Four-level N -scheme crossover resonances in Rb saturation spectroscopy in magnetic fields

Abstract

We perform saturated absorption spectroscopy on the ${D}_{2}$ line for room temperature rubidium atoms immersed in magnetic fields within the 0.05--0.13 T range. At those medium-high field values the hyperfine structure in the excited state is broken by the Zeeman effect, while in the ground-state hyperfine structure and Zeeman shifts are comparable. The observed spectra are composed by a large number of absorption lines. We identify them as saturated absorptions on two-level systems, on three-level systems in a $\mathsf{V}$ configuration, and on four-level systems in an $\mathsf{N}$ or double-$\mathsf{N}$ configuration where two optical transitions not sharing a common level are coupled by spontaneous emission decays. We analyze the intensity of all those transitions within a unified simple theoretical model. We concentrate our attention on the double-$\mathsf{N}$ crossovers signals whose intensity is very large because of the symmetry in the branching ratios of the four levels. We point out that these structures, present in all alkali-metal atoms at medium-high magnetic fields, have interesting properties for electromagnetically induced transparency and slow light applications.