Refractive index control in bicomponent polymer films for integrated thermo-optical applications

Optical properties of transparent polymer thin films, produced by spin-coating on silicon and constituted of polycarbonate PC , poly- methyl methacrylate PMMA , and PC/PMMA, were investigated with regard to integrated thermo-optical TO device applications. Refractive index dependences on wavelength, temperature, and film composition were measured by spectroscopic ellipsometry with a dedicated autocontrolled heater setup, in the ranges of 400 to 800 nm, 25 to 85 °C and 0 to 100 wt% PC, respectively, with determination of Cauchy and Lorentz-Lorenz parameters. Within these intervals, thermomechanical compatibility and pronounced index contrast of around 0.12 between PC and PMMA, as well as their TO coefficients one order of magnitude higher than that of silica, allow convenient tailoring for specific TO requirements. In addition, wide-range fine-tuning of refractive index variation is found to be facilitated by the weak dependence of isothermal dispersion curves and TO coefficients on film composition