Effect of refractive index mismatch on multi-photon direct laser writing

This work reports how the process of three-dimensional multi-photon direct laser writing (mpDLW) is affected when there is a small mismatch in refractive index between the material being patterned and the medium in which the focusing objective is immersed. Suspended-line microstructures were fabricated by mpDLW in the cross-linkable epoxide SU-8 as a function of focus depth and average incident power. It is found that even a small refractive index contrast of Δn = + 0.08 causes significant variation in feature width and height throughout the depth of the material. In particular, both the width and height of features can either increase or decrease with depth, depending upon how much the average incident laser power exceeds the threshold for writing. Vectorial diffraction theory is used to obtain insight into the origin of the effect and how to compensate for it. We demonstrate that varying the average focused power is a practical means for controlling the variation in feature size with focal depth.