Cold-atom confinement in an all-optical dark ring trap

We demonstrate confinement of ⁸⁵Rb atoms in a dark, toroidal optical trap. We use a spatial light modulator to convert a single blue-detuned Gaussian laser beam to a superposition of Laguerre-Gaussian modes that forms a ring-shaped intensity null bounded harmonically in all directions. We measure a $1∕e$ spin-relaxation lifetime of ≈1.50.3em0exs for a trap detuning of 4.00.3em0exnm. For smaller detunings, a time-dependent relaxation rate is observed. We use these relaxation rate measurements and imaging diagnostics to optimize trap alignment in a programmable manner with the modulator. The results are compared with numerical simulations.