Active Tuning of LSPR and SLR for Au Nanoring Metasurfaces and Hybrids via Flexible Plasmonics

Abstract

Recent advances in nanofabrication have stimulated research efforts in the field of flexible plasmonics by integrating functional metasurfaces onto mechanically flexible substrates. In this thesis, we report on the fabrication of flexible metasurfaces composed of gold regular and elliptical nanoring arrays embedded in polydimethylsiloxane (PDMS), using state-of-the-art electron beam lithography and wet-etching transfer techniques. In-situ dark-field reflection spectra are monitored on the flexible systems by implementing a homemade micro-stretcher inside the spectroscope. The feasibility of pattern transfer and reliability of optical measurement are further confirmed by subsequent SEM characterizations on PDMS. The spectral behavior of thin-width nanoring square arrays exhibits a significant shift towards longer wavelengths due to in-situ shape changes under strain. The shape-altering ability is carefully demonstrated through optical/SEM measurements and numerical simulations, which is further understood by a purposed squeezing mechanism. On the other hand, the spectral evolution of elliptical nanorings in square and triangular arrays presents interesting polarization dependence and spectral blueshift under strain. The square array subjected to high strain values exhibits also surface lattice resonances with Fano features due to the coupling between the grating and plasmonic modes. Additionally, we demonstrate Fano resonances in ring-disc-pair hybrid systems on a rigid substrate. The ring-disc-pair system shows significantly enhanced Fano features and surface-enhanced Raman signals with a decreasing gap, predicting well an active spectral tuning once they are transferred onto flexible substrates in future work. In general, this thesis expands the possibilities of conventional gap-altering flexible plasmonics by investigating plasmonic spectral shifts corresponding to NPs shape-altering, surface lattice resonances, and Fano coupling under strain. It provides valuable insights into strain sensing, flexible color displays, and wearable electronics with high sensitivity and selectivity.