Abstract

We numerically study the effect of two coherent copropagating pulses incident on an optically dense collection of four-level atoms. The injected pulses have different frequencies that are nearly resonant with the allowed dipole transitions. Because the transient solution is paramount, we cannot use the steady-state absorption spectrum as an appropriate indicator of probe absorption or amplification. We use instead the imaginary part of the polarization at the probe's frequency as a time-dependent measure of the probe field's gain. Because the gain is a function of time, this polarization by itself is not sufficient to describe whether the probe is amplified. Therefore, we consider the integrated energy density of the probe field to assess the parameters for which the energy at the probe frequency increases. Furthermore, we can drop the weak-coupling and weak-probe-field approximations and study regimes that have not been addressed before, including strong probe-field strengths for which standard linear treatments are not valid and optically dense mediums for which local-field effects cannot be ignored.

© 1995 Optical Society of America