In a recent study, researchers at Harvard John A. Paulson School of Engineering and Applied Sciences developed a new metasurface called "metalens" that uses extremely deep, very narrow holes instead of extremely tall pillars to focus light into a single site.
A Science Direct report describes metasurfaces as nanoscale structures that interact with light. At present, most metasurfaces are using monolith-like nanopillars to focus, shape, and control light.
The taller the nanopillar, the longer it takes for light to pass through the nanostructure, offering the metasurface more versatile control of every color of the light.
Nonetheless, each tall pillars are inclined to cling or fall together. The question, what if, rather than constructing tall structures, one went the other way, arises.
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New Metasurface with More than 12 Million Needle-Like Holes
In this study published in Nano Letters, the study authors showed the new metasurface using over 12 million needle-like holes into a five-micrometer silicon membrane, approximately one out of 20 hair thickness.
As the researchers wrote, these long, thin holes' diameter is only a few hundred nanometers, generating the aspect ratio. Specifically, the height-to-width ratio is almost 30:1.
As described in the study, this is the first time holes with an extremely high aspect ratio have been employed in meta-optics.
According to the senior author of the paper, Federico Capasso, this method may be used to develop large achromatic metalenses that focus different colors of light to the same focal spot, giving way to a generation of high-aspect-ratio flat optics, which include large-area broadband achromatic metalenses.
Capasso is also the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS.
Meanwhile, SEAS graduate student and the paper's co-first author Daniel Lim said, if one would attempt to make pillars with this aspect ratio, "they would fall over."
As reported in a similar Phys.org report, he added that the holey platform augments the optical nanostructures' accessible aspect ratio minus sacrificing mechanical strength.
And, similar to nanopillars which vary in size to focus light, the holey metalens comprises holes of changing size positioned over the two-millimeter lens diameter. The said variation of the hole size is bending the light towards the lens focus.
Essentially, according to postdoctoral at ESA, Maryna Meretska, who's also co-author of the paper, holey metasurfaces are adding a new dimension to lens design by regulating the light's "confinement and propagation" over a wide parameter space and develop new functionalities possible.
The co-author added that holes could be filled in with nonlinear optical materials, resulting in the light's multi-wavelength generation and manipulation or with liquid crystals for the active modulation of properties of light.
Use of Conventional Semiconductor Processes and Standard Materials
The metalenses were fabricated using conventional semiconductor industry procedures and standard materials, making them at scale in the future.
In connection to this, the Harvard Office of Technology Development has protected the intellectual property associated with this work and is exploring more opportunities for commercialization.
The project is backed by the Defense Advanced Research Projects Agency or DARPA. Meanwhile, Lim has gained support from A*STAR Singapore through the national Space Science Scholarship Scheme. On the other hand, Meretska has the backing of NWO Rubicon Gant from the Dutch Funding Agency NWO.
Related information about nanostructures and metalens is shown on AR MR XR's YouTube video below:
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