Broadband optical antireflective structures with continuous graded index profile and their fabrication and use

Abstract

The application discloses a wide-spectrum optical anti-reflection structure with a continuous gradient refractive index distribution and preparation and application thereof. The wide-spectrum optical anti-reflection structure comprises a substrate made of a first optical material, and a microsphere array arranged on the surface of the substrate and made of a second optical material, wherein the refractive index of the second optical material is between the refractive index of an incident medium and the refractive index of the first optical material; the microsphere array is a single-layer array, the microsphere array contacts or is embedded into the surface of the substrate to form a composite interface structure; and in the depth direction, the composite interface structure presents a continuous gradient refractive index distribution from the refractive index of the incident medium to the refractive index of the second optical material. By adjusting the microsphere size and embedding depth, the equivalent refractive index distribution can be flexibly regulated, reflection suppression or absorption enhancement in a wide-angle and wide-spectrum range can be realized, and the wide-spectrum optical anti-reflection structure has both biomimetic characteristics and adjustable optical performance.

Description

Technical Field

[0001] This invention mainly relates to the field of optical antireflection and anti-reflection films and structures, and in particular to a broadband optical antireflection structure with a continuously gradient refractive index distribution, its preparation and application. Background Technology

[0002] As the global energy structure accelerates its transformation towards cleaner and lower-carbon energy, and as optoelectronic functional devices continue to evolve towards higher integration and performance, technological innovation in core areas such as photovoltaic power generation, photoelectric detection, photocatalysis, and photothermal conversion is entering a period of rapid development. To achieve higher light energy utilization efficiency, the industry's demand for functional structures with efficient light energy regulation capabilities across a wide spectral range is becoming increasingly urgent.

[0003] Since Lord Rayleigh proposed the principle of reducing glass surface reflection using oxide layers in the early 20th century, anti-reflective (AR) coating technology has been continuously developed. This type of technology improves light transmittance and reduces reflection loss by controlling the reflection and interference behavior of light at the interface between different media. After more than a century of evolution, AR coatings have developed from early single-layer films to multi-layer interference film systems and are widely used in key optical devices such as optical lenses, solar cells, displays, and biosensors.

[0004] However, the design concept of traditional multilayer interference-type AR films mainly relies on the alternating stacking of dielectric materials with different refractive indices, utilizing the interference principle of a quarter-wavelength thickness to suppress reflection in specific wavelength bands and improve transmittance. Chinese patent CN202510262295.8 discloses a wide-angle optical anti-reflection method based on equivalent gradient refractive index matching and phase angle constraints; Chinese patent CN202421531661.2 discloses a high-transmittance base film. Although these structures exhibit good optical performance in specific wavelength bands, they face several limitations in practical applications:

[0005] On the one hand, the preparation process of multilayer alternating structures is complex, and the requirements for film thickness and uniformity are extremely high, resulting in high production costs;

[0006] On the other hand, abrupt changes in refractive index between layers can easily cause interface reflection, reducing the overall transmission performance. At the same time, it is highly sensitive to the incident angle, and its optical performance degrades sharply under wide-angle incident conditions.

[0007] Therefore, traditional interference-type AR films cannot simultaneously meet the comprehensive optical characteristics requirements of wide band, wide angle and high performance.

[0008] Jin et al. fabricated high-performance long-pass optical interference filters (OIFs) using inkjet printing technology under ambient pressure and combined with digital control of printing resolution, utilizing a non-periodic multilayer structure. They achieved an average transmittance of 91.7% in the 540–800 nm wavelength range (Nature Communications, 2024, 15(1): 3372). However, for multi-spectral anti-reflection scenarios (requiring anti-reflection coverage across the entire visible light spectrum, across different wavelengths, or over wide angles), the precise collaborative design and large-scale application of multilayer thin-film systems still require further breakthroughs.

[0009] In recent years, biomimetic structures (such as moth-eye structures) have become a research hotspot due to their natural broadband antireflective properties. These structures achieve excellent antireflective performance in the visible to infrared range by constructing subwavelength-scale periodic or quasi-periodic micro-nano structures on the material surface, allowing for a gradual transition in the interfacial refractive index. Sun et al. replicated a biomimetic moth-eye structure with a period of approximately 200–300 nm on a transparent polycarbonate substrate using Roll-to-Plate ultraviolet nanoimprinting (UV-NIL). The average reflectance of this structure in the 380–760 nm wavelength range was only 1.21%, and the reflectance was still below 4% at an incident angle of 50°, significantly better than untreated samples (Scientific reports, 2018, 8(1): 1–10). However, existing moth-eye structures typically rely on nanoimprinting, etching, or template replication processes, which are complex, have limited parameter controllability, and are costly to produce. Furthermore, the obtained structures are typically optimized only for transparent media, and their adaptability to lossy, high-absorption substrates (such as metals, semiconductors, or MXene-like materials) is insufficient, failing to simultaneously achieve both anti-reflection and absorption modulation. The refractive index gradient distribution of the fixed-morphology moth-eye array is limited across different wavelengths, and its broadband modulation capability remains inadequate.

[0010] Therefore, designing a structure that is simple in structure, has a continuously adjustable refractive index, and can achieve broadband optical control in both non-destructive and destructive material systems has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0011] In view of the problems existing in the above-mentioned background technology, the purpose of this invention is to provide a broadband optical antireflection structure with a continuously graded refractive index distribution, its preparation and application. The structure of this invention can achieve efficient light reflection suppression and absorption regulation over a wide angle and a wide spectral range, and can be widely used in photovoltaic power generation, photoelectric detection, photocatalysis and photothermal conversion, etc., to effectively improve light energy utilization efficiency.

[0012] The objective of this invention can be achieved using the following technical solutions:

[0013] I. A broadband optical antireflection structure with a continuously graded refractive index distribution

[0014] The broadband optical anti-reflection structure includes:

[0015] The substrate is made of a first optical material;

[0016] A microsphere array, disposed on the surface of the substrate, is made of a second optical material, the refractive index of which is between that of the incident medium and that of the first optical material;

[0017] The microsphere array is a single-layer array, and the microsphere array contacts or is embedded in the surface of the substrate to form a composite interface structure.

