  A transient and high sensitivity sensor based on high-Q microcavity is
proposed and studied theoretically. There are two ways to realize the transient
sensor: monitor the spectrum by fast scanning of probe laser frequency or
monitor the transmitted light with fixed laser frequency. For both methods, the
non-equilibrium response not only tells the ultrafast environment variance, but
also enable higher sensitivity. As examples of application, the transient
sensor for nanoparticles adhering and passing by the microcavity is studied.
It's demonstrated that the transient sensor can sense coupling region, external
linear variation together with the speed and the size of a nanoparticle. We
believe that our researches will open a door to the fast dynamic sensing by
microcavity.
