Metallens

Metallens, more commonly called a metalens or metasurface lens, is a flat optical element that focuses light using a two-dimensional array of subwavelength nanostructures. Rather than relying on gradual phase accumulation in bulk glass, a metalens manipulates the phase, amplitude, and sometimes polarization of incident light at each nanoscale site to reproduce a desired wavefront.

Principle of operation: Each nanopillar or nanoantenna acts as a local optical element that shifts the phase of transmitted or reflected light. By engineering a spatially varying arrangement of these elements, the metalens can mimic the focusing function of conventional curved lenses. Two common approaches are propagation-phase designs, which rely on the geometry to control phase, and Pancharatnam–Berry (geometric) phase designs, which exploit polarization changes to achieve the requested phase profile.

Materials and fabrication: Early metalenses often used plasmonic metals such as gold or silver, but high losses in the visible have shifted attention to dielectric materials like titanium dioxide, silicon, or gallium nitride for higher efficiency. Fabrication typically involves lithography (electron-beam or nanoimprint) and reactive ion etching to create nanoscale features on a substrate such as glass or sapphire.

Advantages and challenges: Metalenses are ultrathin and lightweight, with the potential for camera miniaturization and integration with compact optics. Challenges include achieving high efficiency across a broad bandwidth, reducing chromatic dispersion, and scaling manufacturing for large-area or mass production.

Applications and outlook: Potential uses include smartphone and aerospace imaging, augmented and virtual reality optics, microscopy, and photonic integration. Ongoing research aims to improve achromatic performance, polarization control, and manufacturability.