[0018] In the depth direction, the composite interface structure exhibits a refractive index distribution that is continuously and gradually varied from the incident medium to the refractive index of the second optical material.

[0019] The surface of the microsphere array that contacts or is embedded in the substrate refers to the depth at which the microsphere array is embedded in the substrate, ranging from 0% to 100% of the microsphere diameter.

[0020] The microsphere array is an ordered or quasi-ordered arrangement, with the microspheres in contact with each other or the spacing between them not exceeding 10% of the microsphere radius;

[0021] The microsphere array is composed of several microspheres of equal size, with the diameter of the microspheres ranging from 0.1 μm to 2 μm, preferably from 0.1 μm to 1 μm.

[0022] The microsphere arrangement period P of the microsphere array satisfies the subwavelength diffraction limit condition, specifically: the arrangement period P is less than the center wavelength λ of the incident light target anti-reflection band and the substrate refractive index n. sub The ratio of the two components is partially embedded in the surface of the substrate, such that the composite interface structure includes a first segment and a second segment in sequence along a direction perpendicular to the substrate surface. The first segment is composed of a microsphere portion exposed on the substrate and an incident medium, and the second segment is composed of a microsphere portion embedded in the substrate and the substrate.

[0023] The microsphere array contacts the surface of the substrate, i.e., the depth of the microsphere array embedded in the substrate is 0% of the microsphere diameter, so that the composite interface structure constructs an effective refractive index gradient that continuously transitions from the refractive index of air to the refractive index of the microsphere material in the direction perpendicular to the substrate surface, and reduces surface Fresnel reflection by utilizing subwavelength geometry.

[0024] The microsphere array is completely embedded in the surface of the substrate, that is, the depth of the microsphere array embedded in the substrate is 100% of the diameter of the microspheres. This results in the formation of a composite optical medium layer on the substrate surface by the composite interface structure, which is composed of microsphere material and substrate material. The effective refractive index of this composite medium layer is lower than the refractive index of the substrate body, thereby reducing the optical reflection coefficient of the substrate surface.

[0025] The depth at which the microsphere array is embedded in the underlying substrate ranges from 20% to 80% of the microsphere diameter.

[0026] The refractive index of the second optical material is greater than that of the incident medium.

[0027] In the depth direction, the composite interface structure includes at least one segment, and the refractive index versus depth curve of each segment can be fitted as a fifth-order polynomial curve.

[0028] II. A method for preparing the above-mentioned broadband optical antireflection structure

[0029] The preparation method includes the following steps:

[0030] Step S1) Prepare a substrate with a smooth surface using a first optical material;

[0031] Step S2) Form a monolayer array of microspheres on the smooth surface of the substrate;

[0032] Step S3) embeds the bottom portion or all of the microsphere array into the substrate surface.

[0033] Furthermore, the preparation method further includes the following steps:

[0034] By increasing the diameter or embedding depth of the microspheres, the refractive index change in each segment of the composite interface structure becomes smoother, thereby suppressing reflection over a wide spectral range.

[0035] By reducing the diameter or embedding depth of the microspheres, the refractive index change in each segment of the composite interface structure becomes steeper, thereby enhancing the local field and absorption of high-frequency light.

[0036] III. An optical device comprising the above-mentioned broadband optical anti-reflection structure

[0037] In the optical device, the broadband optical antireflection structure is used to achieve reflection suppression or absorption enhancement over a wide angle and a wide spectrum.

[0038] When both the first and second optical materials are substantially transparent media within the target operating wavelength band, the broadband optical antireflection structure is used to suppress reflection, and the optical device is a photovoltaic cell, a display panel, or a transparent electrode; "substantially transparent" means that the extinction coefficient k of the material within the target operating wavelength band approaches 0 (e.g., k < 10). -3 This is insufficient to produce significant light absorption loss, causing light waves to primarily undergo refraction and transmission.

[0039] When the first optical material is an optically detrimental material with absorption properties within the target operating wavelength band, a broadband optical antireflection structure is used to achieve absorption enhancement, and the optical device is a photothermal conversion, photocatalysis, or infrared absorption device; the "absorption properties" refers to the material having a non-zero and significant extinction coefficient (e.g., k>10). -2 This is sufficient to significantly attenuate the incident light energy within the subwavelength or micrometer scale and convert it into thermal energy or electron-hole pairs, enabling the light waves entering the structure to generate photon energy transfer.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] (1) The structure is simple and the preparation process is simple. The present invention adopts a structure of a single layer of closely arranged microsphere array and a bottom substrate, which can achieve a smooth transition of refractive index between the incident medium and the high refractive index substrate without the need for multi-layer thin film stacking, thus significantly simplifying the preparation process.

[0042] (2) Achieve continuous gradient refractive index control and excellent optical performance. The refractive index of the microsphere array material is between that of the incident medium and the substrate, so that the structure forms a gradient refractive index distribution without abrupt changes in the thickness direction, which can effectively reduce Fresnel reflection and interface loss, and achieve high transmission and low reflection in a wide band.

[0043] (3) It has dual-mode optical function, which can take into account both anti-reflection and absorption. When both the microsphere and the substrate are non-destructive materials (k=0), the structure can be used as an anti-reflection layer; when the substrate is a destructive material (k≠0), the structure can achieve absorption enhancement through localized light field enhancement and optical path extension, thus having the dual functions of broadband anti-reflection and high-efficiency absorption.

[0044] (4) Adjustable optical performance, adaptable to multiple application scenarios. By adjusting the particle size and embedding depth of the microsphere array, the equivalent refractive index distribution of the structure can be flexibly controlled, thereby achieving reflection suppression or absorption enhancement effects for different targets in a wide angle and wide spectral range, which is suitable for photovoltaic power generation, photoelectric detection, photocatalysis and photothermal conversion and other fields. Attached Figure Description

[0045] Figure 1This is a model diagram of a broadband optical antireflection structure with a continuously gradient refractive index distribution according to the present invention.

