Diffraction has defined the fundamental limits of optical resolution for more than a century, shaping the capabilities of microscopes, cameras, and imaging systems. Today, metasurfaces are transforming this long-standing limitation. Metasurfaces enable advances in super-resolution imaging and advanced photonic technologies by controlling the phase, amplitude, and polarization of light at subwavelength scales.1-4
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Diffractive Optics in HRI
High-resolution imaging (HRI) captures images with an extremely high degree of detail, exceeding what the human eye can perceive. This capability enables improved precision, clarity, and depth in visual data, making HRI crucial in medical, scientific, technological, and industrial fields.1
Diffractive optical elements (DOEs) use diffraction to manipulate light, employing structures patterned on the scale of the light’s wavelength to perform specific optical functions. They control the amplitude, polarization, or phase of light in an extremely customizable manner, making them suitable for applications needing focusing, beam splitting, or wavefront shaping.1-3
In HRI, DOEs offer lightweight design and compactness, and perform complex optical functions within a single component, thereby curtailing the need for bulky lens systems. Although DOEs are powerful, metalenses (MLs) represent a notable development in the domain of optical design, particularly for HRI.1,2
Meta-optics has transformed HRI by enabling exceptional control of light at the subwavelength scale through metasurfaces. MLs, a novel class of metasurfaces and a key component of meta-optics, focus light with extreme precision and correct chromatic aberrations, exceeding the limitations of conventional optics.1
Their ability to manipulate different polarizations and wavelengths of light, along with their compact design, enable lightweight, ultra-thin imaging systems with improved resolution. MLs are two-dimensional (2D) arrays of subwavelength structures made from metallic or dielectric materials.1
Unlike conventional DOEs that depend on phase modulation through diffraction, MLs produce spatially varying optical responses at the subwavelength scale by exploiting the interaction between nanostructures and light. This enables more compact and efficient optical devices with higher control over the phase, amplitude, and polarization of light.1,2
Additionally, MLs provide almost ideal diffraction efficiency over a wide range of wavelengths and correct chromatic aberrations, addressing a major limitation of DOEs. MLs could be engineered to operate simultaneously across multiple frequencies.1,2
Better integration can be achieved in miniaturized, modern optical systems like augmented reality displays or smartphone cameras using MLs. Hence, MLs surpass DOEs in versatility and performance, particularly in HRI, where precise control over light is the key to achieving the best image quality.1
Metasurfaces provide a compact solution for imaging polarimeters, although previous designs have limited imaging resolution. A recent study proposed an interleaved MLs design in which three-row metasurface units within each group were customized to interact with three pairs of orthogonal polarization channels for polarization imaging.1
This configuration enabled the synthesis of an ML polarimeter with a practically unlimited numerical aperture by ensuring nearly identical optical paths between the object and nearby metasurfaces, thereby improving resolution.1
Researchers fabricated an ML polarimeter containing crystalline silicon nanostructures. It achieved a 0.51 numerical aperture at 632.8 nm and provided imaging resolution up to 1.2x the wavelength.1
Findings from polarimetric microscopy showed that the proposed ML polarimeters could enable high-resolution polarization imaging across diverse microscopic samples. These advances are making imaging technologies more effective in sensing, microscopy, and consumer electronics applications.1
Metasurfaces exhibit similar electromagnetic responses and corresponding device functions for both forward and backward incidence, except for specially designed Janus metasurfaces. If a monochromatic light is incident at a certain angle, the actual wavefront always differs from the ideal spherical wavefront.2
This happens due to discrepancies between the actual electromagnetic response and the design target, the impact of discretizing the continuous phase distribution determined by the lattice constant, and differences between the ideal phase distribution and the target phase distribution used in the design, leading to fluctuating focusing effects that impact imaging resolution.2
This can be determined by the Strehl ratio, point spread function (PSF), and modulation transfer function (MTF). The PSF is the image formed through the optical system by a point object, while the Strehl ratio is the ratio of the PSF peak of the actual lens to that of an ideal lens. Finally, the MTF is the magnitude of the Fourier transform of the PSF.2
A paper published in Nanomaterials proposed a design method of diffraction structure based on a metasurface for high-resolution spectroscopy. Researchers proposed the method for simultaneous light splitting and focusing.4
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Initially, they established a light-field calculation model based on Fresnel diffraction and computed the transmittance function of the diffraction structure using the phase distribution of the designed structure. The rules for selecting the structure parameters were elaborated on this basis, and the resolution equation of the structure was obtained.4
Subsequently, a diffraction structure with a 60 μm diameter, a 10° off-axis angle, and designed for operation at 1550 nm was simulated to confirm the viability of the structure design method.4
Among the three types of metasurface used to achieve phase modulation in the 0~2π range, the researchers used a transport-phase type metasurface in this work to achieve phase coverage from 0 to 2π by selecting a cell structure with different structural parameters.4
Using the proposed metasurface-based diffraction-structure design approach, a bilayer model was designed. The bottom surface is a linear subwavelength grating for light splitting by controlling the phase distribution, while the top surface is an off-axis ML for focusing and partial light splitting.4
Simulation results demonstrated that the proposed structure achieved higher resolution than the off-axis ML at the same off-axis angle and had a wider working band than the off-axis ML with similar resolution. Additionally, the imaging quality of the structure was better than the off-axis ML with the same parameters.4
Thus, the proposed method could be effectively deployed in wide-band, high-resolution, ultra-compact spectrometer systems.4
Metasurfaces are redefining optical resolution beyond diffraction limits, enabling high-resolution imaging systems. By manipulating phase, amplitude, and polarization at subwavelength scales, they outperform conventional DOEs. MLs provide compact, efficient control of light, correcting aberrations and enhancing imaging.
Applications in polarization imaging and spectroscopy demonstrate improved resolution, numerical aperture, and wide-band performance for advanced optical technologies, including ultra-compact device integration.
References and Further Reading
- Kazanskiy, N. L., Khonina, S. N., & Butt, M. A. (2025). Transforming high-resolution imaging: A comprehensive review of advances in metasurfaces and metalenses. Materials Today Physics, 50, 101628. DOI:10.1016/j.mtphys.2024.101628, https://www.sciencedirect.com/science/article/abs/pii/S2542529324003043
- Gao, Y., & Ma, Y. (2025). Fundamentals to emerging concepts and applications of metasurfaces for flat optics: a tutorial. Advances in Optics and Photonics, 17(4), 789-1058. DOI: 10.1364/AOP.541854, https://opg.optica.org/aop/fulltext.cfm?uri=aop-17-4-789
- Zhang, Q. et al. (2023). Diffractive optical elements 75 years on: from micro-optics to metasurfaces. Photonics Insights, 2(4), R09-R09. DOI: 10.3788/PI.2023.R09, https://www.spiedigitallibrary.org/journals/photonics-insights/volume-2/issue-4/R09/Diffractive-optical-elements-75-years-on--from-micro-optics/10.3788/PI.2023.R09.full
- Hu, J., Wang, L., Zhao, S., & Ye, H. (2023). A Design Method of Diffraction Structure Based on Metasurface for High-Resolution Spectroscopy. Nanomaterials, 13(18), 2503. DOI: 10.3390/nano13182503, https://www.mdpi.com/2079-4991/13/18/2503
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