UC San Diego

Nonlinear active metamaterial surfaces are constructed of planar periodic engineered structures in the sub-wavelength (lambda/4) scale on which the nonlinear circuit components have been populated. Unusual electromagnetic properties of the metamaterials derived by the resonant behavior of the constitutive unit cells have produced remarkable effects such as negative index of refraction, cloaking, and an electromagnetic band gap due to high impedance, while the implementations are restricted in bandwidth and polarization.

The added nonlinearity from the nonlinear components can give the degree of freedom to achieve the unique and useful functionalities which could not be realized with linear and passive metamaterials. This thesis studies the theory, characterization, and capability of nonlinear active metasurfaces. The primary application of the invented metasurfaces is focused on exploring a new type of microwave absorbing structure, and the other unique electromagnetic properties.

The state-of-the-art nonlinear circuits deployed on the metasurface offer the adaptive capabilities to tune the inherited electromagnetic properties. First, the theoretical limitations of the linear lossy coating are addressed to establish the necessity for the need for advanced absorbing structures. Subsequent chapters introduce the invented nonlinear metasurfaces with the different tunabilities, specifically the switchable metasurfaces that selectively absorb high power signals to avoid destructive interference to sensitive electronic devices. The self-tuning metasurfaces adaptively tune the resonance frequencies to match the frequencies of the incident waves for broadband absorbing bandwidth. The reconfigurable impedance surface has octave tunability maintaining the artificial magnetic conductor property to support an extreme broadband antenna system, and the omni-directional metamaterial surface to response all-directional incoming waves to minimize unexpected scattering effects in oblique angles.