion between the waveguide structure and the flexible substrate, as well as the transmission loss of the waveguide structure.

[0017] Preferably, the gradient development includes:

[0018] Immerse the flexible substrate with waveguide structure in propylene glycol methyl ether acetate for 5 min to 10 min.

[0019] The flexible substrate with waveguide structure was transferred and immersed in isopropanol for 2 to 3 minutes.

[0020] Preferably, when the waveguide structure includes a high aspect ratio waveguide structure, the developing process further includes: transferring the flexible substrate with the waveguide structure to n-hexane for 2 minutes to reduce the risk of collapse and damage to the waveguide structure.

[0021] Preferably, after gradient development and drying, the flexible substrate with waveguide structure is cured at low temperature, which can further crosslink the unreacted active groups in the waveguide structure, thereby improving its degree of polymerization, mechanical strength and long-term chemical stability.

[0022] Preferably, the method for obtaining the scan path file includes:

[0023] The three-dimensional model of the waveguide structure is sliced ​​in layers with the Z-axis as the slicing direction to obtain multi-layer cross-sectional profile data.

[0024] The cross-sectional profile data of each layer are processed to divide the long cross-sectional profile in each layer into multiple sub-cross-sectional profiles along its own length direction, and to connect two adjacent sub-cross-sectional profiles through an S-shaped cross-sectional profile.

[0025] The contours of each section are scanned and filled to obtain the scan path file.

[0026] Preferably, the flexible substrate is a PDMS substrate, a PI substrate, a PET substrate, or a TPU substrate.

[0027] The present invention also provides an optical waveguide, which is prepared by the above-described flexible optical waveguide preparation method.

[0028] As described above, the flexible optical waveguide fabrication method and flexible optical waveguide provided by the present invention have the following beneficial effects:

[0029] This invention activates the surface of a flexible substrate through oxygen plasma treatment and / or ultraviolet ozone treatment, and constructs molecular bridges using silane coupling agent grafting technology. The methoxy group (-OCH3) at one end of the silane coupling agent undergoes a hydrolytic condensation reaction with the hydroxyl group (-OH) on the surface of the flexible substrate to form a stable Si-O-Si covalent bond. Meanwhile, the organic functional group (such as methacrylate) at the other end of the silane coupling agent forms a molecular-level adhesion promoting layer that can cross-link with the photoresist molecular chain. This significantly increases the interfacial bonding force between the flexible substrate and the photoresist, effectively reducing the risk of the waveguide structure fabricated by the photoresist detaching from the flexible substrate.

[0030] A two-photon polymerization process is used to scan and expose photoresist coated on a flexible substrate to fabricate waveguide structures, effectively ensuring the fabrication accuracy of the waveguide structures. To further reduce the risk of the fabricated waveguide structures detaching from the flexible substrate and improve yield and reliability, this invention optimizes the original monolithic long cross-sectional contours of each layer after slicing the 3D model into segmented cross-sectional contours formed by connecting shorter sub-cross-sectional contours and S-shaped cross-sectional contours. A scan path file is generated based on the segmented cross-sectional contours, transforming the continuous scanning fabrication of the original long waveguide structure into segmented scanning fabrication of short waveguide segments and S-shaped waveguide segments, thereby reducing unidirectional and continuous stress. The accumulated stress is transformed into discrete, releaseable local stress, avoiding the problem of overall warping caused by the continuous accumulation of internal stress during processing and subsequent development to release the internal stress. Simultaneously, the formed S-shaped waveguide segment is more prone to deformation than a straight waveguide segment. When the flexible optical waveguide bends, it can absorb and release energy by changing its radius of curvature, reducing the risk of waveguide structure breakage. Furthermore, this invention enables dynamic modulation of laser power and scanning speed during the scanning exposure process, and automatically reduces the laser power at the start, end, and turning points of the scanning trajectory, avoiding "overexposure" caused by the sample platform accelerating or decelerating during laser scanning, thus ensuring processing accuracy.

[0031] This invention employs a gradient development method to remove unexposed photoresist, addressing substrate swelling and waveguide structure collapse / damage issues during development, thus ensuring the bonding stability between the waveguide structure and the flexible substrate. Furthermore, the low-temperature curing following development allows for further cross-linking of unreacted active groups within the waveguide structure, thereby enhancing its polymerization degree, mechanical strength, and long-term chemical stability, ultimately improving the reliability of the flexible optical waveguide.

