Optical Precursors with Electromagnetically Induced Transparency in Cold Atoms

We report the observation of Sommerfeld-Brillouin optical precursors generated from a long square-modulated laser pulse propagating through a cold atomic ensemble with electromagnetically induced transparency. The optical depth (${\alpha }_{0}L$) of the medium can be varied from 0 up to 50. We demonstrated that the step-on rising and step-off falling edges propagate with the speed of light in vacuum without a slow light effect. At high ${\alpha }_{0}L$, the precursor is separated from the delayed main pulse at the rising edge, while at the falling edge, we observe damped oscillatory structures resulting from the interference between the precursor and main field.

Figure 1

(a) ${}^{85}\mathrm{Rb}$ energy level diagram of a three-level EIT system. (b) Experimental configuration.

Figure 2

(a) Probe laser EIT transmission profile. (b) Group delay as a function of the probe detuning $\Delta {\omega }_p$. ${\alpha }_{0}L=45$, ${\Omega }_c=4{\gamma }_{13}$.

Figure 3

Observation of optical precursors from a square-modulated pulse propagating through cold atoms. (a) The reference square pulse with no cold atoms present. (b) ${\alpha }_{0}L=18$, coupling laser is switched off (${\Omega }_c=0$). (c) ${\alpha }_{0}L=18$, coupling laser is on (${\Omega }_c=4{\gamma }_{13}$). The theoretical curves (red solid line) in (b) and (c) are obtained using FFT by taking into account the finite rise and fall time.

Figure 4

Optical precursors at the rising edge [(a1)–(a3)] and falling edge [(b1)–(b3)] from a two-level system at different optical depth. The coupling laser is switched off (${\Omega }_c=0$). The circular (blue) are experimental data. The solid (red) curves are obtained using FFT by taking into account the finite rise and fall time. The dotted (green) points are obtained using hybrid-asymptotic analysis.

Figure 5

Optical precursors at the rising edge [(a1)–(a3)] and falling edge [(b1)–(b3)] from a three-level EIT system at different optical depth and with the same coupling laser power (${\Omega }_c=4{\gamma }_{13}$). The circular (blue) are experimental data. The solid (red) curves are obtained using FFT by taking into account the finite rise and fall time. The dotted (green) points are obtained using hybrid-asymptotic analysis.