Tuning the refractive index in 3D direct laser writing lithography: towards GRIN microoptics

We report a study of the determination of polymer cross-linking, namely the degree of conversion and refractive index of the microstructures created by two-photon polymerization (TPP). The influence of TPP processing parameters such as laser intensity and scanning velocity is investigated. The degree of conversion is analyzed via Raman microspectroscopy and the refractive index is measured with the interferometric technique employing a Michelson interferometer. Moreover, the relationship between these two properties is revealed and details are discussed. The largest refractive index change that we have obtained is of the order of 10(-2). Finally, we propose and demonstrate experimentally the realization of the gradientindex (GRIN) structure, resulting from a laser-induced local refractive index modification due to monomer cross-linking, i.e. degree of conversion. This work implies that the TPP technique is a valuable tool for the fabrication of GRIN microoptics for (in)homogeneous molding of light flow at the micrometer scale.