New resonances, gratings, and slow-wave structures based on 2D periodic structures

Author(s): Li, Xiaoqiang | Advisor(s): Itoh, Tatsuo | Abstract: Periodic structure is made of a finite or infinite repetition of a unit cell in one, two or three dimensions. It has been intensively investigated in electromagnetics and widely used in metamaterials, metasurfaces, photonic crystals, frequency selective surfaces (FSS), slow-wave structures, diffraction gratings, etc. Metamaterials and metasurfaces are subsets of periodic structures whose unit cell is much smaller than the wavelength. By proper design of the sub-wavelength unit cell of a periodic structure, its macro properties can be engineered as we wish. This dissertation introduces three novel devices/phenomena which are based on 2D metamaterials/metasurfaces. This dissertation has three sections.In section I, a new resonance phenomenon is demonstrated in waveguide cavities, which simultaneously uses two orthogonal modes (polarizations). This resonance is formed by bouncing waves with similar handedness, between two simple anisotropic metasurfaces having a relative rotation angle. The tilted anisotropic metasurfaces can cross-couple the waves from one polarization to the other at the cavity end. The field profile of the resonant mode does not exhibit nodes and antinodes, thus the resonant frequency is not solely determined by the cavity length. The resonance condition is theoretically demonstrated from both field and transmission-line perspectives, and is validated by simulations showing the existence of the new resonance. The concept is experimentally demonstrated in an X-band dual-mode circular waveguide, demonstrating a very short cavity resonance. The concept brings new possibilities for resonator design with size and tunability benefits at different frequencies.In section II, a metasurface diffraction grating for circularly polarized (CP) incident wave operating at 10 GHz is introduced. Such structure can provide strong auto-collimation for the CP incident wave. Unlike the conventional scatterer, which would typically reverse the handedness of the incident CP wave upon reflection, the proposed grating can preserve the handedness. strong auto-collimation blazing effect and handedness preservation are achieved with specially designed reactive impedance surface, which allows independent control of the responses to transverse electric (TE) and transverse magnetic (TM) polarization. Simulation and measurement are carried out to demonstrate such effect, and comparison with conventional sawtooth grating is made.In section III, a novel slow-wave substrate integrated waveguide (SIW) is proposed and investigated. The slow-wave effect is achieved by the enhanced capacitance between the signal trace grid and periodic grounded patches on the same top layer. Such slow-wave effect can provide more than 40% size reduction in lateral dimension compared with the conventional SIW with the same cutoff frequency. Meanwhile, the longitudinal dimension can also be reduced by more than 40%. Two-pole bandpass filters (BPF) built from conventional SIW and proposed slow-wave SIW (SW-SIW) are compared through simulations and experiments to demonstrate the applications of our SW-SIW. The BPF implemented with SW-SIW shows a size reduction of 58.8%. Its measured unloaded quality factor Qu is up to 120.2.