Dual control of slow light in reciprocal electromagnetically-induced-transparency conditions

Electromagnetically-induced transparency (EIT) of atomic medium interacting with two laser beams can exhibit effects of reciprocal influence of both beams on optical properties of the medium. If the probe beam reaches instantaneous intensity comparable to that of the control beam, considerable energy is transferred to the latter. Resulting increase in the control-beam intensity temporarily modifies EIT conditions and consequently affects propagation of probe-beam pulses. The effect is particularly strong in atomic media exhibiting significant dissipation of radiation energy due to relaxation processes efficiently counteracting the optical pumping. Coherent interaction of atoms with both beams causes reduction of dissipation of the control beam (reciprocal EIT conditions). Our experiments with hot rubidium vapor demonstrate the possibility of dual control over slow propagation of light pulses in such media. Besides the well-known dependence on input control-beam intensity, the delays of output probe pulses depended significantly also on the energy of input probe pulses. Moreover, for relatively short input pulses, very specific shape of output pulses was observed, invariant in wide ranges of input probe pulse energies and control-beam intensities. The shapes of the output pulses were also the same for short input pulses of different shapes as well as for pulses temporarily stored in the medium by switching off the control beam during their propagation.