Three-dimensional and nonlinear metamaterials at terahertz frequencies
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During the past decade metamaterials have emerged as a unifying theme across a large swath of the electromagnetic spectrum. Scale invariance of the underlying equations enables translation of phenomena realized in one region of the spectrum to others. To date, the majority of metamaterials studies focus in the microwave, and infrared to visible regimes, while leaving a span in between. This region, called terahertz regime from 0.3 to 10 terahertz, is of particular interest because of its increasing technological importance, which includes as examples, security screening and embedded imaging. This lag in development of metamaterials is due to enormous challenges with two most important being fabrication strategies and available terahertz sources and detectors. This, in turn, restricts multifunctional responses of metamaterials that are particularly important for implementing dynamic devices at terahertz frequencies. The object of this thesis is to describe our progress on developing a fabrication strategy to construct three-dimensional metamaterials and taking advantage of recent advances in high field terahertz generation to realize nonlinear metamaterials. The first part of this thesis details the developed multilayer electroplating technique for fabrication of stand-up metamaterials on rigid and conformally flexible substrate. The strong resonance resulting from the coupling to the incident magnetic field indicates a significant magnetic response and negative permeability of metamaterials at terahertz frequencies. Extending our fabrication technique, we also experimentally demonstrated broadband three-dimensional metamaterials with dynamic tuning range over 30%. Through photoexcitation of active medium of silicon that is incorporated in the metamaterial active region, the resonant frequency can be effectively tuned. Next, with the judicious incorporation of field enhancement of metamaterials with state-of-the-art technique of high field terahertz generation, field-dependent nonlinear metamaterials fabricated on semiconductors are presented. Modeling and numerical simulations indicate that the origin of nonlinearity arises from nonequilibrium carrier transport within the capacitive regions of resonators. With increasing field, the retrieved off-resonance permittivity exhibits tinting between negative and positive values. Our innovative work opens up numerous possibilities for nonlinear metamaterials at terahertz frequencies. This thesis provides a route forward to create novel metamaterial-based devices for sensing and manipulating electromagnetic waves.
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