1. Introduction

Compared with traditional bulk optical devices, optical metasurfaces can effectively tailor the propagation behaviors of the amplitude, phase, and polarization of the electromagnetic field locally through a thin subwavelength structure [1–3]. Owing to their excellent characteristics and performance, metasurfaces have been investigated for versatile functions such as polarization control and conversion [4–6], optical holography [7–9], structural coloring [10,11], and light beam shaping [12,13]. Focusing and imaging with metalenses has also attracted significant attention in recent years [14–17], in which different types of metalenses based on different mechanisms have been proposed, such as V-shaped [18], cross-shaped [19], and nanorod [20] plasmonic metalenses as well as silicon [21], silicon nitride [22], and titanium dioxide (TiO₂) dielectric metalenses [23,24]. Achromatic metalenses at several discrete wavelengths in the near-infrared wavelength range [25–27] or within a continuous narrow band of 60 nm in the visible wavelength range [28] were first proposed with different arrays of combined nanoresonators, and different types of achromatic metalenses with a broadband wavelength range were proposed and demonstrated. In 2017, Wang et al. proposed and demonstrated an achromatic metalens consisting of an array of Au-SiO₂-Au nanostructures with different combinations of nanorods of different shapes, lengths, widths, and gaps, from which a chromatic aberration over a continuous wavelength region from 1200 to 1680 nm is eliminated for circularly polarized incidences in a reflection scheme [29]. Pancharatnam–Berry (P-B) phases from the rotation of different unit cells coupled with a carefully designed linear dispersion (phase as a function of frequency or wavenumber) of nanostructures generated by localized plasmonic resonances are employed in the achromatic metalens. With a similar idea, in 2018, Chen et al demonstrated a transmissive achromatic metalens that consisted of one or more TiO₂ nanofins of varying lengths, widths, and gaps over a continuous wavelength region from 470 to 670 nm for circularly polarized incidences [30]. In the same year, Wang et al. also demonstrated a transmissive achromatic metalens that consists of gallium nitride (GaN) nanopillars with different shapes and dimensions over a continuous wavelength region from 400 to 660 nm for circularly polarized incidences [31]. In 2019, the GaN nanopillar-based achromatic metalens was further developed into a metalens array. It was used to capture full-color light-field information and eliminate the chromatic aberration over the wavelength region from 400 to 660 nm [32]. In 2021, Li et al. demonstrated a millimeter-scale diameter and high numerical aperture (NA) achromatic TiO₂ metalens at three wavelengths of 488 nm, 532 nm and 658 nm for virtual reality, in which dispersion engineering, zone interference, and circularly polarized incidence are employed [33].

Polarization-insensitive broadband achromatic metalenses have also been proposed with unpolarized incident light. In 2018, Shrestha et al. developed an achromatic metalens consisting of an array of Si nanopillars in transmission mode over the 1200–1650 nm wavelength range [34]. A designed linear dispersion relationship between the phase of nanostructures and the incident light frequency must also be employed to achieve an achromatic operation, in which the phases originate from different and complex isotropic cross-sectional geometries owing to different dielectric waveguide modes with frequency-dependent effective refractive indices. Similarly, in 2019, Fan et al. demonstrated a polarization-insensitive broadband achromatic metalens array using complex isotropic silicon nitride nanopillars with different geometries for aberration-free integral imaging over the wavelength region from 430 nm to 780 nm [35]. In 2019, Chen et al. proposed a polarization-insensitive achromatic metalens from 460 nm to 700 nm using otherwise anisotropic TiO₂ nanofins, in which unpolarized light is considered as a combination of a pair of orthogonal circularly polarized components (i.e., left-handed and right-handed circular polarization) with the same designed linear dispersion slope [36]. In 2021, Huang et al. numerically designed a double-layer polarization-insensitive achromatic metalens made of TiO₂ nanorod with different heights and radii to satisfy the required linear dispersion for achromatic imaging over a field of view of 60° in the visible light range from 470 nm to 650 nm [37]. In 2020, Balli et al. proposed a hybrid achromatic metalens consisting of a layered converging phase plate and a diverging metalens working in the range from 1000 nm to 1800 nm, which was designed by combining recursive ray-tracing algorithms and simulated phase libraries [38]. Very recently, this group further expanded their achromatic bandwidth from visible to shortwave infrared (450-1700nm) by using a hybrid 3D architecture which combines nanoholes with a phase plate [39]. Computational visible broadband metalens imaging with cubic, logarithmic-aspherical, or shifted axicon phase functions was also investigated, in which the characteristic of a spectrally invariant point spread function or extended focal depth enables computational reconstruction of captured images with a digital filter [40,41]. The phase distributions in the plane of these metalenses, however, are limited by fixed mathematical expressions, resulting in limited bandwidth and lack of flexibility in the design. In 2020, Presutti et al. discussed how the product between achievable time delay and bandwidth is limited in any time-invariant system, and theoretically analyzed the bandwidth limit of metalenses regardless of their implementation and verified the theory with previous results [42]. From the above discussion, it can be seen that most presently demonstrated achromatic metalenses are mainly based on the compensation of a designed linear phase dispersion with different nanostructures of different shapes and dimensions, in which a larger slope of dispersion requires more complicated or higher aspect ratio nanostructures, which makes design and fabrication significantly more difficult, if not impossible.

