First Flat Lens for Immersion Microscope

Flat metalens: The array of titanium dioxide nanofins can be tailored for any immersion liquid. (Source: Capasso Lab, SEAS)

A team of researchers from the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS has developed the first flat lens for immer­sion micro­scopy. This lens, which can be designed for any liquid, may provide a cost-effective and easy-to-manu­facture alter­native to the expensive, centuries-old technique of hand polishing lenses for immer­sion ob­jectives. “This new lens has the potential to overcome the drawbacks and chal­lenges of lens-polishing tech­niques that have been used for centuries,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics at SEAS.

When light hits an object, it scatters. Optical micro­scopes work by col­lecting that scattered light through a series of lenses and recon­structing it into an image. However, the fine detailed geometrical infor­mation of an object is carried by the portion of scattered light propa­gating with angles too large to be collected. Immer­sing the object in a liquid reduces the angles and allows for the capturing of light that was pre­viously impossible, improving the resolving power of the micro­scope.

Based on this principle, immer­sion micro­scopes use a layer of liquid between the specimen slide and the objective lens. These liquids have higher refrac­tive indices compared to free space so the spatial reso­lution is increased by a factor equal to the refrac­tive index of the liquid used. Immersion microscopes, like all micro­scopes, are comprised of a series of cascading lenses. The first, known as the front lens, is the smallest and most important component. Only a few milli­meters in size, these semi­circular lenses look like perfectly preserved rain drops.

Because of their distinctive shape, most front lenses of high-end micro­scopes produced today are hand polished. This process, not surpri­singly, is expen­sive and time-consuming and produces lenses that only work within a few specific refrac­tive indices of immersion liquids. So, if one specimen is under blood and another under­water, you would need to hand-craft two different lenses. To simplify and speed-up this process, SEAS researchers used nano­technology to design a front planar lens that can 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.

“These lenses are made using a single layer of litho­graphy, a technique widely used in industry,” said Wei Ting Chen, post­doctoral fellow at SEAS. “They can be mass-produced with existing foundry technology or nano­imprinting for cost-effective high-end immer­sion optics.” Using this process, the team designed meta­lenses that can not only be tailored for any immersion liquid but also for multiple layers of different refrac­tive indices. This is especially important for imaging biolo­gical material, such as skin.

“Our immer­sion meta-lens 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 Alexander Zhu, graduate student at SEAS. “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 Capasso. (Source: Harvard SEAS)

Reference: W. T. Chen et al.: Immersion Meta-Lenses at Visible Wavelengths for Nanoscale Imaging, Nano Lett. 17, 3188 (2017); DOI: 10.1021/acs.nanolett.7b00717

Link: Capasso group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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