Abstract
Metamaterials, artificially structured composite materials with subwavelength unit cells, exhibit exotic properties not easily obtainable or unavailable in nature. Motivated by the promising applications of metamaterials at optical wavelengths in areas such as sensing and bio-image, researchers have devoted considerable efforts to advancing the science and engineering of optical metamaterials. In the optical regime, plasmonic effects can play an important role when metals serve as one of the components in a metamaterial assembly. The objective of this dissertation is to demonstrate the ultra-sensitive refractive index sensor and enhanced intracellular bio-image from the interplay between metamaterials and plasmonics. In the dissertation, we design an X-shaped plasmonic sensor (XPS) that supports plasmonic resonances of quadrupole modes at near-infrared region, combining with our common-path optical system and phase-contrast algorithm to boost the sensing resolution of refractive-index plasmonic sensors. The measured sensing resolution shows two orders greater than that of the conventional plasmonic refractive-index sensors. In fact, there exist two critical demands to optimize the sensitivity of a plasmonic sensor in phase-interrogation measurements. One is to break the symmetry of the plasmonic structure, such that the corresponding resonant wavelengths for s-polarized and p-polarized modes become different, elevating the phase contrast between the s-polarized and p-polarized modes. The other is to employ high-order resonance modes that allow better sensing capability due to their greater quality factors. The phase change of a resonance mode with a high quality factor is sharper than a resonance mode with a low quality factor, which helps to increase the sensitivities for phase interrogation. Therefore, high-order modes are certainly more sensitive, but their scattering cross section is typically too weak to provide a detectable signal level or a stable signal-to-noise ratio. We meet these two criteria to show an ultra-sensitive refractive index sensor benefitted by the designed X-shaped plasmonic metamaterial and custom-built phase-interrogation system. The experimental measurement shows that the sensing resolution of the XPS reaches 1.15×10-6 RIU, not only two orders of magnitude greater than the result of the controlled extinction measurement (i.e., 9.90×10-5 RIU), but also superior than current reported plasmonic sensors. In addition, we also develop a compact plasmonic bio-images based on split-ring resonators (SRRs). Owning advantages such as label-free, coupler-free, tunable spectrum range (from MIR to VIS) and longer detection length, the SRR microscopy (SRRM) is a strong competitor compared to the surface plasmon resonance microscopy (SPRM) for observing bio-target. Our experimental results have successfully demonstrated its capability of constructing the refractive index distribution images of human bone marrow-derived mesenchymal stem cells (hMSCs) and meanwhile, obtaining the information of functional groups from the target cells. Therefore, we expect that the SRR microscopy (SRRM) delivers much simple optical configuration and better penetration depth for truly whole-cell imaging applications.