[0046] Figure 2 This is a schematic diagram illustrating the control principle of a broadband optical antireflection structure with a continuously gradient refractive index distribution according to the present invention.

[0047] Figure 3 These are physical images of the samples prepared in the embodiments and comparative examples of this invention; wherein, a is the Ti3C2T prepared in Example 1. x Thin film, b is the thin film with anti-reflection coating on the surface in Comparative Example 1, and c is the broadband optical anti-reflection structure with a continuous gradient refractive index distribution prepared in Example 1.

[0048] Figure 4 This is a SEM image of the cross-section of a broadband optical antireflection structure with a continuously gradient refractive index distribution prepared in Example 1 of the present invention.

[0049] Figure 5 This is a SEM image of the single-layer closely packed silica microsphere array with a continuous gradient refractive index distribution, prepared in Example 1 of the present invention, after being embedded in a thin film substrate to form pits.

[0050] Figure 6 This is a comparison of the absorption spectrum curves of three samples: a broadband optical antireflection structure with a continuous gradient refractive index distribution prepared in Example 1 of the present invention, a thin film, and a thin film with an antireflection coating on its surface in Comparative Example 1.

[0051] Figure 7 The above are comparison diagrams of absorption spectrum curves of the broadband optical antireflection structure with a continuous gradient refractive index distribution prepared in Example 1 of the present invention. Among them, a is a comparison diagram of absorption spectrum curves after adjusting the particle size of the monolayer tightly packed silica microsphere array, and b is a comparison diagram of absorption spectrum curves after adjusting the hot pressing process pressure to control the embedding depth of the microspheres in the substrate.

[0052] Figure 8 The absorption spectrum of the material prepared in Example 1 of this invention at different incident angles is shown, where a is Ti3C2T x Thin film and b are broadband optical antireflection structures with a continuously gradient refractive index distribution.

[0053] Figure 9 The images show the light absorption characteristics of broadband optical antireflection structures with different microsphere sizes prepared in Example 1 of this invention. In the image, a is a comparison of absorption spectrum curves, and b is a graph showing the average absorptivity-particle size relationship in the 0.2~2.5μm band.

[0054] Figure 10The images show the light absorption characteristics of broadband optical antireflection structures with different embedding depths prepared in Example 1 of this invention. In the image, a is a comparison of absorption spectrum curves, and b is a graph showing the average absorptivity-particle size relationship in the 0.2~2.5μm band.

[0055] Figure 11 The image shows the front view of the broadband optical antireflection structure with a continuously gradient refractive index distribution prepared in Example 2 of this invention.

[0056] Figure 12 This is a comparison of the absorption spectrum curves of the broadband optical antireflection structure with continuously graded refractive index prepared in Example 2 of the present invention and the acrylic gel.

[0057] Figure 13 The images show physical representations of the acrylic gel and the broadband optical antireflection structure with a continuously gradient refractive index prepared in Example 2, where a is the acrylic gel and b is the broadband optical antireflection structure.

[0058] Figure 14 This is a comparison of the absorption spectrum curves of the broadband optical antireflection structure with continuously graded refractive index prepared in Example 3 of the present invention and the MXene thin film.

[0059] Figure 15 This is a comparison of the absorption spectrum curves of the broadband optical antireflection structure with continuously graded refractive index and the polyethersulfone film prepared in Example 5 of the present invention.

[0060] Figure 16 This is a comparison of the absorption spectrum curves of the broadband optical antireflection structure with continuously graded refractive index prepared in Example 6 of the present invention and the epoxy resin film.

[0061] In the figure, 10 is a broadband optical antireflection structure, 11 is a microsphere array, and 12 is a substrate. Detailed Implementation

[0062] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The accompanying drawings illustrate some embodiments and are intended to explain the implementation cases involved in the present invention, but are not limited to such implementation cases. The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0063] like Figure 1 As shown, the broadband optical antireflection structure 10 with a continuously gradient refractive index distribution provided by the present invention includes:

[0064] Substrate 12 is made of a first optical material;

[0065] The microsphere array 11 is disposed on the surface of the substrate 12 and is made of a second optical material, the refractive index of which is between the refractive index of the incident medium and the refractive index of the first optical material.

[0066] The broadband optical control structure of this invention has a continuously varying effective refractive index distribution along the thickness direction. Its refractive index gradually transitions from the upper incident medium to the lower high-refractive-index substrate, forming a two-virtual-refractive-index structure without abrupt changes. This structure effectively smooths the light propagation path at the interface, reduces Fresnel reflection, and achieves efficient transmission and energy coupling of incident light, providing a novel solution for optical control over a wide wavelength and angle range.

[0067] The microsphere array 11 is a single-layer array that contacts or embeds into the surface of the substrate 12, thereby forming a composite interface structure integrated with the substrate 12. In the depth (thickness) direction from the incident medium to the substrate 12, the composite interface structure exhibits a continuously gradually changing refractive index distribution from the incident medium to the second optical material, achieving a smooth transition of refractive index during light incidence, thereby reducing interface reflection or enhancing absorption. It can be seen that the tightly packed single-layer microsphere array 11 contacts the surface of the underlying substrate 12, or is wholly or partially embedded in the underlying substrate 12. Since the refractive index of the microsphere material is between that of the incident medium and the substrate material, the overall structure forms multiple equivalent medium layers in the thickness direction, and the refractive index of each equivalent medium layer changes continuously from top to bottom, thus constructing an effective optical transition layer with continuously gradually changing refractive index characteristics.

[0068] The microsphere array 11 is an ordered arrangement (including but not limited to tetragonal and hexagonal lattices) or a quasi-ordered arrangement. In the microsphere array 11, adjacent microspheres are in contact with each other or the spacing between them does not exceed 10% of the microsphere radius, forming a continuous contact interface with the surface of the substrate 12. The microsphere array 11 is embedded in the underlying substrate 12, and the bottom of the microspheres forms a continuous contact interface with the surface of the substrate 12 without gaps or physical separation, constituting a smooth, gradual refractive index change without any breaks or abrupt changes. Specifically, the refractive index of the microspheres transitions smoothly with the refractive index of the substrate 12, ensuring that the refractive index gradient changes continuously throughout the structure without significant abrupt changes or abrupt changes in refractive index. This gradual refractive index structure effectively reduces light reflection at the interface, improves light transmittance, and avoids the breakpoint or abrupt effect common in traditional structures.

