Metalenses enable precise control of light at sub-wavelength scales with a flat architecture.1
Advanced Photonics, Volume. 7, Issue 4, 046006(2025)
High-resolution and wide-field microscopic imaging with a monolithic meta-doublet under annular illumination
Metalenses have exhibited significant promise across various applications due to their ultrathin, lightweight, and flat architecture, which allows for integration with microelectronic devices. However, their overall imaging capabilities, particularly in microscopy, are hindered by substantial off-axis aberrations that limit both the field of view (FOV) and resolution. To address these issues, we introduce a meta-microscope that utilizes a metalens doublet incorporated with annular illumination, enabling wide FOV and high-resolution imaging in a compact design. The metalens-doublet effectively mitigates off-axis aberrations, whereas annular illumination boosts resolution. To validate this design, we constructed and tested the meta-microscope system, attaining a record resolution of 310 nm (for metalens image) with a 150 μm FOV at 470 nm wavelength. Moreover, by utilizing the integration of metasurface, we implemented a compact prototype achieving an impressive 1-mm FOV with a resolution of 620 nm. Our experimental results demonstrate high-quality microscopic bio-images that are comparable to those obtained from traditional microscopes within a compact prototype, highlighting its potential applications in portable and convenient settings, such as biomedical imaging, mobile monitoring, and outdoor research.
1 Introduction
Metalenses enable precise control of light at sub-wavelength scales with a flat architecture.1
In fact, inspired by traditional lens groups, stacking multiple metalenses would be a promising solution to expand the FOV in far-field imaging.35
In this work, we propose an efficient approach to achieving both high resolution and a wide FOV with a metalens doublet incorporating an annular illumination, as schematically shown in Fig. 1. The system consists of a metalens doublet as the objective made of silicon nitride nano-fins, positioned on both sides of a silica substrate. As illuminated by annular oblique incidence, which is generated by a Köhler illumination path, the microscopy resolution is further enhanced due to the expansion of
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Figure 1.Schematic of the meta-microscope based on metalens doublet and annular illumination. The optimized doublet enlarges the FOV and the annular illumination improves the resolution. Inset is the implemented meta-microscope prototype in a very compact form.
2 Materials and Methods
2.1 Design of Metalens Doublet
Figure 2(a) schematically illustrates the design of metalens-doublet, which is composed of metalens-I facing the object and metalens-II in the backside, with a separation distance determined by the substrate. According to the experiments, the substrate is defined as fused silica with a thickness of
Figure 2.Optical design of the metalens doublet. (a) Ray tracing simulation schematic diagram of the metalens doublet of
The phase profiles of the metalenses were chosen as follows:
By employing the optimized phase distribution, we computed the point spread function (PSF) at various focal points corresponding to different object heights through Rayleigh–Sommerfeld diffraction integration in MATLAB, illustrated in Fig. 2(c). The findings indicate that the metalens doublet achieves nearly diffraction-limited focusing for object heights reaching up to
2.2 Imaging Setup of Metalens Doublet and Annular Illumination
To characterize the imaging performance of the metalens doublet, we designed, fabricated, and tested the doublet at the wavelength of 470 nm. Each nano-fin of the metalens is crafted as a high-aspect-ratio square with a fixed height of
Figure 3.Characterization of the metalens doublet. (a) Phase and transmittance of meta-atoms with 12 different structural parameters, simulated by FDTD solutions. The sizes (in nanometers) of the nano-fins are marked along the phase distribution line. (b) Schematic of a single unit cell, the width and length of the nano fins are 90 nm and 230 nm, respectively. (c) Optical and top-view scanning electron microscope (SEM) images of the fabricated
Following the basic characterization of the metalens doublet, the imaging capabilities of the meta-microscope are further examined in cooperation with the annular illumination system. As previously discussed, many studies on meta-microscopy encounter challenges in achieving a high resolution as designed with a large NA. This difficulty primarily arises from the steep increase in the phase gradient at the periphery of the metalens as the NA increases, leading to inadequate sampling frequency at the edges. Furthermore, the angular dispersion of sub-wavelength structures at the metalens edge becomes more pronounced with higher NA. Therefore, unlike traditional refractive lenses, the NA of metalenses cannot increase ideally with improved resolution as expected. This limitation is a key reason why the ultra-high resolution of meta-microscopy is rarely reported. Consequently, the effective FOV and resolution of most meta-microscope configurations tend to fall short of optimal performance, with SBP typically in the range of only
Fortunately, the resolution of an imaging system is not exclusively determined by the lens’s NA. Enhancing resolution by utilizing illumination to complement the NA is a promising strategy43
In our experimental demonstrations, we built a Köhler illumination to provide annular illumination, consisting of an LED light source, three focusing lenses, and two apertures. The illumination NA was adjustable by modifying the field aperture size (see more details in Note 2 in the Supplementary Material). Leveraging this system, we introduced annular illumination to enhance resolution, as illustrated schematically in Fig. 3(e). In this setup, the Köhler illumination system provided tailored lighting for the sample, where
3 Results
3.1 Imaging under Annular Illumination
We characterized the imaging performance of the meta-microscope system utilizing a 1951 United States Air Force (USAF) resolution test chart as the object. For comparison, we also fabricated and tested a single-layer, spherical-aberration-free metalens with the same aperture diameter and focal length. The results reveal that the singlet metalens produces significant blurriness at an object height of
Figure 4.Microscopic imaging performance. (a) Image of the whole FOV captured by the CMOS. Scale bar is
More importantly, our meta-microscope system has achieved an unprecedented resolution. To date, there have been no prior reports of metalenses capable of resolving group 10 of the resolution chart. The detailed image of groups 10 and 11 is extracted to emphasize the resolution capabilities. As shown in Fig. 4(b), element 5, group 10 is resolvable from the imaging result and normalized intensity profiles of group 10, corresponding to a resolution of 310 nm. To be mentioned, the auxiliary objective lens used in the system does not influence the intrinsic resolution of the device, which is primarily determined by the object NA of the metalens doublet and the illumination condition. Furthermore, our meta-microscope system consistently demonstrates high resolution, exceeding element 2, group 10, across the entire FOV. To be more specific, we moved the resolution chart along the
3.2 Meta-microscope Prototype with an Large FOV
Although the above meta-microscope system design achieves an unprecedented combination of high resolution and a relatively large FOV, it remains more adaptable for general applications where a larger FOV is usually required. Even employing the double-layer metalens, the large effective NA of 0.5 still limits the effective FOV to hundreds of microns. To further demonstrate the flexibility and superiority of our approach, we designed and fabricated another compact meta-microscope prototype with the effective FOV expanding to a millimeter scale.
