Multistable elastic pixel (MEP) based reconfigurable electromagnetic devices
Abstract
Scientific Objectives: We propose a versatile approach to making multistable metasurfaces that can change reversibly from one state to another with large changes in electromagnetic (EM) response. Our approach relies on multistable elastic pixels (MEPs), discrete nematic liquid crystal (NLC)-filled volumes containing a colloid whose position is controlled by the NLC elastic energy landscape. MEPs will have two or more stable sites for colloids with well-defined elastic energy minima, separated by significant energy barriers. The colloid can be placed stably in one location and moved to another by application of an external electric or magnetic switching field. Upon removal of the switching field, the system will persist in the new state. Colloids will serve as scatterers within reconfigurable metasurfaces. Metasurfaces with judiciously placed MEPs and fixed scatterers will have strong changes in EM response, achieved upon change of colloid position within the device close to resonance. Since the colloids can be returned to their original positions by reversing the switching field, the changes in EM responses are reversible. We design and fabricate a number of metasurfaces, including narrow band filters, broad band filters, metasurfaces with switchable polarizing responses, and gradient metasurfaces. We focus on micron-scale colloids for proof of concept, so these metasurfaces will operate in the IR range. The timescales to switch from one state to another, anticipated to be on the order of seconds to tens of seconds with required field strengths on the order of 1kV/m, will be studied in our research. This approach is of interest in systems in which long-lived multistable configurations are desirable, e.g. reconfigurable windows to control energy flux. Methods to be employed: We propose three aims: Aim 1: Design of bistable MEPs: MEP geometry and anchoring energies will be designed, guided by simulation using the Landau-de Gennes free energy functional to find the NLC elastic energy landscape that defines colloid stable states. MEPs will be fabricated using lithographic methods, and colloids within them moved from one location to another via external switching field. The (dynamics of) colloid response will be observed by optical microscopy. MEPs switching times and field strengths will be optimized via consideration of viscous drag, and NLC elastic energy landscapes. Aim 2: Reconfigurable, reversible periodic metasurfaces will be designed to exploit these reconfigurable elements. Responses to EM waves will be simulated using the finite element methods. Periodic structures are modeled as unit cells excited with Floquet ports. The distribution of EM fields and transmission/reflection coefficients are used to characterize the metasurfaces. Metasurfaces incorporating MEPs and fixed scatterers will be fabricated lithographically, and filled with NLC and a single colloid in each pixel by capillarity, and probed via reflection-mode darkfield spectroscopy to characterize the interaction between the incident IR wave and the structure. Aim 3: Gradient metasurface design: Metasurfaces will be designed to support advanced spatial wavefront shaping through careful adjustment of non-similar MEPs of different shapes, sizes, and orientations, guided by inverse optimization methods. Significance of the proposed effort to the advancement of knowledge: This proposal will lead to fundamental knowledge regarding the design of tunable, reconfigurable metasurfaces, e.g. to control thermal fluxes and to change signatures of EM fields. The strategy of exploiting colloidal physics to control colloid location in pixelated structures could be broadly applied in the design of EM devices. Furthermore, we can envision individually addressable pixels, MEPs with multistable structures, and complex colloids. This new toolkit can be combined in a variety of ways to tailor the response of reconfigurable materials.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 11, 2022
- Source ID
- W911NF2210205
Entities
People
- Kathleen J. Stebe
Organizations
- Army Contracting Command
- United States Army
- University of Pennsylvania