Modified needle method with simultaneous thickness and refractive-index refinement for the synthesis of inhomogeneous and multilayer optical thin films

Useful features of an efficient thin-film synthesis technique based on the refinement of inhomogeneous films are adapted to the needle method. As in the former approach, all the layer refractive indices and thicknesses are refined simultaneously, and special attention is given to computation speed and accuracy. In particular, key parameters are calculated from analytical formulas, namely, the gradients used in the optimization routines and the optimum needle positions and refractive indices. Inhomogeneous as well as multilayer solutions are possible. The efficiency of the modified approach is demonstrated by the solution of a complex nonpolarizing, wide-angle antireflection coating problem. The results are compared with those obtained in the past by the conventional needle method and by refinement of inhomogeneous films.