[0032] The preparation method of this invention has the advantages of low processing control difficulty, high processing accuracy and high yield. The flexible optical waveguide it produces has a complete waveguide structure, a tight bond between the waveguide structure and the flexible substrate, and can maintain low transmission loss even after multiple bending (500 bending), which is convenient for promotion and application. Attached Figure Description

[0033] Figure 1 This is a flowchart of a flexible optical waveguide fabrication method according to an embodiment of the present invention.

[0034] Figure 2 This is a schematic diagram of the two-photon polymerization direct writing system involved in this invention.

[0035] Figure 3 This is a schematic diagram illustrating the principle of optimizing a long cross-sectional profile in a certain layer of cross-sectional profile data into a segmented cross-sectional profile.

[0036] Figure 4 This is a three-dimensional view of the flexible optical waveguide fabricated based on the flexible optical waveguide fabrication method of the present invention.

[0037] Figure 5 This is a scanning electron microscope (SEM) schematic diagram of the fabricated flexible optical waveguide.

[0038] Explanation of reference numerals in the attached figures

[0039] Stage 1, objective lens 2, laser 3, beam splitter 4, reflector 5, CCD camera 6, long side profile 7, sub-profile 8, S-shaped profile 9, long waveguide structure 10, short waveguide structure 11. Detailed Implementation

[0040] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0041] Please see Figures 1 to 4It should be understood that the structures, proportions, sizes, etc., illustrated in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and to facilitate understanding and reading. They are not intended to limit the scope of the invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and purpose of the invention, should still fall within the scope of the technical content disclosed in this invention. Furthermore, the terms such as "upper," "lower," "left," "right," "middle," and "one" used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention's implementation.

[0042] like Figure 1 As shown, the flexible optical waveguide fabrication method provided by the present invention includes:

[0043] S1. A flexible substrate is provided, and the flexible substrate is pretreated to form an adhesion promoting layer on the surface of the flexible substrate.

[0044] It should be noted that the flexible substrate is made of organic polymers with high optical transparency, good mechanical flexibility and chemical stability, such as PDMS (polydimethylsiloxane), PI (polyimide), PET (polyethylene terephthalate), or TPU (thermoplastic polyurethane), and there is no limitation on the type of substrate. In this embodiment, the flexible substrate is preferably a PDMS substrate.

[0045] It is understood that PDMS substrates can be purchased directly or made by oneself; in this embodiment, the PDMS substrate has a size of 1cm×1cm and a thickness of 500μm.

[0046] The fabrication method of PDMS substrate is as follows:

[0047] 1) Mix the PDMS matrix and the corresponding curing agent at a mass ratio of 10:1 to form a mixed solution;

[0048] 2) The mixed solution is degassed under vacuum, and the degassed mixed solution is poured into a mold made of glass or silicon wafer for curing. The curing conditions are 2 to 4 hours at 80°C. In this embodiment, the curing time is preferably 2 hours.

[0049] 3) Demolding yields the PDMS substrate.

[0050] In step S1, the pretreatment of the flexible substrate includes surface activation treatment and silane coupling agent grafting; wherein, the surface activation treatment includes oxygen plasma treatment and / or ultraviolet ozone treatment.

[0051] The specific steps of oxygen plasma treatment are as follows: A clean flexible substrate is placed in an oxygen plasma cleaner for treatment. The treatment parameters are: RF power 50 W, treatment time 60 seconds. During this process, high-energy oxygen plasma bombards the surface of the flexible substrate, achieving both physical etching to create nanoscale pits on the originally smooth, inert surface, increasing the specific surface area, and chemical modification to generate active hydrophilic groups such as hydroxyl (-OH) and carboxyl (-COOH) groups on the surface of the flexible substrate. Contact angle measurements showed that the water contact angle of the flexible substrate decreased from 110° before treatment to below 40°, indicating a significant increase in the surface energy of the oxygen plasma-treated flexible substrate, laying the foundation for subsequent chemical bonding.

