Confinement-enhanced polarization spectroscopy in ultrathin alkali vapors.

We report measurements of the anisotropic optical response of an atomic medium confined in submicron-thick vapor cells using polarization spectroscopy. The technique enables systematic characterization of anisotropy as a function of cell thickness, temperature, and probe intensity. Owing to the extremely low optical density of these submicron cells, the medium remains optically transparent even at temperatures up to 250 °C, allowing operation far beyond the conventional thermal limits of vapor-cell spectroscopy. Under these conditions, we observe a dramatic enhancement of the anisotropic signal, reaching up to three orders of magnitude compared to cells of identical thickness operated at 150 °C. This giant anisotropy gives rise to pronounced sub-Doppler spectral features in the probe transmission, persisting even for subwavelength cell dimensions (L⁄λ≈0.64). The observed behavior is attributed to the evolution of Zeeman population imbalance among ground-state sublevels in the strongly confined geometry. Our results demonstrate that submicron vapor cells provide a powerful platform for high-contrast anisotropy spectroscopy and for studying coherence phenomena in extreme confinement regimes.