Challenges and Opportunities in 3D Laser Printing Based on (1 + 1)-Photon Absorption

Most light-based 3D printing methods rely on optical or chemical nonlinearities to spatially confine the polymerization reaction. In 3D micro- and nanoprinting, this nonlinearity can be provided by two-photon absorption, which describes the simultaneous absorption of two photons. To achieve comparable absorption cross sections for two-photon as in one-photon absorption, short and intense laser pulses with intensities in the range of 1 TW/cm 2 are typically required. Herein, we review three emerging excitation processes that provide a quadratic nonlinearity versus intensity without relying on two-photon absorption: upconversion luminescence, two-step absorption, and triplet–triplet annihilation. We term these “(1 + 1)-photon absorption”. Such processes allow for using continuous-wave lasers at much lower peak laser powers and at much lower cost than those typical for two-photon absorption. We review recent progress, describe current challenges, and outline future perspectives.