Metasurface Lens

Replace a curved chunk of glass with a flat sheet of nano-pillars — and focus light just as sharply.

Replaces the smooth curved surface of a refractive lens with a subwavelength array of nanostructures (typically TiO₂, silicon, or GaN pillars on glass) that imposes a custom phase profile on the transmitted light. Choosing pillar height, width, and orientation locally produces any desired phase profile, including the hyperbolic profile required for diffraction-limited focusing. A single TiO₂ metalens can reach NA = 0.85 with >86% transmission at visible wavelengths and image quality comparable to commercial objectives.

Each nanopillar is a truncated waveguide: light passing through accumulates a phase delay determined by the pillar's effective refractive index and geometry. For TiO₂ (n ~ 2.3–2.5 at 532 nm), pillars about 600 nm tall with diameters from ~100 to ~350 nm cover a 0–2π phase range with high transmission. The metalens designer assigns each unit cell — a square lattice of pitch ~250–425 nm — the pillar geometry that contributes the right phase to bend light toward the focal point. For a lens of focal length f, the phase profile is φ(x,y) = −(2π/λ)(√(x² + y² + f²) − f). Achromatic metalenses balance phase and group delay across pillar libraries to reduce chromatic aberration. Fabrication uses atomic-layer-deposition TiO₂ into electron-beam-lithographed resist, followed by lift-off — yielding nearly vertical sidewalls and high aspect ratios.

In plain terms

A normal glass lens is thick and curved because it slows light down gradually as it enters, bending the beam smoothly. A metalens does the same job, but like a sheet of music: each tiny pillar on the surface is tuned to shift the incoming light wave by exactly the right amount, from zero up to one full wave. When all those tiny shifts add up, the light converges to a perfect focus. Because the lens is just a flat surface covered in nanoscale 'notes,' it can be a few micrometers thick — thinner than a red blood cell — yet work as precisely as a high-end microscope objective.

Also called: metalens, flat lens, dielectric metasurface optic.

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A short history

The idea of a flat lens built from subwavelength resonant elements emerged from plasmonic-metasurface work in the early 2000s. In 2016, Khorasaninejad, Chen, Devlin, Zhu, and Capasso at Harvard published the TiO₂ dielectric metalens in Science, demonstrating diffraction-limited NA = 0.8 focusing at visible wavelengths. Achromatic metalenses followed in 2018 (Capasso group, Science). Centimeter-scale metalenses manufactured by deep-UV lithography arrived by 2022. Commercial development followed via the Metalenz spinoff. The 2017 Science review by Khorasaninejad and Capasso established field terminology and roadmap.

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