Advances in Manipulating ElectromagneticWaves using Paired-Metasurface Devices

Abstract

Metasurfaces are two-dimensional arrays of sub-wavelength scattering elements, which aggregately manipulate an electromagnetic wave to perform a desired function. The elements, or unit cells, are designed to impart a local field manipulation to modify the amplitude, phase, or polarization of an incident wave upon transmission or reflection. This property has allowed metasurfaces to demonstrate a wide variety of functions, such as focusing, polarization conversion, radiation pattern control, and holography with devices that are substantially more compact than traditional components. However, individual metasurfaces have relied on loss-based methods to control the transmitted amplitude of the wave. Such methods include reflection, absorption, or polarization conversion loss, significantly lowering the achievable efficiency. Many applications require high efficiency, so alternative approaches are necessary to realize the full application space of metasurface-based devices.

In this dissertation, methods for designing paired-metasurface devices are developed and demonstrated for amplitude and phase control with high efficiency. By spatially separating multiple metasurfaces along a common axis, wave propagation between the metasurfaces is harnessed to achieve lossless amplitude control. The configuration is analogous to compound lenses, so is termed a compound meta-optic. First, a novel approach is developed for designing lossless paired-metasurface devices. An example meta-optic is designed to realize a Dolph-Chebyshev radiation pattern from a simple Gaussian beam source, showing that radiation pattern control usually implemented through phased arrays is achievable using compound meta-optics. Next, the paired-metasurface design approach is extended to all-dielectric optical metasurfaces to implement wavefront shaping functions at optical wavelengths with high efficiency. Beam forming and splitting is demonstrated to convert a simple uniform illumination to multiple Gaussian and Bessel beams with arbitrarily defined propagation directions and peak intensities. Additionally, three-dimensional holography is demonstrated by generating complex-valued computer-generated holograms from a simple uniform illumination. High efficiency is verified in simulation, where the device designs exhibit efficiencies greater than 80% in each case. Proof-of-concept devices operating at near-infrared wavelengths are also fabricated and measured, verifying the design process and performance. Each device exhibits a measured efficiency between 65%-75%, verifying high-efficiency amplitude and phase control. Finally, aspects of microwave and optical metasurface implementations are combined to design dual-band transmit arrays with applications in mechanical beam-steering at microwave frequencies. Simulation results verify that a collimated beam can be steered toward any direction within a conical volume having an apex angle of 40 degrees, controlled by the relative rotation between two refracting transmit arrays, with efficiency greater than 80%. Overall, simulations and experimental validation reinforce the suitability of paired-metasurface devices for a variety of applications requiring high-efficiency performance.

This dissertation addresses existing limitations of metasurface devices in manipulating the amplitude and phase of electromagnetic waves with high efficiency. These findings expand the application space available to metasurface-based devices by providing design strategies and experimental proof-of-concepts which achieve high efficiency while controlling the amplitude, phase, and polarization of electromagnetic waves. This knowledge will contribute to future development of multi-metasurface devices, which will achieve advanced performance in many scientific, industrial, and consumer applications.