  Optical tweezers enable non-contact trapping of micro-scale objects using
light. Despite their widespread use, it is currently not known how tightly it
is possible to three-dimensionally trap micro-particles with a given photon
budget. Reaching this elusive limit would enable maximally-stiff particle
trapping for precision measurements on the nanoscale, and photon-efficient
tweezing of light-sensitive objects. Here we solve this problem by customising
a trapping light field to suit a specific particle, with the aim of
simultaneously optimising trap stiffness in all three dimensions. Initially
taking a theoretical approach, we develop an efficient multi-parameter
optimisation routine to design bespoke optical traps for a wide range of
micro-particles. We show that the confinement volume of micro-spheres held in
these sculpted traps can be reduced by one-to-two orders-of-magnitude in
comparison to a conventional optical tweezer of the same power. We go on to
conduct proof-of-principle experiments, and use a wavefront shaping inspired
strategy to suppress the Brownian fluctuations of optically trapped
micro-spheres in every direction concurrently, thus demonstrating
order-of-magnitude reductions in their confinement volumes. Our work paves the
way towards the fundamental limits of optical control over the mesoscopic
realm.