[0052] The specific steps of ultraviolet ozone treatment are as follows: A clean flexible substrate is placed in an ultraviolet ozone cleaner for treatment. The treatment parameters can be set as follows: ultraviolet lamp power 15W, treatment time 20 minutes. Ultraviolet ozone treatment can form a silica-like (SiOx) thin layer on the surface of the flexible substrate. This thin layer has high surface energy and can generate active hydrophilic groups such as hydroxyl (-OH) groups on the surface of the flexible substrate, thereby significantly improving the surface hydrophilicity. Tests using a contact angle meter showed that the water contact angle of the flexible substrate decreased from 110° before treatment to below 40°, indicating that the hydrophilicity of the flexible substrate treated with ultraviolet ozone was significantly improved.

[0053] Silane coupling agent grafting refers to the process by which a hydrolyzable alkoxy group (such as trimethoxysilyl) at one end of a silane coupling agent undergoes a hydrolytic condensation reaction with a hydroxyl group on the surface of a flexible substrate through a gas-phase or wet process, forming a stable covalent bond (such as Si-O-Si). Meanwhile, the organic functional group (such as methacrylate) at the other end of the silane coupling agent extends outward to form an adhesion-promoting layer that can cross-link with the photoresist molecular chain. To ensure grafting uniformity, this embodiment preferably uses a gas-phase method for silane coupling agent grafting.

[0054] The silane coupling agent can be any silane coupling agent suitable for photoresist systems, such as methacryloxysilane or vinylsilane. There is no limitation on this, as long as the selected silane coupling agent is compatible with the selected photoresist. In this embodiment, the preferred silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane, which is methacryloxysilane with the model number KH-570.

[0055] For ease of understanding, the specific steps of vapor-phase grafting are described below: The surface-activated flexible substrate is rapidly (preferably within 5 minutes) transferred to a sealed glass desiccator, and a petri dish containing an ethanol solution of γ-methacryloxypropyltrimethoxysilane at a volume ratio of 0.8%–1.2% (preferably 1%) is placed at the bottom of the desiccator. The substrate is then allowed to stand at room temperature for 30 minutes to complete the vapor-phase grafting. During this process, the methoxy (-OCH3) group at one end of the γ-methacryloxypropyltrimethoxysilane undergoes hydrolytic condensation with the hydroxyl groups on the surface of the flexible substrate, forming a strong Si-O-Si covalent bond. Simultaneously, the methacrylate group at the other end of the γ-methacryloxypropyltrimethoxysilane extends outward, forming a dense molecular-level adhesion-promoting layer on the surface of the flexible substrate, providing active sites for subsequent photoresist polymerization.

[0056] This invention significantly increases the interfacial bonding force between the flexible substrate and the subsequently coated photoresist by performing surface activation treatment and silane coupling agent grafting on the flexible substrate. This reduces the possibility of the waveguide structure prepared by the photoresist detaching from the flexible substrate during subsequent development or use. This not only helps to increase the yield of flexible optical waveguides, but also lays the foundation for improving the service life of flexible optical waveguides.

[0057] In an optional embodiment, the pretreatment of the flexible substrate further includes ultrasonic cleaning, which is performed before the surface activation treatment. The specific steps of ultrasonic cleaning are as follows: the flexible substrate is ultrasonically cleaned in acetone, ethanol and ultrapure water for 5-10 minutes each, and then dried with nitrogen gas to remove organic pollutants, inorganic dust particles and other impurities adsorbed during the preparation and storage of the flexible substrate, thereby improving the cleanliness of the flexible substrate and avoiding affecting the adhesion between the waveguide structure made of photoresist and the flexible substrate and the transmission loss of the waveguide structure.

[0058] S2. Coat the adhesion-promoting layer of the flexible substrate with uncured photoresist, and place the photoresist-coated flexible substrate on the stage of the two-photon polymerization direct writing system.

[0059] It should be noted that the photoresist can be any existing negative photoresist for two-photon printing, such as IP-DIP, YF-TPP, MON-DIP, or ATE-DIP (produced by Yantai Magic Nanotechnology), and there is no limitation on the type, as long as it is compatible with the selected silane coupling agent; in this embodiment, the photoresist is preferably ATE-DIP photoresist.

