Electromagnetically induced absorption and transparency in a ladder-type atomic system with different dressing mechanisms

In a Doppler-broadened ladder-type Rubidium atomic system $5S_{1/2}$–$5P_{3/2}$–$5D_{5/2}$, three-photon electromagnetically induced absorption (EIA) and transparency (EIT) in two schemes of two coupling lasers with different propagation directions are comprehensively studied. Two coupling fields are entangled tightly with each other and induce constructive interference such that we observe EIA as two coupling fields are counter-propagating. Two coupling fields are independent and cause destructive interference such that only enhanced EIT occurs when two coupling fields are co-propagating, although all of the Doppler-broadened atoms satisfy the condition for three-photon resonance. The experimental results are coincided with the numerically calculated spectra using the dressed perturbation method. The mechanisms of EIT and EIA could also be understood by the dressed-state pictures.