Confined Gaussian-distributed electromagnetic field of tin(II) chloride-sensitized surface-enhanced Raman scattering (SERS) optical fiber probe: From localized surface plasmon resonance (LSPR) to waveguide propagation.

Surface-enhanced Raman scattering (SERS) induced by largely enhanced electromagnetic (EM) field provides a solid and promising avenue for ultrasensitive molecular detection. Here, a confined Gaussian-distributed EM field for SERS fiber probe with two influencing factors (localized surface plasmon resonance (LSPR) of silver and waveguide propagation of optical fiber) are proposed for the first time. SERS fiber probes with high sensitivity and good reproducibility were synthesized via a novel SnCl2 sensitization aided solvothermal method. The influencing factors and EM field distribution are investigated experimentally and theoretically. The LSPR-induced EM enhancement is observed. By introducing a sensitization procedure, silver particles show smaller sizes and narrower interparticle gaps, significantly influencing the LSPR and EM enhancement of the SERS fiber probe. Moreover, a unique waveguide-propagation-induced EM enhancement is brought up. Waveguide propagation modes of optical fibers influence the intensity and enhancement area of EM field. Further, the EM field distribution of SERS fiber probe is studied. It exhibits a concentrically-increased intensity gradient that is confined in core area with maximum enhancement at fiber core center. This confined Gaussian-distributed configuration of EM field on SERS fiber probe facet is induced by the LSPR of plasmons and waveguide propagation of optical fiber.

Investigation on single taper-based all-solid photonic bandgap fiber modal interferometers.

We demonstrate a single taper-based all-solid photonic bandgap (AS-PBG) fiber modal interferometer that consists of a central tapered fiber region connected to the untapered via two abrupt transitions. Modal interference is given by superimposing the bandgap-guided fundamental core mode with a lower effective index and a specific index-guided cladding supermode with a higher effective index. A series of interferometers with taper diameter of 50µm ~60µm and device length of ~3mm are fabricated and studied in contrast to the conventional counterparts. The temperature coefficient of the interferometer is closely determined by the fraction of the cladding supermode energy localized within the index-raised regions of the fiber. The refractive index (RI) responsivities associated to fiber taper sizes are investigated. The measured maximal RI sensitivity is ~3512.36nm/RIU at the taper diameter of 50µm around RI = 1.423. This research gives a deep understanding to the modal-interferometric AS-PBG structure, which we believe to be valuable for the future application of the related device.