Optical trapping is a non-invasive single-particle analytical technique that has found applications in numerous scientific areas, from single cell manipulation to ultra-precise sensing. Compared to single beam traps, counter-propagating dual-beam traps offer enhanced trap stiffness along the direction of laser propagation. In this presentation we show how, by adding a transverse offset between the counter-propagating beams in a lensed, dual-beam trap made of a pair of focused, near-IR laser beams, the optical force field can be engineered to induce a stable periodic orbital motion of a dielectric microsphere. Key relationships between particle dynamics and optical parameters have been evaluated both experimentally and theoretically. We find that the power spectral distribution of scattered light from an orbiting particle has narrow linewidths ((∂ν/ν = 10<sup>-3</sup>), at least two orders of magnitude lower than the linewidths accessible with traditional trapping schemes in air, which makes them better candidates for sensing applications. With the help of numerical simulations, we discuss how the orbital trapping technique can be further optimized to enhance the sensitivity of a particle to changes in the physical parameters of the surrounding medium. The evolution of orbital frequencies can be a useful signature in analyzing the kinetics of deposition or loss of materials from the surface of levitated particles in a controlled environment. Hence, we predict that our experimental scheme could find application as an <i>in-situ</i> single-particle technique for probing reactions relevant to atmospheric chemistry.

Orbital optical trapping of microspheres at atmospheric pressure

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ACS Fall 2022

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