[0060] In step S2, uncured photoresist is spin-coated onto the flexible substrate using a spin coater to form a uniform, defect-free photoresist layer on the flexible substrate. Of course, for flexible substrates that are not easy to spin-coat (such as extremely soft PDMS substrates), slot coating or dip coating methods can be used to complete the coating.

[0061] It should be noted that the two-photon polymerization direct writing system is existing technology, which includes a stage 1, objective lens 2, laser 3, beam splitter 4, reflector 5, and CCD camera 6. Specifically, existing two-photon 3D printing systems such as the GT2 two-photon micro-nano 3D printing system launched by Nanoscribe, the MPO100 two-photon 3D printing equipment provided by Heidelberg GmbH, the D100 two-photon micro-nano processing system launched by Shenzhen Phoenix Technology Co., Ltd., or the Yuzhiquan two-photon three-dimensional direct writing lithography machine can be selected. There is no limitation on this. In this embodiment, the GT2 two-photon polymerization direct writing system is preferred.

[0062] S3. Construct a three-dimensional model of the waveguide structure.

[0063] In this embodiment, the waveguide structure includes a long waveguide structure and / or a short waveguide structure that is a strip waveguide. In this application, the long waveguide structure is a strip waveguide with a length of not less than 400 μm, and the short waveguide structure is a strip waveguide with a length of not more than 200 μm.

[0064] It should be noted that the 3D model of the waveguide structure can be constructed using various existing 3D modeling software such as SolidWorks, Pro / E, and Catia, and there are no restrictions on which one is used; after the 3D model is constructed, it needs to be saved as a printable file in STL format.

[0065] Before saving, you need to use the "Check Entity" function built into the 3D modeling software to confirm that the constructed 3D model is a closed, error-free entity.

[0066] When saving, set the "Resolution" to "Custom" and adjust the "Tolerance" and "Angle Tolerance" to "High" or equivalent levels to ensure that the generated STL file has enough triangular facets to accurately approximate the designed waveguide structure and avoid contour step distortion in subsequent slicing due to insufficient facets.

[0067] S4. The three-dimensional model of the waveguide structure is sliced ​​and processed in layers with the Z-axis as the slicing direction to obtain the scan path file.

[0068] The specific steps to obtain the scan path file include:

[0069] S41. Import the STL format print file into the slicing software, and set the slicing direction and slice height to slice the three-dimensional model of the waveguide structure into layers and obtain multi-layer cross-sectional profile data of the waveguide structure.

[0070] It should be noted that the slicing software can be commercial software such as Cura, Simplify3D, or Describe, or self-programmed software; there are no restrictions on this.

[0071] In this embodiment, the slicing direction is the Z-axis direction.

[0072] It should be noted that the slice layer height is determined according to the actual situation and is not limited thereto, as long as it is less than the height of the waveguide structure; in this embodiment, the slice layer height is 0.2μm to ensure that there are enough processing layers in the Z-axis direction to form a smooth sidewall.

[0073] It should be noted that after the waveguide structure is sliced, the long waveguide structure is sliced ​​into multiple long cross-sectional profiles, and the short waveguide structure is sliced ​​into multiple short cross-sectional profiles; both the long and short cross-sectional profiles are rectangular.

[0074] S42. Process the profile data of each layer to divide the long profile of each layer into multiple sub-profiles along its own length direction, and connect adjacent sub-profiles through S-shaped profiles.

[0075] Specifically, such as Figure 3 As shown, the long side profile 7 of the long cross-section profile is divided into multiple shorter sub-profiles 8 along its own length direction. Adjacent sub-profiles 8 are connected by S-shaped profiles 9, so as to optimize the original integral long cross-section profile into a segmented cross-section profile formed by connecting sub-section profiles and S-shaped cross-section profiles, thereby facilitating subsequent segmented scanning.

[0076] It should be noted that this step can be implemented programmatically using the built-in Python script editor in the slicing software.

[0077] It should be noted that the length of the sub-contour line 8 is 10μm-50μm to avoid long-distance accumulation of internal stress during subsequent processing; in this embodiment, the length of the sub-contour line 8 is preferably 20μm.

