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.

This dissertation discusses how generative-type artificial neural network (ANNs) enables the real-time inverse design process of reconfigurable EM megastructures. Specifically, we demonstrate two designs: 1. a 2-D beamformer with rotatable dielectric slabs and 2. a conformal EM coating responding to free-form design goals on reflection pattern in a dynamic environment, the latter being an ultimate ambition of EM scattering control and nearly impossible to realize with conventional methods. These two examples demonstrate the superiority of ANN methods in dealing with high-nonlinear EM design problems requiring fast responses. Furthermore, the proposed data-driven free-form inverse-design approach can be accommodated for other science/engineering tasks.

There has long been speculation of inter-cellular communication through weak electromagnetic (EM) fields playing a functional role in biological systems. The predominant theory for how a cell could ever generate electric fields is mechanical vibration of charged or polar biomolecules such as cell membranes or microtubules. The challenge to this theory is explaining how high-frequency vibrations would not be over-damped by surrounding biological media. As many of these suspected resonators are too large for atomistic molecular dynamics simulations, accurately modeling biological resonators remains an ongoing challenge. Starting with energy transfer expressions from coupled-mode theory, we derive expressions for the minimum quality factor required to sustain communication for both near- and far-field interactions. Next, we analyze microtubules as candidate ``antennas", using a custom electromagnetic simulation technique to assess the field strength from vibrating microtubules. Building off the difficulties of simulating the microtubule in conventional molecular simulation techniques, we develop our own method of simulating electrodynamic interactions in biological systems and implement it for a model of linear DNA. Lastly, we discuss experimental work with our collaborators in the biological sciences. We developed a series of band pass filter frequency selective surfaces to narrow down the frequency of signaling in barrier experiments, then followed up with listening and stimulation type experiments in the biologically active frequency spectrum. Overall, our modeling suggests that any potential biological EM signaling could not overcome the damping from surrounding viscous media, at least based on identified biological resonances. Biological resonators contain insufficient charge or dipole moments to produce significant fields to overcome background thermal energy. Electrodynamic interactions seem to require electric fields or velocities unrealistic in biological systems. Our experimental results are mixed in their findings, giving indirect evidence of biological signaling, but providing no direct measurement of EM signals.

Conversion of photons to electrical energy has a wide variety of applications including imaging, solar energy harvesting, and infrared detection. The coupling of electromagnetic radiation to free electron oscillations at a metal interface results in interesting properties including enhanced electric fields tightly confined to the surface. Taking advantage of this nonlinear light-matter interaction, this work presents a variety of resonant surfaces optimized for combining electrical and photonic excitations in order to liberate electrons in a vacuum-channel device for applications ranging from enhanced photoemission, optical rectification, and infrared photodetection.

Periodic structure has been extensively applied in many applications, such as wireless communication, sensing, and advanced applied electromagnetics such as invisible cloaking and superlens. However, as broadband systems have rapidly advanced, the bandwidth of the periodic structure has become a bottleneck. In particular, the high dispersion of the periodic structure prevents it from being applied to future wide-band practical applications. Extensive research efforts have been done on the bandwidth problem of the periodic structure, and these have concluded that there is a fundamental bandwidth limitation for passive electromagnetic structures. Nevertheless, great interest still remains in finding alternative solutions to increase the bandwidth of the periodic structures.

All conventional periodic structures are based on a repeatedly patterned passive metallic structure, which can be characterized as passive impedance that follows the Foster's theorem, resulting in a fundamental bandwidth limitation. In contrast, an active periodic structure is composed of unit cells that violate Foster's theorem. Thus, they are potentially capable of breaking the bandwidth limitation.

This thesis studies the non-Foster impedance loaded periodic structures and their broadband electromagnetic fast and slow wave propagation applications. To be more specific, a non-Foster impedance, realized by an active feedback circuit, is applied to the unit cell of the periodic structures in order to increase the bandwidth of the unit cell and eventually broaden the bandwidth of the periodic structures.

The thesis first introduces the fundamental principle of the non-Foster impedance, explains its difference to the passive impedance, and presents its possible active circuit implementation: a negative impedance converter based non-Foster circuit, which covers the derivation of the non-Foster input impedance, its stability, and a design example. Then, the thesis presents broadband fast and slow wave propagation based on the non-Foster impedance loaded periodic structures. For fast wave propagation, a periodically non-Foster impedance loaded waveguide is designed, fabricated, measured, and demonstrated for broadband dispersion-reduced fast wave propagation. The non-Foster fast wave propagation study extends to the observation of the dispersionless superluminal pulse transmission, and a fundamental limitation of a periodic non-Foster loading-based waveguide. On the other hand, this thesis also presents an extremely low-profile slow wave supported electromagnetic surface structure by loading non-Foster impedance to a conventional artificial impedance surface. All the applications presented in the thesis include the principle of loading non-Foster impedance to the conventional passive periodic electromagnetic structures, the design of the non-Foster impedance circuits, and the broadband measurement demonstration.