Method for manufacturing a broadband variable focus superlens

Abstract

The application provides a wide-band variable-focus superlens preparation method, which comprises the following steps: spin coating photoresist on a high-transmittance thin substrate; forming a hole structure with a designed array pattern in the photoresist of the substrate with the spin-coated photoresist by means of exposure and development; placing the sample of the high-transmittance glass with the patterned hole photoresist in an atomic layer deposition system, filling the sample surface with uniform material by means of deposition, and obtaining a sample with a completely filled patterned structure and an overall flat surface; and placing the completely filled sample in a dry etching system, etching off the flat material layer on the photoresist to expose the patterned structure and the photoresist, and then completely removing the photoresist on the sample to obtain a superlens capable of realizing zooming by changing the orbital angular momentum carried by the incident vortex light beam. Thus, the focusing at different spatial positions under the incidence of vortex light with different angular momenta is realized.

Description

Technical Field

[0001] This application relates to the fields of micro-nano technology and three-dimensional integrated metasurface devices, and in particular to a method for fabricating a wide-band variable-focus superlens. Background Technology

[0002] In traditional optical systems, achieving simple zoom functionality requires the coordination of lens groups, based on the interplay of the focal length and spatial position of each lens. However, metasurfaces, constructed using nanotechnology with periodic or aperiodic subwavelength unit structures arranged in a specific pattern, are superlenses capable of modulating electromagnetic waves. Extensive experimental results demonstrate that artificially fabricated subwavelength micro / nanoscale structures can arbitrarily control the polarization, amplitude, phase, and transmission mode of electromagnetic waves. Furthermore, metasurfaces offer advantages such as high modulation efficiency, the ability to achieve complex optical field modulation, and ease of integration, providing a possibility for replacing and enhancing the functionality of traditional optical devices and achieving miniaturization of optical systems.

[0003] Currently, the rich modulation of electromagnetic waves through fabricated micro / nano subwavelength structures has been widely applied, enabling functions that are difficult to achieve in traditional optical systems, such as holographic imaging, information encryption, and achromatic lenses. However, traditional optical imaging systems are limited by their own ability to modulate light fields, often limiting each component to a single function. This reduces the practical requirements for complex applications while maintaining system simplicity and portability. Meanwhile, currently fabricated superlenses offer limited functionality and cannot be autonomously controlled, and their small size hinders their integration into integrated optical devices. Summary of the Invention

[0004] To overcome or at least partially solve the aforementioned technical problems, this application is proposed. Embodiments of this application provide a method for fabricating a wide-band variable-focus superlens, comprising:

[0005] Step S1: Spin-coat photoresist onto a pre-set high-transmittance substrate;

[0006] Step S2: A hole structure with a designed array pattern is formed inside the photoresist by exposure, resulting in a sample with a patterned periodic arrangement structure.

[0007] Step S3: Deposit material in the hole structure of the photoresist using a deposition device to completely fill the hole structure, and deposit material on the surface of the photoresist to obtain a flat layer with a smooth overall surface, thereby obtaining a fully filled sample;

[0008] Step S4: Place the fully filled sample in a dry etching system. First, etch away the planar layer on top of the photoresist to expose the patterned periodic arrangement structure and the photoresist. Then, remove all the photoresist from the sample to obtain a lens that can achieve zoom using the orbital angular momentum of a vortex beam. Here, high transmittance means that the ability to transmit light is at least greater than 95%, or even close to 100%.

[0009] In one embodiment, the high-transmittance substrate includes a high-transmittance thin silicon dioxide substrate, a high-transmittance thin silicon nitride substrate, or a high-transmittance thin aluminum oxide substrate.

[0010] In one embodiment, the photoresist is electron beam photoresist PMMA, Zep photoresist, ultraviolet photoresist AZ photoresist, or SU8 photoresist.

[0011] In one embodiment, the exposure method includes electron beam exposure technology or ultraviolet exposure technology.

[0012] In one embodiment, the patterned periodic arrangement structure is an arrangement structure in which all elements are of the same size and shape, an arrangement structure with the same structure but different sizes, or an arrangement structure with different shapes and sizes.

[0013] In one embodiment, in step S3, the deposition is achieved by atomic layer deposition, magnetron sputtering, electron beam evaporation, thermal evaporation, or chemical hydrothermal method.

