Improving Metalenses with Liquid Crystals

This illustration shows how a metalens refracts light. (Source: G. Strangi & F. Capasso, Harvard U.)

For more than 500 years, humans have mastered the art of refrac­ting light by shaping glass into lenses, then bending or combining those lenses to amplify and clarify images either close-up and far-off. But in the last decade or so, a group led by scientist Federico Capasso at Harvard University has begun to transform the field of optics by engineering flat optics meta­surfaces, employing an array of millions of tiny micro­scopically thin and transparent quartz pillars to diffract and mold the flow of light in much the same way as a glass lens, but without the aber­rations that naturally limit the glass.

Increa­singly smaller, clearer lenses would soon begin to be seen in camera phones, sensors, optical-fiber lines and medical-imaging devices, such as endo­scopes. “Making the lenses used by mobile phones, computers and other electronic devices smaller has been beyond the capa­bilities of traditional glass cutting and glass curving techniques,” according to the World Economic Forum. “…These tiny, thin, flat lenses could replace existing bulky glass lenses and allow further minia­turization in sensors and medical imaging devices.” Now, Case Western Reserve University physics professor Giuseppe Strangi and colla­borators at Harvard have taken a step toward making these metalenses even more useful – by making them recon­figurable.

They did this by harnessing nanoscale forces to infiltrate liquid crystals between those micro­scopic pillars, allowing them to shape and diffract the light in completely new ways – tuning the focusing power, Strangi said. Liquid crystals are especially useful because can be mani­pulated thermally, elec­trically, magne­tically or optically, which creates the potential for the flexible or reconfigurable lenses. “We believe that this holds the promise to revo­lutionize optics as we know it since the 16th century,” said Strangi, whose Nanoplasm Lab at Case Western Reserve inves­tigates extreme optics and the inter­action of light and matter at nanoscale.

Until recently, once a glass lens was shaped into a rigid curve, it could only bend the light in one way, unless combined with other lenses or physi­cally moved, Strangi said. Metalenses changed that, since they allow to engineer the wavefront by control­ling phase, amplitude and polari­zation of the light. Now, by controlling the liquid crystal, the researchers have been able move these new class of metalenses towards new scientific and techno­logical endeavors to generate recon­figurable structured light .

“This is just the first step, but there are many possi­bilities for using these lenses, and we have already been contacted by companies interested in this tech­nology,” Strangi said. Strangi colla­borated with several other researchers in the United States and Europe, including fellow Case Western Reserve researchers Andrew Lininger and Jonathan Boyd; Giovanna Palermo of Universita’ della Calabria in Italy; and Capasso, Alexander Zhu and Joon-Suh Park of the John A. Paulson School of Engineering and Applied Sciences at Harvard Univer­sity.

Lininger said part of the problem with current appli­cations of meta­surfaces is that their shape is fixed at the point of production, but “by enabling recon­figurability in the metasurface, these limi­tations can be overcome.” Capasso, who pioneered the flat optics research field and in 2014 first published research on meta­lenses, credited Strangi for the idea to infiltrate the metalenses with liquid crystals and said this inno­vation represents a step toward even bigger things. “Our ability to repro­ducibly infil­trate with liquid crystals state-of-the art meta­lenses made of over 150 million nanoscale diameter glass pillars and to significantly change their focusing properties is a portent of the exciting science and techno­logy I expect to come out of recon­figurable flat optics in the future,” Capasso said. (Source: Case Western Reserve U.)

Reference: A. Lininger et al.: Optical properties of metasurfaces infiltrated with liquid crystals, Proc. Nat. Ac. Sc., online 10 August 2020; DOI: 10.1073/pnas.2006336117

Link: Dept. of Physics, Case Western Reserve University, Cleveland, USA

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