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

TLDR

The study investigates how two‑photon polymerization parameters affect polymer cross‑linking and refractive index, and demonstrates a gradient‑index microoptical structure. Laser intensity and scanning velocity were varied while degree of conversion was measured by Raman microspectroscopy and refractive index by Michelson interferometry, enabling local refractive index modification through monomer cross‑linking. A strong correlation between conversion and refractive index was found, achieving a maximum change of ~10⁻², confirming TPP as a powerful method for fabricating GRIN microoptics that control light flow at the micrometer scale. Image included.

Abstract

Abstract 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 gradient‐index (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. image