Ponderomotive spectroscopy: Driving Rydberg transitions using harmonics and magic wavelengths of an intensity-modulated optical lattice

We describe recent developments in a novel spectroscopic method that couples Rydberg states using an intensity-modulated optical lattice. The method is fundamentally different from traditional microwave spectroscopy: it engages the $\mathbf{A} \cdot \mathbf{A}$ (ponderomotive) term rather than the $\mathbf{A} \cdot \mathbf{p}$ term of the atom-field interaction Hamiltonian, allowing us to drive microwave transitions between Rydberg states with optical spatial resolution, free from electric dipole selection rules.\footnote{KR Moore, SE Anderson, G Raithel, Nat Comm, 6:6090 (2015)} Experimentally, cold Rb Rydberg atoms are confined in a 1064nm optical lattice.\footnote{SE Anderson, KC Younge, G Raithel, PRL 107:263001 (2011)} Transitions are driven by modulating the lattice intensity using a tunable electro-optic fiber modulator. Recently we have driven dipole-forbidden transitions in third and fifth order, at frequencies up to 94 GHz, using temporal harmonics in the intensity-modulated lattice. We also demonstrate, for two separate transitions, the novel use of a magic wavelength condition in ponderomotive spectroscopy. We discuss experimental results and propose applications of this method to a precision measurement of the Rydberg constant using circular-state Rydberg atoms.