Microwave Dressing of NaCs Molecules*

We report on studies of microwave-dressed ultracold sodium-cesium (NaCs) molecules. Using a phased antenna array, we dress the NaCs molecules with strong and circularly polarized microwave fields. We present the design of the array, with which we achieve up to 46 MHz Rabi coupling between the lowest two rotational states, and 4 degree polarization ellipticity. We demonstrate that, under the dressing field, the properties of the molecules fundamentally change. The inelastic loss is reduced by a factor of 200. The elastic cross-section increases because of the dipolar nature of the collisions. The optical ac polarizability of molecules changes. This change creates a magic transition between the two dressed states, even though no such magic transition exists in the bare molecule. These studies demonstrate that dressed molecules are fundamentally different from undressed ones, and encourage further investigations of this kind.