Flexibly Tunable Surface Waveguide Resonances in Cylindrical Waveguide-Metal-Waveguide Configuration Assisted by Tilted Fiber Grating

Surface waveguide modes/resonances (SWRs) are a promising and relatively new research route toward advanced photonic applications ranging from spectroscopy and sensing to communications. However, the conventional planar setups for exciting the SWRs suffer from its intrinsic shortcomings. Herein, we propose and demonstrate a new cylindrical waveguide-metal-waveguide (WMW) configuration consisting of a tilted fiber grating (TFG) coated with nanometric-scale gold and silicon films for exciting fiber-optic SWRs. The investigations are performed using the ray-optic approach and rigorous coupled mode theory. It is found that the surface waveguide modes essentially stem from polarization-dependent mode transition induced by the dielectric silicon film, including inter mode transition dependent on two adjacent modes for transverse magnetic (TM) polarization and intra mode transition related to only the mode itself for transverse electric (TE) polarization. This is confirmed by the dispersion characteristics, field localization and reflectivity/absorptivity of the metal-silicon composite. Tuned by the silicon film, different surface waveguide modes from low- to high-order are excited. When these surface waveguide modes are phase-matched with the core mode in the TFG, flexibly tunable TM-/TE-polarized SWRs are attained in a wide wavelength range. This work provides new prospects and general guidelines for developing novel robust fiber-optic SWR-based devices with huge potential applications.