Spectroscopy of Rydberg atoms in a ponderomotive optical lattice

We study Rydberg atoms in ponderomotive optical lattices. Unlike for ground-state atoms, for Rydberg atoms in an optical lattice, the extent of the electronic wave-function can approach the lattice period. This leads to state-dependent adiabatic trapping potentials that are unique to Rydberg atoms. We use microwave spectroscopy to experimentally demonstrate and investigate the state-dependence of the adiabatic potentials of S_1/2 Rydberg states of rubidium. The observed microwave spectra depend strongly on both the principal quantum number and the depth of the lattice. A semi-classical simulation is used to interpret the features seen in the microwave spectra. A large fraction of the Rydberg atoms can be trapped in the ponderomotive optical lattice.