We compare the behavior of absorption and of resonance fluorescence spectra in an extremely thin Rb vapor cell as a function of the ratio of $L∕\lambda$, with $L$ the cell thickness $(L\sim 150–1800\mathrm{nm})$ and $\lambda$ the wavelength of the Rb $D_2$ line $(\lambda =780\mathrm{mn})$. The Dicke-type coherent narrowing [G. Dutier et al., Europhys. Lett. 63, 35 (2003)] is observed only in transmission measurements, in the linear regime, with its typical collapse and revival, which reaches a maximum for $L=(2n+1)\lambda ∕2$ ($n$ integer). It is shown not to appear in fluorescence, whose behavior-amplitude, and spectral width, is more monotonic with $L$. Conversely, at high-intensity, the sub-Doppler saturation effects are shown to be the most visible in transmission around $L=n\lambda$.

• Received 12 December 2003

DOI:https://doi.org/10.1103/PhysRevA.69.065802