Nonlinearity Matters in Light–Matter Interaction: Multiphoton 3D Lithography

Multiphoton 3D lithography (MP3DL) based on ultrafast nonlinear light‐matter interactions to enable micro‐/nano‐structuring with resolution beyond the diffraction limit, yet the physical origin of the nonlinear energy deposition that initiates polymerization in photoresists remains ambiguous. Here, we investigate the optical nonlinearities governing MP3 DL using SZ2080 TM , a widely used hybrid organic–inorganic photoresist in micro‐/nano‐fabrication. Both the pure resist and formulations containing the photoinitiators/photosensitizers Irgacure369 and 4,4′‐bis(diethylamino)benzophenone are characterized by Z‐scan technique. The nonlinear absorption and refraction coefficients are experimentally determined at commonly used femtosecond laser wavelengths (515, 800, and 1030 nm) under different focusing configurations. These coefficients reveal energy deposition particularities, indicating that the majority of deposited energy originated from the resist itself and can be further enhanced through strong interaction with the photoinitiator, particularly via charge transfer effects. Additionally, 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 for improving MP3DL efficiency. 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, offering predictive guidelines of polymerization conditions and accelerating the design and optimization of advanced micro‐/nano‐fabrication.