Design And Fabrication Of THz Functional Devices Based On Metamaterials

Terahertz (THz) science and technology have drawn tremendous research interest in the past few decades because of their importance in the medical, security, wireless communication and manufacturing sectors. However, most of naturally existing materials inherently do not respond to THz radiation, resulting in the lack of functional devices, including sources, lenses, switches, modulators and detectors, which are necessary building blocks in constructing the world of THz science and technology. Considerable efforts have been made to fill the THz gap (0.1-10 THz) in view of the important promising applications of THz radiation. Moderate progress has been achieved in THz detection and generation, typified with THz quantum cascade lasers. However, devices to control and manipulate THz waves have been lagged behind.In the search for materials to overcome the accessibility difficulties in the THz region, a class of composite artificial materials termed electromagnetic metamaterials has emerged. With their customizable electromagnetic properties, metamaterials could theoretically be manufactured to respond to any electromagnetic frequency region. Therefore, metamaterial-based functional structures and devices are quite potential in realizing control and manipulation of THz waves that cannot be obtained with natural materials. This thesis will focus on two works, in which we demonstrate active THz modulators and asymmetric polarization rotators by utilizing artificially designed metamaterials. The main results are highlighted as below:1. Given that the resonance properties of metamaterials can be affected by external stimulus, like light, electrical field, magnetic field, temperature, or mechanical strain, their electromagnetic response can then be modified to enable modulation of THz radiation in amplitude, phase or frequency as it propagates through the system. Therefore, engineering metamaterials with tunable resonances are of great importance for improving the functionality and flexibility of THz systems. An ongoing challenge in THz science and technology is to create large-area active metamaterials as building blocks to enable efficient and precise control of THz signals. Here, an active metamaterial device based on enhancement-mode transparent amorphous oxide thin-film transistor arrays for THz modulation is demonstrated. Analytical modeling based on full-wave techniques and multipole theory exhibits excellent consistent with the experimental observations and reveals that the intrinsic resonance mode at 0.75 THz is dominated by an electric response. The resonant behavior can be effectively tuned by controlling the channel conductivity through an external bias. Such metal/oxide thin-film transistor based controllable metamaterials are energy saving, low cost, large area and ready for mass-production, which are expected to be widely used in future THz imaging, sensing, communications and other applications.2. Manipulation of the polarization states is essential for polarization-sensitive devices, having important applications in displays, polarization controllers, and microscopies. More importantly, logical operation of THz linear polarizations is a critical issue for future data coding and processing based on THz waves. However, in THz region, chiral materials with large optical activity are not available in nature, and achieving strongly asymmetric rotation of different linear polarizations is also challenging. In this work, we have designed and fabricated a double-layer bi-anisotropic metamaterial that enables strong asymmetric polarization rotation around the frequency of 0.53 THz. It is found a basic CNOT gate logical transformation can be realized through this system that operates on two orthogonal linear polarizations. Such a polarization processor architecture in THz region is very promising for feasible, robust and energy-efficient THz polarizations controlling and processing, and also provides a potential access to future optical supercomputing generation.