The light confinement properties of high quality (Q) factor microtoroid whispering-gallery mode (WGM) optical resonators prevent efficient coupling between far-field radiation and the WGM. Instead, light is most commonly evanescently coupled to the WGM using optical fibers that have been tapered to micron-scale thickness. These tapers, however, break easily and are sensitive to environmental vibrations and fluid flow fluctuations. This limits their effectiveness in mass-produced and/or field-portable biochemical sensing applications. Here we present a gold nanorod grating as an experimentally-feasible alternative for robust coupling of free-space light to a microtoroid resonator, and we simulate its performance with a novel finite-element 3D beam envelope method. 3D simulations of the full system are not tractable due to its large size. Previously, simulations of nanostructures on microtoroids have been performed on a thin wedge of the 3D system with perfect electrical conductor (mirror) boundary conditions. While these simulations provided some insight, they do not accurately model typical travelling-wave WGM experiments because they can only simulate standing waves. The standing wave nodes and antinodes significantly alter interactions between the WGM and the nanostructure. In our new method, we use a small wedge domain with custom boundary conditions that accurately simulate the travelling wave and nanophotonic interactions. Using this approach, we have designed and simulated a grating for far-field WGM coupling. With the grating, it is possible to maintain a high Q-factor of 3×10^6. We anticipate that our proposed modeling approach can solve a variety of other nanoparticle-microtoroid coupled systems in the future.
Subwavelength systems such as optical nanoantennas are widely used for optical sensing due to their ultrahigh field localization. Compared to isolated nanoantennas, hybrid sensor systems composed of optical nanocavities and microcavities enjoy higher quality factor (Q) plasmonic-cavity modes, as well as larger resonance shifts for any given sensing target. We have shown that rational engineering of the coupling between nanoantennas can maximize the system’s sensitivity. This can be achieved through near-field optimization of the system to maximize the field enhancement and suppression of the far-field radiation to maintain the highest possible Q. Finite element eigenvalue analysis shows that a trimer plasmonic nanoantenna coupled to a whispering gallery mode (WGM) of a microtoroid cavity supports higher Q and field enhancement than single nanorods that are randomly scattered on the surface of microcavity. We have studied the robustness of this system against any possible perturbation in geometry of trimers such as length, angle or gap between the nanoantennas. On the basis of this study, a general design approach is introduced, which helps engineers to enhance the efficacy of plasmonic-photonics based biosensors.
Local field enhancement of plasmonic nanoantennas below the diffraction limit plays an important role in a variety of applications, including surface-enhanced Raman scattering, spontaneous emission enhancement, nanolasing, enhanced nonlinear effects and biosensing. Yet due to the radiation and ohmic loss of these nanocavities, their quality factor (Q) is much smaller than a typical optical microcavity Q factor, such as that of a microsphere or microtoroid. Coupling a nanoantenna to an optical microcavity increases the Q of the hybrid plasmonic-photonic system, however, this dramatically degrades the Q of the original microcavity. Here, we propose a judicious hybridization of a plasmonic dark mode of a gold nanoring and whispering gallery mode (WGM) of a microtoroid. It is shown through finite element simulation that the hybridized WGM and dark mode of the proposed plasmonic gold nanoring solves the aforementioned issues in two ways. First, the small radiation loss of the dark mode minimizes Q degradation and allows the system to maintain its ultra-high Q. Second, the nanoring enhances the field on the microcavity’s surface which in turn increases the interaction between, for example, a biomolecular target and the WGM. We have shown that the proposed system generates larger resonance shifts compared to a microcavity loaded with same volume of conventional linear gold nanoantennas . This results in significant enhancement in the system’s sensitivity. We have repeated the same simulations for different materials and volumes.