Nonlinearity matters in light-matter interaction: multi-photon 3D lithography

Multi-photon 3D lithography (MP3DL) based on ultrafast nonlinear light-matter interactions to enable micro- and nano-structuring with resolution beyond the diffraction limit, yet the physical origin of the nonlinear energy deposition that initiates polymerization in resins/resists remains ambiguous. Here, we present a comprehensive investigation of the optical nonlinearities governing MP3DL using SZ2080 ^TM photoresist, a widely used hybrid organic–inorganic photoresist in 3D micro-/nano-fabrication. Both the pure resist, and its formulations containing the photoinitiators/photosensitizers IRG369 and BIS (Irgacure369 and 4,4'-bis(diethylamino)benzophenone or Michler’s ketone) are characterized by Z-scan technique. The nonlinear absorption and refraction coefficients at all commonly used laser wavelengths (515, 800, and 1030 nm) of fs laser sources under different focusing configurations are experimentally determined. These coefficients reveal energy deposition particularities, indicating that the majority of deposited energy during 3D printing originated from the resist itself and can be further enhanced through strong interaction with the photoinitiator, particularly via charge transfer effects, under all excitation wavelengths. In addition, we show that nonlinear energy deposition can be substantially increased by extending the interaction volume through an increased beam Rayleigh length, providing an alternative route to more efficient multi-photon lithography. Our findings challenge the conventional assumption that photo-polymerization is initiated exclusively by photoinitiators and establish the fundamental basis for photo-initiator-free 3D laser lithography. More broadly, this work allows for the precise control and modelling of energy deposition, offering predictive guidelines of polymerization conditions without extensive empirical trials, thus accelerating the design and optimization of 3D micro- and nano-fabrication.