In this study, we propose and experimentally demonstrate a polarization-insensitive achromatic metalens (PIA-ML) based on computational wavefront coding in the near-infrared region covering the full optical communication band from 1300 to 1700 nm. The proposed PIA-ML consists of an array of simple circular or square Si nanopillars; the nanopillars are individually coded such that the focal depths at the wavelengths at both ends of the achromatic bandwidth of interest overlap at the desired focal plane. Unlike most of the previously reported achromatic metalenses, our proposed achromatic metalens method does not require a linear phase dispersion; it does this by focusing light of different wavelengths onto a designed focal plane by extending the focal depth of different wavelengths by only using simple-shaped nanostructures regardless of linear or non-linear phase dispersion. The computationally coded phase pattern in the plane of the metalens is calculated for each individual pixel rather than a fixed phase profile, and it is optimized using the particle swarm optimization (PSO) algorithm based on an abrupt phase change of waveguide-like modes inside the Si nanopillars, which exhibit high design flexibility for a wide achromatic imaging bandwidth. Experimental results show that both focusing and imaging of the fabricated metalens are consistent with the theoretical predictions within the broadband wavelength range, which provides a new idea for ultra-broadband (400 nm) achromatic imaging that can also be extended to other wavelength bands, such as visible or mid-infrared bands.

2. Principle and design of PIA-ML

Owing to the strong dispersion by a conventional metalens (C-ML), the focal length decreases and the focal point approaches the metalens as the incident wavelength increases, resulting in a large chromatic aberration and a diffused point spread function (PSF) at non-designed wavelengths on the designed focal plane, as shown in Fig. 1(a). The proposed PIA-ML based on computational wavefront coding is designed with an extended depth of focus (E-DOF) and a rotationally symmetric PSF (S-PSF); this ensures that focal depths at wavelengths at both ends of the achromatic bandwidth overlap, and hence, achromatize at the designed focal plane. The E-DOF and S-PSF are determined by the phase distribution in the plane of the designed PIA-ML, which is tailored and optimized by the PSO algorithm [43]. As shown in Fig. 1(b), the focal depth of the proposed PIA-ML is expanded owing to the coded wavefront, and a high degree of consistency of the light intensity distributions of different wavelengths along the z-axis is maintained, from which the dependence of the focal point on the wavelength is greatly reduced, and the chromatic aberration can be eliminated.

 

Fig. 1. Schematic of a PIA-ML based on computational wavefront coding. (a) C-ML with a diffused light intensity distribution at different wavelengths on the focal plane; (b) PIA-ML with focused light spots at all different wavelengths on the focal plane.

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The rotationally symmetric phase distribution φ(r, λ₀) in the plane of a non-dispersion thin lens with a focal length f₀ at the designed wavelength λ₀ is described as

