Lensing effect of electromagnetically induced transparency involving a Rydberg state

We study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud, and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is de-focused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations.