Engineering optical responses of plasmonic-metal–dielectric composite nanosystems

Recent technological advancement in engineering of materials at nanoscale has reinforced the interests towards exploring endless opportunities in an entirely new world where metal–dielectric nanostructures are the ‘Lego’ building blocks. These nanoscopicmaterials exhibit fascinating optical responses by virtue of their localized-surface-plasmon resonance (LSPR), which arises from the collective excitation of electrons at metal–dielectric interface coherently oscillating with incident electromagnetic radiation. Despite having possibilities in wide range of applications, engineering of plasmonic nanosystems is hindered due to lack of comprehensive modelling techniques and simple design guidelines. There is a constant need of developing novel plasmonic-metal–dielectric composite nanosystems which could outperform the prevailing ones and unveil new horizons of exciting applications. A thorough analysis of surface plasmon (SP) characteristics can only could provide simple design guidelines for easy tuning of LSPRenhanced optical responses at design as well as operational phases of the plasmonic nanosystems. This thesis, therefore, contributes towards these objectives and investigates various novel plasmonic-metal–dielectric composite nanosystems operating in visible and near-infrared regions, for devising new pathways of tailoring their optical responses and SP characteristics. The research comprehensively provides simple analytical models and design rules, besides proposing novel nanostructures and their applications in the futuristic ultra-compact ultrafast nanoscale circuits and systems—based on theoretical formulations, numerical calculations, and computational simulations in the realms of classical electrodynamic theory. Design, modelling, and analyses of freestanding, spherical and cuboid shaped composite plasmonic nanostructures are reported exhibiting tailorable exotic plasmon-enhanced optical phenomena such as ultra-narrowband scattering, ultra-sensitive sensing, and ultra-wideband absorption. The effects of substrate on their optical responses are further evaluated and substrate-induced tenability is presented considering both infinitely-large and finite dielectric substrate. The optical responses of the dielectric nanocuboids, introduced as finite substrates, are then evaluated in detail, which unveiled these optically-resonant magneto– electric nanoparticles in ultra-directional scattering applications. The unidirectional scattering profiles are easily tuneable with design parameters of such nanoparticles ensemble. Light absorption and scattering properties of ensembles of weakly-interacting plasmonic-metal–dielectric nanostructures are also investigated considering nearly-identical nanoparticles. Ensembles of size-distributed nanoparticles are also studied, prescribing optimal size-distribution parameters to maximize light absorption or scattering in various applications. Further, optical responses of strongly-interacting, size-distributed, plasmonic nanoparticle ensembles are thoroughly investigated. ‘Plasmene’ is introduced as graphene-counterpart from plasmonic nanoparticle family—in the form of a freestanding, one-particle-thick, superlattice sheet of nanoparticles, which possesses both functionalities of localized gap-plasmons and propagating plasmons. By introducing mechanical stress on plasmene sheets, their stretching-induced tunability is reported as new means of engineering their optical responses. Plasmene nanoribbons, extracted from plasmene sheets, reveal width-dependent gap -plasmon resonances besides supporting propagating plasmons along surface and edge. Effect of folding of these nanoribbons is also evaluated on their optical responses. Folding-induced effects lay the foundation for next-generation foldable plasmonic devices and novel plasmonic superstructures. Investigation of unfolded plasmene sheets with different building blocks evidences tunability in their optical responses and in near-field enhancement as functions of structural parameters and excitation laser wavelength, which allows engineering plasmene-based superstructures for ultra-sensitive detection and sensing applications.