Optical trapping of mesoscopic particles can be used to make high exquisite, precision measurements. This field has seen a surge of activity in the last few decades and spans studies across the sciences. The paper by Raj, Schaich and Dragnea specifically explores the orbiting motion of particles in a counter-propagating trap geometry. Such orbiting can arise due to an angular or translational offset between the beams. Analysis of the orbital motion can be used for high resolution studies in aerosol science.

A key to this advance is the use of an original trapping scheme that uses lenses instead of fibers, which gives enhanced control over the distribution of optical forces. This lensed dual-beam trap geometry allows the particle to perform orbits that are largely insensitive to initial trapping parameters (such as where the particle is inserted and its initial velocity). The authors discovered that spectral features associated with such orbital trapping showed narrower orbital frequencies than the frequencies associated with more conventional trapping geometries. Any change in orbital frequency, which varied nearly linearly with particle size, is indicative of miniscule changes of the properties of the aerosol particle. This may thus be used to probe the real time kinetics of surface reactions on a particle levitated in a gas. An example is the highly sensitive in-situ detection of mass deposited upon or removed from a particle: the authors’ studies showed how a particle that changes its radius by a mere 0.5 nm (0.06% change in mass) could be discerned. The study is an exciting step forward for the use of trapping in precision measurements for aerosol science.

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