Metasurface Imaging Systems

Key Points:

• Metasurfaces offer opportunities in imaging applications in terms of size, weight, and NA, but these considerations need to be balanced with tradeoffs in efficiency, bandwidth, and stray light
• With a single metasurface, aberration correction is limited but is useful in niche applications where size is critical. With small changes in the optical system design (e.g., by using a telecentric meta-optic), large improvements in MTF and relative illumination are possible
• Multiple metasurfaces, including using folded optics, offer additional opportunities to improve aberration correction. Hybrid refractive-metasurface systems also have merit, but there are a number of practical limitations to these systems as well

Post Updated April 27th, 2026

Introduction

In this article, we’ll discuss some of the major considerations in terms of design parameters and constraints for lens design based on metasurface components. We’ll primarily consider geometric aberrations and metasurface-only solutions here, but we’ll briefly touch on chromatic aberrations as well as the potential for hybrid refractive-metasurface systems.

A common mistake we see in imaging system design is when given a handful of specifications, a designer quickly jumps into a commercial lens design software (e.g., Zemax, Code V, or an alternative) and quickly begins optimizing and testing different options. On the one hand, this can be useful to get some rough intuition about feasibility; however, it’s often the case that a customer has provided an incomplete or even inconsistent specification. Furthermore, even if a specification is self consistent, there are often a number of tradeoffs in terms of track length, chief ray incidence angle, and other system parameters that come into play that can become major determinants of performance.

While it sounds obvious, it’s crucial to understand what the overall goal(s) of the design are first before optimizing anything, otherwise it’s not uncommon for a designer to spend a substantial amount of time optimizing something that nobody wants. Understand the problem first, then design toward the relevant goal(s). To appropriately understand and constrain a design problem first, it’s often helpful to first start at a higher level, examining geometric tradeoffs (e.g., even back-of-the-envelope math) before transitioning to running any detailed simulations.

Key Parameters and Considerations

When considering a simplified lens system comprising a single metasurface, there are only a few key parameters to evaluate, including field of view, image resolution (e.g., in pixels), pixel size, f/#, and track length. It’s also important to know what sensor is being used, as this has implications for the type(s) of color filters, chief ray incidence angles of the pixel array, as well as the presence and properties of any sensor coverglass. Another crucial parameter to know is the operating wavelength and desired bandwidth, as this will dictate the metasurface material(s) to use and will impose constraints on achievable performance due to chromatic aberrations.

As diffractive elements, metasurfaces exhibit chromatic aberrations. In the case of a metalens, this manifests as a chromatic focal shift. For example, with a metalens designed to focus green light, if it is instead illuminated with blue light, the focal length will increase, whereas if illuminated with red light the focal length will decrease.

It is a common misconception, even within the metasurface community, that the chromaticity of the lens is due to the resonant properties of the scatterers. While the scatterers themselves do exhibit wavelength dependence, the overall focal shift behavior is not explained by purely a local change in the transmission coefficient of the scatterers due to any resonant phenomenon. It is instead a global effect across the whole surface arising from the positions of discontinuities in the phase function that only are present when an off-design wavelength illuminates the device. These discontinuities arise at points where the phase wraps across a full period. Ultimately, this leads to color-based blur in images with these components.

There are a couple categories of workarounds to chromatic aberration, including dispersion engineering methods and computational imaging techniques, though both of these have notable limits in applicability as well. These will be topics of future articles. Here, we’ll focus primarily on the correction of geometric aberrations and some basic considerations around different types of lenses.

Single-side Metalens Design (Entrance Pupil in the Plane of the Meta-optic)

The simplest metalens design is one where the entrance pupil coincides with the plane of the metasurface itself. In optical design, the entrance pupil is defined as the image of the most limiting optical aperture when the lens is viewed along the optical axis from the object side. In more complicated lens systems (e.g., a smartphone main camera often comprising 6 or more lenses), the entrance pupil does not necessarily coincide with a physical aperture, but in the case of our simple metalens system consisting of 1 optical element, the entrance pupil is in the same plane as the most limiting stop which also happens to be the plane of the metalens itself. Overwhelmingly, most metalenses reported in the literature are of this nature, sometimes with modified phase masks, but ultimately with the layer of nanoposts in the same plane as that of the entrance pupil.

