An ultrathin dielectric metalens has been created that improves focusing capa­bilities but can also be scaled down to a tiny size for integration with photonics equipment. This metalens uses a meta­surface to manipulate light. A flat lens, it offers a lightweight way to reduce the distortion often found in a curved lens. The reflective lens, created by the KAUST team led by Xiaohang Li, was designed and fabri­cated from a custom-designed array of TiO₂ nano­pillars atop a Distributed Bragg Reflector (DBR). The DBR consists of a sandwich of alter­nating layers of SiO₂ and TiO₂.

The resulting metalens is just 300 micro­meters in diameter and has a numerical aperture of 0.6 and a focal length of 200 micro­meters. “This metalens is a special arrangement of nanounit cells at the interface of the DBR, which is designed to reshape the reflected light wavefront by adjusting the location and refrac­tive index of the surface nanounit cells,” explains Zahrah Alnakhli. “This all-dielectric reflec­tive metalens has negligible intrinsic loss and is easy to fabricate.”

The research showed that the metalens can efficiently focus red light, with a wavelength of 633 nanometers, to a high-quality point for an incident beam with a normal angle of incidence. Impor­tantly, and in contrast to many other lens designs, the focusing quality of the lens does not degrade signi­ficantly, even when the angle of the incident beam reaches up to 30 degrees. This enhanced off-axis focusing is signi­ficant as it could benefit a wide range of optical tasks spanning from optical tweezers to imaging.

“The off-axis focusing has many applications for optics of systems based on semi­conductor LEDs, lasers and photo­detectors for communi­cation, display, imaging and others,” says Alnakhli. “One application is a super-long working distance that uses mirrors to increase the optical micro­scopy working distance. This is an important property of microscopic uses in industrial and biological inspec­tions, where the working distance of traditional optical micro­scopy is challenging.” To date, the team has only made lenses that operate at visible wavelengths, but Alnakhli says that in principle the approach can be applied to other spectral regions.

“It is also possible to fabricate a DBR-metalens to operate in the infrared band of the spectrum but with different materials, such as germanium,” says Alnakhli. “Criti­cally, the choice of nano­pillar material depends on the complex refractive index of the material.” In future, the team plans to further improve their metalens per­formance. “I am working to improve the focusing effi­ciency of the lens so that I can integrate it with other opto­electronic devices. This could enhance the direc­tionality of the emitted beam for off-axis appli­cations,” says Alnakhli. (Source: KAUST)

Reference: Z. Alnakhli et al.: Reflective metalens with an enhanced off-axis focusing performance, Opt. Exp. 30, 34117 (2022); DOI: 10.1364/OE.468316

Link: Advanced Semiconductor Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia