Dye‐Laser 4f–4f‐Line Probing of Crystal‐Field Fluctuations in ZnS:Sm 3+ Polycrystalline Films

Using the dye‐laser site‐selective excitation technique, a study is made of 4 G 5/2 → 6 H 9/2 , 6 H 7/2 inter‐Stark level transitions for a special type of Sm 3+ centers in ZnS polycrystalline films. It is found that the emission lines become narrowed compared with the case of “nonselective” excitation, with the peak positions and intensities depending on the location of the dye‐laser energy within the 4f–4f excitation line profiles. The results can be quantitatively described by a simple model which involves small centre‐to‐centre fluctuations of crystal‐field parameters. A comparison with the (much sharper) lines measured for the same type of Sm centers in ZnS bulk crystals shows that these fluctuations have a Gaussian distribution with maximum corresponding to the bulk crystal situation. An explanation can be given on the basis of random strain originating from high dislocation densities. A discussion is made of whether the linewidths are affected by interactions between the Sm centres and by electric fields due to dislocation charge.