Here, a PIA-ML with a diameter of 0.49 mm, a focal length of 1 mm (i.e., NA = 0.238), and achromatic working bandwidth from 1300 nm to 1700 nm is designed and compared with a C-ML with the same diameter and focal length. Corresponding to the desired working wavelength range, the variation of the focal length Δf is ∼0.29 mm between 1300 nm and 1700 nm; therefore, a DOF of ∼0.55 mm (∼2 times of Δf) at the designing wavelength of 1550 nm, i.e., from 0.75 mm to 1.3 mm along the z-axis, must be obtained. The optimized rotationally symmetric coding phase distribution φ(r) with a pixel size of 700 nm along the radial direction and the light intensity profile I(z) along the z-axis in the imaging space of the proposed PIA-ML are achieved using the PSO algorithm and vectorial angular spectrum diffraction theory [44,45], respectively. In the optimization process, the coding phase distribution φ(r) of the proposed PIL-ML is iterated at an incident wavelength of 1550 nm, and the corresponding light intensity profile I(z) is defined as the non-optimal function (NOF(z)), as shown in Fig. 2(a). The desired ideal light intensity profile along the z-axis in the imaging space is defined as the objective function (OF(z)), which is only continuous and uniform (after normalization) between z_min (0.75 mm) and z_max (1.3 mm) and is zero in all other z positions. The DOF of the proposed PIA-ML is determined by the two z positions z_min and z_max. The evaluation function (EF(z)) is defined as the square of the modulus of the difference between OF(z) and NOF(z), that is, EF(z) = |OF(z)-NOF(z)|², and then, the PSO algorithm is used to find the optimal coding phase distribution φ(r) of the proposed PIA-ML to minimize the value of the sum of EF(z) at different z positions, resulting in the extension of the depth of focus and elimination of the chromatic aberration of the proposed PIA-ML. The details of the optimization and calculation process are given in Supplement 1.

 

Fig. 2. The optimized phase distribution of the proposed PIA-ML and its comparison with a C-ML. (a) Schematics of the optimization algorithm of the phase and light intensity profiles; (b) Optimized phase distribution of the PIA-ML with a diameter of 0.49 mm and 1 mm focal length; (c) phase distribution of a C-ML with the same dimensional parameters as the proposed PIA-ML. Insets of (b) and (c) show the magnified phase profiles at different locations; (d)-(f) and (g)-(i) show the self-normalized intensity distributions of light fields on the PIA-ML and C-ML at different wavelengths, respectively, in the x-z plane (d) and (g), x-y plane (e) and (h), and the linescan along the x-axis in the focal plane (f) and (i). The white dotted lines in (d) and (g) show the designed focal plane (z = 1 mm).

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Figures 2(b-i) show the optimized phase distribution of the proposed PIA-ML with a diameter of 0.49 mm and a focal length of 1 mm and its comparison with a C-ML with the same dimensional parameters. Figure 2(b) is the optimized phase distribution of the PIA-ML and Fig. 2(c) shows the phase distribution of a C-ML of the same dimensional parameters as the PIA-ML. Compared with the C-ML, the phase gradient of the optimized PIA-ML in Fig. 2(b) breaks the conventional profile of the Fresnel lens shown in Fig. 2(c). Figures 2(d-f) and Figs. 2(g-i) show the self-normalized light intensity distributions in the x-z plane, x-y plane, and linescan along the x-axis in the designed focal plane of the proposed PIA-ML and C-ML, respectively, at wavelengths from 1300 nm to 1700 nm with a step of 50 nm, which are numerically calculated using vectorial angular spectrum diffraction theory. Figure 2(f) also shows the full width at half maximum values of the calculated focus spots at different wavelengths, which are close to the diffraction limited values (λ/2/NA, 3.3µm@1550 nm).

It is also noted that relatively large sidelobes exist in the focus spots, which may be originated from the optimization process in that the intensity of the light field on the optical axis (i.e., z-axis) is optimized only. This trade-off between the extended focal depth and the relatively large sidelobes could be improved by setting a comprehensive optimization target containing both the factors of intensity on axis and the sidelobes off axis, which will be dealt separately in other paper. Figures 2(d) and (g) show that the designed focal plane (i.e., z = 1 mm, the white dotted line) of the designed PIA-ML exhibits strong focusing across the whole wavelength range from 1300 nm and 1700 nm, while the C-ML focal spot deviates rapidly from the focal plane at different wavelengths. From Fig. 2(g), it is obvious that the intensities at the focal plane at different wavelengths are different, in which the intensities at wavelengths other than 1550 nm rapidly decrease. To show the relative intensities at different wavelengths at the focal plane when compared with that at 1550 nm, an amplification coefficient is given in the bottom of each figure in Fig. 2(h), which mean that an amplification coefficient should be multiplied at the focal plane at different wavelengths in order to have the same peak intensity as that at 1550 nm.