[0014] In one embodiment, in step S3, the material being deposited is a dielectric material, a metallic element, or a composite material with suitable optical refractive index parameters. Here, the suitable optical refractive index parameter refers to high transmittance or a good refractive index for the required operating wavelength; the dielectric material is a material with dielectric properties, a large refractive index n and a small loss k, generally n is greater than k, which modulates the applied electromagnetic wave; the metallic element has a very small refractive index n and a large loss k, usually n is less than k, and the applied electromagnetic wave will induce a current in the metal structure; the composite material refers to a multilayer composite material formed by sequentially filling multiple materials, which can be a multilayer dielectric material, a multilayer metallic material, or a composite material of multiple dielectrics and metals.

[0015] In one embodiment, the dielectric material includes titanium oxide, hafnium oxide, or aluminum oxide, or the elemental metallic material includes gold or silver.

[0016] In one embodiment, in step S4, the etching is achieved by dry etching technology or wet etching technology, wherein the dry etching technology includes ion milling etching, reactive ion etching, or inductively coupled plasma reactive ion etching; and the chemical solution used in the wet etching technology includes hydrofluoric acid or potassium hydroxide.

[0017] In one embodiment, the patterned periodic arrangement structure can phase modulate the geometric phase of light over the entire 2π range.

[0018] In one embodiment, the modulation of the geometric phase of the emitted light is achieved by changing the structural morphology, structural dimensions, or the rotation of the structure itself at a specific location.

[0019] In one embodiment, the patterned periodic arrangement structure has a preset polarization value for incident light of a specific wavelength, which can change the polarization characteristics of the outgoing light.

[0020] In one embodiment, the preset polarization capability value can be freely determined, generally at least greater than 60%, preferably greater than 80%, and more preferably greater than 90%.

[0021] The principle of the fabrication method of the wide-band variable focal length superlens provided by the present invention is to achieve phase modulation at the same time as generating cross-polarized transmitted light in the incident light field through the optical refractive index of the material. Moreover, due to the C4 symmetry of the nanofin structure itself, it can add a geometric phase of twice the rotation angle to the outgoing light when it rotates, thereby realizing phase modulation of the outgoing light phase within the entire 2π range using only a single structure.

[0022] The method for fabricating a broadband variable-focus superlens provided in this application has at least the following technical advantages:

[0023] 1) The preparation process is short, the method is simple and the yield is high. At the same time, it has high integration density and can realize the function of traditional large lenses in a very small area (a few millimeters or even hundreds of micrometers in diameter). In addition, it can achieve spatial zoom capability by simply using the change of orbital angular momentum in the incident light signal, which greatly improves the performance of the lens and reduces the size of the imaging system.

[0024] 2) In this invention, photoresist is exposed, which can quickly obtain precise periodically arranged subwavelength unit structures (tens of nanometers to several micrometers), and has the characteristics of flexibility and ease of operation; at the same time, by using deposition methods, especially atomic layer deposition methods, dense and uniform dielectric materials can be obtained, ensuring the overall device processing quality and the realization of good functions.

[0025] 3) The superlens prepared in this invention has a unit structure with a high aspect ratio, and the large-size, high-density fabrication of the whole sample makes it possible to realize its application in integrated optical devices. Attached Figure Description

[0026] The above and other objects, features, and advantages of this application will become more apparent from the more detailed description of the embodiments of this application in conjunction with the accompanying drawings. The drawings are provided to further illustrate the embodiments of this application and form part of the specification. They are used together with the embodiments of this application to explain this application and do not constitute a limitation thereof. In the drawings, the same reference numerals generally represent the same components or steps.

[0027] Figure 1 This is a flowchart of a method for fabricating a wideband variable focal length superlens according to an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram illustrating the fabrication process of a wideband variable focal length superlens according to an embodiment of the present invention;

[0029] Figure 3 This is an optical microscope image of a broadband variable-focus superlens prepared according to an embodiment of the present invention;

[0030] Figure 4 This is a scanning electron microscope image of a broadband variable focal length superlens prepared according to an embodiment of the present invention.

[0031] Figure 5 This is the light intensity distribution at four focusing positions achieved by a broadband variable focal length superlens prepared according to an embodiment of the present invention when the incident light is 532nm.

[0032] Figure 6 This describes the light intensity distribution at four focusing positions achieved by a broadband variable focal length superlens prepared according to an embodiment of the present invention when the incident light is 633 nm. Detailed Implementation

[0033] Hereinafter, exemplary embodiments according to this application will be described in detail with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments of this application. It should be understood that this application is not limited to the exemplary embodiments described herein.

