Hybrid Metallic-dielectric Micro-and Nanostructures For Photonic Components

Miniaturization is one of the current trends in photonic technology and components/devices, in which micro-/nanoscale waveguide structures is a hot topic in the frontier of micro-/nanophotonics. Usually, dielectric waveguides exhibit favorable optical properties such as low loss, however, the confinement is constrained by diffraction limits and the propagating optical field is easily influenced by dielectric substrates. Metallic waveguides, owing to their unique properties of confining light to deep subwavelength scale and the potential to simultaneously carry optical and electrical signals, have spurred substantial interest in nanophotonics. However, metallic waveguides with tight confinement usually suffer from high losses due to the internal damping of radiation in metals, resulting in serious signal attenuation and heat generation. The simultaneous realization of strong confinement and low propagation loss is therefore one of the critical issues faced by deep subwavelength scale waveguides. With this regard, we propose to integrate metallic and dielectric structures, including metallic rod supported microfiber loop resonator and evanescently coupled matalic-dielectric nanowires, for reducing the losses while maintaining the tight confinement. Furthermore, we successfully realize the hybrid nanophotonic components/devices, such as optical sensors, splitters, interferometers, and ring resonators.In the first part of the work, we demonstrate a robust copper rod supported microfiber loop resonator. By tuning the coupling coefficient to balance the internal losses, extinction ratios of about 30 dB have been obtained in the vicinity of the critical coupling point, demonstrating that the hybrid structure of using metal to support the dielectric micro-/nanowaveguide is an effective way to reduce the substrate-induced effects. Quality factors of higher than 4000 are obtained in these 480-μm-diameter loops, and the resonance wavelength of the loop can be tuned by applying an electric current through the copper rod.As an application of the above-mentioned structure, we demonstrate refractive-index sensing using copper-rod-supported microfiber loops. A change of the surrounding refractive index changes the effective index of the propagating mode guided along the microfiber, and subsequently shift the resonance peak for retrieving the information of the specimens. Sensitivities of refractive-index measurement of 1.1×10-4 and 1.8×10-5 are obtained in a low-concentration ethanol solution and a high-concentration glycerol solution respectively, showing great promise for refractive index sensing in liquid with high stability, high sensitivity and large dynamic range.In the second part of the work, we propose to realize strong comfinement and low loss in a hybrid approach by evanescent coupling dielectric and metallic nanowires. Due to the considerable overlap between propagating modes of the dielectric and plasmonic waveguides, highly efficient coupling of glass nanofibers, semiconductor nanowires and metallic nanowires is achieved. For example, coupling efficiency of about 82% is realized between a ZnO nanowire and a Ag nanowire with a coupling length of merely 220 nm. In addition, we demonstrate a coupling scheme of exciting propagating plasmons in multiple Ag nanowires attached to a single ZnO nanowire.Hybrid nanophotonic components, including polarization splitters, Mach-Zehnder interferometers and micro-ring cavities, are fabricated out of coupled Ag and ZnO nanowires. Extinction ratio of about 12.7 dB is obtained for the polarization splitter with a coupling length down to 200 nm. Clear interference fringes are observed in a hybrid Mach-Zehnder interferometer with a size of 58μm. Hybrid micro-ring resonator with a size of 36μm, exhibits a Q-factor of higher than 500, showing great potential of developing novel nanophotonic devices and simultaneously realizing tight confinement and low loss. Moreover, the integration of multifunctional materials on the micro/nano scale may open new opportunities to realize high-efficiency plasmonic excitation, loss compensation or amplification of plasmonic structures, ultrafast optical modulation, high sensitive optical sensing, as well as to explore cavity quantum electrodynamical phenomena.