[0069] Among them, the continuous contact interface means that all microspheres are attached to the surface of the substrate 12, that is, the embedding depth of each microsphere is basically consistent within the process error range, so that the bonding surface between the microsphere and the substrate forms a virtual iso-depth surface parallel to the substrate surface on a macroscopic scale.

[0070] The microsphere array 11 is composed of microspheres of equal size, with the diameter of the microspheres ranging from 0.1 μm to 2 μm, preferably from 0.1 μm to 1 μm.

[0071] Furthermore, the diameter of the microspheres can be set according to parameters such as the center wavelength of the incident light.

[0072] The refractive index of the second optical material lies between that of the incident medium and the first optical material. The substrate material can be any material with a refractive index greater than 1, including optically transparent and opaque materials, such as glass, quartz, metal oxides, non-metal oxides, metal fluorides, organic polymers, ceramics, composite materials, and light-absorbing materials for photothermal and optoelectronic applications. The microsphere material can be inorganic non-metallic microspheres or organic polymer microspheres, such as silica (SiO2) microspheres or polystyrene (PS) microspheres. By selecting different substrates and microsphere materials, a continuous refractive index gradient transition layer is formed between the incident medium and the substrate.

[0073] The bottom of the microsphere array 11 is in contact with or embedded in the underlying substrate 12. The depth at which the microsphere array is embedded in the underlying substrate ranges from 0% to 100% of the microsphere diameter, preferably from 20% to 80% of the microsphere diameter, so as to form a composite interface structure 10 integrated with the substrate, thereby establishing a continuous and smooth gradient refractive index transition layer between the incident medium and the substrate.

[0074] like Figure 2 As shown, the structural mechanism of this invention is based on the principle of refractive index gradient guidance and optical field matching. When light rays are incident from the incident medium to the surface of the structure, they first come into contact with the microsphere array 11. Since the equivalent refractive index of the microsphere layer is between that of the incident medium and the substrate, the incident light undergoes a continuous transition from a low refractive index (incident medium) to a high refractive index (substrate) during propagation, thereby avoiding the abrupt refractive index difference at the traditional incident medium / substrate interface. The existence of this refractive index gradient achieves smooth matching of optical wave impedance, effectively weakens Fresnel reflection, significantly reduces interface reflection loss, and thus improves the light transmittance and incident energy coupling efficiency.

[0075] According to the effective medium theory, when the characteristic size of the microsphere array is smaller than the ratio of the center wavelength of the anti-reflection band of the incident light to the refractive index of the substrate, the entire microsphere layer can be regarded as an equivalent medium layer with a gradually changing refractive index along the thickness direction. The propagation of light in this medium layer no longer exhibits reflection and refraction at the interface, but rather propagates along a slowly varying refractive index field. The local phase and amplitude of the light wave change continuously along the propagation path, achieving natural refractive index matching from the incident medium to the substrate, thus obtaining optical response characteristics of low reflection and high transmission.

[0076] Furthermore, the tightly embedded interface between the microspheres and the substrate (composite interface structure) forms an optical transition region. The dielectric constant and refractive index of this region change continuously with depth, which can further optimize the spatial uniformity of the light field distribution at the interface. This transition layer not only weakens the coherent interference effect of light at the interface and reduces energy loss caused by electric field reflection, but also enhances the coupling ability of the local light field.

[0077] When the period of the microspheres is less than the ratio of the center wavelength of the antireflection band of the incident light to the refractive index of the substrate, the microsphere array no longer behaves optically as discrete scattering units, but is equivalent to a uniform gradient medium layer with a continuously varying refractive index along the depth direction. This smooth transition eliminates the abrupt refractive index interface that causes Fresnel reflection, making the antireflection effect no longer dependent on the destructive interference condition at a specific wavelength, thus achieving low reflection over an extremely wide wavelength range. Furthermore, thanks to the wide-angle optical impedance matching characteristics provided by the gradient refractive index layer, incident light can smoothly enter the substrate at different angles, effectively overcoming the problem of reflectivity spikes caused by changes in optical path difference under large-angle incident light in traditional interference films. In addition, the geometric symmetry of the microsphere structure and its refractive focusing effect on obliquely incident light further enhance the structure's ability to capture and couple large-angle incident light. The entire structure eliminates the abrupt optical interface change between air and the substrate, thus maintaining stable low reflection and high transmission performance under wide wavelength and wide incident angle conditions.

[0078] In the depth (thickness) direction from the incident medium to the substrate 12, the composite interface structure includes at least one segment, and the relationship curve between the equivalent refractive index and the depth (thickness) of each segment exhibits a fifth-order polynomial gradient distribution.

[0079] For example, when the microsphere array 11 is partially embedded in the surface of the substrate 12, the equivalent refractive index distributions of the first segment and the second segment correspond to the first fifth-order polynomial function and the second fifth-order polynomial function, respectively.

[0080] A continuously varying refractive index distribution refers to the highly smooth curve of the equivalent refractive index versus depth (thickness) of a composite interface structure. Specifically, the first and second derivatives of this curve are continuous, and the derivatives at the air-microsphere interface and the microsphere-substrate interface approach zero or transition smoothly. This double-ended smoothness eliminates the refractive index inflection points present at the interface in linear gradients (such as conical structures), thereby suppressing Fresnel reflection to the greatest extent.

[0081] A fifth-order polynomial-like function is a mathematical model or fitted curve describing the nonlinear variation of the equivalent refractive index with depth, exhibiting an "S"-shaped geometric distribution. The core characteristic of this function is that both its first and second derivatives are continuous and satisfy specific boundary conditions: at the starting point (e.g., the air interface) and the ending point (e.g., the substrate interface) of the refractive index change, the first derivative (slope) of the function is zero or close to zero. This mathematical property ensures that the rate of change of refractive index is gradual, rather than abrupt, as light waves enter and leave the complex interface structure.