[0034] The meanings of some terms mentioned in this article are defined as follows:

[0035] Cross-polarized light refers to a light polarization mode whose polarization mode is opposite to that of the incident light. For example, the incident light is left-handed, but the transmitted light is right-handed.

[0036] Left-handed vortex light refers to left-handed polarized light that carries information about a specific orbital angular momentum.

[0037] Geometric phase refers to the phase information resulting from the additional phase modulation caused by the rotation of the nanostructure itself.

[0038] In a method for fabricating a wideband variable-focus superlens according to an embodiment of this application, the polarization mode and phase of the outgoing light are modulated by utilizing the optical properties of the material. Combined with a periodic arrangement to achieve focusing by realizing focusing and decoding the orbital angular momentum information carried by the incident light, the ability to focus at different positions in space is achieved by changing the orbital angular momentum information of the vortex light is realized.

[0039] like Figure 1 As shown, a method for fabricating a broadband variable-focus superlens according to an embodiment of this application may specifically include the following steps:

[0040] Step S1: Spin-coat photoresist onto a pre-set high-transmittance substrate;

[0041] The high-transmittance substrate includes, but is not limited to, one of the following: high-transmittance thin silicon dioxide substrate, high-transmittance thin silicon nitride substrate, and high-transmittance thin alumina substrate. High-transmittance thin substrates can be obtained commercially or prepared by growth methods, including but not limited to atomic layer deposition, thermal evaporation deposition, or electron beam deposition.

[0042] Photoresist can be any common type of photoresist, such as positive photoresist like PMMA (polymethylmethacrylate) or negative photoresist like HSQ; it can also be positive photoresist like AZ or negative photoresist like SU8; or it can be photoresist from other exposure methods.

[0043] Step S2: Form a hole structure with a designed array pattern inside the spin-coated photoresist by exposure, and obtain a sample with a patterned periodic arrangement structure.

[0044] Exposure can be optical exposure techniques such as ultraviolet light exposure technology and laser direct writing technology, or electronic exposure techniques such as electron beam exposure technology.

[0045] The resulting patterned periodic arrangement structure can be an arrangement structure with all members of the same size and shape, or an arrangement structure with the same structure but different sizes, or an arrangement structure with different structural morphology and size. The patterned periodic arrangement structure can be a single-exposure planar structure, or a multi-layer design planar structure obtained through overlay; this invention does not limit this.

[0046] Step S3: By placing the sample in the deposition system, material is deposited in the photoresist holes and on the surface of the sample, so that the patterned periodic arrangement structure of the sample is completely filled and a flat layer with a smooth overall surface is obtained.

[0047] Optionally, when depositing materials on a sample with a periodic porous structure inside the photoresist, atomic layer deposition can be preferred to fill the porous structure, or materials can be deposited on the sample using physical deposition methods such as magnetron sputtering, electron beam evaporation deposition and thermal evaporation, or chemical deposition techniques such as chemical vapor deposition.

[0048] Step S4: Place the fully filled sample in a dry etching system for etching. First, etch away the planar layer on top of the photoresist to expose the patterned periodic arrangement structure and the photoresist. Then, remove all the photoresist from the sample to finally obtain a lens that can achieve zoom by utilizing the orbital angular momentum of a vortex beam.

[0049] When etching the surface of the deposited material, dry etching techniques or physicochemical etching can be used. For example, dry etching techniques include ion milling, reactive ion etching, and inductively coupled plasma reactive ion etching, or wet etching methods using chemical solutions such as hydrofluoric acid and potassium hydroxide.

[0050] In the obtained lens, the material of the patterned periodic arrangement structure has the ability to efficiently repolarize incident light of a specific wavelength, thus altering the polarization characteristics of the outgoing light. Simultaneously, the patterned periodic arrangement structure has the ability to change the phase of the outgoing light, enabling phase modulation of the geometric phase of the outgoing light over the entire 2π range. This ability to control the phase of the outgoing light can be achieved by changing the structural morphology, structural dimensions, and rotation of the structure itself at specific positions. In practical applications, suitable materials, structural dimensions, and structural morphologies can be selected based on the lens's operating wavelength.

