Meridian whispering gallery modes sensing in a sessile microdroplet on micro/nanostructured superhydrophobic chip surfaces

A liquid microdroplet could be a naturally simple, miniaturized and effective optical cavity by itself due to the intriguing optofluidic properties associated with its surface tension-induced spherical shape. It had been shown that optical whispering gallery modes (WGMs) can be present along the circular rim in the equatorial plane of a sessile microdroplet, and this phenomenon had been leveraged for biosensing demonstrations. However, optical coupling to such equatorial modes for their excitation and monitoring is mostly based on either tapered fiber coupling or free-space beam coupling, each of which demandingly requires precise alignment of the tapered fiber or the free-space beam adjacent to the equatorial surface of the resonator. In this paper, we show that WGMs could also be stimulated along the meridian plane of a liquid microdroplet resting on a properly designed nanostructured chip surface. The geometrical morphology and optical characteristics of a microdroplet cavity are critical to achieve a high-quality Q factor and therefore to realize high-resolution in situ and in vivo monitoring of trace analytes. The unavoidable deformation along the meridian rim of the sessile microdroplet can be controlled and regulated by tailoring the nanopillar structures and their associated hydrophobicity. The nanostructured superhydrophobic chip surface and its impact on the microdroplet morphology are modeled by Surface Evolver, which is subsequently validated by the Cassie–Wenzel theory of wetting. The influence of the microdroplet morphology on the optical characteristics of WGMs is further numerically studied using the finite-difference time-domain method and it is found that meridian WGMs with intrinsic quality factor Q exceeding 104 can exist. Importantly, such meridian WGMs can be efficiently excited by a waveguiding structure embedded in the planar chip, which could significantly reduce the overall system complexity by eliminating conventional mechanical coupling parts. Our simulation results also demonstrate that this optofluidic resonator can achieve a sensitivity as high as 530 nm/RIU. This on-chip coupling scheme could pave the way for developing lab-on-a-chip resonators for high-resolution sensing of trace analytes in various applications ranging from chemical detections, biological reaction processes to environmental protection.