[0082] As a preferred implementation method or theoretical design model, the aforementioned effective refractive index distribution curve can be fitted to a quintic polynomial-like function. Its normalized refractive index distribution n(u) can be approximately represented by the following quintic basis polynomial:

[0083]

[0084] In the formula, n(u) represents the equation for the change of the equivalent refractive index with depth, n s n represents the refractive index of substrate 12. f The refractive index of the incident medium or microsphere is represented by u, which represents the normalized depth coordinate (u∈[0,1], where 0 represents the top of the interface and 1 represents the bottom of the interface).

[0085] It should be noted that the "quasi-quintic polynomial" described in this invention is not limited to the specific coefficients mentioned above. Any smooth high-order polynomial curve or S-shaped function curve (such as the sine square function, etc.) that satisfies the condition that the derivatives at both ends are zero and exhibit monotonical changes is covered within the scope of this definition.

[0086] This invention also provides a method for fabricating a broadband optical antireflection structure, comprising the following steps:

[0087] Step S1) Prepare a substrate 12 with a smooth surface using a first optical material;

[0088] Step S2) A monolayer microsphere array 11 is formed on the smooth surface of the substrate 12;

[0089] This step can be achieved using methods such as self-assembly, template-assisted method, external force-assisted method, or sol-gel method;

[0090] Step S3) embed the bottom portion or all of the microsphere array 11 into the surface of the substrate 12 to obtain a composite interface structure with a gradient refractive index transition layer.

[0091] This step can be achieved using methods such as physical pressure embedding, thermally driven embedding, chemically assisted embedding, heat treatment, solvent wetting, or ion etching.

[0092] The microsphere array in the broadband optical antireflection structure of this invention exhibits a periodically distributed layer of dielectric particles on a macroscopic scale, while on a microscopic scale, the subwavelength size of the microspheres (smaller than the incident light wavelength) creates a subwavelength-level refractive index gradient. According to the theory of effective media, light propagation within this structure no longer follows the traditional refraction / reflection laws of a single interface, but rather primarily follows a slowly varying refractive index propagation mode within an equivalent medium. By continuously adjusting the local dielectric constant of the microsphere array, the phase-matching condition of the incident light wave gradually changes along the propagation path, thereby weakening interface reflection interference and naturally forming a low-reflection, high-transmittance optical response, laying the foundation for broadband control.

[0093] In this invention, the microsphere array in the broadband optical antireflection structure forms an optical transition layer in the close contact area with the underlying substrate, enhancing the uniformity of the local distribution of the light field at the interface and reducing energy loss caused by light field reflection interference. When the microsphere arrangement period is smaller than the ratio of the center wavelength of the incident light target antireflection band (such as the visible to near-infrared band) to the refractive index of the substrate, the structure as a whole behaves as an isotropic equivalent medium, achieving a stable antireflection effect over a wide angle (0~60°) and a wide wavelength range (300~2500 nm).

[0094] When light enters the structure from the incident medium, it first comes into contact with the microsphere array within the structure. Since the equivalent refractive index of the microsphere array is between that of the incident medium and the underlying substrate, the incident light undergoes a smooth transition from a low refractive index (incident medium) to a high refractive index (substrate) during propagation. This avoids the abrupt refractive index difference at the traditional incident medium / substrate interface, effectively suppressing Fresnel reflection, reducing interface reflection loss, and significantly improving the transmittance of the incident light.

[0095] Furthermore, by adjusting the particle size of the microsphere array and its embedding depth in the substrate, the rate of change and thickness range of the graded refractive index distribution can be flexibly controlled.

[0096] As the diameter of the microspheres increases or the embedding depth increases, the thickness of the gradient layer also increases, and the refractive index changes more gradually, thereby achieving a stronger reflection suppression effect over a wider spectral range.

[0097] Conversely, when the size of the microspheres is reduced or the embedding depth is shallower, the structure's response to high-frequency light is enhanced, which can strengthen the local field and improve absorption.

[0098] Therefore, the structure of this invention not only possesses excellent broadband anti-reflection properties, but also allows for the customization of optical functions in different spectral bands through adjustment of geometric parameters.

[0099] This invention also provides an application of a broadband optical antireflection structure in optical devices. The broadband optical antireflection structure is used to achieve reflection suppression or absorption enhancement over a wide angle and a wide spectral range.

[0100] In this invention, the complex refractive index of the microspheres and the substrate material determines the optical operating mode of the structure:

[0101] When both the microspheres and the substrate are substantially transparent media within the target operating wavelength range, the resulting gradient refractive index transition layer (composite interface structure) effectively suppresses Fresnel reflection, achieving high transmission and low reflection over a wide wavelength range. This transition layer smoothly bridges the refractive index difference between the incident medium (low refractive index) and the high refractive index substrate, avoiding the abrupt refractive index changes at the traditional incident medium / substrate interface. This significantly suppresses Fresnel reflection, reducing interface reflection loss by more than 5%, thus achieving high transmission and low reflection optical characteristics over a wide wavelength range without the need for an additional antireflection coating. This approach is suitable for applications requiring high transmittance, such as photovoltaic cells, display panels, and transparent electrodes.

[0102] When the substrate is an optically lossy material with absorption properties within the target operating wavelength range, a certain amount of absorption loss exists in the structure. A gradient refractive index distribution can induce multiple scattering, localized standing wave enhancement, and optical path extension effects on incident light within the structure. The repeated scattering of incident light by the microsphere array prolongs the light propagation path within the structure. The gradient refractive index change induces localized convergence of the light field and forms a standing wave peak at the microsphere / substrate interface to increase the local optical energy density. The superposition of scattering and standing waves further increases the interaction time between light and the lossy material. The synergistic effect of these three factors can improve the light-trapping capability by more than 10%, achieving efficient absorption and energy conversion over a wide spectral range. This mode can be used in fields requiring high absorption rates, such as photothermal conversion, photocatalysis, and infrared absorption devices.

