Dispersionless absorption, dispersionless emission, and two-photon dispersion in a double-lambda-type four-level system under Doppler-free condition

Under the Doppler-free condition and by using semiclassical approach, the optical Bloch equations involving the density matrix elements with variable coefficients are obtained for double-lambda-type four-level system. The steady-state solutions for the density matrix elements are utilized to obtain the dispersive signal lineshapes for various pump positions and intensities. The shifts of anomalous dispersion are attributed by light shift. For the off-resonant pump, the exhibited two-photon dispersion is greatly influenced by pump Rabi-frequency. The dispersion corresponding to Autler-Townes splitting is exhibited. The nonlinear polarization involving the dipole-forbidden levels exhibits dispersionless absorption and dispersionless emission and is attributed by the quantum coherence. The dispersionless absorption and dispersionless emission for the double-lambda-type four-level systems are reported for the first time. The dispersionless absorption is exactly opposite in nature to those of electromagnetically induced transparency (EIT) and is identified as the inverse EIT.