Plasmonics and epsilon-near-zero photonics in optical fiber and thin film platforms : from extreme light nanofocusing to enhanced spontaneous emission.

The major challenges in the study of light-matter interaction in the deep subwavelength regime are the inefficient conversion of nearfield to farfield energy, low signal-to-noise ratio, complicated device designs requiring complex multi-step fabrication processes. Metallic nanowires supporting surface plasmon polaritons (SPP) can localize optical fields at nanoscale tapered ends for near-field imaging. Similarly, epsilon-near-zero (ENZ) resonance, which is the behavior of light inside the medium with vanishing permittivity, in transparent conducting oxide thin films possesses strong light confinement properties. In addition, both SPP and ENZ resonances enhance the local density of optical states. Due to this property, dipole emitters near the plasmonic and ENZ medium experience greatly enhanced spontaneous emission. In this dissertation, I have applied these unique optical properties to overcome the current challenges in the field of nanoscale light-matter interaction such as nearfield scanning probe spectroscopy, nanoscale waveguiding and enhancement of light emission. I have reported four main results in this dissertation. Firstly, I have developed a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot at the metallic tip apex. Secondly, as a proof-of-concept demonstration of the probe described above, I have fabricated a plasmonic nanoantenna on the end facet of a photonic crystal fiber and subsequently demonstrated the coupling of light from the fiber waveguide mode to the nanoantenna plasmonic mode. Thirdly, I have designed a novel optical waveguide of a hollow step index fiber modified with a thin layer of indium tin oxide (ITO) that supports highly confined waveguide mode at the ENZ wavelength of ITO. Lastly, I have observed the room temperature photoluminescence (PL) enhancement of molybdenum disulfide monolayers on epitaxial titanium nitride (TiN) thin films at excitation wavelengths covering the ENZ regime where TiN films transition from dielectric to plasmonic. The first two results provide an important step toward widespread application of optical fibers incorporated with plasmonic tips in nearfield spectroscopic techniques such as tip-enhanced Raman and fluorescence microscopy, single photon excitation and quantum sensors, nanoscale optical lithography, and lab-on-fiber devices. The third result provides new understanding of coupling between ENZ and fiber waveguide modes which has potential applications in nonlinear and magneto-optics, in-fiber light manipulation, and biosensing. The fourth result enriches the fundamental understanding of how emission properties are modulated by ENZ substrates that could be important for the development of advanced nanoscale lasers and light sources, bio-sensors, and nano-optoelectronic devices.