[0103] Specific embodiments of the present invention are as follows:

[0104] Example 1

[0105] In this embodiment, the characteristics and working principle of the broadband optical antireflection structure with a continuously graded refractive index distribution are as described above. (Reference) Figure 1 The overall structure includes a bottom substrate 12 and a single-layer tightly packed microsphere array 11 embedded in the upper surface of the bottom substrate.

[0106] In this embodiment, the bottom substrate 12 is made of selectively absorbing material MXene, specifically Ti3C2T. x ,like Figure 3As shown in Figure a, this material exhibits excellent photothermal conversion performance and high solar energy absorption rate. The microsphere array 11 is a monolayer of tightly packed 400 nm silica microspheres. The bottom of the microsphere array is embedded in the upper surface of the underlying substrate, and the structure forms a continuous gradient refractive index distribution in the thickness direction, resulting in a broadband optical antireflection structure with a continuous gradient refractive index distribution. (See figure a for details.) Figure 3 As shown in c.

[0107] The fabrication process of the overall structure is as follows:

[0108] First, silica microspheres with a particle size of 400 nm were dispersed in methanol solvent under ultrasonic assistance to prepare a microsphere dispersion with a mass concentration of 20 wt%. Then, the surfactant PEO-TDE was added to the dispersion, with the surfactant mass fraction controlled at 0.25 wt%, and ultrasonic dispersion was continued to obtain a uniform and stable colloidal particle dispersion. Next, a 3*3 cm glass slide was used as a substrate and ultrasonically cleaned sequentially with acetone, isopropanol, ethanol, and deionized water for 10 min each to remove surface impurities and organic residues. After cleaning, the glass substrate was dried for later use. Then, approximately 50 μL of the above colloidal particle dispersion was dropped onto the clean and dried glass substrate and spin-coated at 2000 rpm for 30 s. After the above process, a monolayer of tightly packed silica microspheres with a particle size of 400 nm can be prepared on the glass substrate surface. This array can serve as the microsphere array layer in this structure.

[0109] Next, 16 mL of a selective solar thermal absorption material solution with a concentration of 0.25 mg / mL was thoroughly mixed. Subsequently, the dispersion was uniformly deposited onto the surface of a porous MCE filter membrane (mixed cellulose ester membrane) using a vacuum-assisted filtration method. After filtration and deposition, the resulting film had a thickness of approximately 2 μm, forming a dense and uniform solar selective absorption layer.

[0110] Finally, the monolayer closely packed silica microsphere array with a particle size of 400 nm prepared above was dried under vacuum for 5 min to reduce the surface energy between the microspheres and the glass substrate, thereby improving the stability of subsequent transfer bonding. Subsequently, the wet film was bonded to the surface of the vacuum-dried microsphere array and hot-pressed at 50°C for 20 min to promote physical interlocking and interfacial bonding between the two. After the treatment was completed, the glass substrate was gently peeled off after the structure had dried naturally. The in-situ formed silica microsphere array remained on the film surface, and the bottom of the microsphere array was embedded in the substrate, thus obtaining a composite optical structure with a monolayer closely packed microsphere array on the surface.

[0111] like Figure 4 and Figure 5 As shown, in the prepared broadband optical antireflection structure with a continuous gradient refractive index distribution, a monolayer of tightly packed silica microspheres is partially embedded in the thin film surface, effectively inducing incident light to smoothly enter the interior of the thin film.

[0112] The refractive index of silica microspheres (n≈1.45) is between that of air (n≈1.0) and that of MXene substrates (n≈2.5~3.0, Ti3C2T). x Between the air and MXene surfaces, a system with a gradually changing refractive index is formed. When light is incident from air onto the surface of this structure, it undergoes a smooth transition from a low refractive index (air) to a high refractive index (MXene) during propagation, forming a smooth, gradually changing refractive index distribution along the thickness direction. This achieves progressive matching of the optical wave impedance, effectively weakening Fresnel reflection at the air / MXene interface, reducing interface reflection loss to below 5%, and significantly improving the incident coupling efficiency of light. Simultaneously, MXene is a typical lossy material (imaginary part of complex refractive index k≠0), and its layered structure imparts high electrical conductivity (≈10). 4 (on the order of S / cm) and intrinsic light absorption characteristics (absorption coefficient in the visible to near-infrared band ≈ 10). 5 cm -1 Simultaneously, the presence of the microsphere array induces multiple scattering and localized optical field enhancement effects on the structural surface, causing incident light to reflect multiple times between the microspheres and the substrate and extending its propagation path. This significantly increases the light's residence time and energy absorption depth, thereby enhancing the MXene layer's capture depth of incident light. At the subwavelength scale, the silica microspheres can also form Mie resonances and interface standing wave modes, with a significant enhancement of the localized electric field at the microsphere-substrate interface. This localized field enhancement effect allows incident energy to be coupled more concentratedly into the MXene layer, further improving the overall absorption efficiency of the structure.

[0113] like Figure 6 As shown, the broadband optical modulation structure prepared in Example 1 exhibits higher absorption rates across the entire visible to near-infrared band (approximately 300–2500 nm). Particularly in the main solar irradiance range of 400–1500 nm, its absorption rate is significantly higher than that of the comparative sample, with an average absorption rate increase of approximately 10% compared to a single-layer film and approximately 3%–5% compared to a film with an anti-reflective coating.

[0114] refer to Figure 7 Based on the working principle described in the specification, the structure of this invention has excellent optically adjustable characteristics. For example... Figure 7 As shown in Figure a, by adjusting the particle size of the monolayer tightly packed silica microsphere array, the equivalent refractive index distribution and gradient change rate of the structure can be altered; and as... Figure 7As shown in b, by adjusting the pressure conditions in the hot-pressing process to control the embedding depth of the microspheres in the substrate, the thickness of the interface transition layer and the light field distribution can be further affected. Adjusting both of these parameters can effectively control the absorption performance, thereby achieving differentiated absorption rate enhancement effects in different wavelength ranges to meet the needs of various optical and energy conversion applications.

