Optical metasurfaces (OMSs) are subwavelength-dense planar arrays of nanostructured components (commonly referred to as meta-atoms) that are used to regulate the local phases and amplitudes of scattered optical fields, allowing for subwavelength manipulation of radiation wavefronts. Free-space wavefront shaping, optical vortex production, adaptable polarization transformations, and optical holography are just a few of the applications that have been proven in the last decade. Most described OMSs, however, are static, with very well optical responses governed by OMS configurations set at manufacture. It would be ideal to develop dynamic OMSs with external monitoring reconfigurable functions for more intelligent and adaptive systems like free-space optical tracking/communications, light detection and ranging (LIDAR), and dynamic display/holography.
The tremendous density of array elements, which are also arrayed in nanometer-thin planar configurations, makes realizing dynamic OMSs extremely difficult. One of the current techniques is to use dynamically regulated constituents with external stimuli that can modify their optical characteristics, tuning their optical responses and reconfiguring the OMS functions. A variety of dynamic OMSs has been demonstrated by using such materials, including liquid crystals (LCs), two-dimensional (2D) materials, phase-change materials, and others. For example, by integrating the OMS into an LC cell, reconfigurable beam steering was realized through electrically rotating the LCs in an addressable manner. Phase-change materials such as Ge2Sb2Te5 or VO2 were also used to construct dynamic OMSs due to their metal-insulator transitions or reversible amorphous-crystalline.
In addition, 2D materials, especially graphene, can also be used to develop dynamic OMSs, since their visual characteristics can be altered impressively with high-speed electricity/chemical doping, thereby making dynamic OMS with possibly high-speed responsiveness available. Despite some progress in these setups, significant problems remain unresolved. LCs therefore intrinsically need polarization-resolved operations, phase-change materials have a relatively low response time, and 2D-based OMSs are relatively efficient in terms of modulation.
Another technique is directed to the production of dynamic OMSs by mechanical actuation to vary its geometrical properties. Initial trials involve elastomerically-built OMSs which are enabled by OMS stretching, using dynamic functionality. Microelectromechanical system (MEMS) provides for faster and more accurate actuation with a precise nanoscale and a resolution, which also provide mature design and manufacturing capabilities. For instance, MEMS-actuated metasurface doubles were created for a varifocal lens which controlled its relative locations by MEMS actuators, thereby leading to the continual tuning of focal length.
Nevertheless, two OMSs and their unique responses are not updated in this setup and are generally hard to apply for dynamic wavefront modulation. Recently, a dynamic 1D wavefront shaping with a high response speed (~1 MHz) was shown through directly structuring OMSs on a moveable Silicon-On-Insulator (SOI) membrane. Direct OMS integration into the MEMS-actuated membrane in this situation results in a set of design restrictions, which impedes polarisation-related performance and 2D wavefront design.
In this context, an electrically connected dynamic MEMS-OMS platform is built to produce efficient and broadband, and quick reflectivity on the 2D wavefront by integrating thin-film piezoelectric MEMS with the gap-surface plasm (GSP) based OMS. The main goal will be to split the typical GSP-based OMS so that the OMS layer comprising metal nano bricks and a back reflector is physically separated by power-driven air gaps.
OMSs and MEMS mirrors are created and manufactured in independent processing paths and integrated so allowing flexibility of conception on both sides and lowering the complexity of production. The selection of piezoelectric MEMS in conjunction with GSP-based OMS is dictated by particular benefits of the former such as consistent out-of-plan actuation and a low voltage / low power operation, which allows continuous tuning/reconfiguration of MEMS OMS components with ultra-compact dimensions and low power usage.
Similar to the conventional GSP-based OMSs, 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 nano bricks on a glass substrate (OMS structure). By activating the MEMS mirror, the air spacer gap ta may be accurately controlled. When the air gap is narrow (ta 200 nm), the GSP excitation and resonance in the MIM configuration, and thus nano brick dimensions, influence the optical responses of OMS unit cells.
Numerous geometrical OMS parameters must be established before moving further with the design of dynamically controlled MEMS-OMSs. To begin, the working wavelength was set to 800 nm, and a 250 nm OMS unit cell size that is significantly smaller than the operational wavelength was selected. The nano brick thickness tm is chosen for the widely phased coverage with high reflex amplitudes, which leads to a choice of tm = 50 nm if the least achievable air gap can be reached between 20 and 50 nm. The side lengths of the nano bricks are chosen to be comparable to enable polarization-independent optical response.
