Designs of Functional Devices Based on Spoof Surface Plasma Polaritons


Student thesis: Doctoral Thesis

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Award date30 Oct 2020


This thesis presents some novel functional devices based on spoof surface plasma polaritons (SPPs) at microwave, millimeter-wave and terahertz frequencies. By employing the strong field confinement characteristics of the 3D groove SPP waveguide, the proposed millimeter-wave and terahertz antennas exhibit wide operating bandwidths, low cross-polarizations, and high gains and efficiencies. With the help of asymmetrical field confinements, we employ the SPP structures in crosstalk reduction and filter applications. The SPP structures with stronger field confinement usually lead to huge insertion loss near the cutoff frequency. By integrating with lossy metal, the tapered SPP structures can absorb the propagating electromagnetic (EM) waves and operate as terahertz wideband and compact loads. 

First, a millimeter-wave SPP rod antenna is developed to generate wideband stable radiation with low cross-polarization. Periodic metallic grooves operate as an SPP transmission line with high EM field confinement. The concept of a dielectric tapered rod antenna is employed to design an SPP rod antenna with linearly-tapered groove depths, which converts the confined energy to radiation mode. The tapered structure only affects the vertical component on the transverse plane and thus it can achieve vertical-linearly polarization with low cross-polarization. The measured |S11| of this prototype is below -15.6 dB from 50 to 75 GHz. Experimental results show that the linearly-tapered antenna can achieve an average gain of 14.95 dBi with ± 1.45 dBi variation and a better than -25.6 dB cross-polarization level in E-plane radiation patterns.

Second, two high-gain millimeter-wave antennas based on spoof SPP waveguide are presented. The dispersion of spoof SPP modes supported by metallic grooves is found to be insensitive to the lateral width. Therefore, we propose two millimeter-wave antennas with wide-groove SPP, namely, an SPP tapered rod antenna and an SPP leaky-wave antenna. The widened grooves enlarge the aperture sizes of the antennas, which in turn enhance the antenna gains. Based on the similar principle of conventional dielectric rod antennas, the proposed SPP rod antenna achieves a gain of 16.06 to 19.3 dBi in 50-75 GHz band. By employing periodic modulation, the SPP leaky-wave antenna operates from 50 to 70 GHz for |S11| below -10 dB. This leaky-wave antenna with 24 modulated periods achieves a gain of 20.1 to 23.9 dBi in the operating band with beam scanning range from -39° to -3°.

Third, we present terahertz circularly- and linearly-polarized leaky-wave antennas with spoof SPP by spin-orbit interaction. Besides the strong field confinement, spoof SPP waveguides inherently have longitudinal and transverse electric fields with phase quadrature, which means that electric fields along spoof SPP waveguides have spin properties. Based on the spin-orbit interaction, two terahertz circularly-polarized leaky-wave antennas are achieved with periodic cylinders: (a) by loading cylinders on top of the spoof SPP waveguide, symmetrical right-hand circularly polarized (RHCP) and left-hand circularly polarized (LHCP) radiations in the transverse plane can be excited; and (b) when the cylinders are loaded along the side of the spoof SPP waveguide, RHCP radiation can be generated in the longitudinal plane; when the cylinders are replaced by cuboids, the RHCP radiation will become right-hand elliptical. Based on field superposition theory, elliptically-polarized waves with opposite handedness can be generated by symmetrically loading periodic cuboids on both sides of the SPP waveguide, leading to linearly-polarized (LP) radiation. A terahertz LP leaky-wave antenna was designed and fabricated by 3D printing with surface metallization. It achieves a gain of 14.5 to 17.5 dBi and a beam-scanning range of -18° to +12° in the operating frequency band of 145-220 GHz with |S11| < -10 dB.

Fourth, a novel compact spoof SPP waveguide integrated with blind vias is proposed. When integrated with blind vias, the conventional U-shaped SPP structure can achieve a smaller size and reduce the cutoff frequency due to the additional shunt equivalent capacitance and inductance. Meanwhile, depending on the groove orientation, this SPP structure leads to asymmetrical EM field confinements in the transverse plane, where the EM field is strongly confined around the groove orientation but much weaker on the opposite side. Due to this strong field confinement, the measured crosstalk between two SPP waveguides with 1 mm edge separation is -25 dB in 3-13 GHz band, a reduction of 18 dB on the average when compared with microstrip line implementation. On the other hand, owing to the weaker field confinement, energy from the SPP waveguide can be easily coupled into other parasitic structures for filter designs. By loading quarter-wavelength resonators with four SPP cells along the main SPP waveguide, the notch numbers and their frequencies can be controlled by SPP dispersion.

Fifth, two wideband and compact SPP loads with nickel (Ni) operating at terahertz frequencies are presented. Since the SPP structure has the strongest subwavelength field confinement and huge metallic loss near the cutoff frequency, the tapered Ni-SPP structures can efficiently absorb the propagating EM wave in a wide frequency band as matching loads. One U-shape Ni-SPP load can achieve VSWR < 1.2 in 150-1000 GHz range with 0.5 λ0 length at 250 GHz, which was demonstrated in a two-way microstrip-type divider. U-shaped SPP structures integrated with folded stubs can achieve a compact geometry by loading an equivalent capacitor, and the new Ni-SPP load is suitable for the terahertz circuits on thick substrates. The VSWR is smaller than 1.2 in 195-500 GHz band with 0.9 λ0 × 0.12 λ0 in size at 300 GHz. The performances of one Goubau-line divider indicate the feasibility of this Ni-SPP load on thick substrate (0.2 λg at 300 GHz, λg is the dielectric wavelength).