We show that the optical trapping of dielectric particles by a single focused beam in front of a weakly ref
lective surface is considerably affected by interference of the incident and ref lected beams, which creates
a standing-wave component of the total field. We use the two-photon-excited f luorescence from a trapped
dyed probe to detect changes in the distance between the trapped beam focus as the focus approaches the
ref lective surface. This procedure enables us to determine the relative strengths of the single-beam and
the standing-wave trapping forces. We demonstrate that, even for ref lection from a glass’water interface,
standing-wave trapping dominates, as far as 5 mm from the surface.