Couches minces optiques

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Design and thin film material optical characterizations

One of the main research interest of the RCMO team is the development of robust and performant methods for the determination of the opto-geometrical parameters of optical thin films from spectrophotometric measurements.
This study is critical as this is a mandatory preliminary step before the design of any multilayer structure. This one is achieved thanks to the developments of optimization methods and dedicated programs that can be implemented for single layers by also for more complex multilayer stacks, i.e. reverse engineering.
These developments have then be applied to the determination of the refractive index and extinction coefficient dispersions of various classical dielectric and metallic materials (Nb2O5, SiO2, Si, Ag, Au, Cr…). A set of parameters that were determined using this method can be found on refractiveindex.info.

The second step towards the fabrication of thin film multilayer filter is the design. It allows determining the thin film sequence (materials and thickness of each layer) that will allow achieving the spectral target.
The designed structure can be based on analytical solutions, i.e. using theoretical formulae with performances meeting or close to the target, or numerical solutions with innovative methods coupled with global or local optimization algorithms. In this case we minimize a figure of merit defined as the distance between the calculated performances and the target. Moreover, it is important to take into account the technological aspect of the problem as the calculated formulae must be compatible with the fabrication tools of the Institut Fresnel.
To achieve, the optimization can be constraint for example with boundaries for the thickness (minimal and maximal) or with optimal monitoring techniques that are expected to be used for the precise control of the thickness of each layer of the stack. Finally, by simulating the errors on the thickness of each layer (typical of the chosen monitoring technique) but also the fluctuations of the refractive indices, it is then possible to predict the feasibility and success chances of a filter, even before any experimental demonstration.