Dicke Coherent Narrowing in Two-Photon and Raman Spectroscopy of Thin   Vapour Cells
  The principle of coherent Dicke narrowing in a thin vapour cell, in which sub-Doppler spectral lineshapes are observed under a normal irradiation for a l/2 thickness, is generalized to two-photon spectroscopy. Only the sum of the two wave vectors must be normal to the cell, making the two-photon scheme highly versatile. A comparison is provided between the Dicke narrowing with copropagating fields, and the residual Doppler-broadening occurring with counterpropagating geometries. The experimental feasibility is discussed on the basis of a first observation of a two-photon resonance in a 300 nm-thick Cs cell. Extension to the Raman situation is finally considered. 
Gabriel Dutier (LPL), Petko Todorov (LPL), Ismah\`ene Hamdi (LPL),   Isabelle Maurin (LPL), Solomon Saltiel (LPL), Daniel Bl...