A multi-focal tunable broadband achromatic superlens

Abstract

This invention proposes a multifocal tunable broadband achromatic superlens with polarization multiplexing functionality. The lens consists of two layers: a SiO2 dielectric layer and a TiO2 array layer from bottom to top. Based on the principle of holographic superposition, this design generates five sets of variable focus arrays within the focal plane, and employs an iterative optimization algorithm to control the intensity distribution of the focal spot. Under x-polarized light incident at a wavelength of 625 nm, the focal spot uniformity reaches 99%. Furthermore, this invention introduces a particle swarm optimization (PSO) algorithm to optimize cross-band phase compensation, significantly reducing phase mismatch errors in the achromatic design and effectively suppressing chromatic aberration in the 550–700 nm wavelength range. Based on this design, a broadband uniform multifocal distribution, near-diffraction-limited focusing performance, and switchable focal length based on the incident light polarization state are achieved. Therefore, this invention has significant application value in fields such as synchronous imaging, polarization-multidimensional optical communication, and integrated quantum computing.

Description

Technical Field

[0001] This invention relates to a multifocal adjustable broadband achromatic superlens, belonging to the field of novel artificial electromagnetic materials and electromagnetic wave control technology. Background Technology

[0002] Metasurfaces, as two-dimensional equivalents of metamaterials, can flexibly control the amplitude, phase, and polarization of light, opening new avenues for cutting-edge fields such as generalized Snell's law, polarization control, beam shaping, and superlenses. Multifocal superlenses, as important functional devices, have been widely applied in imaging and optical communication due to their multifocal characteristics. However, traditional devices mainly focus on phase modulation, making it difficult to achieve uniform distribution of focal spot intensity. Furthermore, the integration of multifocal and polarization functions suffers from high design complexity, narrow operating bandwidth, and severe crosstalk, thus hindering their further development. Therefore, designing a multifocal superlens with wide-band response, multi-polarization compatibility, and high focal uniformity has become an urgent need to drive the development of related applications.

[0003] Using rectangular cross-section nanopillars as the basic structural unit, compared to traditional circular or irregular cross-section structures, rectangular cross-section nanopillars exhibit stronger birefringence and polarization response characteristics. They can achieve independent and continuous phase and amplitude modulation in orthogonal polarization directions and easily cover the full phase range from 0 to 2π. Simultaneously, the rectangular profile is highly compatible with existing photolithography and etching processes, with steep sidewalls and excellent dimensional uniformity. Even under high aspect ratio conditions, they maintain excellent structural stability and mechanical strength, which is beneficial for improving the yield and reliability of device fabrication. Furthermore, while ensuring high transmission efficiency and low optical loss, rectangular units facilitate multi-band, multi-polarization state, and multi-functional integration, thus adapting to the high-performance and integrated application requirements of micro-nano optoelectronic devices such as superlenses, beam shaping, holographic imaging, and polarization manipulation.

[0004] In 2020, Huang et al. designed a visible-light multifocal superlens based on computer-generated holograms (CGH) and geometric phase principles. However, this design suffers from crosstalk between different wavelengths, affecting image quality; furthermore, the structure lacks polarization multiplexing capability, limiting its functional scalability. In 2024, Lu et al. optimized the multifocal intensity uniformity using an inverse design method. In 2025, Shi et al. achieved a focal uniformity as high as 99.07% by combining iterative optimization algorithms, but this research had a narrow bandwidth, applicable only to a single wavelength, thus limiting its application in broadband systems. Although research on multifocal superlenses has made some progress, the aforementioned devices suffer from problems such as narrow operating wavelengths, low tunability, and poor focal uniformity. Therefore, designing a multifocal tunable broadband achromatic superlens has significant academic and application value. Summary of the Invention

[0005] This invention relates to a multifocal adjustable broadband achromatic superlens, which has a simple structure and is easy to manufacture.

[0006] This invention is achieved through the following technical solution:

[0007] The multifocal adjustable broadband achromatic superlens described in this invention, such as Figure 1 As shown in (a), a dielectric layer and a TiO2 array layer made of SiO2 material are arranged sequentially from bottom to top; wherein, the TiO2 array layer is composed of 61×61 rectangular cross-section unit structures, and the length and width of each unit structure vary depending on its position in the array.

[0008] The unit structure is as follows Figure 1 As shown in (b), the size parameters are all determined by filtering from the database; the database is established by the vertical axis dimension L and the horizontal axis dimension W of the scanning unit structure (both scanning ranges are 50nm-350nm).

[0009] When the length and width of the unit structure increase from 50 nm to 350 nm, the dielectric unit exhibits an approximately linear smooth phase gradient, which can completely cover the 2π phase range and has good transmittance.

[0010] The superlens designed in this invention uses the particle swarm optimization (PSO) algorithm to reduce phase matching error, thereby achieving achromatic aberration function. After optimization, the total phase error is reduced by 30%.

[0011] The superlens designed in this invention achieves achromatic function in the wavelength range of 550~700nm, and exhibits different focal length characteristics under illumination by incident light with different polarization states, such as... Figure 2 As shown in (a) and (b).

[0012] Compared with traditional superlenses, the superlens designed in this invention has better focal uniformity, as shown in the specific comparison below. Figure 3 As shown.

[0013] Compared with traditional superlenses, the superlens designed in this invention achieves a focal point uniformity of up to 99%, exhibiting excellent uniformity, such as... Figure 4 As shown.

[0014] The superlens designed in this invention has superior focal length stability compared to traditional superlenses, see... Figure 5 (a); Length, width, and half-height are close to the diffraction limit, see Figure 5 (b)

[0015] The boundary conditions for the simulation experiment of this invention are set as a perfectly matched layer (PML), and the light source wavelengths used are 550nm, 600nm, 650nm and 750nm, respectively, with the incident mode being positive incident along the z-axis.

