Density-ratio dependence of spin polarization and spatial homogeneity in hybrid Rb–Cs vapor cells

Hybrid spin-exchange optical pumping (hybrid SEOP) using multiple alkali-metal species is commonly employed to improve the spatial homogeneity of alkali spin polarization in atomic magnetometers and comagnetometers. We investigate the dependence of spin polarization and its spatial homogeneity on the alkali density ratio (Formula presented) (Formula presented) in Rb–Cs hybrid vapor cells. A series of cells with controlled D_r were fabricated and characterized using absolute electron paramagnetic resonance polarimetry at multiple positions along the cell axis. The results show that the density ratio strongly influences both the magnitude and spatial uniformity of alkali polarization through the competition among optical pumping, spin exchange, and spin-destruction processes. An optimal density ratio is observed near (Formula presented) (Formula presented), where the polarization reaches its maximum while maintaining high spatial homogeneity (end-to-end ratio ∼89% and RMS homogeneity (Formula presented) (Formula presented) ). Under typical operating conditions, the optimal density ratio lies in the range (Formula presented) (Formula presented) –10. These results clarify the role of the alkali density ratio in Rb–Cs hybrid SEOP and provide practical guidance for optimizing hybrid vapor cells used in precision atomic magnetometers and comagnetometers.