  The optical properties of a weak probe field in a duplicated two-level system
are investigated in multiphoton resonance (MPR) condition and beyond it. It is
shown that, by changing the relative phase of applied fields, the absorption
switches to the amplification without inversion in MPR condition. By applying
the Floquet decomposition to the equation of motion beyond MPR condition, it is
shown that the phase-dependent behavior is valid only in MPR condition.
Moreover, it is demonstrated that the group velocity of light pulse can be
controlled by the intensity of applied fields and the gain-assisted
superluminal light propagation (fast light) is obtained in this system. In
addition, the optical bistability (OB) behavior of the system is studied beyond
MPR condition. We apply an indirect incoherent pumping field to the system and
it is found that the group velocity and OB behavior of the system can be
controlled by incoherent pumping rate.
