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

Abstract

The invention discloses an achromatic metamaterial lens based on a long focal depth light beam and an imaging method, and the method comprises the steps: generating a zero-order Bessel light beam and a high-order Bessel light beam through a polarization multiplexing metamaterial lens, carrying out the imaging of an object, obtaining a zero-order image and a high-order image, subtracting the high-order image from the zero-order image, and obtaining a high-order image; and obtaining an achromatic image of the to-be-measured object. According to the invention, a large focusing depth of field is obtained through a conical phase, and then broadband imaging is obtained by using a focal depth overlapping region in a broadband, so that achromatic imaging of a visible light waveband is realized; the lens size is not limited, the area can reach the centimeter level or above, meanwhile, image blurring caused by side lobes is effectively eliminated through image subtraction, and the imaging quality is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metasurfaces, in particular to an achromatic metasurface lens based on a long depth-of-field beam and an imaging method. Background Art

[0002] Chromatic aberration of lenses causes light of different wavelengths to not be focused on the same plane, resulting in image blurring and color distortion. Correcting chromatic aberration over a wide bandwidth is a prerequisite for full-color imaging. However, traditional refractive achromatic elements (e.g., cemented lenses) are usually large in size, costly, and difficult to manufacture with high precision. Metasurface lenses are a new type of diffractive lens composed of subwavelength unit structures, with a thickness on the order of optical wavelengths, having the advantages of being light and thin. They provide more precise and efficient wavefront control, offering a solution for compact and lightweight achromatic optical elements.

[0003] However, current achromatic metasurface lenses are usually very small in size, with an aperture in the range of dozens to hundreds of micrometers, and the dispersion regulation ability of the metasurface unit structure itself for achieving the achromatic function is insufficient. In principle, they do not have the ability to construct a large-aperture and high-efficiency achromatic lens. Therefore, the achromatic ability of existing metasurface lenses is not sufficient for full-color imaging applications. Summary of the Invention

[0004] Object of the Invention: The object of the present invention is to provide an achromatic metasurface lens based on a long depth-of-field beam and an imaging method, to solve the problems of poor dispersion regulation ability, blurred images, and inapplicability to full-color far-field imaging of existing achromatic metasurface lenses.

[0005] Technical Solution: The achromatic metasurface lens of the present invention is a polarization multiplexed metasurface lens, which generates the point spread functions of a zero-order Bessel beam and a high-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 zero-order Bessel beam is:

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

[0008] where x and y are coordinates in the plane, NA is the numerical aperture, and λ is the central wavelength; is the helical phase term of the annular main lobe for generating the high-order Bessel beam, is the azimuth angle, and n (n≥1) is the order of the high-order Bessel beam.

[0009] Furthermore, a polarization multiplexed metasurface lens is designed by using a method of jointly regulating geometric phase and propagation phase or polarization-sensitive propagation phase.

[0010] Further, the polarization multiplexed metasurface lens selects any two independently controllable polarization channels. For example, orthogonal circular polarization, orthogonal linear polarization, or co-polarization is selected as the two polarization channels.

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

[0012] The imaging method based on the long depth-of-field beam according to the present invention includes the following steps:

[0013] The incident light irradiates the object to be measured and then enters the metasurface lens. By switching the polarization state of the incident light, a zero-order image formed by the zero-order Bessel beam and a high-order image formed by the high-order Bessel beam are obtained;

[0014] Subtract the high-order image from the zero-order image to obtain the achromatic image of the object to be measured.

[0015] Further, the object to be measured is a colored object.

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

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

[0018] The incident light irradiates the object to be measured and then enters the first metasurface lens to obtain a zero-order image formed by the zero-order Bessel beam; record the position coordinates of the first metasurface lens, and switch the second metasurface lens to the same position; the incident light irradiates the object to be measured and then enters the second metasurface lens to obtain a high-order image formed by the high-order Bessel beam; subtract the high-order image from the zero-order image to obtain the achromatic image of the object to be measured;

[0019] The phase of the first metasurface lens is:

[0020] The phase of the second metasurface lens is:

[0021] where x and y are coordinates in the plane, NA is the numerical aperture, and λ is the central wavelength; is the helical phase term of the annular main lobe for generating the high-order Bessel beam, is the azimuth angle, and n (n≥1) is the order of the high-order Bessel beam.

