Enhanced self-action effects by electromagnetically induced transparency in the two-level atom

Abstract

Electromagnetically induced transparency ~EIT! has been studied primarily within the context of multilevel atomic systems. We show that EIT can occur also in a two-level atomic system and can lead to strong self-action and slow-light effects that are not hampered by material absorption, with important potential implications for processes such as squeezed-light generation and the propagation of optical solitons. Electromagnetically induced transparency ~EIT! is a powerful technique that can be used to render a material system transparent to resonant laser radiation, while retaining the large and desirable nonlinear optical properties associated with the resonant response of a material system @1#. EIT has been observed in several different experimental configurations @2,3#, and its occurrence in still other configurations has been predicted theoretically @4#. Laboratory studies have confirmed that EIT can be used to enhance the efficiency of physical processes including nonlinear frequency conversion @3,5# and optical phase conjugation @6#. In addition, it has been predicted that EIT can enhance the properties of a much broader range of processes including squeezed-light generation @7# and low-light-level photonic switching @8#. EIT has been observed both in atomic vapors @2,3,5,6,9# and in solids @10#, and it plays a key role in the generation of slow light @11# and its concomitant production of extremely large nonlinear optical effects @12#. Intimately related to EIT is coherent population trapping @13#, and the establishment of high refractive indices @14#. Most previous work on EIT has dealt with multilevel atomic systems. In this article, we present a theoretical analysis of EIT effects in the two-level atomic system. The analysis of EIT within the context of a much simpler level scheme leads to additional insight into the origin of EIT. Our analysis confirms previous work showing that the presence of a strong control field can establish frequencies at which the atomic absorption vanishes or is negative, and, most importantly, that the third-order nonlinear optical response leading to self-action effects can be quite large at these particular frequencies @15#. Thus two-level-atom EIT effects hold considerable promise for applications such as spatial soliton propagation @16#, squeezed-light generation by selfphase modulation @19#, and many types of optical switching devices @20#. We also find that low group velocities ~slow light! can occur within the context of the strongly driven two-level atom, and that in some ways the large nonlinear optical response can be understood as a direct consequence of the slowness of the light propagation. The origin of EIT in the two-level atom can be understood in terms of the energy-level diagram shown in Fig. 1, which shows a strong control field of frequency v c and amplitude Ec interacting with the atomic system. Population is coherently cycled between the lower level a and the upper level b