Achromatic metasurface lens based on long focal depth light beam and imaging method

Abstract

This invention discloses an achromatic metalens based on a long depth-of-focus beam and its imaging method. The method utilizes a polarization-multiplexed metalens to generate a zero-order Bessel beam and a higher-order Bessel beam, which are used to image an object, obtaining a zero-order image and a higher-order image respectively. The higher-order image is then subtracted from the zero-order image to obtain an achromatic image of the object under test. This invention achieves a large depth of field through a conical phase and utilizes the overlapping area of ​​the depth of field within a wide wavelength band to obtain broadband imaging, realizing achromatic imaging in the visible light band. The lens size is not limited, and the area can reach the centimeter level or above. Simultaneously, image subtraction effectively eliminates image blurring caused by side lobes, significantly improving image quality.

Description

Technical Field

[0001] This invention relates to the field of meta-lens technology, and in particular to achromatic meta-lens based on long focal depth beams and imaging methods. Background Technology

[0002] Chromatic aberration in lenses prevents light of different wavelengths from focusing on the same plane, resulting in image blurring and color distortion. Correcting chromatic aberration over a wide bandwidth is essential for achieving full-color imaging. However, traditional refractive achromatic elements (e.g., cemented lenses) are typically bulky, expensive, and difficult to manufacture with high precision. Metalenses, a new type of diffractive lens composed of subwavelength unit structures, are on the order of optical wavelengths in thickness, offering advantages such as lightness and thinness. They provide more precise and efficient wavefront control, offering a solution for compact and lightweight achromatic optics.

[0003] However, current achromatic metalenses are usually very small in size, with apertures ranging from tens to hundreds of micrometers. Furthermore, the metacell structure used to achieve achromatic function has insufficient dispersion control capability, and in principle, it does not have the ability to construct large-aperture, high-efficiency achromatic lenses. Therefore, the existing achromatic capabilities of metalenses are insufficient for full-color imaging applications. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide an achromatic metalens based on a long focal depth beam and an imaging method, which solves the problems of poor dispersion control, blurry images, and unsuitability for full-color far-field imaging of existing achromatic metalenses.

[0005] Technical solution: The achromatic metalens of the present invention is a polarization multiplexing metalens, which generates the point spread function of zero-order Bessel beam and higher-order Bessel beam at the same position on the same focal plane by switching the polarization state of the incident light.

[0006] The phase of the zeroth-order Bessel beam is:

[0007] The phase of the higher-order Bessel beam is:

[0008] Where x and y are in-plane coordinates, NA is numerical aperture, and λ is center wavelength; The spiral phase term for the annular main lobe of a higher-order Bessel beam is used to generate the beam. Let be the azimuth angle, and n (n≥1) be the order of the higher-order Bessel beam.

[0009] Furthermore, polarization-multiplexed metalenses are designed using a method that combines geometric phase and propagation phase modulation or polarization-sensitive propagation phase modulation.

[0010] Furthermore, the polarization-multiplexing metalens can select any two independently adjustable polarization channels. For example, orthogonal circular polarization, orthogonal linear polarization, or co-polarization can be selected as the two polarization channels.

[0011] Furthermore, the side lobes of the zero-order Bessel beam and the higher-order Bessel beam coincide.

[0012] The imaging method based on long depth-of-focus beams described in this invention includes the following steps:

[0013] After incident light illuminates the object under test, it enters the metalens. By switching the polarization state of the incident light, the zero-order image of the zero-order Bessel beam imaging and the high-order image of the high-order Bessel beam imaging are obtained.

[0014] The achromatic image of the object under test is obtained by subtracting the higher-order image from the zero-order image.

[0015] Furthermore, the object to be tested is a colored object.

[0016] Furthermore, the incident light is incoherent light.

[0017] Another imaging method based on a long depth-of-focus beam according to the present invention includes the following steps:

[0018] After incident light illuminates the object under test, it enters the first metalens to obtain a zero-order image of a zero-order Bessel beam. The position coordinates of the first metalens are recorded, and the second metalens is switched to the same position. After incident light illuminates the object under test, it enters the second metalens to obtain a high-order image of a high-order Bessel beam. The achromatic image of the object under test is obtained by subtracting the high-order image from the zero-order image.

