Comparative study of SiO₂, Si₃N₄ and TiO₂ thin films as passivation layers for quantum cascade lasers

The aim of this article is to determine the best dielectric between SiO₂, Si₃N₄ and TiO₂ for quantum cascade laser (QCL) passivation layers depending on the operation wavelength. It relies on both Mueller ellipsometry measurement to accurately determine the optical constants (the refractive index n and the extinction coefficient k) of the three dielectrics, and optical simulations to determine the mode overlap with the dielectric and furthermore the modal losses in the passivation layer. The impact of dielectric thermal conductivities are taken into account and shown to be not critical on the laser performances.