The proposed PIA-ML's optimized phase profile shown in Fig. 2 can be implemented using an array of simple Si nanopillars with a gradient distribution on a sapphire (Al₂O₃) substrate. Si nanopillars are arranged according to the designed phase patterns in Fig. 2 with a periodic square lattice (P = 700 nm) and varying combinations of geometries (i.e., square and circle) and dimensions (the heights of all the nanopillars were fixed at H = 1500 nm). Figure 3 shows the schematics of the unit cells of Si nanopillars and the corresponding transmitted light fields at the designed wavelength of 1550 nm calculated using the finite-difference time-domain method (FDTD Solutions, Lumerical, Canada). The 3D simulation model contains a semi-infinitely thick Al₂O₃ substrate and a silicon nanopillar (square/circle) sitting on the substrate. An x- or y- linearly polarized light source with a wavelength of 1550 nm is placed in the substrate. The perfectly matched layers along z direction and the periodic boundary conditions along x and y directions (period P = 700 nm) are assumed. The dielectric properties of Si and Al₂O₃ given in the software’s database are employed. Figures 3(a-c) and 3(d-f) show the Si nanopillar’s square and circular unit cell structures, respectively, and their phase and transmission optical responses for different side lengths (diameters). A phase modulation that covers multiple 2π and a near-unit transmission can be achieved with different dimensions of the square or circular nanopillars, which provides a data library for the design of the proposed PIA-ML. The small difference in the optical responses between the square and circular Si nanopillars shown in Figs. 3(b-c) and Figs. 3(e-f) renders a finer gradient of phase for a finer manipulation in phase pattern of the designed PIA-ML, in which unit cells with a transmission of higher than 75% and a phase resolution (a maximum difference from the theoretical value) of 0.18 rad are employed in the design of the proposed PIA-ML. The phases originating from the Si nanopillars can be explained by the waveguide-like modes, which are dependent on the geometry and dimensions of the Si nanopillars [31–34]. Figure 3(g) and Fig. 3(h) show the normalized distribution of the magnetic field intensity within the unit cells of the square and circular Si nanopillars, respectively, with different sizes (side lengths or diameters) from 150 nm to 500 nm with a step of 50 nm at the designed wavelength of 1550 nm. The white dotted lines represent the boundaries of the Si nanopaillars. It can be seen that all the magnetic fields are well confined to the inside of the nanopillars, which show different intensity distributions along the z-axis with different side lengths (diameters), corresponding to different waveguide-like modes and transmitted amplitudes and phases. It is noted that the polarization insensitivity of the proposed PIA-ML is naturally derived from the inherent symmetry of the Si nanopillars (circular or square) and also the symmetric patterning of these Si nanopillars in the metalens, which is different from those based on P-B phase with the requirements of anisotropic unit structure and circular polarization incidences.

 

Fig. 3. 3D schematics of the Si nanopillars and the corresponding transmitted light fields at the designed wavelength of 1550 nm. (a) unit cell of the square Si nanopillar; (b)-(c) phase and transmission with side dimension of the square nanopillar; (d) unit cell of the circular Si nanopillar; (e)-(f) phase and transmission with diameter of the circular nanopillar; (g)-(h) normalized distribution of the magnetic field intensity within the unit cell of square and circular Si nanopillars with different side lengths and diameters, respectively. The period of the unit cell and the height of nanopillars are all fixed at P = 700 nm and H = 1500 nm. The white dotted lines in (g) and (h) are the boundary of the Si nanopillars.

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3. Fabrication and imaging of metalens

3.1 Fabrication of metalens

The proposed polarization-insensitive achromatic metalens based on computational wavefront coding with a diameter of 0.49 mm and focal length of 1 mm was fabricated and demonstrated. The structural pattern of the designed PIA-ML was fabricated using electron beam lithography (EBL). A ∼250 nm thick e-beam resist (polymethyl methacrylate, PMMA) film was spin-coated (4000 r/min) on the crystalline silicon (c-Si) on a sapphire wafer, in which the thicknesses of the c-Si and sapphire substrate were 1500 nm and 500 µm, respectively, and a pattern of PMMA was created by the EBL with a dose of 130 µC/cm² at a voltage of 10 kV and a current of 3.7 pA. The PMMA was developed in IPA:DI (3:1) for 100 s, and a 40 nm thick hard etching film (chromium, Cr) was deposited by electron beam evaporation, and then a hard etching mask was defined by the lift-off process. The Si nanopillars of the proposed PIA-ML were formed through inductively coupled plasma (ICP) etching in a mixed gas of C₄F₈ (80 sccm), SF₆ (60 sccm), and O₂ (5 sccm) with an ICP power of 800 W and an RF power of 35 W for 150 s. Finally, the Si metal was obtained after the removal of the Cr hard mask by chromium etching solution. Figure 4(a) shows an optical microscope image of the entire fabricated PIA-ML. Figures 4(b-e) show the scanning electron microscope (SEM) images of the PIA-ML with different magnifications of different regions. Figures 4(b-c) show the top-view SEM image of the center and the magnified edge part of the PIA-ML, and Figs. 4(d-e) show the typical 45°-view SEM images of the center and edge parts of the fabricated PIA-ML.