[0022] Further, the first metasurface lens and the second metasurface lens are controlled by geometric phase or propagation phase. Among them, the rotation angle of the meta-atoms in the first metasurface lens based on geometric phase satisfies: The rotation angle of the meta-atoms in the second metasurface lens based on geometric phase satisfies:

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

[0024] Advantages: Compared with the prior art, the advantages of the present invention are as follows: (1) The present invention realizes achromatic imaging in the visible light band through a Bessel beam with a long depth of focus. Its imaging principle is different from the traditional achromatic metasurface lens based on the regulation of unit dispersion. Instead, it obtains a large focusing depth of field through a conical phase, and then uses the overlapping region of the depth of focus within a wide band to obtain broadband imaging, solving the chromatic aberration problem caused by the diffraction effect of the traditional focusing metasurface lens; at the same time, the size of the lens of the present invention is not limited, and the area can reach more than centimeter level. (2) Aiming at the influence of the long depth-of-focus phase side lobes, the lens composed of the zero-order and high-order Bessel phases is used for image subtraction in the present invention, effectively eliminating the image blurring caused by the side lobes and significantly improving the clarity and contrast of the imaging. Description of the Drawings

[0025] Figure 1 It is a diagram of the phase, transmittance, and polarization conversion efficiency of metaatoms with different sizes in Embodiment 1 of the present invention.

[0026] Figure 2 It is an electron microscope image and SEM image of the sample in Embodiment 2 of the present invention.

[0027] Figure 3 It is a longitudinal light 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 It is a schematic diagram of subtracting the zero-order Bessel beam and the second-order Bessel beam in Embodiment 2 of the present invention.

[0029] Figure 5 It is a diagram of the imaging results and subtraction results of the imaging object in different imaging planes in Embodiment 2 of the present invention.

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

[0031] Figure 7 It is a diagram of the imaging results and subtraction results of the imaging object at z = 0.3 mm in Embodiment 2 of the present invention.

[0032] Figure 8 It is a diagram of 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 It is an electron micrograph of the sample in Embodiment 3 of the present invention.

[0034] Figure 10Longitudinal optical field distribution diagrams of the zero-order Bessel beam and the second-order Bessel beam of Embodiment 3 of the present invention.

[0035] Figure 11 Schematic diagram of subtracting the zero-order Bessel beam and the second-order Bessel beam of Embodiment 3 of the present invention. Detailed implementation manners

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

[0037] Embodiment 1

[0038] In this embodiment, a geometric phase and propagation phase combined modulation method is adopted to design a polarization multiplexed metasurface lens. By switching the polarization state of the incident light, two Bessel beams can be generated at the same position respectively, and more accurate alignment of the PSF and the image can be achieved.

[0039] First, determine the phase of the metasurface lens. In order to achieve the achromatic function of the lens, the present invention selects a Bessel beam with long depth of focus characteristics. The phase of the zero-order Bessel beam is: The phase of the high-order Bessel beam is:

[0040] Then, determine the design parameters of the lens, including the central wavelength, numerical aperture, lens radius, substrate material, unit structure material, and unit structure period, etc.

[0041] Based on the above determined parameters, a structure library of unit period metaatoms is obtained by the method of simulation scanning, including information such as size, phase, transmittance, etc. In order to construct the structure library, the finite-difference time-domain method (FDTD) is used to sweep the parameters of the periodic nanostructure. The scanning ranges of the length L and width W of the metaatom are both set to be 80 - 260 nm, and periodic boundary conditions are set in both the x and y directions. The target bandwidth of far-field imaging is the entire visible light band. Here, the central wavelength λ = 530 nm is selected, and the period P of the unit structure x = P y = 340 nm. The substrate material of the metasurface lens is silica (SiO 2 ), and the unit metaatom is a rectangular silicon nitride (SiN x ) with a height of 1200 nm. The phase (Phase), transmittance (T), and polarization conversion efficiency (PCR) of metaatoms with different sizes are obtained, as shown in (a)-(c) of Figure 1 respectively.

[0042] In order to determine the order of the high-order Bessel beam, it is necessary to select structures with higher transmittance and polarization conversion efficiency from the structure library to satisfy the phases of the zero-order and high-order Bessel beams of different orders in two orthogonal polarization states respectively, and perform simulation.

[0043] Extract the PSF at the same focal plane from the results of the above simulation, compare the overlapping degree of the side lobes of the high-order Bessel beam and the zero-order Bessel beam, and select the order number n with the highest overlapping degree from them, so as to obtain a metasurface lens that can generate polarization multiplexing of the zero-order Bessel beam and the high-order Bessel beam.

