Metalenses

Metalenses are planar optical elements composed of metasurfaces—two-dimensional arrays of subwavelength nanostructures that impart spatially varying phase to transmitted light. By engineering the geometry of each scattering element, a metalens can produce the same wavefront as a conventional curved lens but in a thin, flat form factor. There are two common phase-control approaches: propagation phase, where the resonant response of a nanoscale pillar provides a local phase shift, and geometric phase (Pancharatnam-Berry phase), where rotation of anisotropic elements converts polarization state into a phase shift.

Fabrication uses nanoscale lithography on substrates such as glass or silicon, with common materials including TiO2, silicon, and GaN for visible and near-IR performance. A metalens is designed to realize a target focal length and numerical aperture by encoding a 2π phase profile across the aperture. Metasurfaces can correct aberrations, support high NA, and, with dispersion engineering, achieve reduced chromatic aberration or achromatic operation over a band.

Applications include compact imaging lenses for smartphone cameras, microscopy, augmented reality, LiDAR, and space- and fiber-optic systems. Advantages over conventional lenses include thinner form factor, potential for mass production and integration with semiconductor technology. Challenges remain, including efficiency losses, limited bandwidth, polarization dependence, and fabrication scalability for large apertures. Research directions focus on broadband and achromatic metalenses, active or tunable metasurfaces, and multi-functional devices that combine focusing with polarization control or spectral filtering.