By balancing the steep linear and nonlinear dispersions of an intracavity electromagnetically induced transparency
medium, the cavity transmission spectra can be significantly modified and controlled. As the cavity
input intensity increases, the cavity linewidth changes from far below to above the empty cavity linewidth,
corresponding to the photon propagation in the cavity from "subluminal" to "superluminal", respectively. A
"white-light cavity" condition has been achieved under certain experimental conditions.