[0051] The broadband variable-focus superlens fabrication method provided by one embodiment of the present invention can fabricate stable and highly uniform high aspect ratio nanostructures. In practical applications, structural parameters and material types can be flexibly replaced according to operational requirements. Furthermore, the large area size is more suitable for practical integrated optical device applications.

[0052] The broadband variable focal length superlens fabrication method provided in this invention can achieve large-area precision processing through exposure, deposition and etching. The unit structure can be fabricated across scales (from nanometers to micrometers), and the overall lens structure can also achieve large-area fabrication control (from tens of micrometers to millimeters or even centimeters).

[0053] An example of the fabrication method of the broadband variable focal length superlens according to the present invention, such as... Figure 2 As shown, in step S1, a high-transmittance thin silicon oxide substrate 1 is prepared in advance, cleaned with isopropanol and deionized water, and then an electron beam photoresist 2 (PMMA495A5) is spin-coated onto the cleaned substrate 1 at a speed of 2000 r / min for 60 s. It is then placed on a hot plate at 180°C and baked for 1 min, and this process is repeated three times. Figure 2 As shown in S1.

[0054] In step S2, the sample obtained in step S1 is exposed using electron beam lithography. The predetermined pattern for exposure is set as a rectangular pattern with C4 symmetry. After exposure, the sample is developed using PMMA 495A5 developer and then fixed using a fixer to obtain a PMMA495A5 photoresist structure with periodically arranged holes, as shown below. Figure 2 As shown in S2.

[0055] In step S3, the sample obtained in step S2 is placed in the chamber of an atomic layer deposition (ALD) system to deposit titanium oxide as the dielectric material. Water is used as the oxygen source, and tetramethylamine titanium precursor is used as the titanium source to avoid chlorine contamination. The sample is heated to the required vapor pressure. Throughout the process, the ALD system reacts under the protection of a continuous Ar carrier gas flow, and the chamber maintains a stable reaction growth temperature. Figure 2 As shown in S3.

[0056] In step S4, the sample obtained in step S3 is placed in an inductively coupled plasma etching system. Using trifluoromethane, argon, and oxygen plasma etching, the excess titanium oxide layer at the top is first etched until the residual photoresist is exposed. Figure 2 As shown in S4(1), the remaining photoresist PMMA495A5 is then completely removed, finally resulting in periodically arranged titanium oxide dielectric pillars standing upright on a high-transmittance thin silicon oxide substrate, as shown in the figure. Figure 2 As shown in S4(2).

[0057] In the above example, the prepared unit structure was chosen to be a rectangular titanium oxide nanopillar with a sample diameter of 500 μm.

[0058] In the above embodiments, the high-transmittance silicon oxide substrate material in step S1 can also be other high-transmittance materials that meet the requirements, and is not limited to the silicon oxide substrate mentioned in the embodiments. The exposure pattern in step S2 can be any pattern designed according to actual needs, and is not limited to the patterns mentioned in the embodiments; at the same time, the patterns in the periodic arrangement can all be the same pattern, or different patterns, or patterns of different sizes. In actual processing, the materials and processing parameters required in each step can be flexibly adjusted according to actual requirements, and even a certain step can be repeated multiple times to achieve the processing of multiple material structures. Therefore, through the processes of exposure, deposition, and etching, we can process and fabricate a lens that achieves variable focus using the orbital angular momentum of a vortex beam. Figure 3 and Figure 4 Optical microscope images and scanning electron microscope images of a broadband variable focal length superlens prepared according to an embodiment of the present invention are shown respectively.

[0059] Figure 5 and Figure 6 The light intensity distributions at four focusing positions achieved by a broadband variable-focus superlens fabricated according to an embodiment of the present invention at incident light wavelengths of 532 nm and 633 nm are shown, and the corresponding measured focal lengths are given. The superlens fabricated according to the present invention utilizes a patterned periodic arrangement structure to achieve high polarization efficiency for incident light of specific wavelengths within a broadband range. It can decode orbital angular momentum information under excitation by both 532 nm and 633 nm incident light, realizing focusing functionality at different spatial positions.

[0060] The method for fabricating a wide-band variable-focus superlens provided in this invention utilizes the refractive index gradient of the material and the environment to achieve cross-polarization conversion and phase control of light. At the same time, it utilizes the stable patterning process of electron beam exposure and combines it with the material homogenization material growth process of atomic layer deposition to obtain the fabrication of a large-size superlens.

