Artificially-made subwavelength-scaled metallic and/or dielectric patterns give metasurfaces their special optical properties, which can be tuned by design by arranging the nano-optical elements on a two-dimensional plane so as to mimic the phase profile of conventional bulk optical lenses.
In their paper, the authors note that although metasurfaces are not new, they tend to be static. Hence mounting them on an electrostatically actuated MEMS platform such as a 2D scanner opens up new fields of application, where heavier traditional lenses are currently being used.
The researchers started with the design of a plasmonic lens producing a line focus, like a cylindrical lens, when illuminated with monochromatic mid-infrared light (at a 4.6μm wavelength). Here the polarization independent design unit cells consisted of disc-shaped 50nm-thick gold resonators of laid on a 400nm-thick silicon dioxide layer on top of a 200nm-thick gold film. Because changing the radius of the disc on the metamaterial also changes the phase of the reflected light, the researchers were able to create a hyperbolic phase profile by spatially distributing discs of carefully chosen sub-wavelength diameters across their planar lens. The resulting 0.8×0.8mm metalens, fabricated using standard photolithography techniques on a silicon-on-insulator (SOI) wafer, was able to focus light incident at an angle of θ = 45° to the lens surface at a focus distance of 5mm.
The authors then proceeded to lift-off and transfer the metalens onto a 2D MEMS scanner capable of controlling the angle of the lens along two orthogonal axes by ±9°, for dynamic beam steering. One application could be to compensate for off-axis incident light, hence correcting comatic aberrations.
The paper reports that for low angular displacements, the integrated lens-on-MEMS system does not affect the mechanical performance of the MEMS actuators and preserves the focused beam profile as well as the measured full width at half maximum.
The researchers note that this proof-of-concept metalens-on-a-MEMS integration could be extended to the visible and other parts of the electromagnetic spectrum, for a wide range of applications including MEMS-based microscope systems, holographic and projection imaging, LIDAR scanners, or laser printing.
But they also envisage that by integrating thousands of such metalenses on large dynamically reconfigurable MEMS micromirrors, with each metalens individually controllable in 2D, they could create a new type of reconfigurable fast digital Spatial Light Modulator with an unprecedented degree of control and manipulation of the optical field. The authors even hope they could make the metalenses themselves reconfigurable, by integrating single metasurface unit cells onto more complex cantilever-based MEMS designs.
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