Logan Hillberry, UT-Austin

"Optically trapped microspheres as sensors of mass and sound: Brownian motion as both signal and noise"

Abstract: Owing to their small size, advanced position detection possibilities, and accurate theoretical description, optically trapped microspheres have become a paradigmatic system for myriad sensing applications. This dissertation reports two air-based experiments that leverage the unique properties of optically-trapped microspheres as measurement tools: inertial mass sensing and sound detection.

We measure the mass of a microsphere in three ways. Careful error analysis allows quantitative comparison between our method and others appearing in the recent literature. As figures of merit, we focus on accuracy, precision, and speed. We find that monitoring the variance of the microsphere's velocity degree of freedom while undergoing equilibrium Brownian motion enables measurement of our microsphere's ~25 picogram mass with 4.3% accuracy and 1.6% precision across 14 vastly different trapping laser powers and using 10x less data than our most accurate (3.2%) and precise (0.9%) method. The more-accurate method is in fact a calibration step that must always be performed initially, but the microsphere's velocity variance may subsequently be monitored, thereby elevating mass to a dynamic measurement variable.

For sound detection, we develop a model for the sensitivity of a microsphere's velocity to an external acoustic perturbation. In this case, the microsphere's Brownian motion is a noise source that must be overcome for a signal to be detectable. We validate our method by comparing measurements of pure-tone bursts between our system and two state-of-the-art, commercially-available acoustic sensors. We then demonstrate the microsphere's advantage in measuring high-frequency-content signals using impulsive sounds generated by laser ablation. We resolve an acoustic rise time of 1 microsecond on the same signal that our high-bandwidth microphone measures a 7 microsecond rise time. At the same time, our microsphere-based sensor's higher bandwidth resolves a nearly 3x larger peak pressure than the microphone.

This dissertation builds towards these two experimental results by first contextualizing them in a non-technical historical review. Key technical background is then developed pedagogically, followed by details of the trapping and detection apparatus. After the experiments are reported, we conclude with a summary of the results and an outlook on the future of optically trapped microspheres as sensitive detectors.