Spectroscopy in an extremely thin vapor cell: Comparing the cell-length dependence in fluorescence and in absorption techniques

We compare the behaviour of absorption and of resonance fluorescence spectra in an extremely thin Rb vapour cell as a function of the ratio of L/ l , with L the cell thickness (L ~ 150-1800 nm) and l the wavelength of the Rb D2 line (780 nm). 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) l/2 (n integer). It is shown not to appear in fluorescence, whose behaviour –amplitude, and 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 = nl.