A self-reference sensing technique for ultra-sensitive chemical and biological detection using whispering gallery microresonators

Ultra-sensitive and label-free chemical and biological sensing devices are of great importance to biomedical research, clinical diagnostics, environmental monitoring, and homeland security applications. Optical sensors based on ultra-high-quality Whispering-Gallery-Mode (WGM) micro-resonators, in which light-matter interactions are significantly enhanced, have shown great promise in achieving compact sensors with high sensitivity and reliability. However, traditional sensing mechanisms based on monitoring the frequency shift of a single resonance faces challenges since the resonant frequency is sensitive not only to the sensing targets but also to many types of disturbances in the environment, such as temperature variation and mechanical instability of the system. The analysis of the signals is also affected by the positions of sensing targets on the resonator. Thus, it is difficult to distinguish signals coming from different sources, which introduces 'false positive' detection. We report a novel self-reference sensing mechanism based on mode splitting, a phenomenon in which a high-quality optical mode in a WGM resonator splits into two modes due to intra-cavity Rayleigh scattering. In particular, we demonstrated that the two split modes that can be induced by a single nanoparticle reside in the same resonator and serve as a reference to each other. As a result, a self-reference sensing scheme is formed. This allows us to develop a position-independent sensing scheme to accurately estimate the sizes of nanoparticles. So far we have achieved position-independent detecting and sizing of single nanoparticles down to 20 nm in radius with a single-shot measurement using an on-chip high-quality WGM microtoroid resonator.