Variable Kerr nonlinearity and optical bistability of a four-level lambda atomic medium

Abstract

Using perturbation theory we obtain expressions for the third-order nonlinear susceptibility and the Kerr nonlinear coefficient of a four-level lambda-type atomic system in the presence of Doppler broadening. The model is applied to the Rb atomic gas medium at room temperature; the investigations demonstrate the ability to enhance and control the Kerr nonlinear coefficient according to laser parameters in single-window and double-window EIT regimes. The amplitude and the sign of the Kerr nonlinear coefficient change sensitively to the driving laser parameters as well as ambient temperature. As a representative application, this Kerr nonlinear medium is applied to optical bistability (OB) and shows that the threshold intensity and the width of OB are also easily changed according to the laser parameters. In particular, in double-window EIT regime the emergence of nonlinear coefficient at different frequency domains that leads to the appearance of OB at multiple frequency domains simultaneously. This study is a good preparation for experimental observations of Kerr nonlinearity and optical bistability.