Dark-line atomic resonances in a submicron-thin Rb vapor layer

TLDR: In this paper, an experimental investigation of the effect of electromagnetically induced transparency (EIT) using bichromatic laser radiation and an extremely thin cell filled with pure Rb with smoothly controllable thickness $L$ of the atomic vapor layer in the range

Abstract: We report an experimental investigation of the effect of electromagnetically induced transparency (EIT) using bichromatic laser radiation and an extremely thin cell filled with pure Rb with smoothly controllable thickness $L$ of the atomic vapor layer in the range $\ensuremath{\sim}780--1600\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, for which $L$ is comparable to the laser wavelength $\ensuremath{\lambda}$ resonant with the ${D}_{2}$ line $(780\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. It is revealed that the transmission spectrum of the probe laser contains two sub-Doppler peaks that have different linewidths. The narrow peak corresponds to EIT, whereas the broader one results from velocity-selective optical pumping (VSOP) and repumping processes. It is demonstrated that in the case of nonzero detuning of the coupling laser, the EIT resonance and VSOP are shifted with respect to each other on the frequency scale, which makes it possible to observe a competition between the two effects (this is not possible to realize in an ordinary cell). Also, the Dicke-type coherent narrowing effect depending on the ratio $L∕\ensuremath{\lambda}$ influences the absorption spectrum of the probe laser. Formation of an EIT resonance is substantially favored for the atoms with slow normal velocity, caused by their longer interaction time with the bichromatic laser field. As a result of the predominant contribution of these atoms, the observed linewidth of the EIT resonance is only $\ensuremath{\sim}9\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, which is more than ten times narrower than the inverse window-to-window flight time. In an external magnetic field, three and five EIT resonances have been observed for the vapor thickness of $L=2\ensuremath{\lambda}$, on $^{87}\mathrm{Rb}$ and $^{85}\mathrm{Rb}$, respectively.