Active metamaterial devices at terahertz frequencies
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Electromagnetic metamaterials have emerged as a powerful tool to tailor the electromagnetic material properties and control wave propagation using artificial sub-wavelength structures. During the past fifteen years, metamaterials have been intensively studied over the electromagnetic spectrum (from microwave to visible), giving rise to extraordinary phenomena including negative refractive index, invisibility cloaking, sub-diffraction-limit focusing, perfect absorption, and numerous novel electromagnetic devices and optical components. The terahertz regime, between 0.3 THz and 10 THz, is of particular interest due to its appealing applications in imaging, chemical and biological sensing and security screening. Metamaterials foster the development of terahertz sources and detectors and expand the potential applications of the terahertz technology through the realization of dynamic and tunable devices. The objective of this thesis is to present different mechanisms to implement active terahertz metamaterial devices by incorporating advanced microelectromechanical system technology. First, an optical mechanism is employed to create tunable metamaterials and perfect absorbers on flexible substrates. A semiconductor transfer technique is developed to transfer split ring resonators on GaAs patches to ultrathin polyimide substrate. Utilizing photo-excited free carriers in the semiconductor patches, a dynamic modulation of the metamaterial is demonstrated. Additionally, this thesis investigates how sufficiently large terahertz electric fields drive free carriers resulting in nonlinear metamaterial perfect absorbers. Second, a mechanically tunable metamaterial based on dual-layer broadside coupled split ring resonators is studied with the help of comb drive actuators. One of the layers is fixed while the other is laterally moved by an electrostatic voltage to control the interlayer coupling factors. As demonstrated, the amplitude and phase of the transmission response can be dynamically modulated. Third, a microcantilever array is used to create a reconfigurable metamaterial, which is fabricated using surface micromachining techniques. The separation distance between suspended beams and underlying capacitive pads can be altered with an electrostatic force, thereby tuning the transmission spectrum. The tuning mechanisms demonstrated in this thesis can be employed to construct devices to facilitate the development and commercialization of new compact and mechanically robust metamaterial-based terahertz technologies.