Determination of nanometric Ag2O film thickness by surface plasmon resonance and optical waveguide mode coupling techniques

There is a continuing need for measuring nanometric film thicknesses for a wide variety of industrial and scientific purposes. Kretschmann-type sensors are well-known multilayer nanometric sensing devices. This work is focused on two objectives: firstly, the design of an Ag2O Kretschmann sensor and, secondly, the development of a measurement protocol for determining silver oxide thickness when a silver film of given initial thickness is gradually converted into silver oxide by exposure to a controlled oxygen-rich atmosphere. The particular characteristics of the reflectivity curves of this multilayer structure are studied for both p- and s-wave polarization as a function of the incidence angle and layer thickness. In the former, the surface plasmon resonance (SPR) dip position as well as its FWHM depends strongly on silver oxide thickness. For s-wave polarization, a broad dip due to optical waveguide mode coupling is observed for angles larger than the total internal reflection (TIR) angle when sufficiently large silver oxide thicknesses are studied. Besides, reflectivity at fixed angles for both polarizations was studied as a function of the silver oxide layer. Each of these relations may be represented by different continuous functions defined for successive ranges that can be used as calibration curves. Taking into account all these features, a measurement protocol is proposed for determining silver oxide thickness when a 45 nm initial silver film is gradually converted into silver oxide by exposure to an oxygen-rich atmosphere. This new approach is an alternative to the traditional methods of full angular interrogation in the Kretschmann configuration. Based on this procedure, it is possible to measure Ag2O thickness in the range 0--70 nm.