An annular layered structure metasurface planar lens and a preparation method thereof

Abstract

The application discloses a kind of annular layered structure metasurface plane lens and preparation method thereof.The annular layered structure metasurface provided by the application is a cylindrical structure composed of multiple periodic layer super units, each periodic layer super unit is composed of several concentric ring layers with different dielectric constants, and the central periodic layer super unit is composed of a central cylinder and several adjacent concentric ring layers;The radial thickness of each concentric ring layer is less than one tenth of the equivalent working wavelength.According to Shell law and metasurface theory, combined with the central symmetry of the structure, each periodic layer super unit will deflect any polarization state electromagnetic wave incident along the axis of the cylinder at the same angle, and focus on the central axis, with a focal length of 0~Nλ, where N represents the number of periodic layer super units, and λ represents the working electromagnetic wave wavelength in vacuum.The plane lens has the advantages of simple and compact structure, polarization insensitivity, free focusing distance, easy integration, etc.

Description

Technical Field

[0001] This invention relates to the field of optical elements and their fabrication technology, and in particular to a metamaterial or metasurface lens and its fabrication method. Background Technology

[0002] Traditional lenses play a vital role in optical systems, but they also have some drawbacks. For example, traditional lenses are usually made of natural materials, and their operating wavelengths and performance are completely limited to a very limited number of materials. Secondly, to adjust the focal length, correct aberrations, or achieve specific optical functions, multiple lenses are often required to be combined, increasing the complexity of the system, losing some light energy, and making the design and adjustment process more difficult. In addition, the manufacture and processing of high-quality lenses requires high-quality materials and precise manufacturing processes, so the cost is relatively high. Furthermore, traditional lenses are relatively large and heavy, have complex structures, and are not easy to integrate.

[0003] Metamaterials (or metasurfaces) are typically composed of subwavelength microstructures. By precisely designing and optimizing the geometry and distribution of these microstructures, the phase and amplitude of electromagnetic waves can be adjusted, thus controlling the wavefront and enabling various optical functions. Metamaterial (or metasurface) optical elements are compact, easy to fabricate, flexible in functional design, and suitable for applications across different wavebands. However, existing metamaterial (or metasurface) lenses are almost all based on adjusting the propagation direction of electromagnetic waves using a single subwavelength microstructure (e.g., cylinders, cuboids, crosses, etc.). The electromagnetic waves are often focused at only a limited number of points. As mentioned earlier, this presents the same problems and difficulties associated with adjusting the focal length to correct aberrations. Furthermore, strong near-field interference exists between subwavelength microstructures, and the dielectric discontinuities between microstructures (usually air) not only exacerbate wavefront distortion but also prevent the lens from focusing or deflecting partially polarized electromagnetic waves. These factors severely impact the performance of metamaterial (or metasurface) lenses. Therefore, truly realizing a lens that is polarization insensitive, requires no focus adjustment, has a simple and compact structure, and is easy to integrate and fabricate remains a key challenge that needs to be addressed. Summary of the Invention

[0004] In view of this, it is necessary to provide a ring-shaped layered metasurface planar lens to address the shortcomings of existing technologies. This invention uses a periodic layered metaunit based on a subwavelength thick-layer unit array to deflect and focus electromagnetic waves onto a single line, achieving true polarization insensitivity.

[0005] To achieve the above objectives, the present invention employs the following technical solution: It comprises multiple periodic layer superunits, each consisting of several concentric ring layers with different dielectric constants. The central periodic layer superunit is composed of a central cylinder and several adjacent concentric ring layers. The radial thickness of each concentric ring layer is less than one-tenth of the equivalent working wavelength. The equivalent working wavelength is the wavelength of the working electromagnetic wave in vacuum divided by the refractive index of the layer with the largest dielectric constant in the periodic layer superunit. The working electromagnetic wave is directed along the axial direction of the cylinder towards the planar lens of the layered structure metasurface.

