2, The Chinese University of Hong Kong, Hong Kong, Shatin N.T., Hong Kong
Two-photon lithography is a polymerization based direct laser writing technique that is capable of printing complex 3D parts with submicron features. During this process, submicron features are written within the interior of a photo-responsive resist via localized polymerization chemistries driven by nonlinear two-photon absorption. This capability has enabled researchers from diverse fields to fabricate functional micro/nano scale structures for applications such as photonic crystals, optical and mechanical metamaterials, microfluidics, miniaturized optics, and flexible electronics. The most widely implemented form of this process involves serially scanning a tightly focused laser spot in space to generate 3D parts. This serial scheme severely limits the scalability of the process. In addition, material scalability is limited by the optical constraints for transparency of the resist and refractive index matching with the focusing objective lens. These optical constraints limit printing of tall millimeter scale structures with submicron features to only a small set of resist materials. Herein, we have overcome rate and material based scalability limitations via (i) parallelization of the process that increases the rate by at least 50 times without adversely affecting the resolution or the depth resolvability observed in serial writing and (ii) modifications to the focusing optics that reduces the sensitivity of the process to index mismatch and resist opacity.
Although past attempts to parallelize two-photon lithography have been successful in increasing the rate of the process, those implementations have reduced the ability to fabricate complex 3D parts. Specifically, past demonstrations have either printed (i) the same feature into a periodic structure or (ii) an extrusion of an arbitrarily complex 2D plane (without depth resolvability). Here, we have overcome this scalability versus part complexity tradeoff by implementing a projection-based parallel writing scheme for printing of arbitrarily complex 3D parts. In this scheme, an image of an array of individually actuated micro-mirrors is projected onto a plane interior to the resist. We have achieved depth resolvability by ensuring that the femtosecond pulsed laser beam is both spatially and temporally focused. In addition, we have modified a commercial objective lens in such a way that the light propagation path length through the resist is only a fraction of the objective’s working distance. This has enabled us to print tall millimeter scale structures with index mismatched resists (such as acrylate monomers with fumed silica particles) via the dip-in printing mode. In combination, our work (i) increases the rate of two-photon lithography by a factor of at least 50 without adversely affecting feature resolution and (ii) broadens the applicability of dip-in printing mode to non-index matched resist materials.
Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-740672.