Optical pumping effect on modulation transfer spectroscopy of the D1 line of ⁸⁵Rb atoms with non-cycling transition

The D_1 line of ^{85}Rb exhibits non-cycling transitions; therefore, only weak modulation transfer spectroscopy (MTS) signals could be obtained \cite{Khan2024}. In this study, however, strong MTS spectra were obtained for the D_1 line via optical pumping (OP) of the D_2 line of $^{85}$Rb. Four linear parallel (lin \parallel lin) and linear perpendicular (lin\perplin), \sigma^+ - \sigma^+, and \sigma^+ - \sigma^- polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), \sigma^+, and \sigma^- polarizations. Under the linear perpendicular configuration, the MTS signal for the F_g=3 \rightarrow F_e=2 transition of the D_1 line exhibited approximately a 41-fold increase in the signal slope in the presence of a linearly-polarized OP beam, compared with the signal slope obtained without OP. The calculated MTS spectra for the \sigma^+ \sigma^- configuration in the presence of \sigma^+- or \sigma^--polarized OP beam as an example matched well with experimental observations.