Subwavelength phenomena in tunable metasurfaces via structural modulations

Abstract

Metasurfaces offer a versatile route to tailor electromagnetic responses through geometric design of subwavelength meta-atoms rather than by modifying constituent materials. In the terahertz (THz) regime, where available material functionalities are limited, structural degrees of freedom provide an effective means to engineer and modulate electromagnetic states. This thesis investigates how geometric tuning governs THz metasurface responses, with an emphasis on subwavelength phenomena including gap plasmon confinement and guided-mode resonances. Three representative platforms – phase change material (PCM) nanogap antenna, mechanically reconfigurable plasmonic nanogaps, and dielectric gratings – are examined to establish a unified view of structure-driven control across distinct material systems. First, a vanadium dioxide (VO2)–Au hybrid nanogap loop antenna was fabricated via atomic layer lithography, enabling direct integration of VO2 thin-film growth within a conventional photolithography process. While a pronounced resonant redshift is not observed, the device exhibits a measurable transmittance modulation associated with the VO2 insulator-to-metal transition (IMT), demonstrating phase-transition-enabled amplitude control in a nanogap geometry. Second, the mechanical and optical behaviors of a stretchable zerogap platform are numerically analyzed. Finite element analysis elucidates trench formation under tensile strain and predicts the attainable strain and gap width range. The temperature dependence of surface-enhanced Raman signals is interpreted in terms of thermally induced gap size variation. Systematic electromagnetic analysis further reveals saturation and thickness-dependent extrema. Finally, radiative loss control of guided-mode resonances (GMR) in THz monolithic intrinsic silicon gratings is examined. Angle-resolved THz transmittance measurements identify the grating period, fill factor, and etching depth as key parameters governing the photonic band structure. As proof of concept, the platform is demonstrated as a refractive index sensor, exhibiting linear resonance shifts with glucose concentration. Overall, the results demonstrate that structural modulation provides a unifying framework for tailoring THz electromagnetic states in subwavelength metasurfaces, enabling continuous and versatile control beyond material-limited approaches.