 

Fig. 4. The optical microscope image and SEM images of the fabricated PIA-ML. (a) Optical brightfield microscope image of the whole metalens; (b) SEM image of top-view of the center; (c) Magnified SEM image of the edge; (d) and (e) PIA-ML SEM images of 45°-view of the center and edge, respectively. Scale bar in (b) to (e) is 4 µm, 1 µm, 1 µm, and 1 µm, respectively.

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3.2 Focusing and imaging experiment of metalens

To verify the achromatic characteristics of the fabricated PIA-ML, the focusing behaviors of the fabricated PIA-ML were measured at different wavelengths and compared with those of a fabricated C-ML with the same structural parameters in the designed near-infrared region using a charge-coupled device (NIR-CCD). The schematics of the experimental focus are shown in Fig. 5(a). The light wave emitted by a super-continuum laser (Fianium, SC450) with different wavelengths passes through an optical long-pass filter (Thorlabs, FELH1300) and is scattered by a diffuser. An achromatic doublet lens (Thorlabs, AC508-075-C-ML) collects the incident light into pinhole 1 (diameter, 70 µm), and the incident light passes through the second identical achromatic doublet lens. Subsequently, the PIA-ML focuses the incident light at the designed focal length (i.e., z = 1 mm), and the focal spot was recorded on a NIR-CCD (XENICS, XEVA-1.7-320) using a 50× objective lens (Mitutoyo, MY50X-825). The objective lens and NIR-CCD were placed on a 3D motorized stage and moved together along the z-axis to capture the focal spot. A diaphragm was used in the experiment to filter out stray light. Figures 5(b-d) show the normalized experimental focusing intensity in the x-z plane, x-y plane, and the linescan along the x-axis at the focal plane of the fabricated PIA-ML at different wavelengths from 1300 nm to 1700 nm with a step of 50 nm. It is seen Fig. 2, all the experimentally focused images using the fabricated PIA-ML agree well with the theoretical expectations. Figure 5(e) shows the normalized experimental focusing intensity of the fabricated C-ML in the x-z plane. The C-ML exhibits a large chromatic aberration, in which the focal length along the z-axis varies rapidly with wavelength. In contrast, strong focusing is maintained at the designed focal plane across the entire wavelength range from 1300 to 1700 nm with the fabricated PIA-ML shown in Fig. 5(c), which means that the chromatic aberration is eliminated in the full wavelength band of optical communication.

 

Fig. 5. (a) Schematics of the experimental measurement setup. (b)-(d) show the normalized experimental focusing intensity of the fabricated PIA-ML at the different wavelengths in the x-z plane (b), x-y plane (c), and linescan along the x-axis at the focal plane (d) and the normalized experimental focusing of the fabricated C-ML in x-z plane at different wavelengths (e). The white dotted lines in (b) and (e) are the designed focal plane (z = 1 mm). The symbols in Fig. 5(a) are the following: SCL, super continuous laser; F, optical long-pass filter; Df, diffuser; Di, diaphragm; L, achromatic doublet lens; P, pinhole; and O, objective lens.

