Spatial correlations between Rydberg atoms in an optical dipole trap

We use direct spatial ion imaging of cold Rb-85 Rydberg atom clouds to measure the Rydberg-Rydberg correlation function, with and without light-shift potentials generated by an optical dipole trap. We find that the blockade radius depends on laser detunings and spatially varying light shifts. At certain laser detunings the probability of exciting Rydberg atoms at particular separations is enhanced, which we interpret to be a result of direct two-photon excitation of Rydberg atom pairs. The results are in accordance with predictions [F. Robicheaux and J. V. Hernandez, Phys. Rev. A 72, 063403 (2005)] and a model we develop that accounts for a one-dimensional dipole-trap potential.