All-fiber Hybrid Photon-plasmon Circuits:Integrating Nanowire Plasmonics With Fiber Optics

In recent decades, optical fiber has promoted the development of global communication network, and the conventional fiber-optic technology have been well-established. With the development of modern photonics and optical information technology, as well as the increasing requirements of high bandwidth and low power consumption and other performance, the miniaturization of photonic components and devices with higher integration density becomes one of the most important trends in photonics. Generally speaking, the confinement of light in optical fiber is constrained by diffraction limits. Employing new structures or mechanisms for manipulating light therefore has been the driving force to push forward the frontier of fiber optics, among which incorporating fiber optics with plasmonics is one of the current trends for exploring new opportunities. Owing to their unique properties of confining light to deep subwavelength scale, metal nanowires have attracted substantial interested in nanophotonics. However, metallic waveguides with tight confinement usually suffer from high losses resulting in serious signal attenuation. With this regard, we demonstrate all-fiber hybrid photon-plasmon circuits by integrating metal nanowires with optical fibers and assemble all-fiber hybrid photon-plasmon resonators and Mach-Zehnder interferometers.In order to systematically study the integrating of nanowire plasmonics in fiber optics, in the first part of the work, we demonstrate a simple, direct measurement of propagation losses in metallic nanowires. Owing to their atomic surface smoothness, uniform diameters and low intrinsic loss, silver nanowires that can be routinely fabricated have been attracting increasing attentions. However, silver is chemically unstable under ambient conditions, readily reacting with oxygen and hydrogen sulfide, which would result in a short lifetime of silver nanowires (typically several days) and prevent their uses in open air and many other atmospheres. Gold, by contrast, provides very high chemical stability and relatively low loss for plasmonic waveguiding, as have been demonstrated for plasmonic applications in various structures and have shown great potential for plasmonic waveguiding in NIR spectral ranges. As the two most commonly used metallic nanoscale waveguide, here we investigate the propagation losses of gold and silver nanowires.By directly measuring the propagation-distance-dependent output of the nanowire via fiber-taper-assistant near-field-coupling technique, a typical propagation loss of0.41dB/μm (for633nm light) in a260nm diameter silver nanowire is obtained. And for gold nanowire, we obtain a typical loss of0.89dB/μm at785-nm wavelength in a210-nm-diameter gold nanowire. We have also investigated the dependences of propagation losses on the gold nanowire diameter and light wavelength, and shown that the propagation loss is averagely increased with decreasing diameter and wavelength. Our results from direct measurements offer an unambiguous loss information of metallic nanowires, as well as a helpful reference for nanowire-based plasmonic circuits and devices.Based on the above work, we demonstrate all-fiber hybrid photon-plasmon circuits by integrating silver nanowires with optical fibers. Relying on near-field coupling, we realize a photon-to-plasmon conversion efficiency up to92%in a fiber-based nanowire plasmonic probe. Around optical communication band, we assemble an all-fiber resonator and a Mach-Zehnder interferometer (MZI) with Q-factor of6×106and extinction ratio up to30dB, respectively. Since the all-fiber circuits are connected by standard rapid interconnection (hot plugging) FC/PC connectors, it is convenient to change the optical path by adding/reducing a certain length of the optical fiber. Using the MZI, we also demonstrate fiber-compatible plasmonic sensing with high sensitivity and low optical power.In this work, we demonstrate all-fiber hybrid photon-plasmon circuits by integrating silver nanowires with optical fibers. Our results of integrating nanowire plasmonics with standard fiber optics may open a variety of new opportunities for both fiber optics and nanowire photonics. Generally, the fiber-optic technology can provide highly flexible circuitry from long-haul to chip-to-chip level, and the nanowire plasmonic waveguiding is ideal for ultra-compact interconnection and redirection from wavelength to sub-wavelength scale. The seamless integration of nanowire plasmonics with fiber optics offers a promising route to bridge light from macroscopic fiber systems to microscopic nanowire plasmonics down to deep-subwavelength level, as well as a convenient platform for exploring novel fiber-compatible plasmonic-based devices.