[0115] like Figure 8 As shown, the left figure is the absorption spectrum of MXene at different incident angles, and the right figure is the absorption spectrum of the structure prepared in Example 1 at different incident angles. It can be seen that the structure has low angle sensitivity.

[0116] like Figure 9 a and Figure 9 As shown in Figure b, MXene was selected as the substrate material, and silica (SiO2) microspheres were used as the top layer. The depth to which the microspheres were embedded in the substrate was uniformly set to 1 / 2 of their diameter (1 / 2D, where D is the diameter of the microsphere). By systematically adjusting the microsphere particle size (0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, 0.9 μm, 1 μm, 2 μm) and conducting light absorption rate tests and analyses, the light absorption characteristics of the material under different particle size conditions were obtained. The results show that the effective control range of the microsphere diameter is 0.1 μm to 2 μm, of which 0.1 μm to 1 μm is the preferred particle size range, which can achieve better light absorption performance.

[0117] like Figure 10 a and Figure 10 As shown in Figure b, MXene was selected as the substrate, and silica (SiO2) microspheres with a diameter of 400 nm were arranged on the top layer. The depth of the microspheres embedded in the substrate (0 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm) was systematically controlled, and the light absorption characteristics of the material corresponding to different embedding depths were clarified by combining light absorption rate testing and analysis. The results show that the effective range of the microsphere array embedding depth in the substrate is 0%~100% of the microsphere diameter, with 20%~80% being the preferred range, within which the material exhibits better light absorption performance.

[0118] In summary, compared with traditional multilayer antireflective coating structures, the broadband optical antireflective structure with a continuous gradient refractive index distribution proposed in this invention has comprehensive advantages such as simple structure, excellent performance, and reliable process. This structure naturally forms a continuous gradient refractive index distribution through the self-assembly of a single-layer microsphere array and a substrate, eliminating the need for additional multilayer film deposition or complex vacuum coating processes, thus significantly simplifying the fabrication process and reducing manufacturing costs. Over a wide wavelength range, the gradient refractive index layer effectively smooths the refractive index transition between air and the high-refractive-index substrate, reducing interface reflection loss. Simultaneously, the multiple scattering and localized light field enhancement effects induced by the microsphere array further promote deep light coupling and energy capture, resulting in overall absorption performance significantly superior to traditional antireflective films. Furthermore, by adjusting the particle size of the microsphere array and its embedding depth in the substrate, the equivalent refractive index distribution of the structure can be flexibly controlled, thereby achieving reflection suppression or absorption enhancement effects for different applications over a wide spectral range. More importantly, the microsphere array and the substrate are integrated in a mortise and tenon manner to form a stable composite interface, which can effectively avoid the peeling and failure of multilayer films in traditional coatings due to thermal stress or insufficient adhesion.

[0119] Therefore, the structure of this invention achieves efficient light energy regulation while also being simple to manufacture, and has broad application and promotion value.

[0120] Comparative Example 1

[0121] The preparation process in this embodiment is similar to that in Example 1, except that the upper monolayer of tightly packed silica microspheres is replaced with a uniform silica layer. Specifically, approximately 300 μL of a 3 wt% polyhydrosilazane (PHPS) solution, using n-butyl ether as a solvent, is sprayed onto the dried film surface. After drying, the PHPS layer transforms into a dense silica layer during oxidation, forming an interference layer with a thickness of approximately 150 nm. Figure 3 As shown in b. (Reference) Figure 6 It is known that this structure can achieve optical absorption enhancement based on the principle of destructive thin-film interference. By introducing a single layer of silicon dioxide interference film on a high-reflectivity substrate, reflection can be suppressed and absorption rate improved at a specific wavelength. However, since the interference layer only works within a limited wavelength range, this structure can only achieve absorption rate enhancement in a specific band and lacks the ability to enhance the entire spectrum. Compared with the broadband optical antireflection structure with a continuously gradient refractive index distribution proposed in this invention, the interference structure obtained in Comparative Example 1 has significant shortcomings in terms of absorption intensity and bandwidth coverage. The experimental results of Comparative Example 1 further highlight the unique advantages exhibited by the structure of this invention under the action of a continuous gradient refractive index distribution, proving the superior performance of this invention in achieving broadband optical control and efficient energy harvesting.

[0122] Example 2

[0123] The preparation process in this embodiment is largely the same as in Example 1, with the key difference being that the material of the bottom substrate 12 is replaced with acrylic gel (AAc, a polymer material). Figure 11 , Figure 13 a and Figure 13 As shown in Figure b, in the fabricated continuous graded refractive index broadband optical antireflection structure, a monolayer of tightly packed silica microspheres is partially embedded in the film surface, effectively guiding incident light smoothly into the film's interior. The transition layer formed by this structure smoothly bridges the refractive index difference between the low-refractive-index incident medium and the high-refractive-index substrate, avoiding the abrupt refractive index change problem at the traditional incident medium / substrate interface, thus significantly suppressing Fresnel reflection and effectively reducing interface loss. Figure 12 It is evident that this broadband optical modulation structure exhibits superior transmittance performance across the entire visible to near-infrared wavelength range (approximately 300–2500 nm).

[0124] Other embodiments of the present invention are shown in the table below:

[0125] Example 3 <![CDATA[Ti3C2T x MXene]]> PS 1 μm Example 4 <![CDATA[Ti3C2T x MXene]]> <![CDATA[SiO2]]> 1 μm Example 5 PES <![CDATA[SiO2]]> 400 nm Example 6 PR <![CDATA[SiO2]]> 400 nm

[0126] Example 3

[0127] The preparation process in this embodiment is largely the same as in Example 1, with the key difference being that the material of the microsphere array 11 is replaced with polystyrene (PS, a polymer material). Figure 14 As shown, this broadband optical modulation structure exhibits increased absorption across the entire visible to near-infrared wavelength range (approximately 200–1800 nm).