[0044] Example 2

[0045] In this example, the metasurface lens in Example 1 is used for far-field imaging. The order number of the high-order Bessel beam is selected as n = 2, and the phase formulas of the two Bessel beams can be written as:

[0046]

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

[0048] Subtraction imaging requires precise alignment of the zero-order image and the second-order image. In the polarization multiplexing metasurface lens used in this example, by switching the polarization state of the incident light, the PSF or image of two beams of light can be generated at the same position on the same focal plane without spatial adjustment of the lens.

[0049] Select circular polarization as the orthogonal basis, where the zero-order and second-order Bessel beams correspond to right-handed circular polarization (RCP) and left-handed circular polarization (LCP), respectively. Then the Jones matrix satisfied by the metaatom is:

[0050]

[0051] where RC is the rotation matrix in the circular basis vector. In this example, an orthogonal polarization channel is selected, that is, RCP is converted to LCP and LCP is converted to RCP. Due to mirror symmetry, ф LR = ф RL = ф, then the phases satisfied by the zero-order Bessel beam and the second-order Bessel beam are and where θ is the rotation angle of the metaatom, and ф is the common phase that both the zero-order Bessel beam and the second-order Bessel beam need to satisfy.

[0052] Select the metaatoms that meet the phase requirements from the structure library of Example 1 to form a metasurface lens. In this example, the diameter D of the processed sample of the polarization multiplexing metasurface lens is 350 μm, Figure 2 are the electron microscope image and SEM image of the sample.

[0053] Irradiate the sample with a laser, and capture the PSF using an objective lens and a color CMOS. Figure 3 The longitudinal light field distributions of the zero-order Bessel beam and the second-order Bessel beam in (a) and (b) of Figure 3 are shown respectively. Their focal depths are 781 μm, 858 μm, and 990 μm at wavelengths λ of 630 nm, 530 nm, and 470 nm respectively. That is, within a quite long focal plane range, light of different wavelengths can achieve good focusing.

[0054] The PSFs at the white dotted lines (z = 0.3 mm) in (a) and (b) of Figure 3 were extracted, and the results are as shown in (a) and (b) of Figure 3 Figure 4 Figure 4 Subtracting the zero-order PSF and the second-order PSF of the same wavelength gives (c) of Figure 4 Figure 4 Figure 4 The normalized light field cross-sectional distribution diagram extracted along the dotted line of the PSF in (d) of Figure 4 is shown. The intensity distributions of the zero-order Bessel beam and the second-order Bessel beam are represented by blue and green dotted lines respectively. The zero-order and second-order Bessel beams are precisely aligned, and the side lobes match well. The intensity distribution diagram after subtraction is represented by a red solid line. The side lobes of the PSF are eliminated, while the main lobe is reduced to a certain extent.

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

[0056] Figure 6 The MTFs at different focal planes at the same wavelength (λ = 530 nm) are shown in (a) of Figure 6 . The MTF shows position invariance, confirming that the Bessel beam has significant advantages in long focal depth imaging. Figure 6 The MTFs at the same focal plane (z = 0.3 mm) at different wavelengths are shown in (b) of Figure 6 , proving the wavelength invariance of the Bessel beam.

[0057] Select the imaging plane z = 0.3 mm, place a wavelength filter behind the white light LED, and obtain the monochromatic light imaging result, as shown in Figure 7 Figure 7 Figure 7 The image of the zero-order Bessel beam is shown in (a) of Figure 7 . Due to the presence of side lobes, the image is blurred at all wavelengths, but the degree of blurring is the same at all wavelengths. Figure 7 The image of the second-order Bessel beam is shown in (b) of Figure 7 . The hollow main lobe and strong side lobes lead to an increase in background blur. Subtracting the image intensities, the subtraction coefficient for each wavelength is s = 0.6, and the final image is as shown in (c) of Figure 7 Figure 7 Figure 7 In (d) of Figure 7 is alongFigure 7 The normalized intensity distribution diagram extracted by the dashed line in (c). The intensity distributions of the zero-order image and the second-order image are represented by blue and green dashed lines respectively, and 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 are the imaging results of two objects under a broadband white light source, showing the same imaging trend as under monochromatic illumination, that is, the 2nd-order image appears more blurred than the 0th-order image, and there is no obvious chromatic aberration, as shown in Figure 8 (a) and (b) in. The clarity of the subtracted image is significantly improved, as shown in Figure 8 (c) in. Figure 8 (d) in is the intensity distribution 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, and 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] In this example, two metasurface lenses are used to generate a zero-order Bessel beam and a high-order Bessel beam respectively. The specific method is to use the geometric phase principle to design two lenses to satisfy the phases of the two Bessel beams respectively, and collect the PSF and images by switching the lenses; the phase of the zero-order Bessel beam is: The phase of the high-order Bessel beam is:

[0061] The incident light of both metasurface lenses is selected as LCP, and the outgoing light is converted to RCP. In order to make the metaatoms achieve the function of a half-wave plate, it is necessary to select a structure with the highest polarization conversion efficiency from the structure library in Example 1 as the unit metaatom constituting the metasurface lens. The selected structure here is a rectangular nanocolumn 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 metasurface lens satisfies: The rotation angle of the metaatoms in the second metasurface lens satisfies: where x, y are the in-plane coordinates of the lens, and n = 2.

