Discerning the transition from electromagnetically induced transparency to Autler–Townes splitting using the decaying dressed state formalism

We use biorthogonal basis consisting of the ‘right’ and ‘left’ eigenvectors of the effective non-Hermitian Hamiltonian of a three-level Λ system driven by a bi-chromatic field, and resolve the probe absorption spectrum into components corresponding to the decaying dressed states of the system. It is observed that the spectrum primarily consists of three components and two of them undergo dramatic changes when the control field Rabi frequency is changed from the low to high field regime. Based on the symmetry properties of a combination of these components, a parameter internal to the dynamics of the system is defined and is used to distinguish between electromagnetically induced transparency (EIT) and Autler–Townes (AT) splitting, and the threshold for transition between them in an objective manner. The formulation is further extended to the resonance fluorescence spectrum on the probe transition in the EIT and AT regimes.