[0078] The S-shaped contour line can be generated using either a cosine or sine function, with no restriction on which; when using a cosine function to generate the S-shaped contour line, the cosine function used is: Where y(x) is the Y-axis coordinate of each point on the S-shaped contour line; x is the X-axis coordinate of each point on the S-shaped contour line; x_start is the X-axis coordinate of the starting point of the S-shaped contour line; L_curve is the length of the S-shaped contour line on the X-axis, which is generally 2μm to 4μm; A is the amplitude, which is generally 1μm to 2μm.

[0079] S43. Scan and fill the contours of each section to obtain the scan path file.

[0080] Different line fill types can be set for the cross-sectional profile, such as spiral fill, straight fill, and cross fill. At the same time, the spacing between two adjacent scan lines within the same cross-sectional profile can be adjusted by setting the scan spacing.

[0081] The scanning spacing should be greater than the diameter of the laser focal spot and less than 1 / 50 of the diameter of the Airy disk. In this embodiment, the scanning spacing is preferably 0.15 μm to ensure appropriate overlap between adjacent scanning lines while taking efficiency into account, so as to facilitate the formation of a continuous and dense structure in subsequent processing.

[0082] The scan path file is in gwl format, which can be directly read by a two-photon aggregation direct-write device.

[0083] This invention, after slicing the three-dimensional model of the waveguide structure, optimizes the original monolithic long cross-sectional profile of each layer into a segmented cross-sectional profile formed by connecting sub-section profiles and S-shaped cross-sectional profiles. This transforms the continuous scanning processing of the original long waveguide structure into segmented scanning processing of short waveguide segments and S-shaped waveguide segments. This facilitates the conversion of unidirectional, continuous stress accumulation into discrete, releaseable local stresses in subsequent processing, avoiding the problem of overall warping caused by the continuous accumulation of internal stress during processing and subsequent development to release internal stress. At the same time, the formed S-shaped waveguide segments are more prone to deformation than straight waveguide segments. They can absorb and release energy by changing their own radius of curvature, thereby protecting the more fragile straight waveguide segments and the adhesion interface between the waveguide structure and the substrate. This further reduces the probability of waveguide structure detachment during subsequent development or use, and improves the adhesion stability of the waveguide structure on the flexible substrate.

[0084] S5. Import the scan path file into the two-photon polymerization direct writing system for scanning exposure to form a waveguide structure on the flexible substrate.

[0085] Zero-point positioning and printing parameter settings are performed before the formal scanning and exposure.

[0086] Zero-point positioning is an existing technology, and its specific steps are as follows: The stage is moved upwards to slowly bring the front end of the objective lens into contact with the photoresist surface until a clear concave meniscus is formed at the objective lens-resist interface; the laser is activated to polymerize the photoresist, and the stage is moved upwards. When the cured photoresist structure is observed to be precisely attached to the flexible substrate plane, the laser focal length is determined to be on the substrate plane; The stage is then moved along the x-axis and y-axis respectively. If no change is observed in the size of the cured photoresist structure, it indicates that the stage has not tilted in any direction, and the laser focal length is consistent with the height of the interface in all directions; if a change in the size of the cured photoresist structure is observed, it indicates that the stage has tilted, and micro-adjustment is required until the laser focal length is consistent with the height of the interface in all directions; Parameters such as brightness and contrast are changed, and the point where the exposure point is clearest in the camera image is the critical line of the substrate-photoresist interface. This position is set as the absolute zero point (Z = 0) for three-dimensional processing.

[0087] Printing parameter settings: Set laser power P to 5mW-10mW, scanning speed v to 10000point / s-50000point / s, and exposure time to 2 ms-5 ms.

[0088] In two-photon polymerization, the transverse diameter D and longitudinal width L of a voxel are proportional to the laser power P and related to the scanning speed. The ratio is inversely proportional; to ensure exposure uniformity, the laser power and scanning speed linkage control function of the two-photon polymerization direct writing system is used during processing to dynamically adjust the laser power and scanning speed, keeping the exposure energy per unit volume stable. Simultaneously, the laser power and scanning speed are reduced 100µm from the starting point of the trajectory, 100µm before the ending point of the trajectory, and at trajectory turns. This effectively eliminates the "overexposure" phenomenon at the endpoints caused by the acceleration, deceleration, and turning of the stage during laser scanning, ensuring exposure uniformity and thus guaranteeing processing accuracy.