Based on the analysis in Note 1 in the Supplementary Material, we appropriately reduce the object NA to 0.3 and enlarge the aperture of lens-doublet, enabling a notable FOV of 1 mm in diameter with a corresponding angular FOV of 54 deg (measured from the center of metalens-I), and achieving a resolution of 620 nm. Leveraging the unique light manipulation capabilities of metasurfaces, we also designed an annular illumination system within a highly compact framework. This was accomplished by fabricating an illumination metasurface composed of six segments 1 mm away from the overall center, each configured to illuminate a 1-mm-diameter area from different directions. The fabrication process for the illumination metasurface is similar to that used for the metalens doublet. The phase modulation of each metasurface takes the form as
Figure 5.Meta-microscope prototype and its application in bio-diagnostics. (a) Photographic image of the meta-microscope prototype, along with the optical and top-view SEM images of the fabricated illumination metasurface. Scale bar is
Moreover, our meta-microscope showcases significant potential in life sciences due to its ability to maintain high resolution across a large FOV. To demonstrate its bio-imaging capabilities, we imaged cervical cancer cells, as shown in Fig. 5(c). Compared with a conventional single-layer, spherical-aberration-free metalens, the integration of the metalens doublet and illuminated metasurfaces significantly expands both resolution and FOV, allowing detailed observation of cellular lesions across a 1-mm range. The insets in areas 1–3 display various stages of cellular canceration within the same FOV. Normal cells exhibit relatively regular circular nuclei with a typical size of
4 Discussion and Conclusion
As previously analyzed, there is an inherent trade-off between the NA and FOV of a metalens. Although multi-layer metalenses can significantly reduce off-axis aberrations, their effectiveness in high-NA systems remains limited. As the NA increases, the sampling frequency at the periphery of a large-aperture metalens often falls short, and larger diffraction angles reduce modulation efficiency, compromising focusing ability and causing deviations between the designed and actual imaging resolution. This challenge has been extensively documented in meta-microscopy research.46,47 To facilitate comparison, we summarize the key parameters from recent studies, with the main findings visually represented in Fig. 6 and detailed parameters provided in Note 7 in the Supplementary Material.
Figure 6.Parameters and performance of some meta-based microscopes reported.
In this work, we developed a meta-microscopic imaging system that combines a
Jiacheng Sun received his bachelor’s degree in physics from Nanjing University. He is currently a PhD candidate at the College of Engineering and Applied Science, Nanjing University. His research interest includes meta-based microscopy, integrated imaging devices, and advanced compound fabrication process.
Chen Chen obtained her bachelor’s and PhD degrees from Nanjing University in 2016 and 2021, respectively. Her research interest includes meta-photonics, metalens imaging technology, and polarization optics. To date, she has published over 30 papers in journals including Light Science & Applications, Optica, Physical Review Letters, Nano Letters, and PhotoniX, with more than 1300 citations.
Tao Li is a professor at the College of Engineering and Applied Sciences at Nanjing University. He received his PhD from Nanjing University in 2005. His research interest includes optical metamaterials, topological photonics, and on-chip photonic integrations.
Biographies of the other authors are not available.
[42] J. W. Goodman. Introduction to Fourier Optics(2005).
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Jiacheng Sun, Wenjing Shen, Junyi Wang, Rongtao Yu, Jian Li, Chunyu Huang, Xin Ye, Zhaoyu Cheng, Jiefu Yu, Peng Wang, Chen Chen, Shining Zhu, Tao Li, "High-resolution and wide-field microscopic imaging with a monolithic meta-doublet under annular illumination," Adv. Photon. 7, 046006 (2025)
Category: Research Articles
Received: Feb. 11, 2025
Accepted: Apr. 28, 2025
Posted: Apr. 28, 2025
Published Online: May. 28, 2025
The Author Email: Chen Chen (chenchen2021@nju.edu.cn), Li Tao (taoli@nju.edu.cn)