Terahertz technologies have been attracting attention over the past few years due to their unique applications in bio-chemical sensing, bio-imaging, spectroscopy, security, and communications. Traditionally, these applications are steering electromagnetic (EM) waves in free space and result in bulky and diffraction-limited systems. The maximum spatial resolution of the system is defined by an inherent constraint, the Abbe diffraction limit. In order to minimize the sizes of the systems and to increase the spatial resolution of the system beyond the diffraction limit, the active and tunable devices for the EM wave steering and confinement with subwavelength beam sizes are studied. The devices are applied on to two major applications, a multifunctional waveguide and a gradient flat lens; however, finalizing the design requires that the constitutive parameters and material properties of unit cells of metamaterials be determined and characterized.;First, investigations are conducted to retrieve the material constitutive parameters of the proposed active metamaterials, such as effective permittivity and permeability. Classical methods for determining these properties are not robust. In this study, improved methods for single-layered and multi-layered MMs are proposed to fully retrieve effective material properties and constitutive parameters of the MMs at all frequencies of the interest, including the values in the resonant band.;Second, a new kind of active tunable MM structures is invented, based on a passive gradient dielectric substrate and partially disconnected electrodes. This creates a system capable of actively reconfiguring a beam profile by tuning the material properties of individual unit cells as desired. This active gradient MMs technology has great potential in multiple fields, such as optical communications, quantum computing, and as an in-situ reconfigurable mask for photolithography.;Finally, a novel subwavelength waveguide and a multifunctional flat lens based on the proposed active tunable MMs are investigated. This new active waveguide design would lead to vastly superior wave couplers, splitters, and phase compensators, able to alter beam profiles, steer beams, act as optical multiplexers, switches, tunable filters and selectable polarizers, etc. The proposed methods and invented active tunable MMs can be adopted in the systems operating at different frequencies from microwaves to optics.
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