An ultra-thin dielectric meta-lens has been created to improve focusing capabilities, but can also be reduced to tiny size for integration with photonics equipment.
A meta lens uses a meta surface to manipulate light. A flat lens, it offers a lightweight way to reduce distortion often found in a curved lens.
The reflective lens, created by the KAUST team led by Xiaohang Li, was designed and manufactured from a range of bespoke TiO2 nanopillars on top of a distributed Bragg reflector (DBR). The DBR consists of a sandwich of alternating layers of SiO2 and TiO2.
The resulting meta-lens is only 300 μm in diameter and has a numerical aperture of 0.6 and a focal length of 200 μm. “This meta-lens is a special arrangement of nano-unit cells at the interface of the DBR, which is designed to reshape the wavefront of reflected light by adjusting the location and refractive index of the nano-unit cells. unit area,” says Ph.D. student Zahra Alnakhli. “This all-dielectric reflective meta-lens has negligible intrinsic loss and is easy to fabricate.”
Posted in Express Optics, research has shown that the meta lens can effectively focus red light, with a wavelength of 633nm, to a high quality point for an incident beam with a normal incident angle. Importantly, and unlike many other lens designs, the focus quality of the lens does not degrade significantly, even when the incident beam angle reaches up to 30 degrees. This improved off-axis focusing is important because it could benefit a wide range of optical tasks ranging from optical tweezers to imaging.
“Off-axis focusing has many applications for system optics based on solid-state LEDs, lasers, and photodetectors for communication, display, imaging, and more,” says Alnakhli. “One application is a super long working distance that uses mirrors to increase the working distance of optical microscopy. This is an important property of microscopic uses in industrial and biological inspections, where the working distance of traditional optical microscopy is difficult.”
To date, the KAUST team has only made lenses that work 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 meta-lens 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 material’s complex refractive index.”
In the future, the KAUST team plans to further improve the performance of its meta lenses. “I’m working on improving the focusing efficiency of the lens so that it can be integrated with other optoelectronic devices. This could improve the directionality of the emitted beam for off-axis applications,” says Alnakhli.
Convex to concave: more metasurface moire gives a wide range lens
Zahrah Alnakhli et al, Reflective Metalens with improved off-axis focus performance, Express Optics (2022). DOI: 10.1364/OE.468316
Provided by King Abdullah University of Science and Technology
Quote: Meta-lens offers superior off-axis focus (2022, October 24) Retrieved October 25, 2022 from https://phys.org/news/2022-10-meta-lens-superior-off-axis-focus. html
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