Femtosecond laser-induced refractive index changes at the surface of dielectrics: quantification based on Newton ring analysis

Abstract

We report on reflectivity and transmission patterns resembling Newton rings at the surface of a broad range of dielectric materials upon irradiation with single femtosecond laser pulses. We demonstrate that the patterns are due to the formation of a submicrometer layer of modified material underneath the laser-irradiated region. This permanent layer acts as a low-finesse micro Fabry–Perot etalon, producing a system of dark and bright rings upon illumination with narrowband light, whose number and optical contrast are related to thickness and optical constants of the layer. We find that the appearance of Newton rings is a universal phenomenon in fs-laser irradiated inorganic dielectrics (amorphous and crystalline), polymers, and semiconductors above the ablation threshold. We demonstrate how this phenomenon can be exploited for characterization of the layer by studying in detail three different dielectric materials as model systems, namely, fused silica, quartz, and phosphate glass, at fluences above and below the ablation threshold. An analysis of the Newton rings allows quantifying in a simple way the sign and amount of the changes in the complex refractive index as well as the thickness of the laser-modified layer. This technique greatly helps in characterizing the often problematic residual surface layer produced in laser structuring applications and has the ability to serve as an in situ, real-time monitor for minimizing its thickness and optical changes.