Far-field single nanoparticle detection and sizing

Whispering gallery modes (WGMs) based optical microcavities are important for highly sensitive optical sensing. However, the current experimental realizations are strongly dependent on the high-resolution tunable lasers and evanescent coupling, which are too cumbersome and too expensive for portable devices. Herein we experimentally demonstrate a cost-effective and robust approach to detect and size a single nanoparticle with far-field laser emissions. By placing a limacon microdisk close to a spiral microdisk, chiral resonances have been successfully generated. In contrast to previous researches, here the internal chirality is strongly correlated with the far-field patterns (FFPs) and thus can be transduced to far-field emissions. Once a nanoparticle is attached onto the limacon microdisk, the asymmetrical backscattering at the notch of spiral can be averaged by the symmetrical scattering of the nanoparticle. Consequently, the internal chirality and the corresponding FFPs are changed. By measuring the far-field directional laser emission, nanoparticles with radius~50 nm have been successfully detected and sized without employing any spectral information for the first time. As narrow-linewidth tunable laser is not used in our experiment and microdisks possibly lase with electrical driven, this research shall route a new way to cost-effective, portable, highly sensitive optical sensors.