Metalens for Immersion Microscope Overcomes Challenges of Hand-Polishing Techniques

A flat lens for immersion microscopy has been developed. The lens, which can be designed for use with any form of liquid, could provide a cost-effective and easy-to-manufacture alternative to hand polishing lenses for immersion objectives.

Because of their distinctive shape, most front lenses of high-end immersion microscopes are hand polished. This process is expensive and time-consuming and produces lenses that only work within a few specific refractive indices of immersion liquids. If one specimen is under blood and another underwater, two different lenses would need to be hand-crafted.

Researchers from the John A. Paulson School of Engineering and Applied Sciences (SEAS) used nanotechnology to design a front planar lens that could be easily tailored and manufactured for different liquids with different refractive indices. The lens is made up of an array of titanium dioxide nanofins and fabricated using a single-step lithographic process.

Harvard researchers integrated an immersion metalens into a commercial scanning confocal microscope, achieving an imaging spatial resolution of approximately 200 nm. Courtesy of the Capasso Lab/Harvard SEAS.

The team demonstrated liquid immersion metalenses free of spherical aberration at various design wavelengths in the visible spectrum. They tested water and oil immersion metalenses of various numerical apertures (NA) up to 1.1. Results showed that the metalenses’ measured focal spot sizes were diffraction-limited with Strehl ratios of approximately 0.9 at 532 nm.

By integrating the oil immersion metalens (NA = 1.1) into a commercial scanning confocal microscope, the team was able to achieve an imaging spatial resolution of approximately 200 nm.

“These lenses are made using a single layer of lithography, a technique widely used in industry,” said researcher Wei Ting Chen. “They can be mass-produced with existing foundry technology or nanoimprinting for cost-effective high-end immersion optics.”

The team used this process to design metalenses that can not only be tailored for any immersion liquid but also for multiple layers of different refractive indices — important for imaging biological material, such as skin.

“Our immersion metalens can take into account the refractive indices of epidermis and dermis to focus light on the tissue under human skin without any additional design or fabrication complexity,” said researcher Alexander Zhu.

The array of titanium dioxide nanofins can be tailored for any immersion liquid. Courtesy of Capasso Lab/Harvard SEAS.

The metalenses can be easily adapted to focus light through multilayers of different refractive indices and mass-produced using modern industrial manufacturing or nanoimprint techniques, leading to cost-effective high-end optics.

“We foresee that immersion metalenses will find many uses not only in biological imaging but will enable entirely new applications and eventually outperform conventional lenses in existing markets,” said professor Federico Capasso.

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.7b00717).