Abstract
Metasurfaces are artificial surfaces with a subwavelength thickness that can provide versatile control of electromagnetic (EM) waves. This thesis presents the discrete metasurface. A general design formulation for discrete metasurface design is proposed. To design a periodic metasurface realizing a desired EM wave functionality, the general design formulation can be used to determine the number of elements and the required reflection/transmission coefficients of the elements. Based on the discrete metasurface design formulation, a series of aggressively discretized metasurface designs are proposed. Aggressive discretization in a metasurface design is to use the least number of elements required. The dramatically decreased number of elements leads to a lower requirement for phase/surface impedance coverage, which simplifies the metasurface design. The enlarged element size allows the use of simple structures with largely relaxed fabrication tolerance, which leads to broadband design. The improved fabrication tolerance can benefit the metasurface design in the high frequency regime.Firstly, aggressive discretization in reflective metasurface design is investigated. (a) An aggressively discretized metasurface realizing anomalous reflection is proposed. The metasurface has two unit cells per period, it is designed to reflect a plane wave from 50° to −22.5° at 24 GHz. The theoretical upper limit for the bandwidth of this metasurface reflecting all the incident power into the desired mode is found to be 67%, and the proposed metasurface can reflect more than 80% of the incident power into the desired anomalous reflection mode over a broad bandwidth of 53.6%. (b) An aggressively discretized metasurface realizing power splitting is proposed. The metasurface has two elements per period and it can equally reflect a normally incident wave into two beams with refraction angles of −38.7° and 38.7° respectively at 24 GHz, with a power efficiency of 96.6%. The bandwidth within which the metasurface equally reflects more than 90% of the incident power into the two beams is 48.8%.
Secondly, an aggressively discretized metasurface realizing anomalous refraction and diffraction mode circulation is proposed. The metasurface has three elements per period, it can realize efficient anomalous refraction by deflecting a normally incident wave by 45° at 28 GHz. Due to aggressive discretization, the metasurface can also efficiently deflect incident waves from −45° and 45° to refracted waves with angles of 0° and −45° respectively, hence achieving diffraction mode circulation. For each anomalous refraction case, the proposed metasurface can refract more than 80% of the incident power into the desired mode, with a 3-dB power efficiency bandwidth of 11%. The aggressive discretization in the metasurface design leads to significant simplicity, with the critical feature size 28-fold increased compared with a corresponding finely discretized metasurface realizing anomalous refraction.
Thirdly, an aggressively discretized metasurface realizing orbital angular momentum (OAM) mode circulation is proposed. The proposed metasurface is designed in a coaxial waveguide, it has three elements and can efficiently convert input waves with OAMs of −1, 0 and 1 to output waves with OAMs of 0, 1 and −1 respectively. The OAM modes are synthesized using the TEM and H11 modes in the coaxial waveguide. A proper cut-off condition is realized by tuning the geometrical parameters of the coaxial waveguide so that only OAM modes with states of −1, 0 and 1 can propagate. The proposed metasurface is designed at 10 GHz. It can realize the circulation of the three OAM modes with near-perfect efficiency.
Finally, terahertz (THz) transmissive focusing metalenses based on standard printed circuit board (PCB) fabrication technology are proposed. The significantly increased feature size in the aggressively discretized metasurface design leads to relaxed fabrication tolerance, which allows us to design THz metalenses with the standard PCB fabrication technology. (a) A linearly-polarized metalens is designed using the unit cell composed of three cascaded rectangular patches. It can realize sub-diffraction focusing with a transmission efficiency of 61% and a focusing efficiency of 41%. (b) A polarization-insensitive metalens is designed using the unit cell composed of three cascaded cross-shaped patches. It can realize sub-diffraction focusing with a transmission efficiency of 66% and a focusing efficiency of 43%. Both metalenses are designed at 300 GHz with a large NA of 0.86. They are designed with a minimum line width and gap distance of 0.1 mm, which can be achieved by the standard PCB fabrication.
| Date of Award | 23 Sept 2022 |
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| Original language | English |
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| Supervisor | Man Hon Alex WONG (Supervisor) & Kwok Wa LEUNG (Supervisor) |