Electromagnetically induced transparency in an inverted-γ system of interacting cold atoms

We study the transmitted intensity and correlation properties of a probe field traveling through a sample of interacting cold Rb-87 atoms driven into the inverted gamma configuration. Two windows of electromagnetically induced transparency are found in the transmission spectrum, which are either sensitive or immune to the input probe intensity. This means that one window is accompanied by cooperative optical nonlinearity while the other is a result of a linear optical response. Moreover, we find sufficiently suppressed two-photon correlation within the nonlinear window and greatly enhanced two-photon correlation between the linear and nonlinear windows. In the case of the degenerate window, however, it is viable to conceal the cooperative optical nonlinearity behind the linear optical response. This then allows us considerable flexibility to manipulate the propagation and evolution of a quantum light field.