Enhanced Spontaneous Emission of Monolayer MoS2 on Epitaxially Grown Titanium Nitride Epsilon-Near-Zero Thin Films.

Room-temperature photoluminescence enhancement of molybdenum disulfide (MoS2) monolayers on epitaxial titanium nitride (TiN) thin films grown by molecular-beam-epitaxy as well as magnetron-sputtered TiN films is observed by a confocal laser scanning microscope with excitation wavelengths covering the transition of TiN's macroscopic optical properties from dielectric to plasmonic. The photoluminescence enhancement increases as TiN becomes more metallic, and strong enhancement is obtained at the excitation wavelengths equal to or longer than the epsilon-near-zero (ENZ) wavelength of TiN films. A good agreement is observed between measured and calculated enhancements. The enhancement is attributed to the increased excitation field in MoS2 at TiN's ENZ wavelength and interference effects for thick spacers that separate the MoS2 flakes from TiN films in the metallic regime. This study enriches the fundamental understanding of emission properties on ENZ substrates that could be important for the development of advanced nanoscale lasers/light sources, optical/biosensors, and nano-optoelectronic devices.

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe: erratum.

In this erratum, the funding section of our paper [Optics Express, 27, 38098- 38108 (2019)] has been updated.

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe.

Metallic nanowires supporting surface plasmon polaritons can localize optical fields at nanoscale tapered ends for near-field imaging. Radially polarized light through the optical fiber or free space efficiently excites the converging radial plasmons at the apex of a sharp tip. However, such radial vector mode excitation through optical fiber requires precise polarization control and strongly polarization maintaining state in optical fiber, thus inducing a complexity for optical applications. In this paper, we propose a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot. The linearly polarized fiber mode is converted to radial surface plasmon polaritons (SPPs) through asymmetric coupling at the base of the metallic nano-tip. The radial SPP's then propagate along the half ellipsoid tip and are focused at the tip apex giving rise to the nanoscale concentration of optical energy. The probe can be implemented with near-field imaging techniques such as tip-enhanced Raman microscopy to obtain topographic and chemical spectroscopic information in atomic resolution for studying light-matter interaction at the nanoscale.

Excitation of epsilon-near-zero resonance in ultra-thin indium tin oxide shell embedded nanostructured optical fiber.

We report a novel optical waveguide design of a hollow step index fiber modified with a thin layer of indium tin oxide (ITO). We show an excitation of highly confined waveguide mode in the proposed fiber near the wavelength where permittivity of ITO approaches zero. Due to the high field confinement within thin ITO shell inside the fiber, the epsilon-near-zero (ENZ) mode can be characterized by a peak in modal loss of the hybrid waveguide. Our results show that such in-fiber excitation of ENZ mode is due to the coupling of the guided core mode to the thin-film ENZ mode. We also show that the phase matching wavelength, where the coupling takes place, varies depending on the refractive index of the constituents inside the central bore of the fiber. These ENZ nanostructured optical fibers have many potential applications, for example, in ENZ nonlinear and magneto-optics, as in-fiber wavelength-dependent filters, and as subwavelength fluid channel for optical and bio-photonic sensing.