[0089] S6. Perform gradient development and drying to remove unexposed photoresist.

[0090] The specific steps of gradient development in this process include:

[0091] S61. Immerse the flexible substrate with waveguide structure horizontally into a glass dish containing propylene glycol methyl ether acetate (PGMEA) and let it stand for 5 to 8 minutes. During this time, the dish can be gently shaken several times to promote the dissolution and diffusion of the uncrosslinked resin. In this embodiment, it is preferable to soak for 5 minutes.

[0092] Propylene glycol methyl ether acetate, as a moderately polar solvent, can gently and effectively dissolve unreacted components of photoresist, while its swelling effect on flexible substrates is far less than that of strongly polar solvents such as acetone.

[0093] S62. Quickly transfer the flexible substrate with waveguide structure into isopropanol (IPA) and immerse it for 2 to 3 minutes.

[0094] Isopropanol is highly polar and can displace and remove residual propylene glycol methyl ether acetate and other trace residues. At the same time, due to its low surface tension and good compatibility with flexible substrates, isopropanol effectively reduces the risk of damage to the waveguide structure during the development process.

[0095] Furthermore, since the long waveguide structure in the waveguide structure is optimized into a segmented connection structure consisting of a straight short waveguide segment and an S-shaped curved waveguide segment, the long waveguide structure can absorb the stress generated by the swelling of the substrate through the expansion and contraction deformation of the S-shaped waveguide segment. This can avoid defects such as cracking or detachment of the waveguide structure above the substrate, thereby improving the yield of flexible optical waveguide fabrication.

[0096] In an optional embodiment, for waveguide structures with high aspect ratios, gradient development further includes: S63, rapidly transferring the flexible substrate with the waveguide structure to n-hexane for 2 minutes to further reduce surface tension and prevent the waveguide structure from collapsing or being damaged during drying.

[0097] After gradient development, the flexible substrate with waveguide structure is dried. The preferred drying method is supercritical drying, which involves placing the flexible substrate with waveguide structure into the sample chamber of a supercritical carbon dioxide dryer, replacing the residual solvent with liquid CO2, and then slowly heating and pressurizing it above the critical point (e.g., 35°C, 80 bar). This state is maintained for 30 minutes, and finally the pressure is slowly released to completely eliminate the gas-liquid interface and the resulting capillary forces, thus preventing the waveguide structure from sticking, bending, or collapsing during the drying process.

[0098] In a further embodiment, the method for preparing the flexible optical waveguide further includes: S7, low-temperature curing, which involves ultraviolet curing or thermal curing at a temperature below 60°C to obtain the final flexible optical waveguide.

[0099] To reduce the risk of thermal deformation of the flexible substrate, the preferred curing temperature is 40°C.

[0100] The specific steps of UV curing are as follows: The flexible substrate with the waveguide structure is placed under a 365nm UV lamp with an intensity of 20 mW / cm² and irradiated in a 40℃ forced-air drying oven for 20 minutes. This low-temperature post-curing process aims to further crosslink the unreacted active groups in the waveguide structure, thereby improving its degree of polymerization, mechanical strength, and long-term chemical stability.

[0101] It should be noted that step S2 can be located between steps S3 and S5.

[0102] This invention also provides a flexible optical waveguide fabricated using the above-described flexible optical waveguide fabrication method, wherein the fabricated flexible optical waveguide is as follows: Figure 4 As shown, a long waveguide structure 10 and multiple short waveguide structures 11 were fabricated on a flexible substrate. Figure 5 This is a scanning electron microscope (SEM) image of the flexible optical waveguide. Figure 5 It can be seen that the waveguide segments in the long waveguide structure 10 are well connected, and both the long waveguide structure 10 and the short waveguide structure 11 are structurally complete; moreover, the waveguide structure is tightly bonded to the interface of the flexible substrate, with no visible gaps or peeling.

[0103] In addition, the sidewalls of the waveguide structure are smooth and vertical, and the surface roughness Ra is less than 8 nm as determined by atomic force microscopy (AFM), which meets the optical requirements for low scattering loss.

