Micro-sized synthesis of customized 3D GRadient INdex elements by femtosecond laser lithography

Increasing interest in micro-scale optics has led to the development of novel miniaturization techniques, among which laser lithography stands out as a powerful approach. Advances in this field have focused not only on improving the spatial resolution, but also on tuning the optical properties of the structures as the refractive index (RI). Specifically, elements with a spatial variation of the local RI can be obtained (GRadient INdex). Knowing the dependence of the RI on exposure parameters enables the realization of 2D and multilayered GRIN structures. In this work, we use a hybrid polymer Ormocomp to fabricate GRIN microstructures via femtosecond laser lithography based on two-photon polymerization (TPP). With our procedure, GRIN distributions in 3-dimensions and with a few micrometers size are possible. We characterize the RI of our samples with phase maps obtained through interferometry and Fourier analysis methods. We complement our results with ray tracing simulations for optimum custom-made GRIN designs. Our approach represents a comprehensive and versatile strategy for the tailored fabrication of 3D GRIN micro-optical systems, in which the downscaling is foreseen.