[0061] The wide-band variable-focus metalens fabricated in this way can overcome the problems of complex lens assembly systems in traditional optical imaging systems and the excessive system size caused by changes in the relative positions of lens groups to change the focal length. At the same time, it is of great significance for the research and application of metasurfaces in realizing functions at different spatial locations, and provides guidance for the future use of metasurfaces as components of functional devices to achieve more efficient and complex functions.

[0062] The fabrication method of the wide-band variable focal length superlens provided in this invention is simple in process, has a high success rate, low cost, low requirements for experimental equipment, and the superlens formed can be modified according to the needs of the working range to meet a wide range of requirements. It will play an important role in the future design and application of metasurface devices.

[0063] The above description has been given for purposes of illustration and description. Furthermore, this description is not intended to limit the embodiments of this application to the forms disclosed herein. Although numerous exemplary aspects and embodiments have been discussed above, those skilled in the art will recognize certain variations, modifications, alterations, additions, and sub-combinations thereof.

Claims

1. A method for fabricating a broadband variable-focus superlens, characterized in that, The method includes: Step S1: Spin-coat photoresist onto a pre-set high-transmittance substrate; Step S2: A hole structure with a designed array pattern is formed inside the photoresist by exposure, resulting in a sample with a patterned periodic arrangement structure. Step S3: Deposit material in the hole structure of the photoresist using an atomic layer deposition device to completely fill the hole structure, and deposit material on the surface of the photoresist to obtain a flat layer with a smooth overall surface, thereby obtaining a fully filled sample; Step S4: Place the fully filled sample in a dry etching system. First, etch away the planar layer on top of the photoresist to expose the patterned periodic arrangement structure and the photoresist. Then, remove all the photoresist from the sample to obtain a lens that can achieve zoom using the orbital angular momentum of a vortex beam. In step S3, the sample obtained in step S2 is placed in the cavity of the atomic layer deposition equipment to deposit titanium oxide medium material. Water is used as the oxygen source, and tetrakis(2-methylamino)titanium precursor is used as the titanium source to avoid chlorine contamination and is heated to the required steam pressure. Throughout the process, the atomic layer deposition equipment is under the protection of continuous Ar carrier gas flow, and the cavity always maintains a stable reaction growth temperature. In step S3, the material deposited on the surface of the photoresist is a dielectric material, a metallic element, or a composite material with suitable optical refractive index parameters. The high-transmittance substrate includes a high-transmittance thin silicon dioxide substrate, a high-transmittance thin silicon nitride substrate, or a high-transmittance thin aluminum oxide substrate.

2. The method as described in claim 1, characterized in that, The photoresist is electron beam photoresist PMMA, Zep photoresist, ultraviolet photoresist AZ photoresist, or SU8 photoresist.

3. The method as described in claim 1, characterized in that, The exposure methods include electron beam exposure technology or ultraviolet exposure technology.

4. The method as described in claim 1, characterized in that, The patterned periodic arrangement structure is an arrangement structure in which all elements are of the same size and shape, an arrangement structure with the same structure but different sizes, or an arrangement structure with different shapes and sizes.

5. The method as described in claim 1, characterized in that, The medium material includes titanium oxide, hafnium oxide, or aluminum oxide, or the elemental metal material includes gold or silver.

6. The method as described in claim 1, characterized in that, The etching is achieved through dry etching technology or wet etching technology. The dry etching technology includes ion milling etching, reactive ion etching, or inductively coupled plasma reactive ion etching. The chemical solution used in the wet etching technology includes hydrofluoric acid or potassium hydroxide.

7. The method as described in claim 1, characterized in that, The patterned periodic arrangement structure can phase modulate the geometric phase of the emitted light over the entire 2π range.

8. The method as described in claim 7, characterized in that, The control of the geometric phase of the emitted light is achieved by changing the structural morphology, structural size, or the rotation of the structure itself at a specific position.

9. The method as described in claim 1, characterized in that, The patterned periodic arrangement structure has a preset polarization value for incident light of a specific wavelength, which can change the polarization characteristics of the outgoing light.

10. The method as described in claim 9, characterized in that, The preset polarization value can be freely determined.

11. The method as described in claim 10, characterized in that, The preset polarization value is greater than 60%.

12. The method as described in claim 11, characterized in that, The preset polarization value is greater than 80%.

13. The method as described in claim 12, characterized in that, The preset polarization value is greater than 90%.

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