Two-photon resonances, ac Stark splittings, and shifts of the resonance peaks in Doppler free absorptive lineshapes of a double-lambda type four level system

Under Doppler free condition and by using semiclassical approach, a double-lambda type four-level system coupled to two electromagnetic fields are investigated. The coupled optical Bloch equations involving the density matrix elements in steady state are solved numerically to investigate the signal lineshapes as the function of signal detuning and pump Rabi-frequencies. The on-resonant pump leads to dynamic Stark splitting. The dent in the absorption lineshape is exhibited for off-resonant pump and is a signature of two-photon resonances. The appearance of extra absorption and emission peaks in the computed lineshapes are attributed by the nonlinear polarizations and hence the spontaneously generated coherences.