Exposure-dependent refractive index of Nanoscribe IP-Dip photoresist layers

The refractive indices of photoresists used for direct laser writing (DLW) have been determined after exposure to ultraviolet (UV) light. However, it was anticipated that the refractive index will differ when applying a two-photon polymerization (TPP) process. In this Letter, we demonstrate that this is indeed the case. Making use of a guided mode coupling approach, we measure the dispersive real part of the refractive index (n) of a commercial photoresist (IP-Dip, Nanoscribe) at very high accuracy. Additionally, the imaginary part of the refractive index (k) is determined from absorption measurements for wavelengths in the range 300 to 1700 nm. TPP layers exhibit a significantly lower refractive index than their UV exposed bulk counterparts (Δn up to 0.01). Furthermore, when fabricating a TPP shell and UV exposing the interior, the refractive index of the shell will not change. This is an important consideration for optical component design and opens the possibility for low refractive index difference wave guiding.