  We demonstrate a bistable optical trap by tightly focusing a vortex laser
beam. The optical potential has the form of a Mexican hat with an additional
minimum at the center. The bistable trapping corresponds to a non-equilibrium
steady state (NESS), where the microsphere continually hops, due to thermal
activation, between an axial equilibrium state and an orbital state driven by
the optical torque. We develop a theoretical model for the optical force field,
based entirely on experimentally accessible parameters, combining a Debye-type
non-paraxial description of the focused vortex beam with Mie scattering by the
microsphere. The theoretical prediction that the microsphere and the annular
laser focal spot should have comparable sizes is confirmed experimentally by
taking different values for the topological charge of the vortex beam.
Spherical aberration introduced by refraction at the interface between the
glass slide and the sample is taken into account and allows to fine tune
between axial, bistable and orbital states as the sample is shifted with
respect to the objective focal plane. We find an overall agreement between
theory and experiment for a rather broad range of topological charges. Our
results open the way for applications in stochastic thermodynamics as it
establishes a new control parameter, the height of the objective focal plane
with respect to the glass slide, that allows to shape the optical force field
in real time and in a controllable way.
