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In this paper, a novel nano sensor structure is proposed, which consists of a metal-insulator-metal waveguide (MIM) with rectangular baffles and a water droplet cavity with rectangular stubs (WDCRS). The WDCRS structure optimizes the sensitivity of a single water droplet cavity and makes the transmission curve clearer and smoother. The transmission characteristics of WDCRS structure were simulated using finite element method (FEM). The transmission characteristics of the exported structure were analyzed in detail. In addition, the influence of structural geometric parameters on sensing performance was also studied, and it was found that the size of the water droplet cavity is a key factor in improving sensitivity. When applied to a refractive index sensor, the structure achieves a sensitivity of up to 2,300 nm/ RIU with a corresponding figure of merit (FOM) of 60.5. These works provide some ideas for the design of high-performance nanostructures and multiple Fano resonance excitation structures. [ABSTRACT FROM AUTHOR]

Abstract We propose and theoretically study a silica double-toroid microcavity to confine the whispering-gallery mode (WGM) in an ultra-small space, which consists of two toroid-to-toroid coupled cavities separated by a nanoscale gap region with low refractive index, such as the air gap. Benefitting from the strong field localization of the "slot" effect, power enhancement of symmetric WGMs in the gap results in the ultra-small mode volume of 4. 8 μ m which is quite smaller than the conventional toroidal microcavity with mode volume of about hundreds of cubic micrometers. The confined modes hold the potential advantages over conventional photonic devices, especially in the applications of the sensing and optical trapping. In the application of refractometer, a high refractive index sensitivity of up to 468 nm per refractive index unit (nm/RIU) can be obtained in the 1000 nm wavelength band. In the optical trapping application, the double-toroid microcavity enables a significant field enhancement in the confined WGM, and the gradient force could reach as high as 22 pN/W for a single nanoparticle with the radius of 5 nm. As the potential advantages, this study of silica double-toroid microcavity provides a good reference for realizing the high-efficient photonic applications such as optical sensing and trapping. Highlights • Confined whispering-gallery mode is proposed in the double-toroid cavity. • A refractometer with the sensitivity of up to 468 nm/RIU is yielded. • The optical trapping with the gradient force of 22 pN/W is proposed. [ABSTRACT FROM AUTHOR]

We propose and numerically investigate a fiber-optic sensor comprised by core-offset splicing up to 20 equal-length single-mode fiber (SMF) segments, which is used for the high-sensitivity refractometric sensing owing to the open cavities with much light propagating outside the optical fiber. The emergence of high-order modes in the silica cladding and air enhances the multi-mode interference, therefore improving the device sensitivity. Because of the small core-offset distance between fiber segments and suitable fiber segment length, enhanced light scattering reduces the degeneracy and overcomes free-spectrum-range limitation in that case of Fabry–Perot interferometer (FPI) with equal fiber segments, resulting in the large wavelength tuning range. Furthermore, silver (Ag) particles, Ag rod and Ag microsphere placed on the core-offset region, respectively, further enhance the light intensity outside the fiber owing to the reconfiguration and refocusing of optical light after the silver particles, then further improving the sensitivity of refractometric sensing. Up to 1078 nm per refractive index unit (nm/RIU) could be obtained based on 6 μ m -diameter Ag microsphere-assisted fiber-optic sensor with 10 fiber segments. • Multiple fiber segments are core-offset spliced for refractometric sensing. • Silver particle are placed in the core-offset region to enhance the RI sensitivity. • The refractometer with the sensitivity of 1078 nm/RIU is yielded. [ABSTRACT FROM AUTHOR]