Functional electromagnetic surfaces on irregular grids

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

For electromagnetic and optical waves, thin surfaces or films incorporating an array of carefully designed small particles can achieve useful functionalities such as focusing, guiding, redirecting, or absorbing power and energy. The state-of-the-art design methodologies utilize analytical and numerical techniques based on periodicity of particle placement for accuracy and efficiency. While they have enabled rapid development and maturation of an important class of devices in recent years over a wide range of frequencies from microwaves to visible light, the periodicity condition limits the surface shape to be planar and the particle positions to be strictly regular. Such restrictions are incompatible with surfaces having curved boundaries or profiles, leading to sub-optimal performances in modern curved platform-conformal applications. In this project, a novel design methodology for such functional surfaces that comprise particles positioned on irregular grids will be investigated, lifting the traditional restrictions and allowing curved conformal surfaces to be accurately designed and realized. Its key foundation is accurate semi-analytical evaluation of the interactions between particles positioned at irregular grid points. This project advances knowledge in wave control methods using thin engineered surfaces. The technique developed in this project can be readily extended to controlling sound or seismic waves. A successful completion will also enhance the electromagnetic signature control capabilities of electronic warfare assets, contributing to national security. Active research participation by undergraduate students and live demonstrations of wave propagation and control mechanisms in front of high school audiences will raise the awareness, interest, and literacy in electromagnetic wave phenomena as well as in STEM disciplines in general. The project will investigate functional electromagnetic surfaces of flat and curved profiles comprising an array of subwavelength elements arranged on an irregular grid. Design techniques will be first established for individual elements on irregular grids for planar transmission-line metamaterials and both reflective and transmissive metasurfaces. In the microwave regime, transformation-optics-based devices in planar transmission-line metamaterials, a subwavelength-thin spherical-shell metasurface cloak for free-standing scatterers, and a transmissive hemispherical-shell metasurface lens will be synthesized, fabricated, and measured based on the new design theory. Overcoming the challenges associated with traditional designs based on periodic boundary conditions, subwavelength elements are placed on irregular grids created by an unstructured mesh generator. For a desired response, the local electric and magnetic fields that excite each individual resonator are accurately evaluated, taking into account mutual coupling effect from all elements. This is achieved by first partitioning the entire surface into near and far regions from a resonator location. Elements in the far region are represented by surface-averaged current densities in evaluating their collective coupling, while those in the near region contribute as individual elements. Then, each resonator is uniquely determined from the fundamental polarization relation. Transformation-optics-based devices will be designed on irregular transmission-line grids, fabricated using the standard printed-circuit technologies, and measured. For a reflective surface, a spherical-shell metasurface cloak will be designed as an array of printed resonators on a grounded spherical dielectric substrate and the prototype will be measured for its cloaking effectiveness. For a transmissive application, a conformal bi-anisotropic metasurface focusing lens comprising multilayer resonators will be designed on an unstructured hemispherical grid, fabricated, and measured. The project advances the understanding of functional surface design and operation, and enables a new class of subwavelength-thin conformal devices optimized for curved boundaries and profiles. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.