MEMS Chips Get Metalenses

Metasurface-based flat lens integrated onto a MEMS scanner: Scanning electron micrograph (left) and optical microscope image (right) of a lens-on-MEMS device. Integration of MEMS devices with metalenses will help to create a new paradigm to manipulate light by combining high-speed dynamic control with precise spatial manipulation of wave-fronts. (Source: Center for Nanoscale Materials, ANL)

Lens techno­logies have advanced across all scales, from digital cameras and high bandwidth in fiber optics to the LIGO lab instru­ments. Now, a new lens technology that could be produced using standard computer-chip tech­nology is emerging and could replace the bulky layers and complex geo­metries of tradi­tional curved lenses. Flat lenses, unlike their tradi­tional counter­parts, are rela­tively light­weight, based on meta­surfaces. When the subwave­length nano­structures of a meta­surface form certain repeated patterns, they mimic the complex curvatures that refract light, but with less bulk and an improved ability to focus light with reduced distortion. However, most of these nano­structured devices are static, which limits their func­tionality.

Federico Capasso, an applied physicist at Harvard Univer­sity who pioneered metalens tech­nology, and Daniel Lopez, group leader of nanofabri­cation and devices at Argonne National Labora­tory and an early developer of micro­electro­mechanical systems (MEMS), brainstormed about adding motion capa­bilities like fast scanning and beam steering to metalenses for new appli­cations. Capasso and Lopez developed a device that inte­grates mid-infrared spectrum meta­lenses onto MEMS.

MEMS is a circuit-based tech­nology that incor­porates micro­electronics and includes mechanical micro­structures like actuators and gears. Ubi­quitous in every­thing from cellphones to airbags, bio­sensing devices, appliances and optics, MEMS are fabri­cated using the same techniques used for inte­grated circuits on typical computer chips. “Dense inte­gration of thousands of indi­vidually controlled lens-on-MEMS devices onto a single silicon chip would allow an unpre­cedented degree of control and mani­pulation of the optical field,” Lopez said.

The researchers formed the meta­surface lens using standard photo­lithography techniques on a silicon-on-insulator wafer with a 2-micron-thick top device layer, a 200-nano­meter buried-oxide layer, and a 600-micron-thick handle layer. Then, they placed the flat lens onto a MEMS scanner, essen­tially a micro­mirror that deflects light for high-speed optical path length modu­lation. They aligned the lens with the MEMS’ central platform and fixed them together by depo­siting small platinum patches.

“Our MEMS-inte­grated meta­surface lens proto­type can be electri­cally controlled to vary the angular rota­tion of a flat lens and can scan the focal spot by several degrees,” Lopez said. “Furthermore, this proof-of-concept inte­gration of meta­surface-based flat lenses with MEMS scanners can be extended to the visible and other parts of the electro­magnetic spectrum, implying the potential for appli­cation across wider fields, such as MEMS-based micro­scope systems, holo­graphic and pro­jection imaging, LIDAR scanners and laser printing.”

When electro­statically actuated, the MEMS platform controls the angle of the lens along two orthogonal axes, allowing the scanning of the flat lens focal spot by about 9 degrees in each direction. The researchers estimate that the focusing effi­ciency is about 85 percent. “Such meta­lenses can be mass produced with the same computer-chip fabri­cation techno­logy and in the future, will replace conven­tional lenses in a wide range of appli­cations,” Capasso said. (Source: AIP)

Reference: T. Roy et al.: Dynamic metasurface lens based on MEMS technology, APL Phot. 3, 021302 (2018); DOI: 10.1063/1.5018865

Link: Nanoscience and Technology Division, Argonne National Laboratory, Lemont, USA • Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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