The coherent superposition of two-atomic levels induced by coherent population trapping is employed in a standard
Λ type scheme to form a tripod-like system. A weak probe pulse scanning across the system is shown to experience a crossover from absorption to transparent and then to amplification. Consequently the group velocity of the probe pulse can be controlled to propagate either as a subluminal, a standard, a superluminal or even a negative speed. It is shown that the propagation behavior of the light field is entirely determined and controlled by the initial states of the coherent superposition.
Abstract We consider propagation of a probe pulse in an atomic medium characterized by a combined tripod and Lambda (Λ) atom-light coupling scheme. The scheme involves three atomic ground states coupled to two excited states by five light fields. It is demonstrated that dark states can be formed for such an atom-light coupling. This is essential for formation of the electromagnetically induced transparency (EIT) and slow light. In the limiting cases the scheme reduces to conventional Λ- or N -type atom-light couplings providing the EIT or absorption, respectively. Thus, the atomic system can experience a transition from the EIT to the absorption by changing the amplitudes or phases of control lasers. Subsequently the scheme is employed to analyze the nonlinear pulse propagation using the coupled Maxwell–Bloch equations. It is shown that a generation of stable slow light optical solitons is possible in such a five-level combined tripod and Λ atomic system.
We propose a scheme to realize storage and retrieval of two-component optical soliton in a coherent atomic system. The system under consideration is a cold, lifetime-broadened four-level atomic gas with a tripod configuration working at the condition of double electromagnetically induced transparency. We show that due to the existence of two dark states, the optical absorption of the probe pulse can be largely reduced. In addition, the two orthogonal polarization components of the probe pulse with a form of vector optical soliton cannot only be slowed down substantially but also be stored and retrieved robustly by switching a control laser field off and on. We also show that it is possible to achieve a memory of
N-component optical soliton by using the
N dark states in a
(N+1)-pod system
(N>2). The results reported here may have promise in applications for light information processing in a weak nonlinear regime.