Results and discussion 11 Tailoring birefringence of LIPSS through thin film coatings A. San Blas1,2, N. Casquero1,2, S. Sánchez1,2, L.M. Sánchez-Brea3, J. Buencuerpo3, A. Rodríguez1,2, S.M. Olaizola1,2 1 Ceit, Manuel Lardizabal 15, 20018 Donostia / San Sebastián, Spain, 2 Universidad de Navarra, Tecnun, Manuel Lardizabal 13, 20018 Donostia / San Sebastián, Spain 3 Applied Optics Complutense Group, Optics Department, Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Ciudad Universitaria s.n., 28040, Madrid, Spain asanblas@ceit.es Introduction Experimental Conclusions Laser-induced periodic surfaces structures (LIPSS) present some degree of birefringence due to the orientation of the nanoripples. Among other properties, this makes LIPSS a good candidate for applications involving polarization changes such as reflective polarization gratings, polarimeters, displays or polarizing beam splitters. SHORTCOMING - Low parameter tunability. Fabrication of LIPSS in a small depth and periodicity range. SOLUTION - Introduction of a new degree of freedom. Thin film dielectric coatings modify the optical properties of the material, specifically, reflectivity and birefringence. Dielectric thin film coatings are a good candidate for the tunability of LIPSS birefringence. Reflectivity can be improved for certain wavelengths by varying coating thickness. LIPSS irregularities need to be fixed or taken into account. Coating structure is conformal rather than uniform. Preliminary study with promising results, further research is being carried out. Acknowledgements Characterization Determination of the reflectivity in a broad spectrum (400-1100 nm) with a supercontinuum laser and an integrating sphere to collect the reflected light with output for a spectrometer. Transversal profile and film thickness analysis by means of SEM (Scanning Electron Microscopy) and FIB (Focused Ion Beam). Platinum is deposited on the sample before the FIB procedure in order to protect the surface of the sample. Fabrication Stainless steel samples processed with femtosecond laser focused on the surface of the sample to fabricate nanostructures (LIPSS) with an estimated period of 600 nm and depth of 300 nm. The sample was moved with X, Y and Z stages at a speed of 1 mm/s, processing lines of 10 μm width in each pass, moving in parallel trajectories until the desired size is achieved. Pulse energy used was 1 μJ. Polarization of the laser was controlled with a half-wave plate so that the orientation of the generated LIPSS were perpendicular to the direction of the processed lines. The samples were cleaned in a 5 minutes ultrasonic bath with acetone. Polarization changes induced by the sample were measured with a polarimeter using a half-wave plate in combination with 633 and 808 nm continuous laser sources at 25º incidence with respect to the surface normal and linear polarization at 45º with respect to LIPSS orientation. Afterwards, thin film coatings of thicknesses 50, 100, 200 and 300 nm of SiO2 or Si3N4 were deposited in the different samples through PECVD (plasma enhanced chemical vapor deposition). Numerical simulations FDTD simulations of the effects on polarization as a function of LIPSS parameters (depth, periodicity) and coating thickness in SiO2. Coating structure and LIPSS characterization In the range of thicknesses used in this experiment, the coatings present good conformality to LIPSS. Promising result regarding the numerical simulations, which show that the structure of the coating is a key parameter. The profile of LIPSS shows irregularities with average depth of 340 nm. LIPSS periodicity of ~630 nm has been measured through 2D-FFT analysis of the SEM images. Reflectivity High reflectivity is very desirable for potential applications. Our results show that thin film coatings modify LIPSS reflectivity in certain regions of the spectrum depending on coating thickness and material, improving LIPSS performance. Birefrin