[0006] When the incident electromagnetic wave of this invention is incident along the axis of the cylinder towards the layered metasurface planar lens, due to the different dielectric constants (i.e., different refractive indices) of the materials in each layer of the periodic metaunit, the transmission phase of the electromagnetic wave of the same wavelength is different when passing through each layer. If the dielectric constants of each layer are appropriately designed, the wavefronts of the transmitted electromagnetic waves can be synchronized, and the electromagnetic waves will propagate in the same direction and be focused on the central axis. The focal length ranges continuously from 0 to fmax, where fmax is described as follows: f max =Nλ=Λcotθ

[0007] Where N is the number of periodic supercells, λ is the operating wavelength in vacuum, Λ is the total thickness of the periodic supercells, and θ is the electromagnetic wave transmission angle. The transmission angle θ can be determined by the following formula.

[0008] Wherein, is the radius of a single periodic layer supercell.

[0009] Furthermore, the initial phase of electromagnetic waves passing through the layers of matter from the inside out in a periodic supercell must change linearly.

[0010] Furthermore, the difference between the maximum and minimum phases of electromagnetic waves passing through each layer of matter in a periodic supercell is an integer multiple of 2π.

[0011] Furthermore, optimization can be achieved by adjusting parameters such as layer thickness, overall planar lens thickness, and material dielectric constant in the simulation software.

[0012] Furthermore, the characteristic is that the constituent materials of the periodic unit structure include, but are not limited to, a combination of one or more substances such as dielectric, doped semiconductor, or metal.

[0013] Furthermore, the electromagnetic wave is a linearly polarized, circularly polarized, or elliptically polarized electromagnetic wave, and the waveband of the electromagnetic wave covers visible light, infrared light, terahertz, and microwave.

[0014] The specific steps of the method for preparing the above-mentioned annular layered metasurface planar lens of the present invention are as follows:

[0015] Step 1: First, grow the cylindrical material at the epitaxial growth center and use it as the first layer material;

[0016] Step 2: Using the first layer as a growth carrier, the second layer is grown by evaporation and sputtering around the central axis, and then milled by focused ion beam.

[0017] Step 3: On the milled surface of the second layer, use the same method to sequentially rotate and vaporize, sputter to grow, and mill the second, third, fourth, and so on layers of material until the first cycle layer supercell is grown.

[0018] Step 4: Based on the previous periodic layer superunit, prepare other periodic layer superunits using the same method until the entire metasurface planar lens is prepared.

[0019] The innovation of the metasurface lens of this invention is mainly reflected in the following three points: (1) There are no gaps between the thick layered structural units of different dielectric subwavelengths, which eliminates near-field interference and avoids the inconsistency of the wavefront of transmitted electromagnetic waves; (2) A periodic layer superunit with a truly high symmetry structure is designed, so that the metasurface lens can have the same deflection or focusing effect on incident electromagnetic waves of any polarization state; (3) The superunit composed of subwavelength unit arrays is selected as the control unit for deflection or focusing of incident electromagnetic waves, so that the metasurface lens is not limited to focusing on a certain point or a few points, but on a line, and its focal length range continuously changes from 0 to Nλ.

[0020] Therefore, compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. Compared to traditional lenses, this invention has a simple and compact structure, is easy to integrate and fabricate, and its operating wavelength can be applied to visible light, infrared light, terahertz, microwave, and other bands if its size is scaled proportionally. Therefore, it not only effectively reduces the manufacturing cost of lenses but also greatly expands its application scenarios.

[0022] 2. Compared to existing metasurface lenses, this invention eliminates gaps between wavelength units, removing near-field interference and avoiding wavefront inconsistencies in transmitted electromagnetic waves, thus improving focusing performance. Furthermore, the high symmetry of the periodic layer metaunit structure ensures that this metasurface lens truly exhibits the same deflection or focusing for incident electromagnetic waves of any polarization state. Therefore, this invention not only improves the performance of existing metasurface lenses but also simplifies the design and operation of subsequent integrated components.

[0023] 3. Selecting a supercell composed of subwavelength unit arrays as the control unit for deflecting or focusing incident electromagnetic waves can effectively reduce the design complexity and application limitations of existing lens focal lengths. While reducing production and manufacturing requirements, it also provides strong support for the future development of integrated optical components. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the cross-sectional (XOY plane) structure of the central periodic layer supercell provided in an embodiment of the present invention.

[0026] Figure 2 The cross-sectional (XOZ plane) structure and focusing imaging schematic diagram of the annular layered metasurface planar lens provided in the embodiment of the present invention.

[0027] Figure 3 The transmittance and transmission phase shift of an incident electromagnetic wave of a certain wavelength as it passes through each subwavelength layer unit are provided in the embodiments of the present invention.