This simple metalens design can work reasonably well for illumination at a single wavelength and when the field of view is limited. As the incidence angle increases, several third-order Zernike terms known as Seidel aberrations, become more significant, including coma and field curvature. Compared to a refractive system that can exhibit a curved surface on one or both sides of a lens, this flat form factor exhibits significant limitations in correcting these aberration terms. With the appropriate phase function, on-axis light can mitigate spherical aberration with a hyperboloid phase, but this only holds for a very narrow field of view.

Telecentric Metalens Design (Entrance Pupil Behind the Meta-optic)

An improvement on this design occurs when the entrance pupil is shifted to another plane, closer to the object side. Here, the limiting stop of the system is still centered on the optical axis but instead is on the other side of metasurface’s substrate, such that light will pass through this pupil and propagate through the substrate before impinging on the nanoposts on the frontside of the substrate. While this change appears small, it has a profound effect on aberration correction.

This change enables the metalens to be image-side telecentric, meaning that the size of the image formed stays the same regardless of the distance of the image (e.g., if the image sensor moves in or out of focus, the magnification will remain the same). This is because a telecentric design ensures that the chief ray of each incident ray bundle remains at a small angle relative to the optical axis on the image side of the system, whereas in the simpler case where the entrance pupil is in the plane of the metasurface, the chief ray incidence angle is essentially unmodified, meaning that the magnification will change linearly with a change in the image sensor distance.

Ultimately, the telecentric nature of this new lens design leads to substantial reductions in Seidel aberrations, with the exception of distortion. For high field angles (e.g., on the order of a 90-degree full diagonal field of view or more), distortion can reach values of 20% or more. Distortion can often be corrected with digital dewarping algorithms, though the designer must carefully evaluate if this is feasible in terms of power budget and latency for the given application.

Another key advantage offered by the telecentric design is that it exhibits very high relative illumination. This comes hand in hand with the reduced chief ray incidence angle. Like with the simple metalens design, however, the telecentric counterpart exhibits significant chromatic aberrations and only functions well for a single wavelength or very narrow bandwidth illumination.

Multi-element Systems and Folded Optics

So far, we have only considered optical designs comprising a single meta-optic, including with adjustments in the position of the entrance pupil leading to enhancements in aberration mitigation. Ultimately, a single meta-optic will be limited in degrees of freedom for optimizing a wavefront over an extended field of view while achieving a desired modulation transfer function (MTF) target. Extending the system to include multiple meta-optics enables additional aberration correction.

Several demonstrations of this in the literature exist and have enabled extension of the field of view relative to a single meta-optic. This is typically accomplished via a frontside-backside alignment process, with the two meta-optics on the same optical axis but on either side of the substrate. An alternative process exploits folded optics, where the two (or more) meta-optics are all located on the same side of a substrate but either via a metallic backreflector or total internal reflection (TIR), light will propagate at angle from one meta-optic to the next, with the substrate serving as a waveguide.

The advantage of folded optics is that the meta-optics can be aligned side by side in a single lithography step. With both approaches, the substrate thickness tolerance can be a critical parameter, especially if the number of meta-optics in a folded geometry increases beyond 2, as discrepancies in thickness may be compounded after passing through each subsequent component.

Another key challenge with having multiple meta-optics is stray light. Each metasurface often will only direct a portion of the incident light to the desired focus, with the remainder being sent to other diffraction orders. Under the right conditions, this can be tolerable; however, as the number of meta-optics increases, the amount of stray light will tend to increase.

Hybrid Refractive-Meta Systems

Another promising avenue for mitigating metalens aberrations is to build systems that comprise both refractive lenses and meta-optics. In this manner, you can exploit advantages of both types of lenses; however, similarly you must also contend with the disadvantages of both types of optics.

There are several manners of combining metasurfaces with refractive elements. A number of designs focus on taking an existing refractive system and incorporating a metasurface element as an additional correcting surface, which is referred to as a “meta-corrector”. This can improve the MTF relative to a refractive objective lens in isolation, but there are caveats.

