Electromagnetically induced absorption (EIA) in a degenerate closed transition was measured under the action of a probe and a coupling laser of various polarizations giving rise to different possible N-systems or multi-level systems. While the probe shows EIA the coupling is mainly characterized by “absorption within transparency”. However, for certain lasers' intensity ranges, a switch between “absorption within transparency” and “transparency within transparency” in the coupling absorption spectra was measured for the first time.
After the discovery of electromagnetically induced absorption (EIA), both theoretical and experimental research was performed to explain not only the probe laser absorption and dispersion spectra, but also those of the coupling laser. The investigations of EIA in degenerate two-level systems have often been realized using two linearly polarized lasers of orthogonal polarization [1]. However, the simplest configuration leading to EIA is the so-called N-system [2] which basically requires the simultaneous adoption of a circularly polarized coupling laser and a linearly polarized probe (here called σ−π). To our knowledge, no experimental measurements of the coupling laser exist in the configurations (σ+−σ−, π−σ− or σ+−π).
We discuss the influence of phase-changing collisions on the group velocities in Doppler-broadened, cycling, degenerate two-level systems where
Fe=Fg+1 and
Fg>0, interacting with pump and probe lasers, that exhibit electromagnetically induced absorption (EIA). Two model systems are considered: the
N system where the pump and probe are polarized perpendicularly, and EIA is due to transfer of coherence (TOC), and the double two-level system (TLS) where both lasers have the same polarization, and EIA is due to transfer of population (TOP). For the case of Doppler-broadened EIA TOC, which occurs at low pump intensity, there is a switch from positive to negative dispersion and group velocity, as the rate of phase-changing collisions is increased. For the case of EIA TOP at low pump intensity, the dispersion and group velocity remain negative even when the collision rate is increased. Pressure-induced narrowing, accompanied by an increase in the magnitude of the negative dispersion and a decrease in the magnitude of the negative group velocity, occurs in both EIA TOC and EIA TOP, at low pump intensity. When the pump intensity is increased, a switch from negative to positive dispersion and group velocity, with increasing collision rate, also occurs in the double TLS system. However, the effect is far smaller than in the case of the
N system at low pump intensity.