Using Optical Tweezers to Simultaneously Trap, Charge, and Measure the Charge of a Microparticle in Air

Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a ∼1  μm SiO_{2} sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.