[0019] The phase of the first meta-lens is:

[0020] The phase of the second meta-lens is:

[0021] Where x and y are in-plane coordinates, NA is numerical aperture, and λ is center wavelength; The spiral phase term for the annular main lobe of a higher-order Bessel beam is used to generate the beam. Let be the azimuth angle, and n (n≥1) be the order of the higher-order Bessel beam.

[0022] Furthermore, the first and second metalenses are modulated by geometric phase or propagation phase, wherein the rotation angle of the metaatoms in the first metalens based on geometric phase satisfies: The rotation angles of the metaatoms in the second metalens based on geometric phase satisfy:

[0023] Furthermore, the object to be tested is a colored object; the incident light is incoherent light.

[0024] Beneficial effects: Compared with the prior art, the advantages of the present invention are: (1) The present invention realizes achromatic imaging in the visible light band by using a long focal depth Bessel beam. Its imaging principle is different from that of traditional achromatic metalenses based on the control of unit dispersion. Instead, it obtains a large focusing depth of field by using a conical phase and then uses the overlapping area of ​​the focal depth in the wide band to obtain broadband imaging, which solves the chromatic aberration problem caused by the diffraction effect of traditional focusing metalenses. At the same time, the lens size of the present invention is not limited and the area can reach the centimeter level or above. (2) In view of the influence of the long focal depth phase side lobe, the present invention uses a lens composed of zero-order and high-order Bessel phases for image subtraction, which effectively eliminates the image blur caused by the side lobe and significantly improves the clarity and contrast of the image. Attached Figure Description

[0025] Figure 1 The diagram shows the phase, transmittance, and polarization conversion efficiency of metaatoms of different sizes in Example 1 of this invention.

[0026] Figure 2 These are electron microscope and SEM images of the sample from Example 2 of the present invention.

[0027] Figure 3 This is a longitudinal optical field distribution diagram of the zero-order Bessel beam and the second-order Bessel beam in Embodiment 2 of the present invention.

[0028] Figure 4 This is a schematic diagram of the subtraction of a zero-order Bessel beam and a second-order Bessel beam in Embodiment 2 of the present invention.

[0029] Figure 5 The images show the imaging results and subtraction results of the imaging object on different imaging planes in Embodiment 2 of the present invention.

[0030] Figure 6 This is the MTF diagram of Embodiment 2 of the present invention.

[0031] Figure 7 The image shows the imaging result and subtraction result of the imaging object at z=0.3mm in Embodiment 2 of the present invention.

[0032] Figure 8 The images shown are the imaging results and subtraction results of different imaging objects under a broadband white light source in Embodiment 2 of the present invention.

[0033] Figure 9 This is an electron micrograph of the sample from Example 3 of the present invention.

[0034] Figure 10The diagram shows the longitudinal optical field distribution of the zero-order Bessel beam and the second-order Bessel beam in Embodiment 3 of the present invention.

[0035] Figure 11 This is a schematic diagram of the subtraction of a zero-order Bessel beam and a second-order Bessel beam in Embodiment 3 of the present invention. Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0037] Example 1

[0038] This embodiment employs a method of joint control of geometric phase and propagation phase to design a polarization-multiplexed metalens. By switching the polarization state of the incident light, two Bessel beams can be generated at the same position, enabling more precise alignment of the PSF and the image.

[0039] First, the phase of the metalens is determined. To achieve the achromatic function of the lens, this invention selects a Bessel beam with long depth of focal length. The phase of the zero-order Bessel beam is: The phase of a higher-order Bessel beam is:

[0040] Then, the design parameters of the lens are determined, including the center wavelength, numerical aperture, lens radius, substrate material, unit structure material, and unit structure period.

[0041] Based on the parameters determined above, a structural library of periodic metaatoms was obtained through simulated scanning, including information such as size, phase, and transmittance. To construct the structural library, the finite-difference time-domain (FDTD) method was used to scan the periodic nanostructures. The scanning range for the metaatomic length L and width W was set to 80–260 nm, and periodic boundary conditions were set in both the x and y directions. The target bandwidth for far-field imaging was the entire visible light band; here, a center wavelength λ = 530 nm was chosen, and the period P of the unit structure was selected. x =P y =340nm. The substrate material of the metalens is silicon dioxide (SiO2), and the unit metaatoms are rectangular silicon nitride (SiN) with a height of 1200nm. x The phase, transmittance (T), and polarization conversion efficiency (PCR) of metaatoms of different sizes were obtained, as shown below. Figure 1 As shown in (a)-(c).