[0104] Under the condition of minimum bending radius R = 3 mm, the prepared flexible optical waveguide was subjected to repeated bending tests, and the transmission loss was tested during the bending test. It was found that after the flexible optical waveguide was repeatedly bent 500 times, no microcracks or structural failures were found. Moreover, after 500 bending cycles, the increase in loss of the flexible optical waveguide was less than 0.2 dB. This fully demonstrates that the flexible optical waveguide prepared by the present invention has excellent mechanical robustness and optical stability.

[0105] In summary, by modifying the flexible substrate and combining it with optimized two-photon polymerization and gradient development techniques, this application significantly improves the interfacial bonding strength between the flexible substrate and the fabricated waveguide structure. This effectively reduces or avoids the problem of the waveguide structure detaching from the flexible substrate during processing and use, thereby improving the yield and reliability of flexible optical waveguide fabrication. This provides an effective approach for achieving high-quality and controllable fabrication of nanoscale flexible optical waveguides.

[0106] Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0107] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for fabricating a flexible optical waveguide, characterized in that, Includes the following steps: A flexible substrate is provided, and the flexible substrate is pretreated to form an adhesion-promoting layer on the surface of the flexible substrate; Uncured photoresist was coated onto the adhesion-promoting layer of a flexible substrate, and the photoresist-coated flexible substrate was placed on the stage of a two-photon polymerization direct writing system. Construct a three-dimensional model of the waveguide structure; The three-dimensional model of the waveguide structure is sliced ​​and processed in layers with the Z-axis as the slicing direction to obtain the scan path file; The scan path file is imported into the two-photon polymerization direct writing system to scan and expose the photoresist on the flexible substrate in order to form a waveguide structure on the flexible substrate. Gradient development and drying are performed to remove unexposed photoresist.

2. The method for fabricating a flexible optical waveguide according to claim 1, characterized in that, The pretreatment of the flexible substrate includes surface activation treatment and silane coupling agent grafting; the surface activation treatment includes oxygen plasma treatment and / or ultraviolet ozone treatment.

3. The method for fabricating a flexible optical waveguide according to claim 2, characterized in that, The silane coupling agent used for grafting is γ-methacryloxypropyltrimethoxysilane.

4. The method for fabricating a flexible optical waveguide according to claim 2, characterized in that, The pretreatment of the flexible substrate also includes ultrasonic cleaning; the steps of ultrasonic cleaning are as follows: the flexible substrate is ultrasonically cleaned for 5 minutes each in acetone, ethanol and ultrapure water, and then dried with nitrogen.

5. The method for fabricating a flexible optical waveguide according to claim 1, characterized in that, The gradient development includes: Immerse the flexible substrate with waveguide structure in propylene glycol methyl ether acetate for 5 min to 10 min. The flexible substrate with waveguide structure was transferred and immersed in isopropanol for 2 to 3 minutes.

6. The method for fabricating a flexible optical waveguide according to claim 1, characterized in that, When the waveguide structure includes a high aspect ratio waveguide structure, the development further includes: transferring the flexible substrate with the waveguide structure to n-hexane for 2 minutes.

7. The method for fabricating a flexible optical waveguide according to claim 1, characterized in that, After gradient development and drying, the flexible substrate with waveguide structure is cured at low temperature.

8. The method for fabricating a flexible optical waveguide according to claim 1, characterized in that, The method for obtaining the scan path file includes: The three-dimensional model of the waveguide structure is sliced ​​in layers with the Z-axis as the slicing direction to obtain multi-layer cross-sectional profile data. The cross-sectional profile data of each layer are processed to divide the long cross-sectional profile in each layer into multiple sub-cross-sectional profiles along its own length direction, and to connect two adjacent sub-cross-sectional profiles through an S-shaped cross-sectional profile. The contours of each section are scanned and filled to obtain the scan path file.

9. The method for fabricating a flexible optical waveguide according to claim 1, wherein the flexible substrate is a PDMS substrate, a PI substrate, a PET substrate, or a TPU substrate.

10. An optical waveguide, characterized in that, The optical waveguide is prepared by the flexible optical waveguide preparation method according to any one of claims 1 to 9.

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