[0016] Compared with the prior art, the advantages of the present invention are:

[0017] By employing the PSO algorithm to optimize the phase matching error and combining iterative optimization algorithms to improve the uniformity of multifocal intensity, this invention can simultaneously achieve broadband operation and uniform multifocal operation.

[0018] This invention can focus x-polarized and y-polarized incident light onto two different focal planes, achieving polarization-dependent multifocal control and expanding the application scenarios of superlenses. Attached Figure Description

[0019] Figure 1 (a) is a schematic diagram of the working principle of the superlens; Figure 1 (b) is a side view of the unit structure; Figure 1 (c) is a side view of the entire superlens.

[0020] Figure 2 (a) and (b) are diagrams showing the focal length variation of the superlens designed in this invention.

[0021] Figure 3 This is a comparison image of the normalized field intensity distribution along the x-direction near the focal point of the superlens designed in this invention and other superlenses with the same conditions but without light intensity uniformity optimization. The embedded image is the field intensity distribution in the xy plane.

[0022] Figure 4 This shows how the focal uniformity changes with wavelength under x-polarized and y-polarized light incidence.

[0023] Figure 5 (a) Comparison of focal length changes of the superlens and chromatic aberration superlens designed in this invention under x-polarized and y-polarized light incidence. Figure 5 (b) shows the variation of the FWHM of the focused spot of the superlens designed in this invention with wavelength. Detailed Implementation

[0024] To better understand the present invention, the present invention will be further explained and described below with reference to the embodiments and accompanying drawings. The following embodiments are only for illustrative purposes and are not intended to limit the present invention.

[0025] like Figure 1 As shown in (a), the superlens consists of a SiO2 dielectric layer and a TiO2 array layer from bottom to top.

[0026] The unit cell structure of the dielectric layer is arranged in a period of 350 nm. Figure 1In (b), L is the longitudinal length of the rectangular cross-section unit structure, and W is the transverse length of the rectangular cross-section unit structure.

[0027] After introducing particle swarm optimization (PSO) algorithm to compensate for the dispersion of the material, the phase at any position on the metasurface can be obtained by the following formula: φ x φ y f and fy represent the ideal phases in the x-polarization state and the y-polarization state, respectively. fx and fy correspond to the design focal lengths in the two polarization states. dij,x and f dij,y The distance from the focal point to the center of the lens corresponds to the two polarization states.

[0028] Photolithography, or photolithography techniques, can be used to obtain structural arrays with rectangular cross-sections. Linearly polarized light enters the metasurface of the SiO2 dielectric layer and propagates along the z-axis.

[0029] The phase distribution obtained by the particle swarm optimization (PSO) algorithm is highly consistent with the ideal phase.

[0030] exist Figure 2 From the intensity distribution of the transmitted field in the x−z plane, it can be seen that when the incident light is x-polarized, the transmitted light is focused at the position z≈18 µm; when the incident light is y-polarized, the transmitted light is focused at the position z≈10.5 µm.

[0031] exist Figure 3 In comparison to the superlens structure without intensity uniformity optimization, the focal intensity uniformity of this design is significantly improved.

[0032] Figure 4 In this invention, excellent focal uniformity is achieved: the average focal uniformity under x-polarization and y-polarization can reach 95% and 93%, respectively.

[0033] Figure 5 In this invention, the focal length changes little with the working wavelength, achieving good achromatic performance; the obtained full width at half maximum (FWHM) of the focal spot is close to the theoretical diffraction limit, indicating that the focusing spot size of the superlens is effectively controlled.

[0034] It should be noted that although the embodiments described above are illustrative, they are not intended to limit the invention. Therefore, the invention is not limited to the specific embodiments described above. Any other embodiments obtained by those skilled in the art under the guidance of this invention without departing from its principles are considered to be within the protection scope of this invention.

Claims

1. A multifocal adjustable broadband achromatic superlens, as shown in Figure 1(a). Its features are: The superlens consists of a SiO2 dielectric layer and a TiO2 array layer from bottom to top.

2. The multifocal adjustable broadband achromatic superlens according to claim 1, characterized in that: The unit structure is shown in Figure 1(b). The height H of the TiO2 unit is set to 600 nm, and the period P of the superlens array is set to 350 nm.

3. The multifocal adjustable broadband achromatic superlens according to claim 1, characterized in that: Each titanium dioxide nanopillar unit that makes up the superlens is selected and matched from a pre-constructed metasurface unit database. The unit database is established by scanning and traversing the vertical axis dimension L and horizontal axis dimension W of the nanopillar units. The scanning values ​​of the vertical axis dimension L and the horizontal axis dimension W are both in the range of 50nm-350nm.

4. The multifocal adjustable broadband achromatic superlens according to claim 1, characterized in that: The particle swarm optimization algorithm was introduced to optimize the phase profile, which effectively compensated for the material dispersion and reduced the matching error by more than 30%.

5. A multifocal adjustable broadband achromatic superlens according to claim 1, characterized in that: The superlens is shown in Figure 1(c). The superlens consists of 61x61 dielectric unit structures and has a diameter of 21μm.

6. The multifocal adjustable broadband achromatic superlens according to claim 1, characterized in that: The superlens can effectively suppress dispersion in the 550nm-700nm broadband band; the incident x-polarized light and y-polarized light can be focused onto two different focal planes by polarization multiplexing, and both polarization states have multi-focal focusing capabilities, while maintaining highly consistent focusing characteristics and focal energy uniformity, as shown in Figures 3 and 4.

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