The role of FA:K+ and FA:Na+ defects in laser light generation and color image formation at the (100) and (110) surface sites of AgCl and AgBr.

Abstract

FA:K+ and FA:Na+ color centers at the low coordination (100) and (110) surface sites of AgCl and AgBr thin films play important roles in providing tunable laser oscillation and color image sensitization. Double-well potentials at these sites are investigated using ab initio molecular electronic structure calculations. Quantum clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that are the nearest neighbors to the FA defect site were allowed to relax to equilibrium. The calculated Stokes shifted optical transition bands suggest that laser light generation is sensitive to (i) the lattice anion, (ii) the coordination number of surface ions, (iii) the dopant cation, (iv) the vibrational coupling mode, and (v) the choice of the basis set centered on the anion vacancy. An attempt has been made to explain these results in terms of Madelung potentials, electron affinities and optical–optical conversion efficiencies. All relaxed excited states of the defect-containing surfaces were deep below the lower edges of the conduction bands of the ground state defect-free surfaces, suggesting that FA:K+ and FA:Na+ are suitable laser defects. The probability of orientational destruction of the two centers, attributed to the assumed saddle point ion configurations along the 〈110〉 axis, was found to be dependent on the lattice anion, the coordination number of surface ions, and the dopant cation. For optical memories, a high recording sensitivity was assigned to FA:Na+ relative to FA:K+, to the (110) surface relative to the (100) surface, and to AgCl relative to AgBr. The dependence of exciton (energy) transfer on the lattice anion, the coordination number of surface ions, and the dopant cation was clarified. The Glasner–Tompkins empirical rule was generalized to include the surface coordination number and the dopant cation in addition to the lattice anion. As far as color image formation is concerned, the examined supersensitizer was found to increase the sensitizing capabilities of two primary dyes in the excited states by increasing the relative yield of quantum efficiency. The (110) surfaces of AgBr and AgCl were more sensitive than the corresponding (100) surfaces, and the AgBr thin film was found to be more sensitive than that of AgCl. Based on quasi Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states was determined.