The simulation of a homogeneous quarterwave layer by a thin-film assembly requires that the complex amplitude reflections of both layer systems have to coincide at the design wavelength. Since the complex amplitude reflection consists of a real and an imaginary part (magnitude and phase), this requirement leads to two conditions for the simulating film assembly. The addition of a third condition, namely, the coincidence of the slopes of the magnitude curves at the design wavelength, yields a good approximation of the complex amplitude reflection of the homogeneous quarterwave layer over a broad wavelength range and is, therefore, called a wideband simulation. The three conditions for the wideband simulation can be fulfilled by three independent parameters. These can be the layer thicknesses of a three-layer system consisting of two materials.
© 1985 Optical Society of America
R. Herrmann, "Quarterwave layers: simulation by three thin layers of two materials," Appl. Opt. 24, 1183-1188 (1985)