High-Q Whispering-Gallery-Mode-Based Plasmonic Fano Resonances in Coupled Metallic Metasurfaces at Near Infrared Frequencies

Fano resonances in metallic and dielectric structures have received much attention in recent years due to their promising applications in surface enhanced phenomena, sensing, and nonlinear optics. The lossless high refractive index dielectric structures have been shown to have ultranarrow resonances at near infrared and optical frequencies. However, it is rather challenging to realize such narrow Fano resonances using metallic nanostructures due to large optical losses in metals that cause significant broadening of the plasmonic resonances. Herein, whispering gallery mode based sharp Fano resonances are demonstrated experimentally and numerically in plasmonic nanostructures at near infrared frequencies with highest quality factor. The design here consists of conductively coupled annular and rectangular aperture arrays that support multiple Fano resonances. The sharp Fano resonances are observed for both the orthogonal polarizations of the incident electric field due to the coupling between quadrupolar resonance of the annular aperture with the dipolar and quadrupolar resonance of the rectangular aperture. The response of the plasmonic metasurface can be switched from single to multiple Fano resonances by switching the polarization of the incident radiation which is highly desirable for hyperspectral sensing and imaging. The spectral tuning of Fano resonances are further achieved by tailoring the coupling between the annular and the rectangular apertures.