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Abstract
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This paper investigates the behavior of linear and nonlinear optical susceptibility of an open
four-level molecular system, under two-step excitation based on electromagnetically induced transparency
(EIT). The system was irradiated with a weak probe field and strong coupling field. It is shown that
the use of a strong coupling field in the triplet states of an alkali-metal dimer can change the spin-orbit
interaction (SOI). The optical response of the system can then be modified in a controllable way. The
electromagnetically induced transparency transforms into electromagnetically induced absorption (EIA)
in the presence of a coupling field. Changing the sign of the dispersion, this region is associated with
switching subluminal and superluminal propagation. Furthermore, for the proper value of the coupling
field, the controllable parameters, enhanced Kerr nonlinearity with reduced linear absorption, can be
obtained under a weak probe field. With this approach, SOI can be controlled by changing only one of
the controllable parameters, using triplet-triplet strong coupling with different spin state. Therefore, the
desired region of the spectra can be obtained, in contrast to the other four-level system, in which at
least two strong fields are used to change optical properties. This mechanism can be suitable in molecular
systems or semiconductors to be used in optical bistability and fast all-optical switching devices.
1 Introduction
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