Controlling the spectrum of light pulses by dynamical Electromagnetically Induced Transparency

We present a theoretical and experimental study on thepossibility of spectral manipulation of weak probe-laser pulses in the presence of dynamical electromagnetically induced transparency. We predict a spectral enlargement or narrowing process depending on whether the probe-laser pulse is overlapped by the rising or the falling edge of the coupling pulse, respectively. The results of an experiment in sodium atomic vapors confirm the theoretical predictions.