Propagation of smooth and discontinuous pulses through materials with very large or very small group velocities

We investigate the propagation of optical pulses through two different solid-state optical materials, ruby and alexandrite, for which the group velocity can be very small (v(g) > c or negative). We find that for smooth pulses the fractional delay or advancement is maximized through the use of pulses with durations comparable to the response time of the physical process - coherent population oscillations - that leads to these extreme group velocities. However, we find that the transmitted pulse shape becomes distorted unless the pulse is much longer or much shorter than this response time. We also investigate the transmission of pulses that possess an abrupt change in pulse amplitude. We find that, to within experimental accuracy, this nearly discontinuous jump propagates at the usual phase velocity of light c/n, even though the smoothly varying portions of the pulse propagate at the group velocity.X112729sciescopu