Three-dimensional printing on the micro/nanoscale is well known to be possible through the use of ultrafast, pulsed lasers taking advantage of a nonlinear multi-photon absorption effect. The feature size and resolution of this technique, often referred to as 2-photon polymerization (2PP), is greatly determined by the individual three-dimensional printing pixel size, called a voxel. Determination of this important feature of fabrication has been primarily explored through experimental means, however numerical models have been developed for the propagation of a single laser pulse through transparent materials, such as fused silica.
In this work, a (2+1) spatiotemporal model is applied to a femtosecond laser pulse at 800nm wavelength propagating through a transparent photoresist. This model takes into account the effects of laser beam diffraction, group velocity dispersion, self-focusing, defocusing, and absorption due to the free electrons and nonlinear photoionization of the valence electrons. Using calculated energy flux and free electron density an absorption profile is determined allowing the prediction of the threshold of polymerization and giving insight into the voxel size. This prediction is then verified experimentally through single pulse experiments and voxel size measured using scanning electron microscopy and atomic force microscopy.
Paul Somers, Xiaolong He, and Xianfan Xu, "Numerical modeling of multi-photon polymerization by ultrafast laser (Conference Presentation)," Proc. SPIE 10519, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII, 1051911 (Presented at SPIE LASE: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290612.5751433014001.
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