Cavity Sensing – Sensor Net

We observed the optomechanical oscillation of a whispering gallery mode (WGM) microcavity, a silica microsphere, in liquid. Thanks to its high quality (Q) factor and small mode volume, the accumulated high intensify of the cavity on optical resonance leads to an radiation force, which is sufficient to amplify the mechanical motion of the cavity and trigger the regenerative oscillation. By keeping the pump laser at a constant optical power within a certain range of wavelength near the resonance, the mechanical oscillation of the optical resonator is achieved under the highly dissipative environment. Furthermore, the spectra indicate a high stability of such an optomechanical oscillation for cavities immersed in either heavy water or buffered solution.

One of the interesting finds about the cavity optomechanical oscillation is that, the optical field inside the cavity is able to produce an effective mechanical rigidity like a spring. Also, our experimental data shows that the effective rigidity of the mechanical mode is dominated by the optically induced spring in the cavity and the intrinsic mechanical rigidity plays a fairly minor role. Therefore, the optomechanical frequency sensitively depends on the laser-cavity detuning. Such a sensitive optical-to-mechanical frequency transduction means that a cavity resonance wavelength shift induced by a particle binding event can be transduced to an mechanical frequency change that is much larger than its oscillation linewidth. By utilizing the optical spring effect, we further enhance the sensing resolution compared with conventional approaches, allowing us to detect single bovine serum albumin proteins at a high signal-to-noise ratio.

Cavity optomechanical spring and its RF spectra in heavy water

Optical resonance shift is transduced to mechanical shift through the optical spring

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