[0128] Example 4

[0129] The preparation process in this embodiment is largely the same as in Example 1, with the key difference being that the particle size of the microsphere array 11 is replaced with 1 μm. For example... Figure 7 As shown in a, this broadband optical modulation structure exhibits increased absorption across the entire visible to near-infrared wavelength range (approximately 0.3–2500 nm).

[0130] Example 5

[0131] The preparation process in this embodiment is largely the same as in Example 1, with the key difference being that the material of the bottom substrate 12 is replaced with polyethersulfone (PES, a polymer material). Figure 15 It is evident that this broadband optical modulation structure exhibits superior transmittance performance across the entire visible to near-infrared wavelength range (approximately 300–2500 nm).

[0132] Example 6

[0133] The preparation process of this embodiment is optimized based on Example 1. The significant difference is that the material of the bottom substrate 12 is changed to epoxy resin polymer (PR), and the composite process of the microsphere array and the substrate is changed from hot pressing to casting. Figure 16 The spectral test results show that the broadband optical modulation structure prepared by this process exhibits superior optical transmittance performance in the entire visible to near-infrared band (300~2500 nm).

[0134] The above are only some embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent material and structural changes made under the inventive concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A broadband optical antireflection structure with a continuously graded refractive index distribution, characterized in that, include: The substrate (12) is made of a first optical material; A microsphere array (11) is disposed on the surface of the substrate (12) and is made of a second optical material, the refractive index of which is between the refractive index of the incident medium and the refractive index of the first optical material; the microsphere array (11) is an ordered or quasi-ordered arrangement structure, with the microspheres in contact with each other or the spacing between them not exceeding 10% of the microsphere radius; the microsphere array (11) is composed of several microspheres of equal size, with the diameter of the microspheres ranging from 0.1 μm to 2 μm; the arrangement period P of the microsphere array (11) is less than the center wavelength λ of the anti-reflection band of the incident light target and the refractive index n of the substrate. sub The ratio; The microsphere array (11) is a single-layer array, and the microsphere array (11) contacts or is embedded in the surface of the substrate (12) to form a composite interface structure; in the depth direction, the composite interface structure presents a continuously gradually changing refractive index distribution from the incident medium to the second optical material. The microsphere array (11) contacts the surface of the substrate (12), so that the composite interface structure constructs an effective refractive index gradient that continuously transitions from the air refractive index to the refractive index of the second optical material in a direction perpendicular to the substrate surface, thereby reducing surface Fresnel reflection by utilizing subwavelength geometry. The microsphere array (11) is fully embedded in the surface of the substrate (12), so that the composite interface structure forms a composite optical medium layer on the surface of the substrate, which is composed of microsphere material and substrate material. The effective refractive index of the composite optical medium layer is lower than the refractive index of the substrate body, thereby reducing the optical reflection coefficient of the substrate surface. The microsphere array (11) is partially embedded in the surface of the substrate (12), such that the composite interface structure includes a first segment and a second segment in sequence along a direction perpendicular to the substrate surface. The first segment is composed of a microsphere portion exposed on the substrate and an incident medium, and the second segment is composed of a microsphere portion embedded in the substrate and the substrate. By increasing the microsphere diameter or embedding depth, the refractive index change of each segment in the composite interface structure becomes smoother, thereby suppressing reflection over a wide spectral range. By decreasing the microsphere diameter or embedding depth, the refractive index change of each segment in the composite interface structure becomes steeper, thereby enhancing the local field and absorption of high-frequency light. When both the first and second optical materials are substantially transparent media materials within the target operating wavelength band, the broadband optical antireflection structure is used to achieve reflection suppression; when the first optical material is an optically detrimental material with absorption characteristics within the target operating wavelength band, the broadband optical antireflection structure is used to achieve absorption enhancement.

2. The broadband optical antireflection structure with a continuously graded refractive index distribution according to claim 1, characterized in that: The microsphere array (11) is embedded in the substrate (12) at a depth ranging from 20% to 80% of the microsphere diameter.

3. The broadband optical antireflection structure with a continuously graded refractive index distribution according to claim 1, characterized in that: The refractive index of the second optical material is greater than that of the incident medium.

4. The broadband optical antireflection structure with a continuously graded refractive index distribution according to claim 1, characterized in that: In the depth direction, the composite interface structure includes at least one segment, and the refractive index versus depth curve of each segment can be fitted as a fifth-order polynomial curve.

5. A method for fabricating a broadband optical antireflection structure as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1) Prepare a substrate (12) with a smooth surface using a first optical material; Step S2) Form a monolayer array of microspheres (11) on the smooth surface of the substrate (12). Step S3) embeds the bottom portion or all of the microsphere array (11) into the surface of the substrate (12).

6. An optical device comprising the broadband optical antireflection structure as described in any one of claims 1 to 4, characterized in that: The broadband optical antireflection structure is used to achieve reflection suppression or absorption enhancement.

7. The optical device according to claim 6, characterized in that: When both the first and second optical materials are substantially transparent media materials within the target operating wavelength band, the broadband optical antireflection structure is used to achieve reflection suppression, and the optical device is a photovoltaic cell, a display panel, or a transparent electrode; when the first optical material is an optically detrimental material with absorption characteristics within the target operating wavelength band, the broadband optical antireflection structure is used to achieve absorption enhancement, and the optical device is a photothermal conversion, photocatalysis, or infrared absorption device.

8. An optical device comprising a broadband optical antireflection structure obtained by the preparation method described in claim 5, characterized in that: The broadband optical antireflection structure is used to achieve reflection suppression or absorption enhancement.

9. The optical device according to claim 8, characterized in that: When both the first and second optical materials are substantially transparent media materials within the target operating wavelength band, the broadband optical antireflection structure is used to achieve reflection suppression, and the optical device is a photovoltaic cell, a display panel, or a transparent electrode; when the first optical material is an optically detrimental material with absorption characteristics within the target operating wavelength band, the broadband optical antireflection structure is used to achieve absorption enhancement, and the optical device is a photothermal conversion, photocatalysis, or infrared absorption device.

Images