[0028] Figure 4 (a) A diagram showing the focusing effect of the transmitted electromagnetic wave in the XOY plane after the electromagnetic wave is incident on the metasurface lens of the present invention; (b) A diagram showing the focusing effect of the transmitted electromagnetic wave in the XOZ plane after the electromagnetic wave is incident on the metasurface lens of the present invention.

[0029] In the figure, 1 is the first layer of material (central cylinder) constructed by material 1 in the central periodic layer superunit; 2, 3, 4, 5, and 6 are the second, third, fourth, fifth, and sixth layers of material (ring layers) constructed by materials 2, 3, 4, 5, and 6 in the central periodic layer superunit, respectively; 7 is the electromagnetic wave incident along the axis of the cylinder; 8 is the wavefront of the incident electromagnetic wave; 9 is the flat metasurface lens of this invention; 10 is the transmitted electromagnetic wave; 11 is the wavefront of the transmitted electromagnetic wave; 12 is the axis of the central cylinder; r is the radius of a single periodic layer superunit; d is the thickness of the flat metasurface lens; |t| and Φ are the transmission coefficient mode and phase of the transmitted electromagnetic wave with a wavelength of 20 micrometers after passing through different material layer thicknesses, respectively; and |E| is the transmitted electric field mode. Detailed Implementation

[0030] 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0032] The central periodic layer superunit of the annular layered metasurface planar lens provided in this embodiment of the invention, such as... Figure 1 As shown, the periodic superunit consists of the first layer (central cylinder) constructed from material 1. Layers 2, 3, 4, 5, and 6 are the second, third, fourth, fifth, and sixth layers (ring layers) constructed from materials 2, 3, 4, 5, and 6 in the central periodic superunit, respectively. All materials are doped GaAs, with doping concentrations of 1 to 6 of 1×10¹⁴ cm⁻³, 1×10¹⁵ cm⁻³, 2.45×10¹⁵ cm⁻³, 3.89×10¹⁵ cm⁻³, 6.31×10¹⁵ cm⁻³, and 8.32×10¹⁵ cm⁻³, respectively. The concentric ring width is 3.6 μm, and the number of periodic superunits is 2. Assuming an electromagnetic wavelength of 395 μm, the calculated dielectric constants of materials 1 to 6 at this wavelength are 12.7, 11.3, 9.1, 6.9, 3.3, and 0.3, respectively. In addition, to maximize transmittance, the thickness of the flat lens was set to 11.2 μm, based on the theory of perfect matching layers.

[0033] The incident electromagnetic wave 7, with its wavefront 8 parallel to the metasurface plane lens 9, experiences a deflection of the wavefront 11 of the transmitted electromagnetic wave 10 relative to the wavefront of the incident electromagnetic wave 7 when it strikes the metasurface plane lens 9. Figure 2 As shown. Simulation calculations show that the phase shift increases sequentially from the inside out in the periodic supercell, with the maximum and minimum phase shifts differing by 2π, and the transmittance is above 0.35 for all layers. Figure 3 As shown. The electromagnetic wave 10 transmitted from each layer has its wavefront 11 deflected by 3.1° relative to the incident electromagnetic wave 7, meaning the propagation direction of the transmitted electromagnetic wave is deflected by 3.1° relative to the propagation direction of the incident electromagnetic wave 7, and finally focuses on the central axis 12. Simulation calculations show that the line length of the transmitted electromagnetic wave focused on the central axis is 780μm, meaning the focal length of the metasurface planar lens 9 ranges from 0 to 780μm. The focusing effect of the transmitted electromagnetic wave in the XOY and XOZ planes is shown in the diagram. Figure 4 As shown.

[0034] In some preferred embodiments, the more layers in the periodic layer supercell, the better the focusing effect.

[0035] In some preferred embodiments, the more periodic layer supercells the metasurface planar lens has, the larger the focal length range.

[0036] In some preferred embodiments, the incident working electromagnetic wave along the z-axis can be a linearly polarized, circularly polarized, or elliptically polarized electromagnetic wave, and the waveband of the electromagnetic wave covers visible light, infrared light, terahertz, and microwave.

[0037] In some preferred embodiments, by proportionally adjusting the shape parameters of the metasurface planar lens structure and selecting appropriate constituent materials, the band of the working electromagnetic wave can cover visible light, infrared light, terahertz, and microwave.

