Multiphoton 3D lithography

Multiphoton 3D lithography (MP3DL) is a mesoscale additive manufacturing technique (product dimensions range from nanometres to centimetres) that uses confined non-linear light-matter interactions to produce 3D structures. The use of ultrafast pulsed lasers to induce photocrosslinking enables rapid optical 3D printing of diverse materials ranging from pure organic natural resins to fully inorganic amorphous and crystalline ceramics. MP3DL allows for the direct writing of unrestricted, true free-form geometries, reaching 100 nm feature size and millimetre-scale object dimensions; further, the dose dependence of the photomodification depth (degree of conversion) allows for 3D greyscale and 4D patterning. The throughput of the technique is constantly improving with the recent development of novel light sources, synthesis of special materials and novel exposure strategies. In this Primer, we introduce the photophysical principles behind the technique, describe experimental methods, highlight the milestones achieved, review promising applications and discuss reproducibility, limitations and upcoming optimizations. Finally, we provide an outlook on future trends and the potential to exploit artificial intelligence for mesoscale multi-material 4D advanced additive manufacturing.