Friday 12th August 2022

Dynamic piezoelectric MEMS-based optical metasurfaces

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INTRODUCTION

Optical metasurfaces (OMSs) represent subwavelength-dense planar arrays of nanostructured elements (often called meta-atoms) designed to control local phases and amplitudes of scattered optical fields, thus being able to manipulate radiation wavefronts at a subwavelength scale (15). Numerous applications have already been demonstrated in the past decade, including free-space wavefront shaping (69), versatile polarization transformations (1013), optical vortex generation (1416), and optical holography (1720), to name a few. However, to date, most reported OMSs are static, featuring well-defined optical responses determined by OMS configurations that are set during fabrication. For more intelligent and adaptive systems, such as light detection and ranging (LIDAR), free-space optical tracking/communications, and dynamic display/holography (2123), it would be highly desirable to develop dynamic OMSs with externally controlled reconfigurable functionalities.

Realization of dynamic OMSs is very challenging because of the high density of array elements that are also arranged in nanometer-thin planar configurations. One of the currently investigated approaches relies on using dynamically controlled constituents, whose optical properties can be adjusted by external stimuli, thereby tuning their optical responses and reconfiguring the OMS functionalities. A variety of dynamic OMSs have been demonstrated by using such materials, including liquid crystals (LCs) (2426), phase-change materials (2731), two-dimensional (2D) materials (3237), and others (3841). For example, by integrating the OMS into an LC cell, reconfigurable beam steering was realized through electrically rotating the LCs in an addressable manner (25). Phase-change materials such as Ge2Sb2Te5 (2730) or VO2 (31) were also used to construct dynamic OMSs due to their reversible amorphous-crystalline or metal-insulator transitions. Furthermore, 2D materials, especially graphene, can be also used to implement dynamic OMSs since their optical properties can be remarkably adjusted through electrical gating/chemical doping with ultrafast switching speed, thus enabling dynamic OMSs with potentially ultrafast response (32, 34). Despite certain progress achieved with these configurations, there are still unresolved critical issues. Thus, LCs inherently require the polarization-resolved operation (2426), phase-change materials feature relatively slow response times (2931), while OMSs based on 2D materials suffer from relatively low modulation efficiencies (35, 36).

Another approach for realizing dynamic OMSs relies on direct modifying their geometrical parameters via mechanical actuations (4252). Initial attempts include OMSs fabricated on elastomeric substrates with dynamic functionalities enabled by OMS stretching (45, 46). Faster and more accurate actuation can be achieved with microelectromechanical systems (MEMS) that allow for electrically controlled actuation with nanometer precision and resolution, featuring also mature design and fabrication techniques (4244, 4752). For example, varifocal lenses were realized with MEMS-actuated metasurface doublets, whose relative positions were controlled by MEMS actuators, resulting in continuous focal length tuning (50). In this configuration, however, the two OMSs and their individual responses are not modified, making it difficult to use for dynamic wavefront manipulation in general. Very recently, through directly structuring OMSs on a movable silicon membrane of a silicon-on-insulator (SOI) wafer, dynamic 1D wavefront shaping with fast response speed (~1 MHz) was demonstrated (51). In this case, direct OMS integration into the MEMS-actuated membrane leads to certain design limitations, resulting in polarization-dependent performance and impeding implementation of 2D wavefront shaping.

Here, by combining a thin-film piezoelectric MEMS (5356) with the gap-surface plasmon (GSP)–based OMS (68, 57), we develop an electrically driven dynamic MEMS-OMS platform for realizing efficient, broadband, and fast 2D wavefront shaping in reflection. The main idea is to split the conventional GSP-based OMS (68, 57), so that an OMS layer containing metal nanobricks and a back reflector is physically separated by an electrically controlled air gap, with an ultraflat MEMS mirror serving as a moveable back reflector (Fig. 1A). OMSs and MEMS mirrors are designed and fabricated in separate processing paths and then combined, ensuring thereby the design freedom on both sides and reducing the fabrication complexity. The choice of the piezoelectric MEMS to be combined with the GSP-based OMS is dictated by specific advantages of the former, including continuous out-of-plane actuation capability and low voltage/power operation (53), which enable the development of continuously tunable/reconfigurable MEMS-OMS components with ultracompact sizes and low power consumption.

<a rel="nofollow" href="https://advances.sciencemag.org/content/advances/7/26/eabg5639/F1.large.jpg?width=800&height=600&carousel=1" title="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." class="fragment-images colorbox-load" rel="gallery-fragment-images-1407384283" data-figure-caption="

Fig. 1 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.

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Fig. 1 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.

With this platform, we experimentally demonstrate dynamic polarization-independent beam steering (Fig. 1B) and reflective 2D focusing (Fig. 1C). By electrically actuating the MEMS mirror and thus modulating the MEMS-OMS distance, polarization-independent dynamic responses with large modulation efficiencies are demonstrated. Specifically, when operating at a wavelength of 800 nm, the beam steering efficiency (in the +1st diffraction order) reaches 40 and 46% for the respective transverse magnetic (TM) and transverse electric (TE) polarizations (electric field parallel/perpendicular to the reflection plane, respectively), where 76 and 78% are expected from simulations, while the beam focusing efficiency reaches 56 and 53% (64 and 66% expected from simulations). Furthermore, the dynamic response of the investigated MEMS-OMSs is characterized with the respective rise/fall times of ~0.4/0.3 ms, characteristics that can be further improved by using MEMS mirrors optimized for bandwidth in the megahertz range. For example, by using MEMS actuated membranes to ensure ~30 MHz of switching speeds (5456).

RESULTS

Operational principle

Similar to the conventional GSP-based OMSs (68, 57), the proposed MEMS-OMS configuration represents a metal-insulator-metal (MIM) structure composed of a bottom thick gold layer atop a silicon substrate (MEMS mirror), an air spacer, and a top layer with 2D arrays of gold nanobricks on a glass substrate (OMS structure). The air spacer gap ta can be finely adjusted by actuating the MEMS mirror (Fig. 2A). When the air gap is small (ta < 200 nm), the optical responses of OMS unit cells are determined by the GSP excitation and resonance in the MIM configuration (57, 58) and thus by nanobrick dimensions (8, 57). To progress further toward the design of dynamically controlled MEMS-OMSs, several geometrical OMS parameters must be determined. First, we set the operating wavelength at 800 nm and choose the OMS unit cell size of 250 nm that should be substantially smaller than the operating wavelength (8, 57). Assuming the smallest achievable air gap is between 20 and 50 nm, the nanobrick thickness tm is then optimized to achieve a wide phase coverage with large reflection amplitudes, resulting in the choice of tm = 50 nm (fig. S1). The nanobrick lateral dimensions, side lengths, are chosen to be equal to ensure the polarization-independent optical response. Analysis of the complex reflection coefficients of the OMS conducted for…

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