Sub-pg mass sensing and measurement with an optomechanical oscillator.

Mass sensing based on mechanical oscillation frequency shift in micro/nano scale mechanical oscillators is a well-known and widely used technique. Piezo-electric, electronic excitation/detection and free-space optical detection are the most common techniques used for monitoring the minute frequency shifts induced by added mass. The advent of optomechanical oscillator (OMO), enabled by strong interaction between circulating optical power and mechanical deformation in high quality factor optical microresonators, has created new possibilities for excitation and interrogation of micro/nanomechanical resonators. In particular, radiation pressure driven optomechanical oscillators (OMOs) are excellent candidates for mass detection/measurement due to their simplicity, sensitivity and all-optical operation. In an OMO, a high quality factor optical mode simultaneously serves as an efficient actuator and a sensitive probe for precise monitoring of the mechanical eigen-frequencies of the cavity structure. Here, we show the narrow linewidth of optomechanical oscillation combined with harmonic optical modulation generated by nonlinear optical transfer function, can result in sub-pg mass sensitivity in large silica microtoroid OMOs. Moreover by carefully studying the impact of mechanical mode selection, device dimensions, mass position and noise mechanisms we explore the performance limits of OMO both as a mass detector and a high resolution mass measurement system. Our analysis shows that femtogram level resolution is within reach even with relatively large OMOs.

Thermo-optomechanical oscillator for sensing applications.

We demonstrate and characterize a thermo-optomechanical oscillator based on a PMMA-coated silica microtoroid and employ it as a sensor. The observed thermo-optomechanical oscillation has a unique waveform that consists of fast and slow oscillation periods. A model based on thermal and optical dynamics of the cavity is used to describe the bi-frequency oscillation and experiments are conducted to validate the theoretical model in order to explore the origin of the two oscillatory phenomena. As opposed to previously shown hybrid toroidal microcavities, the excessive PMMA coating boosts the thermo-mechanical (expansion) effect that results in bi-frequency oscillation when coupled with the thermo-optical effect. The influences of the input power, quality factor, and wavelength detuning on oscillation frequencies are studied experimentally and verified theoretically. Finally the application of this oscillator as a sensor is explored by demonstrating the sensitivity of oscillation frequency to humidity changes.

Application of dynamic line narrowing in resonant optical sensing.

We propose a dynamic operational mode and the resulting dynamic line narrowing as a method for enhancing the resolution and the detection limit of high-quality (high-Q) resonant optical sensors. Using a silica microtoroid as an experimental platform, we demonstrate that dynamic line narrowing through the thermo-optic effect can significantly improve the detection limit in both resonant shift and resonance splitting operating modes.