Graphene-based devices for terahertz and beyond

Reconfigurable electromagnetic and optical devices are important for nonlinear op tical and next-generation communication systems. This thesis analyzes graphene based tunable structures for the control of radiation patterns in terahertz antennas and for enhanced nonlinear optical generation in hybrid nonlinear metasurfaces. Three different graphene-enabled platforms are explored: electrostatically pro grammable antennas, magnetically biased reconfigurable antennas, and graphene lithium niobate nonlinear metasurfaces. First, a transparent disc-shaped programmable antenna working at sub-terahertz frequencies is proposed. The antenna consists of two orthogonal dipoles integrated with eight fan-blade-shaped graphene parasitic elements on a polyimide substrate. By adjusting the chemical potential of graphene, the current distribution across the radiating structure is modified, enabling programmable beam steering. Single, dual, and quad-beam radiation patterns can be generated by the antenna with dis crete beam steering across the full 360 degree azimuthal plane. The proposed design obtains a maximum gain of 2 dBi for single-beam configuration, while maintaining good impedance matching with a minimum reflection coefficient of-36.4 dB at 200 GHz and a-10 dB bandwidth ranging from 187 GHz to 214 GHz. The second part of the thesis introduces a graphene-based dipole antenna whose radiation characteristics are reconfigured through magnetic biasing. Unlike conven tional electrical tuning methods, this mechanism exploits the anisotropic conduc tivity induced in graphene by applying in-plane and out-of-plane magnetic fields. A set of graphene parasitic elements placed near the dipole arms enables beam steering through magnetic-field-controlled current redistribution without the need for direct electrical contacts. The antenna is designed on a SiO2 substrate and op timized via parametric studies to enhance impedance matching, realized gain, and radiation efficiency. Fourteen magnetic bias configurations are analyzed, each pro ducing a distinct radiation pattern. The results demonstrate improved realized gain and pattern reconfigurability at 260 GHz, indicating the potential of magnetically controlled graphene antennas for compact terahertz communications systems. Finally, this thesis investigates nonlinear optical processes in a hybrid graphene lithium niobate metasurface designed for simultaneous second- and third-harmonic generation. The metasurface consists of graphene-LiNbO3 pillars arranged on a SiO2 substrate and exploits the strong second-order nonlinearity of lithium niobate together with the third-order nonlinear responses of graphene and SiO2. Guided mode resonances supported by the periodic structure are identified through nu merical simulations and analytical effective-index modeling. The results show that both second-harmonic generation (SHG) and third-harmonic generation (THG) ef ficiencies are enhanced at resonance conditions, with simultaneous enhancement occurring near the crossing of TE0 and TM1 guided modes. Overall, the results presented in this thesis demonstrate the versatility of graphene for enabling tunable electromagnetic and nonlinear optical devices. The proposed antenna designs provide effective radiation pattern reconfigurability in the sub terahertz regime, while the hybrid metasurface enables multi-resonant nonlinear optical generation via geometrical control of guided-mode resonances.