Optical dispersion of homogeneously mixed ZnSMgF2 films

Mixed films of ZnSMgF₂ were prepared by co-evaporation from a single source. The dispersion of the optical constants was obtained from the reflectance and transmittance data of the films for various compositions in the spectral range 2000–7000 Å. The values of the optical constants of the mixed films at any wavelength lie between the values of the optical constants of pure ZnS and MgF₂ films. The refractive index of the mixed films closely obeys the Lorentz-Lorenz relation at wavelengths at which the absorption is small. The fundamental absorption edge of the mixed films shifts from that of MgF₂ to that of ZnS continuously but non-linearly on increasing the concentration of ZnS in MgF₂. Thus it is possible to use these films as variable frequency cut-off filters. The fundamental absorption edge of the mixed films follows the (hv-E_g)² relation, indicating an indirect optical transition, in sharp contrast to the direct optical transition observed in bulk ZnS.