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

We demonstrate confinement of $^{85}\mathrm{Rb}$ 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 $\ensuremath{\approx}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for a trap detuning of $4.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. 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.