[0042] To determine the order of a higher-order Bessel beam, it is necessary to select a structure with high transmittance and polarization conversion efficiency from the structure library so that it satisfies the phase of the zero-order and different-order higher-order Bessel beams under two orthogonal polarization states, and then perform simulation.

[0043] The PSF at the same focal plane is extracted from the simulation results above. The degree of overlap between the side lobes of the higher-order Bessel beam and the zero-order Bessel beam is compared. The order n with the highest degree of overlap is selected to obtain a polarization multiplexing metalens that can generate both zero-order and higher-order Bessel beams.

[0044] Example 2

[0045] This embodiment utilizes the metalens from Embodiment 1 for far-field imaging. The order of the higher-order Bessel beam is chosen to be n=2. Therefore, the phase formulas for the two Bessel beams can be written as follows:

[0046]

[0047] Where x and y are in-plane coordinates; NA is the numerical aperture, which is set to 0.2 here; λ is the center wavelength; The azimuth angle is used to generate a hollow PSF.

[0048] Subtractive imaging requires precise alignment of the zero-order and second-order images. The polarization multiplexing metalens used in this embodiment can generate two beams of light PSF or images at the same position on the same focal plane by switching the polarization state of the incident light, without the need for spatial adjustment of the lens.

[0049] Choosing circular polarization as the orthogonal basis, where the zeroth-order and second-order Bessel beams correspond to right-hand circular polarization (RCP) and left-hand circular polarization (LCP), respectively, the Jones matrix satisfied by the metaatoms is:

[0050]

[0051] Where RC is the rotation matrix in the circular basis vectors. In this embodiment, orthogonal polarization channels are selected, i.e., RCP to LCP and LCP to RCP transformations. Due to mirror symmetry, ф LR =ф RL =ф, then the phase satisfied by the zeroth-order Bessel beam and the second-order Bessel beam is and Where θ is the rotation angle of the meta-atom, and ф is the common phase that both the zero-order Bessel beam and the second-order Bessel beam must satisfy.

[0052] Meta-lenses were constructed from the structure library of Example 1, selecting meta-atoms that met the phase requirements. In this embodiment, the sample diameter of the fabricated polarization multiplexing meta-lens was D = 350 μm. Figure 2 These are electron microscope and SEM images of the sample.

[0053] The sample was illuminated with a laser, and the PSF was captured using an objective lens and a color CMOS sensor. Figure 3 (a) and (b) in the figure show the longitudinal optical field distribution of the zero-order Bessel beam and the second-order Bessel beam, respectively. Their depths of focus are 781 μm, 858 μm and 990 μm at wavelengths λ of 630 nm, 530 nm and 470 nm, respectively, which means that light of different wavelengths can be well focused within a fairly long focal plane range.

[0054] Extracted Figure 3 The PSF at the white dashed line (z = 0.3 mm) in (a) and (b) are shown in the following results. Figure 4 As shown in (a) and (b) in the figure. Subtracting the zero-order PSF and the second-order PSF for the same wavelength yields the result. Figure 4 (c) in the middle. Figure 4 In Figure (d), the normalized optical field cross-section distribution extracted along the dashed line of the PSF is shown. The intensity distributions of the zero-order and second-order Bessel beams are represented by blue and green dashed lines, respectively. The zero-order and second-order Bessel beams are precisely aligned, and the sidelobes are well matched. The intensity distribution after subtraction is represented by a solid red line. The sidelobes of the PSF are eliminated, while the main lobe is reduced to some extent.

[0055] In this embodiment, the negative plate of the USAF1951 resolution plate is used as the imaging object, and an LED is selected as the light source. Figure 5 Figures (a) and (b) show the imaging results and subtraction results of the two imaging channels on different imaging planes at a center wavelength of 530 nm. The subtraction imaging performance remains consistent across different imaging planes.

[0056] Figure 6 In Figure (a), the MTF (Mean Transmission Flow) is displayed at different focal planes at the same wavelength (λ = 530 nm). The MTF exhibits position invariance, confirming the significant advantage of Bessel beams in long depth-of-focus imaging. Figure 6 In Figure (b), the MTF of the same focal plane (z = 0.3 mm) at different wavelengths is shown, which proves the wavelength invariance of the Bessel beam.

