The physics of 3D printing with light

The goal of 3D printing is to realize complex 3D structures by locally adding material in small volume elements called voxels — in contrast to successively subtracting material by etching, milling or machining. This field started with optics-based proposals in the 1970s. Progress has required breakthroughs in physics, chemistry, materials science, laser science and engineering. This Review focuses on the physics underlying optics-based approaches, including interference lithography, tomographic volumetric additive manufacturing, stereolithography, continuous liquid-interface printing, light-sheet printing, parallelized spatiotemporal focusing and (multi-)focus scanning. Light–matter interactions that are discussed include one-photon, two-photon, multi-photon or cascaded nonlinear optical absorption processes for excitation and stimulated-emission depletion or excited-state absorption followed by reverse intersystem crossing for de-excitation. The future physics challenges lie in further boosting three metrics: spatial resolution, rate of voxel creation and range of available dissimilar material properties. Engineering challenges lie in achieving these metrics in compact, low-cost and low-energy-consumption instruments and in identifying new applications. This Review categorizes the physics of many different light-based 3D printing modalities and expounds on the light–matter interactions required for the creation of (multi-)material 3D structures. An outlook is provided regarding key printing performance parameters and future directions.