The operating bandwidth of the design is critical, and if too wide the metasurface may reduce chromatic performance; however, if designed appropriately and with a custom dispersion function, it can correct some level of chromatic aberration—this approach exploits dispersion engineering and for this to work well, the metasurface aperture in the hybrid system in practice needs to be limited to a small size in order to be feasible.

Alternatively, one can design a completely new hybrid lens system, rather than simply leveraging a meta-optic as an additional correcting surface. In this case, it’s important to understand the design goals, because as the number of elements increase, there are many different ways of positioning the meta-optics relative to the refractives and material selection can become more complicated, requiring answers to a number of design questions:

• What plastics or glasses will the refractives be made of?
• How effectively can a meta-optic or meta-optics compensate for the dispersion of the other elements?
• What is the optimal position of the meta-optic from a design perspective as well as from an opto-mechanical integration perspective (these are not necessarily the same)?

Of particular interest are refractive lens designs exploiting conformal meta-optics, where the metasurface is fabricated on a flexible substrate that can adhere to the curved morphology of a refractive lens.

With any of these approaches, there are some important design challenges and practical considerations. First, from a design perspective, it’s necessary to utilize an appropriate optical model. At the level of a lens and not just the scatterer physics, meta-optics often rely on wave-based models such as Fourier optics. Refractive lenses, however, heavily rely on geometric, ray-based methods. There are tools that can model both, and in some cases together, but such state-of-the-art tools do not yet accurately predict efficiencies or stray light currently for meta-optics. It’s critical to understand the limitations of the optical solver used and what approximations are utilized.

From a practical perspective, it’s also not always clear that adding meta-optics to a system will be advantageous. While meta-correctors can improve MTF, in some cases this must come at the cost of increased total track length, and the enhanced aberration correction behavior in some instances may be equally possible with the addition of a refractive surface or other type of optic rather than a meta-optic. And from a cost perspective, if a complex multi-element lens design exploits both refractives and meta-optics, products based on this design must have manufacturing processes and supply chains for both types of optics, which currently do not overlap with one another.

Altogether, these considerations can make hybrid systems impractical for many real-world design problems, especially if high volumes are required, unless a substantial performance enhancement is possible with the meta-optic that would otherwise be impossible or infeasible.

Summary

Metasurfaces offer several advantages for imaging systems, primarily related to their flat and lightweight form factor, and large space-bandwidth product that can enable high-NA imaging. With their flat nature, metasurfaces are limited in their aberration-correcting ability relative to the curved geometries possible with refractives, but under the appropriate design conditions (e.g., constrained operating bandwidth and field of view), they have significant merits for imaging applications.

Small adjustments in the optical system, such as shifting the entrance pupil to realize a telecentric system, can enhance performance, whereas extending to multi-element systems (e.g., folded optics or hybrid meta-refractive assemblies) bring additional complexity but have the potential to substantially improve image quality.

It’s critical that a designer has a deep understanding of their use case and their customer’s design goals as there are a number of performance and practical constraints when using meta-optics for imaging.

References

Shalaginov, Mikhail Y., et al. “Single-element diffraction-limited fisheye metalens.” Nano Letters 20.10 (2020): 7429-7437.

Arbabi, Ehsan, et al. “Controlling the sign of chromatic dispersion in diffractive optics with dielectric metasurfaces.” Optica 4.6 (2017): 625-632.

Colburn, Shane, Alan Zhan, and Arka Majumdar. “Metasurface optics for full-color computational imaging.” Science advances 4.2 (2018): eaar2114.

Arbabi, Amir, et al. “Miniature optical planar camera based on a wide-angle metasurface doublet corrected for monochromatic aberrations.” Nature communications 7.1 (2016): 13682.

Kim, Youngjin, et al. “Metasurface folded lens system for ultrathin cameras.” Science Advances 10.44 (2024): eadr2319.

Arbabi, Amir, et al. “Planar metasurface retroreflector.” Nature Photonics 11.7 (2017): 415-420.

Chen, Wei Ting, et al. “Broadband achromatic metasurface-refractive optics.” Nano letters 18.12 (2018): 7801-7808.

Harrison, Kramer, et al. “Optiland: Open-Source Optical Design Software in Python.”