Techniques for Mass Determination of Airborne Micro-Particles in an Optical Dipole Force Trap

We present the development of a new technique for rapidly measuring the masses of airborne particles confined in free space optical dipole force traps. This technique employs an ultrafast CMOS sensor with a wide field of view to image the real time motion of trapped particles on timescales in which diffusive Brownian motion predicted by Einstein makes a transition to ballistic motion. The technique relies on direct imaging of drop-and-restore experiments without the need for a vacuum environment. In these drop-and-restore experiments, the trapping light is rapidly shuttered with an acousto-optic modulator causing the particle to be released from and subsequently recaptured by the trapping force. The trajectories from the falls and restorations are combined to infer the particle mass, which we also corroborate using an analysis of position autocorrelation functions of the trapped particles. Using the drop-and-restore technique, we report a statistical uncertainty of less than 2% for masses on the order of 510^(14) kg, using a data acquisition time of approximately 90 seconds. We also show that the measurement of a ballistic mean-squared displacement can be used for a preliminary estimate of the mass of a trapped micro-particle. Furthermore, we show that even the capacity to detect the timescale at which the transition to ballistic motion occurs, in combination with a measurement of particle size, can provide a similar mass estimate, consistent with the more rigorous mass determinations developed in this thesis. Ultimately, the methods presented here constitute simple, effective, and competitive alternatives for characterizing trapped particles and measuring their masses, in comparison with more elaborate standard techniques.