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Two-photon polymerization (TPP) has emerged as a favored advanced manufacturing tool for creating complex 3D structures in the sub-micron regime. However, the widescale implementation of this technique is limited partly due to the cost of a high-power femtosecond laser. In this work, a method is proposed to reduce the femtosecond laser 3D printing power by as much as 50% using a combination of two-photon absorption from an 800 nm femtosecond laser and single photon absorption from a 532 nm nanosecond laser. The underlying photochemical process is explained with modeling of the photopolymerization reaction. The results show that incorporating single-photon absorption from a visible wavelength laser efficiently reduces inhibitor concentration, resulting in a decreased requirement for femtosecond laser power. The radical to macroradical conversion is dominated by the reduction in oxygen concentration, while the reduction in photoinitiator concentration limits the threshold power reduction of the femtosecond laser. 

A mathematical model is developed to describe the photochemical processes in two-photon nanolithography, including two-step absorption leading to initiation and self-deactivation of the photoinitiator by laser irradiance, polymer chain propagation, termination, inhibition, and inhibitor and photoinitiator diffusion. This model is solved numerically to obtain the concentrations of the reaction species as a function of time and space as a laser beam is scanned through a volume of photoresist, from which a voxel size or linewidth is determined. The most impactful process parameters are determined by fitting the model to experimentally measured linewidths for a range of laser powers and scanning speeds, while also obtaining effective nonlinearities that are similar to previously measured values. The effects and sensitivities of the different process parameters are examined. It is shown that the photopolymerization process is dominated by diffusion of photoinitiators and oxygen inhibitors, and that self-deactivation can lead to higher effective nonlinearities in two-photon nanolithography.