Terahertz Metasurfaces
Creators: name: Cole, Michael
Publisher: name: The Australian National University
Year: 2019

description: Terahertz radiation has many unique properties not found in other parts of the electromagnetic spectrum. Terahertz radiation is non-ionizing, many common materials are transparent to it, and a significant amount of materials have unique spectral signatures. These properties have the potential to provide significant advances in current technologies, such as imaging, security, biomedical analysis, and communications. However, there are still challenges that need to be overcome within the field of terahertz research; metamaterials and metasurfaces offer the possibility to make further advancement. Three particular terahertz challenges that will be addressed with metamaterials and metasurfaces in this thesis are perfect absorption of terahertz radiation, manipulation of terahertz polarization, and phase control of terahertz waves for large angle refraction. Metamaterials and metasurfaces are used in devices for controlling wave transmission, reflection, and absorption from microwave to optical wavelengths. There are vast applications of absorbers; for example, they are used in anti-reflective coatings and in detectors. Efficient detectors are still lacking in this part of the spectrum and efficient absorbers are a crucial component of any detector. In this work, an all-dielectric metamaterial was proposed that acts as a perfect terahertz absorber. The unit cell consists of a dielectric cylinder embedded in a low index material. To achieve near-perfect terahertz absorption (99.5%) impedance matching of the electric and magnetic resonances was employed within the cylinders of the Huygens' metasurface. Changing the aspect ratio between the height and diameter of the cylinder grants control of the impedance matching. It was shown that the absorption resonance can be tuned to particular frequencies from 0.2 to 2.0 THz by changing the geometric parameters of the structure, while keeping the aspect ratio of the cylinders nearly constant. In addition to perfect absorption, it is shown here that terahertz chiral metamaterials achieve resonant transmission and strong optical activity. This response is realized in a metasurface coupled to its Babinet complement, with additional twist. Uniquely, the optical activity achieved in this type of metamaterial is weakly dispersive around the resonant transmission maxima, but it can be highly dispersive around the transmission minima. It was recently shown that this unique optical activity response is closely related to zeros in the transmission spectra of circular polarizations through the Kramers-Kronig relations and strong resonant features in the optical activity spectrum corresponding to the Blaschke phase terms. It is demonstrated here that by modifying the meta-atom geometry greatly affects the location and magnitude of these Blaschke phase terms. Three different meta-atoms were studied, which are variations on a simple cross structure. The responses of each structure were measured using terahertz time-domain spectroscopy and analyzed via numerical simulations. For operation in transmission, Huygens' metasurfaces are commonly used, since their favorable impedance matching to the surrounding media minimizes reflections and maximizes transmission. However, recent research has shown that Huygens' metasurfaces are non-optimal, particularly for large angles of refraction, and that to eliminate reflections and spurious beams it is necessary to use a bianisotropic metasurface. In this work two concrete terahertz metasurface designs are compared based on the Huygens' and Omega-type bianisotropic approaches, demonstrating anomalous refraction angles for 55, and 70 degrees. It is shown here that for the lower angle of 55 degrees, there is no significant improvement when using the bianisotropic design; whereas, for refraction at 70 degrees the bianisotropic design shows much higher efficiency and fidelity of refraction into the designed direction.