Two-Color Optical Nonlinearity in an Ultracold Rydberg Atom Gas Mixture

We report the experimental observation of strong two-color optical nonlinearity in an ultracold gas of ${}^{85}\mathrm{R}\mathrm{b}$-${}^{87}\mathrm{R}\mathrm{b}$ atom mixture. By simultaneously coupling two probe transitions of ${}^{85}$Rb and ${}^{87}$Rb atoms to Rydberg states in electromagnetically induced transparency (EIT) configurations, we observe significant suppression of the transparency resonance for one probe field when the second probe field is detuned at $\sim 1\mathrm{G}\mathrm{H}\mathrm{z}$ and hitting the EIT resonance of the other isotope. Such a cross-absorption modulation to the beam propagation dynamics can be described by two coupled nonlinear wave equations we develope. We further demonstrate that the two-color optical nonlinearity can be tuned by varying the density ratio of different atomic isotopes, which highlights its potential for exploring strongly interacting multi-component fluids of light.