[0038] Based on the above description, those skilled in the art should have a clear understanding of the multifunctional layered metamaterial optical element provided in the embodiments of the present invention.

[0039] This invention provides a method for fabricating a metasurface planar lens, the specific steps of which are as follows:

[0040] Step 1: First, GaAs is doped into an epitaxially grown central cylindrical layer as the first layer material, with a radius or thickness of 3.6 μm and a height of 11.2 μm, and then milled by focused ion beam.

[0041] Step 2: Using the first layer of doped GaAs as the growth carrier, the second layer of doped GaAs is grown by rotating it around the central axis by vapor deposition and sputtering. The thickness of the second layer is the same as that of the first layer. The second layer is then milled by focused ion beam.

[0042] Step 3: On the milled surface of the second layer, use the same method to sequentially rotate and vaporize, sputter to grow, and mill the second, third, fourth, etc. layers of material, with the thickness of each layer being the same as the thickness of the first layer, until the first cycle layer supercell is grown.

[0043] Step 4: Based on the previous periodic layer superunit, prepare other periodic layer superunits using the same method until the entire metasurface planar lens is prepared.

[0044] In summary, by utilizing electromagnetic wave interference effect, Shell's law, perfect matching layer theory, and mature layered material processing technology, the metasurface planar lens provided by this invention is not only simple and compact in structure, but also has advantages such as polarization insensitivity and no need for focus adjustment. It can not only improve the performance of existing metasurface lenses, but also simplify the design and operation of subsequent integrated components, thereby reducing production and manufacturing requirements and greatly expanding its application scenarios.

[0045] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0046] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A ring-shaped layered metasurface planar lens, characterized in that, It comprises multiple periodic layer superunits, each of which consists of several concentric ring layers with different dielectric constants. The central periodic layer superunit consists of a central cylinder and several adjacent concentric ring layers, with no gaps between the adjacent concentric ring layers. The radial thickness of each concentric ring layer is less than one-tenth of the equivalent working wavelength, which is the wavelength of the working electromagnetic wave in vacuum divided by the refractive index of the layer with the largest dielectric constant in the periodic layer superunit. The working electromagnetic wave is directed along the axial direction of the central cylinder towards the annular layered structure metasurface planar lens. The initial phase of the electromagnetic wave changes linearly when passing through each layer of material in the periodic layer superunit from the inside out, and the difference between the maximum and minimum phases when passing through each layer is an integer multiple of 2π, so that the transmitted electromagnetic wave is focused into a line along the central axis.

2. The annular layered metasurface planar lens as described in claim 1, characterized in that, The number of concentric ring layers in the periodic superunit is 6 or more.

3. The annular layered metasurface planar lens as described in claim 1, characterized in that, The thickness of each concentric ring layer in the periodic superunit may be equal or unequal.

4. The annular layered metasurface planar lens as described in claim 1, characterized in that, The constituent materials of the periodic layer supercell structure include, but are not limited to, a combination of any one or more of dielectric materials and doped semiconductor materials.

5. The annular layered metasurface planar lens as described in claim 1, characterized in that, The working electromagnetic wave is a linearly polarized, circularly polarized, or elliptically polarized electromagnetic wave, and the waveband of the working electromagnetic wave covers visible light, infrared light, terahertz, and microwave.

6. The annular layered metasurface planar lens as described in claim 1, characterized in that, The focal length of the transmitted electromagnetic wave varies continuously from 0 to Nλ, where N represents the number of periodic layer supercells and λ represents the wavelength of the working electromagnetic wave in vacuum.

7. A method for fabricating a ring-shaped layered metasurface planar lens as described in any one of claims 1-6, characterized in that, include: First, a cylindrical layer of material is grown epitaxially, which is then used as the first layer. Using the first layer as a growth carrier, the second layer is grown by rotating and sputtering around the central axis, and then milled. On the milled surface of the previous layer, subsequent layers are grown by rotating and sputtering, and then milled, until the first periodic superunit is grown. Based on the previous periodic superunit, other periodic superunits are prepared using the same method until the entire metasurface planar lens is prepared.

8. The method for preparing the annular layered metasurface planar lens as described in claim 7, characterized in that, The milling is performed using focused ion beam milling.

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