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Experimental imaging with the fabricated PIA-ML in the designed wavelength range was further performed by replacing the pinhole with different targets (Fig. 5(a)). Figures 6(a-d) show the experimental images of the United States Air Force (USAF) resolution test target, object patterns of atoms, flowers, and butterflies generated by the fabricated PIA-ML, which are reconstructed using the alternating direction method of multipliers (ADMM) algorithm to reduce the background noise, at the wavelengths 1300, 1350, 1400, 1450, 1500, 1550, 1600, 1650, and 1700nm [46,47]. The ADMM algorithm is widely applied in the field of image restoration due to its good performance in image denoising and image deblurring, which is helpful to improve the image quality by deconvoluting the direct captured images in this work. During the imaging process, all the positions of the object patterns and the NIR-CCD camera were fixed when the wavelength was changed. In Fig. 6, the ringing effect and unevenness of the images are due to the non-uniformity (large side-lobes) of the captured PSF patterns in Fig. 5(c) caused by imperfections in fabrication and/or the low spatial resolution of the NIR-CCD, for which the pixel size is 30 µm. Note that the white lines in the images at a wavelength of 1700 nm are electronic noise caused by the long integration time. As shown in Fig. 6, light waves of different wavelengths within a wide bandwidth can be clearly imaged at the design focal plane, in which the chromatic aberration is eliminated. It should be noted that the image resolution shown in Fig. 6 is well close to the theoretical resolution limit. In the experiment, the line width of part 3 and part 4 of Group 1 in the USAF test resolution target is 198 µm and 177 µm, respectively. For the fabricated metalens with an aperture of 0.49 mm and focal length of 1 mm, the resolution limit in terms of the line width is ∼204 µm, which is calculated by M×λ/(2NA), where M is ratio of the focal length of applied doublet lens in the measurement (75 mm, i.e., L₂ in Fig. 5) and the focal length of the proposed metalens (1 mm). The non-uniformity in the experimental images (e.g., not-well resolved digits in the marginal area in Fig. 6(a)) can be attributed to the side-effect of the large sidelobes, which results in the non-uniform light field in the image plane.

 

Fig. 6. Experimental images deconvoluted by ADMM algorithm with the fabricated PIA-ML at different wavelengths. (a) USAF resolution test target; (b) atoms; (c) flower; and (d) butterfly. Scale bar is 1 mm.

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Ultra-broadband imaging with a mixed polychromatic light from 1300 to 1700 nm of the fabricated PIA-ML was also experimentally demonstrated by replacing the super-continuum laser with a broadband halogen lamp light source (Zolix, GLORIA-T150A). Figures 7(a-d) show the broadband images of different targets with the fabricated PIA-ML, in which well-resolved and high-contrast images with a certain degree of non-uniformity caused by the sidelobes can be observed. The focus efficiency was measured using the experimental setup shown in Fig. 5(a). The focusing intensity (I₁, sum of intensity from all pixels) of the PIA-ML was captured by the NIR-CCD. Then, the transmitted intensity (I₂) was captured with the same incident laser energy and integration time of the camera by replacing the metalens with pinhole 2 (diameter, 95µm). The focus efficiency is calculated as follows: Focus efficiency = 26.6×I₁/I₂. The coefficient of 26.6 represents the ratio of the area of the PIA-ML to pinhole 2. The characteristics of the focus efficiency are shown in Fig. 7(e), and the average measured focus efficiency of the fabricated PIA-ML is approximately 25% over the entire working wavelength band, which is roughly the same as those reported in previous studies [29–32,36].

 

Fig. 7. Ultra-broadband experimental images by the fabricated PIA-ML with a mixed polychromatic light from 1300 nm to 1700 nm. (a) USAF resolution test target; (b) atoms; (c) flower; (d) butterfly; and (e) measured focus efficiency of the PIA-ML at different wavelengths from 1300 nm to 1700 nm. Scale bar is 1 mm.

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4. Conclusion

In summary, we propose and demonstrate a polarization-insensitive achromatic metalens based on computational wavefront coding. In contrast to all the reported achromatic methodologies that require a linear phase dispersion with the inverse of wavelength, the proposed achromatic method focuses light of different wavelengths onto a designed focal plane by extending the focal depth of different wavelengths with coded nanostructures, which removes the limitation of requiring a linear phase dispersion. The nanopillars were individually coded such that the focal depths at the wavelengths at both ends of the achromatic bandwidth of interest overlap at the desired focal plane. Using the PSO algorithm, an optimized phase pattern of a PIA-ML consisting of an array of simple circular or square Si nanopillars that can work in the full optical communication band from 1300 nm to 1700 nm have been obtained. Experimental results show that both focusing and imaging of the fabricated metalens are consistent with the theoretical predictions within the broadband wavelength range, which provides a new methodology for ultra-broadband achromatic imaging with simple-shaped nanostructures, in contrast to the complicated nanostructures required by the linear phase dispersion compensation method. The proposed method can be easily scaled to large diameter and can also be extended to other wavelength bands such as visible or mid-infrared bands, and is potentially applicable to the meta-optical system in bio-imaging applications [48,49].