Piezoelectric microelectromechanical system-based optical metasurfaces

Optical metasurfaces

Optical metasurfaces typically represent sub-wavelength dense planar arrays of nanostructured elements also known as meta-atoms that are designed to offer scattered optical fields and local phase regulation. Numerous applications in the past have demonstrated free-space wavefront shaping, versatile polarization transformations, optical vortex generation and optical holography. For more intelligent and adaptive systems including light detection and ranging (LIDAR) as well as free-space optical tracking and communications, or dynamic display and holography, it is highly desirable to develop with reconfigurable functionalities. In this work, Chao Meng and a team of scientists combined a thin-film piezoelectric MEMS (micromechanical system) with the gap-surface plasmon-based optical metasurface (OMS) to develop an electrically-driven dynamic MEMS-OMS . In the main idea, they facilitated the conventional gap surface plasmon-based optical metasurface to form a moveable back reflector. The scientists designed and developed the OMS and MEMS mirrors to discern the processing paths and then combined them to ensure design freedom on both sides with reduced complexity during development. The work offered a continuously tunable and reconfigurable MEMS-OMS platform with ultracompact dimensions and low power consumption.

The experiments

Using this platform, Meng et al. experimentally showed dynamic polarization-independent beam steering and reflective 2D focusing. They electrically actuated the MEMS mirror to regulate the MEMS-CMS distance, and showed polarization-independent dynamic responses with large modulation efficiencies. The device functioned at a wavelength of 800 nm with a beam steering efficiency reaching 40 to 46 percent for transverse magnetic (TM) and transverse electric (TE) polarizations. The proposed device maintained a metal-insulator-metal structure composed of a thick gold layer placed on top of a silicon substrate to form the microelectromechanical systems mirror, while 2D arrays of gold nanobricks on a glass substrate formed the optical (OMS) structure. The scientists facilitated the proposed functional wavelength in the device and observed the transformation of the reflection phase response to indicate a simple and straightforward approach to realize a MEMS-OMS chip.

2D wavefront shaping with the MEMS-OMS. (A) Schematic of mirror-like light reflection by the MEMS-OMS before the actuation, i.e., with the initial gap of ~350 nm between the OMS nanobrick arrays and MEMS mirror. Incident light is specularly reflected by the MEMS-OMS regardless the OMS design. (B and C) Schematic of demonstrated functionalities, (B) anomalous reflection and (C) focusing (depending on the OMS design), activated by bringing the MEMS mirror close to the OMS surface, i.e., by decreasing the air gap to ~20 nm. Credit: Science Advances, 10.1126/sciadv.abg5639