Magnetically induced separation and enhancement of optical precursors via electromagnetically induced transparency

Based on magnetically-induced coherence, a scheme for separating and enhancing optical precursors is proposed in a four-level atomic system. An rf field induces a magnetic dipole transition, which consists of a loop system with other optical fields. Under conditions of electromagnetically induced transparency, optical precursors could be separated from the delayed main signal when the relative phase of the system is 0. While, by changing the phase from 0 to π, the phase-dependent dispersion strengthens the constructive interference between the precursors and main signal, thus the precursor pulse is much enhanced. This is attributed to the competition between magnetically induced slow- and fast-light effects.