Detection of plasmonic nanoparticles using whispering gallery mode resonators

Summary form only given. The emergence of whispering gallery mode (WGM) resonators as miniaturized environmental sensors has enabled ultra-sensitive detection of nanoparticles and un-labeled biomolecules in solution [1]. Although one report has demonstrated single molecule sensitivity [1], the mechanism by which this was achieved is not fully understood and the results have not been repeated [2]. Thus many efforts have been made to either enhance the signal (e.g., via plasmonic enhancement [2]) or decrease noise (e.g., with interferometry [3]) in order to achieve single molecule detection. Our work focuses on plasmonic enhancement to enhance signal-to-noise (SNR). In this paper, we demonstrate real-time detection of gold (Au) nanorods with a silica microtoroid stabilized using the well-known Pound-Drever-Hall (PDH) technique [4]. The PDH technique utilizes nulled lock-in detection, and is insensitive to amplitude noise from the laser. In this scheme, 10 nm nanorods (aspect ratio of 4:1) can be detected with a SNR of 11.25, even with a relatively low resonator Q factor of 6 × 10⁵. The volume of these nanoparticles are comparable with the smallest nanoparticles detected to date using WGM devices [3].