[0062] In this implementation case, the diameter D of the processed polarization multiplexed metasurface lens sample is 350 μm, Figure 9 is the electron microscope image of the sample.

[0063] A point light source is constructed with a laser and the sample is irradiated, and the PSF is captured with an objective lens and a color CMOS.Figure 10 Among them, (a) and (b) are the longitudinal optical field distributions of the zero-order Bessel beam and the second-order Bessel beam, respectively. It can be seen that after replacing the parallel light incidence with a point light source, the depth of focus of the Bessel beam is elongated, and the maximum depth of focus at 530 nm is 1.19 mm, as shown by the orange dashed line.

[0064] The Figure 10 PSFs at the white dashed line (z = 0.3 mm) in (a) and (b) in were extracted, and the results are as Figure 11 shown in (a) and (b) in. Subtracting the zero-order PSF and the second-order PSF of the same wavelength gives Figure 11 (c) in. Figure 11 (d) in is the normalized intensity distribution diagram 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 side lobes match well. The intensity distribution diagram after subtraction is represented by a red solid line. The side lobes of the PSF are eliminated, and the main lobe is also reduced to a certain extent.

Claims

1. An achromatic meta-lens based on a long focal depth beam, characterized in that: The meta-lens is a polarization multiplexing meta-lens, which generates a point spread function of a zero-order Bessel beam and a high-order Bessel beam at the same position on the focal plane by switching the polarization state of the incident light; The phase of the zero-order Bessel beam is: The phase of the high-order Bessel beam is: Where x and y are the coordinates in the plane, NA is the numerical aperture, and λ is the central wavelength; is the spiral phase term that generates the annular main lobe of the high-order Bessel beam, is the azimuth angle, and n (n ≥ 1) is the order of the high-order Bessel beam.

2. The achromatic meta-lens based on a long focal depth beam according to claim 1, characterized in that: Polarization-multiplexing meta-lens is designed by jointly controlling geometric phase and propagation phase or polarization-sensitive propagation phase.

3. The achromatic meta-lens based on a long focal depth beam according to claim 1, characterized in that: The meta-lens selects any two independently adjustable polarization channels.

4. The achromatic meta-lens based on a long focal depth beam according to claim 1, characterized in that: The side lobes of the zero-order Bessel beam and the high-order Bessel beam overlap.

5. An imaging method based on a long focal depth beam based on the metalens according to any one of claims 1 to 4, characterized in that: The steps include: The incident light illuminates the object to be measured and then enters the meta-lens. By switching the polarization state of the incident light, a zero-order image of the zero-order Bessel beam imaging and a high-order image of the high-order Bessel beam imaging are obtained. The high-order image is subtracted from the zero-order image to obtain the achromatic image of the object to be measured.

6. The imaging method according to claim 5, characterized in that: The object to be measured is a colored object.

7. The imaging method according to claim 5, characterized in that: The incident light is incoherent light.

8. An imaging method based on a long focal depth beam, characterized in that: The steps include: The incident light irradiates the object to be measured and then enters the first meta-lens to obtain a zero-order image of the zero-order Bessel beam imaging; the position coordinates of the first meta-lens are recorded, and the second meta-lens is switched to the same position; The incident light illuminates the object to be measured and then enters the second meta-lens to obtain a high-order image of the high-order Bessel beam imaging; Subtract the high-order image from the zero-order image to obtain an achromatic image of the object to be measured; The phase of the first meta-lens is: The phase of the second metalens is: Where x and y are the coordinates in the plane, NA is the numerical aperture, and λ is the central wavelength; is the spiral phase term that generates the annular main lobe of the high-order Bessel beam, is the azimuth angle, and n (n ≥ 1) is the order of the high-order Bessel beam.

9. The imaging method according to claim 8, characterized in that: The first metalens and the second metalens are regulated by a polarization-sensitive geometric phase or a polarization-insensitive propagation phase, wherein the rotation angle of the metaatom in the first metalens based on the geometric phase satisfies: The rotation angle of the meta-atom in the second meta-lens based on geometric phase satisfies:

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

Images