Free-space coupling of nanoantennas and whispering-gallery microcavities with narrowed linewidth and enhanced sensitivity

Due to the broad scattering spectral profiles, localized surface plasmon resonances (LSPRs) of Pd nanoparticles have low resolution and limited sensitivity for hydrogen detection. In this work, we use a simple light-irradiation method to demonstrate that free-space light can be efficiently coupled into and from the microfiber whispering-gallery modes (WGMs) by the Pd nanoantennas. The nanoantenna-microfiber cavity system provides strong intermodal coupling between LSPRs and WGMs, and induces significant modulation of the scattering spectra. A measured full width at half-maximum of 3.2 nm at 622.7 nm is obtained, which is the narrowest in Pd nanoparticle-based LSPR structures reported up to now. The ultranarrow resonances offer enhanced sensitivity to hydrogen gas detection with a figure of merit reaching similar to 2.22. Other advantages of the Pd nanoantenna-microfiber cavity system including independence of precise alignment of excitation light, large tunability of the resonant wavelengths, easy and low-cost fabrication of the system, have also been demonstrated.