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Simultaneous slow and fast light

Dispersion properties of an optical medium can be greatly changed using electromagnetically induced transparency (EIT) and other nonlinear effects. Thus, the group velocity of light pulse can be much smaller (greater or even negative) than light speed in vacuum, which is called slow (fast) light. Slow and fast light are very useful in optical processing, such as optical buffer, switching, and storage devices. Usually, slow and fast light cannot be generated simultaneously in the same medium. In this paper, we propose applicable level structures to generate controllable slow and fast light simultaneously at two frequencies, with the same polarization.