Direct measurement of two-photon absorption and refraction properties of SZ2080TM-based resists at 515 nm: insights into 3D printing

Accurate knowledge of nonlinear optical parameters is essential for optimizing energy deposition in ultrafast laser 3D printing, yet these values remain undetermined for many commonly used materials. In this study, we address this gap by experimentally determining the two-photon absorption (TPA) and non-linear refraction coefficients (β and n 2) of the widely used SZ2080TM resist with the photo-initiators (PI) IRG369 and BIS (Irgacure 369 and 4,4′ bis(diethylamino)-benzophenone or Michler’s ketone). Using the Z-scan method at 515 nm with a low repetition rate (1 kHz) to avoid thermal accumulation, we found that the nonlinear response of the host polymer has a considerable contribution to energy deposition despite the addition of the PI, as the host polymer makes up the majority of 99 % in the solution. The TPA cross section σ were 5.7 ± 0.4 GM (1 GM = 10−50 cm4 s photon−1) for pure SZ2080TM, ∼40 $\sim 40$ GM for IRG and ∼87 $\sim 87$ GM for BIS at 515 nm. The nonlinear refractive index n 2 for pure polymer was (85.3 ± 6) × 10−5 cm2/TW, favoring a self-focusing, and was larger than that for PIs: ∼16×10−5 $\sim 16{\times}1{0}^{-5}$ cm2/TW (IRG369) and ∼2.8×10−5 $\sim 2.8{\times}1{0}^{-5}$ cm2/TW (BIS). Hence, the properties of the host material govern non-linear light propagation, although, in high numerical aperture focusing, self-focusing has a minor contribution to the variation of refractive index. Crucially, the determined TPA coefficients for pure SZ2080TM provide experimental evidence that it can initiate polymerization without PIs, enabling a more sustainable and environmentally friendly fabrication route by avoiding the use of toxic additive compounds. These findings will allow for the estimation of exact energy deposition in 3D laser printing using ultrashort laser pulses and support the development of an initiator-free additive manufacturing approach.