Ultra-high resolution mass sensing based on an optomechanical nonlinearity.

Ultra-high resolution mass sensing used to be realized by measuring the changed mechanical oscillation frequency by a small mass that should be detected. In this work we present a different approach of mass sensing without directly measuring such mechanical frequency change but relying on the modified light field due to a previously less explored nonlinear mechanism of optomechanical interaction. The concerned optomechanical setup used for the mass sensing is driven by a sufficiently strong two-tone field satisfying a condition that the difference of these two drive frequencies matches the frequency of the mechanical oscillation, so that a nonlinear effect will come into being and lock the mechanical motion under the radiation pressure into a series of fixed orbits. A small mass attached to the mechanical resonator slightly changes the mechanical frequency, thus violating the exact frequency match condition. Such small change can be detected by the amplitude modification on the higher order sidebands of the cavity field. Even given a moderate mechanical quality factor for the setup, the added mass can still be detected to the levels corresponding to a mechanical frequency shift from 5 to 7 order less than the mechanical damping rate. Because the output cavity field difference for very close values of mechanical frequency is not blurred by thermal noise, such mass sensing can be well performed at room temperature. The previous tough requirements for ultra-high resolution mass sensing can be significantly relaxed by the method.