Enhanced Kerr Nonlinearity via Atomic Coherence in a Three-Level Atomic System

We measure the Kerr-nonlinear index of refraction of a three-level $\Lambda$-type atomic system inside an optical ring cavity. The Kerr nonlinearity is modified and greatly enhanced near atomic resonant conditions for both probe and coupling beams. The Kerr nonlinear coefficient $n_2$ changes sign when the coupling beam frequency detuning switches sign, which can lead to interesting applications in optical devices such as all-optical switches.