[0057] By selecting an imaging plane of z = 0.3 mm and placing a wavelength filter behind the white LED, a monochromatic light imaging result is obtained, as shown below. Figure 7 As shown. Figure 7 Image (a) shows an image of a zero-order Bessel beam. Due to the presence of side lobes, the image is blurred at all wavelengths, but the degree of blurring is consistent across all wavelengths. Figure 7 Image (b) shows an image of a second-order Bessel beam. The hollow main lobe and strong side lobes increase background blur. Subtracting the image intensities, with a subtraction factor of s = 0.6 for each wavelength, yields the final image as shown below. Figure 7 As shown in (c). Figure 7 (d) is along Figure 7In (c), the normalized intensity distribution map is extracted by the dashed line. The intensity distributions of the zero-order and second-order images are represented by blue and green dashed lines, respectively, while the intensity of the subtracted image is represented by a red solid line. Compared with the original zero-order image, the subtracted image effectively suppresses background blur and significantly improves the signal-to-noise ratio.

[0058] Figure 8 The imaging results for the two objects under broadband white light source show the same trend as under monochromatic illumination: the level 2 image appears more blurred than the level 0 image, and there is no obvious color difference. Figure 8 As shown in (a) and (b), the image sharpness is significantly improved after subtraction, as... Figure 8 As shown in (c). Figure 8 In the diagram (d), the intensity distribution is shown along the white dashed line. The intensity distributions of the zero-order and second-order images are represented by blue and green dashed lines, respectively, while the intensity of the subtracted image is represented by a red solid line. It can be seen that although the subtraction operation reduces the overall image intensity, it increases the relative difference between the signal and the background without introducing additional chromatic aberration.

[0059] Example 3

[0060] This embodiment uses two metalenses to generate a zero-order Bessel beam and a higher-order Bessel beam, respectively. Specifically, it employs the geometric phase principle, designing two lenses to satisfy the phase requirements of the two Bessel beams, and then acquiring the PSF and image by switching the lenses. The phase of the zero-order Bessel beam is: The phase of a higher-order Bessel beam is:

[0061] Both metalenses use LCP as the incident light and convert the outgoing light to RCP. To enable the metaatoms to function as half-wave plates, the structure with the highest polarization conversion efficiency is selected from the structure library of Example 1 as the unit metaatoms constituting the metalenses. The selected structure is a rectangular nanopillar with a length of 255 nm, a width of 100 nm, and a height of 1200 nm. According to the geometric phase principle, the rotation angle of the metaatoms in the first metalens satisfies: The rotation angle of the meta-atoms in the second metalens satisfies: Where x and y are the coordinates in the plane of the lens, and n = 2.

[0062] In this embodiment, the sample diameter of the fabricated polarization multiplexing metalens is D = 350 μm. Figure 9 This is an electron microscope image of the sample.

[0063] A point light source was constructed using a laser to illuminate the sample, and the PSF was captured using an objective lens and a color CMOS sensor. Figure 10(a) and (b) in the figure show the longitudinal light field distribution of the zero-order Bessel beam and the second-order Bessel beam, respectively. It can be seen that after replacing the parallel light incident with a point source, the focal depth of the Bessel beam is lengthened, with a maximum focal depth of 1.19 mm at 530 nm, as shown by the orange dashed line.

[0064] Extracted Figure 10 The PSF at the white dashed line (z = 0.3 mm) in (a) and (b) are shown in the following results. Figure 11 As shown in (a) and (b) in the figure. Subtracting the zero-order PSF and the second-order PSF for the same wavelength yields the result. Figure 11 (c) in the middle. Figure 11 In Figure (d), the normalized intensity distribution is extracted along the dashed line of the PSF. The intensity distributions of the zero-order Bessel beam and the second-order Bessel beam are represented by blue and green dashed lines, respectively. Consistent with the results in Example 2, the zero-order and second-order Bessel beams are precisely aligned, and the sidelobes are well matched. The intensity distribution after subtraction is represented by a solid red line. The sidelobes of the PSF are eliminated, and the main lobe is also reduced to some extent.

Claims

1. An achromatic meta-lens based on long focal depth light beams, characterized in that, The metalens is a polarization multiplexing metalens, which generates the point spread function of a zero-order Bessel beam and a higher-order Bessel beam at the same position on the focal plane by switching the polarization state of the incident light; the target bandwidth of far-field imaging is the entire visible light band. The phase of the zeroth order Bessel beam is: ; The phase of the high-order Bessel beam is: ; in and Coordinates in the plane Numerical aperture, The center wavelength; The spiral phase term for the annular main lobe of a higher-order Bessel beam is used to generate the beam. It is the azimuth angle. The order of a higher-order Bessel beam. ; The method for determining the order includes: determining the design parameters of the lens; based on the parameters, obtaining a structure library of unit-periodic metaatoms through a simulation scanning method, and obtaining the phase, transmittance, and polarization conversion efficiency of metaatoms of different sizes; selecting structures with high transmittance and polarization conversion efficiency from the structure library so that they satisfy the phase of zero-order and higher-order Bessel beams of different orders under two orthogonal polarization states, and performing simulation; extracting the PSF at the same focal plane from the simulation results, comparing the degree of overlap between the side lobes of the higher-order Bessel beam and the zero-order Bessel beam, and selecting the order n with the highest overlap.

2. The apochromatic meta-lens based on long focal depth light beams according to claim 1, characterized in that, Polarization-multiplexed metalenses are designed by using a method that combines geometric phase and propagation phase control or polarization-sensitive propagation phase control.

3. The apochromatic meta-lens based on long focal depth light beams according to claim 1, characterized in that, The meta-lens can select any two independently adjustable polarization channels.

4. The apochromatic meta-lens based on long focal depth light beams according to claim 1, characterized in that, The side lobes of the zero-order Bessel beam and the higher-order Bessel beam overlap.

5. An imaging method based on a long depth-of-focus beam using a metalens according to any one of claims 1-4, characterized in that, Includes the following steps: After incident light illuminates the object under test, it enters the metalens. By switching the polarization state of the incident light, the zero-order image of the zero-order Bessel beam imaging and the high-order image of the high-order Bessel beam imaging are obtained. The achromatic image of the object under test is obtained by subtracting the higher-order image from the zero-order image.

6. The imaging method of claim 5, wherein, The object to be tested is a colored object.

7. The imaging method of claim 5, wherein, The incident light is incoherent light.

8. A method of imaging based on long depth of focus light beams, characterized in that, Includes the following steps: After the incident light illuminates the object under test, it enters the first metalens to obtain a zero-order image of the zero-order Bessel beam; the position coordinates of the first metalens are recorded, and the second metalens is switched to the same position; After the incident light illuminates the object under test, it enters the second metalens to obtain a high-order image of a high-order Bessel beam imaging; The achromatic image of the object under test is obtained by subtracting the higher-order image from the zero-order image; the target bandwidth of far-field imaging is the entire visible light band. The phase of the first superlens is: ; The phase of the second meta-lens is: ; wherein and is an in-plane coordinate, is a numerical aperture, is a center wavelength; is a spiral phase term that generates a donut-shaped main lobe of the high-order Bessel beam, is an azimuthal angle, is an order of the high-order Bessel beam, ; The method for determining the order includes: determining the design parameters of the lens; based on the parameters, obtaining a structure library of unit-periodic metaatoms through a simulation scanning method, and obtaining the phase, transmittance, and polarization conversion efficiency of metaatoms of different sizes; selecting structures with high transmittance and polarization conversion efficiency from the structure library so that they satisfy the phase of zero-order and higher-order Bessel beams of different orders under two orthogonal polarization states, and performing simulation; extracting the PSF at the same focal plane from the simulation results, comparing the degree of overlap between the side lobes of the higher-order Bessel beam and the zero-order Bessel beam, and selecting the order n with the highest overlap.

9. The imaging method of claim 8, wherein, The first and second metasurface lenses are regulated by a polarization-sensitive geometric phase or a polarization-insensitive propagation phase, wherein the rotation angle of the superatom in the first metasurface lens based on the geometric phase satisfies: ; The rotation angle of the superatom in the second metasurface lens based on geometric phase satisfies: .

10. The imaging method according to claim 8, characterized in that, The object to be tested is a colored object; the incident light is incoherent light.

Images