Observation of the discrete transition of optically trapped particle position in the vicinity of an interface

Discrete transitions in the position of a particle optically trapped in the vicinity of an interface are observed using a nanometer position sensing system. These discrete transitions in the position with distance from the interface are attributed to interference effects due to reflection and scattering of the trapping laser beam from the interface and the particle. The discrete transitions occurred at uniform intervals of $∼400 nm,$ corresponding to the spacing of standing wave nodes of the trapping laser beam.