Study of complex OMS reflectance coefficients for increasing air vacuums demonstrates the progressive decrease in phase gradient for various nano brick side lengths, with the amplitude and reflection phase becoming independent on the nano brick length at an air gap of ta = 350 nm. This abrupt change in optical responses is linked to high GSP (at normal incidence) dependence and GSP reflections on the air gap at nano brick terminations: the two quickly reduce due to increased air gap, thereby smoothing out and eventually removing the GSP resonance.
The detected modification of the reflection phase response indicates a straightforward and uncomplicated technique to build dynamically controlled MEMS-OMSs: Any conceivable GSP-based OMS can be designed for a particular smallest air gap (for example, 20 nm), and its functionality can then be turned on and off by adjusting the MEMS mirror. Hereafter, this approach was shown by accomplishing dynamically controlled ‘Polarization-independent beam steering’ and ‘Polarization-independent dynamic reflective 2D focusing’.
An independently produced OMS, an ultra-flat MEMS mirror, and a printed circuit board were used to create the MEMS-OMS for polarization-independent dynamic beam steering. The fabrication and characterization techniques for the MEMS-OMS for polarization-independent dynamic reflective 2D focusing were comparable to those utilized for the dynamic beam steering MEMS-OMS.
Pairing a thin-film piezoelectric MEMS mirror with a GSP-based OMS, we created the electrically driven dynamic MEMS-OMS platform. By carefully actuating the MEMS mirror, this platform allows for adjustable phase and amplitude modulation of reflected light. MEMS-OMS devices, both efficient (~50 percent), broadband (~20 percent near the operational length of 800 nm), and fast (<0.4 ms) have been developed and experimentally proven for functioning in the near-infrared wavelength range for dynamic polarization-independent beam steering and reflective 2D focusing. Note that when employing the circularly polarized light, the operating bandwidth can be increased significantly, using the geometrical (Pancharatnam-Berry) phase as its transformation is based on OMS.
When modifying the MEMS-OMS separation by altering the applied voltage of ~4V, both devices rely on the phase response transformation. A MEMS-OMS design for dynamically checking all features for traditional GSP-based OMS, from polarization control/detection to vector/vortex beam generation, can be done by the same principle of operation: For a smaller air gap, the GSP based OMS is designed with a necessary capability that can then be enabled and disabled by moving the MEMS mirror from and to the OMS surface.
Furthermore, the significant adjustment of the size-dependent phase response with the MEMS-OMS separation, which can be precisely adjusted by electrical MEMS actuation, implies that more sophisticated dynamic functionalities could be realized. The ability to switch between various diffraction orders to provide the quasi-continuous beam steering is one of the functions of special interest to industrial applications (for use in, e.g., LIDAR applications).
Thus, corresponding to reflection angles of 0°, 5.2°, and 10.5° in glass (i.e., 0°, 7.7°, and 15.5° in the air) under naturally incident light with 800-nm wavelength, the MEMS-OMS device have also been designed and experimentally demonstrated for polarization-independent dynamic beam steering between three (0th, 1st, and 2nd) diffraction orders. The OMS configuration comprised of two OMSs with various supercells with Λsc1= 12Λ and Λsc2= 24Λ optimized at two discrete air gaps and interleaved by using a random-interleaving method. The experimental evaluation verified the desired dynamic MEMS-OMS response: when the actuation voltage increased, the +1st and +2nd diffraction orders appeared, succeeding one another, as predicted.
The need to close the MEMS mirror (~100 nm) to the surface of the OMS is also a viable guideline for further study and development. Localized plasm resonances may be used for larger MEMS-OMS separations due to stimulation of polaritons in thin metal sheets from a short-range surface plasm (SR-SPPs). In our first simulations, SR-SPP resonances hybridize with the Fabry-Pérot resonances, which are enabled by wavelength wide air gaps and open a comparable approach to the above-noted route to change the response of the OMS phase by regulating the air gap.
The reflected phase gets independent of the nano brick size at certain air gaps (separated by half the wavelength), resulting in the mirror-like behavior. There are spaces between these air gaps where the phase does depend on the nano brick size (note a dotted line at the 1250 nm gap). The nano brick sizes at these gaps can be set in such a way that a phase-gradient metasurface can be realized. Thus, transitioning between these two separate air gaps leads to the transitioning between mirror-like and gradient metasurface behavior, related to the shifting between the same kinds of GSP-based metasurface responses.
As shown by our simulations of dynamic beam steering, the MEMS-OMS can be performed near an air gap of 1 m or more using this approach, avoiding the challenge of obtaining nanometer-sized air gaps. All in all, we presume that the established MEMS-OMS platform can recognize a wide range of functionalities with dynamically reconfigurable performances, opening up exciting possibilities for the successful realization of elevated dynamically controlled devices with useful applications in the future reconfigurable/adaptive optical systems.
Research paper: Dynamic piezoelectric MEMS-based optical metasurfaces