Linear and nonlinear applications of plasmonics based on optical planar waveguides

• Main Author: Wu, Tingting
• Other Authors: Wei Lei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2019
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/103435; http://hdl.handle.net/10220/48610
• Institution: Nanyang Technological University

Plasmonics is the intersection research area of science and technology which studies the behavior of surface plasmons and it seems that plasmonics is the only viable way toward the realization of nanophotonics. Plasmons are quantized charge density waves which are usually described as collective oscillations of electrons and are broadly divided into two categories: propagating surface plasmons and localized surface plasmons. Plasmons are able to localize the electric field of light to their proximity and provide accordingly high field enhancement in the immediate vicinity of the metal-dielectric interface. In this thesis, we mainly apply plasmonics in linear and nonlinear optics to study plasmonic structures and explore the advantages in them. Nonlinear optics describe the light behavior in a nonlinear medium where the dielectric polarization depends nonlinearly on the light field and its impact on technology and industrial applications are excellent. Nonlinear optics processes have been ubiquitously used in the modern scientific and technological applications, which facilitates diverse phenomena such as ultra-short pulsed lasers, optical signal processing, imaging, sensing, and many others. However, strong nonlinear optical effects generally require giant optical fields interacting with the nonlinear media. Plasmonics can provide the giant optical fields. Theoretically, a fully nonlinear coupled mode differential equation model for lossy plasmonic waveguides is proposed and used to investigate efficient third-order wavelength conversion in various designed plasmonic structures. Specifically, two different kinds of plasmonic waveguides are studied: metallic plasmonic waveguide and graphene plasmonic waveguide. Experimentally achieved efficient third harmonic generation from a bowtie silicon hybrid plasmonic system is also presented. The obtained plasmonic enhanced third harmonic generation facilitate the future development of new frequency generators and signal processing at mid-infrared and terahertz frequencies. Metallic plasmons possess large losses in the frequency regimes of interest which motivated us to explore other available material supporting plasmons like graphene. Graphene is a truly two-dimensional crystalline material and its tunable Fermi energy property and optical transparency have been widely utilized on optics besides its high mobility, flexibility, and environmental stability. Doped graphene supports electrically tunable long-lived plasmons with low loss and significant wave localization. Generally, the interaction between the infrared light and the nanometric-scale molecules is very weak, the high field localization and enhancement of the graphene plasmonic device enable a tunable biosensor with high-sensitivity for label-free detection of the nanometer molecules. The doping level of graphene can be tuned to dynamically modify the plasmonic resonance so as to selectively identify the molecules. We develop three graphene plasmonic systems to detect the molecule chemical fingerprints with high accuracy. The greatly enhanced light-matter interaction and the broadband tunability of the localized graphene plasmonic resonance enable accurate label-free identification of the molecular vibrational modes at subwavelength scale. The high sensitivity may accelerate the further development of novel cost-effective biosensors with superior molecular chemical fingerprints sensitivity in an active graphene plasmonic device. To analytically study a general class of plasmonic structures to explore the key features linking the far-field energy to the near-field energy and attaining considerable field confinement and enhancement, we introduce the transformation optics technique. Transformation optics is a technique which can be used to control the trajectories of light rays by warping space and simplify the modelling process of plasmonic devices by transforming the coordinate system. We introduce a fully analytical model with transformation optics to analytically resolve the essence of the significant enhancement of the molecule Raman emission in instability regime. The exact analytical calculations go beyond the description by the quantum-mechanical model and confirm the efficient Raman emission happens at a near constant laser blue-detuning from the plasmonic resonance for molecule vibrational frequency less than half the plasmonic linewidth. The analytical model provides a computational framework that allows the predictions of all the spectral properties of the molecule Raman emission and facilitates the experimental achievement of single molecule detection as well as other nonlinear processes.