Massive scale-up of nanoscale 3D printing via projection two-photon lithography
Two-photon lithography (TPL) is a nanoscale additive manufacturing technique for the fabrication of arbitrarily complex 3D structures. Although TPL uses light to polymerize material, it can print sub-diffraction 3D volumetric pixels smaller than 200 nm. This capability makes it particularly attractive for printing of optical and mechanical metamaterials, photonic crystals, micromachines, and micro-optics. However, the slow speed of the process (~0.01 mm3/hr) makes it impractical for many applications. Past scale-up attempts have failed to achieve printing of fine submicron features. We have increased the processing rate by more than a thousand times by using time-domain focusing of femtosecond light. We demonstrated this by printing, within single-digit millisecond time scales, nanowires with widths smaller than 175 nanometers over an area one million times larger than the cross-sectional area. Our current work in this area includes elucidating the process mechanisms to enable high-fidelity and deterministic printing of 3D structures, generating physics-based and data-driven predictive models of photo-polymerization, and expanding the material palette beyond polymers.
Publication: Saha et al., Science, 2019. https://doi.org/10.1126/science.aax8760
Affordable nanoscale additive manufacturing via superluminescent light projection
The high cost of nanoscale additive manufacturing equipment (relative to commercial hobbyist 3D printers) is a significant barrier to its widespread adoption in real-world applications. A recurring theme in nanoscale patterning is the tradeoff that exists between precision (i.e., a surrogate for performance) and equipment cost. We have demonstrated how one may break this tradeoff through a novel light projection technique that leverages the advantageous coherence properties of superluminescent light. This has enabled us to reduce the cost of nanoscale metal printing by more than 35 times, while simultaneously enabling sub-diffraction printing with light that is a billion times less intense than the light required for two-photon lithography. Our current work in this area includes developing novel illumination techniques to overcome the practical rate limits, expanding the capability to all three dimensions, and understanding the underlying processing mechanisms for printing metals (i.e., nucleation and crystallization) and polymers (i.e., photo-curing).
Publication: Choi and Saha, Advanced Materials, 2024. https://doi.org/10.1002/adma.202308112
Scalable manufacturing of spherical foam targets for inertial fusion energy
Nuclear fusion promises clean, reliable, safe, and abundant energy with minimal radiation risks. It is the same process that powers the sun and generates the sunlight received on earth. However, achieving controlled fusion on earth is a major challenge. Inertial fusion is one way to achieve and control fusion. It requires holding the nuclear fuel in small pea-sized capsules, called targets. For inertial fusion energy (IFE) to be a practical source of electricity, the cost of making these capsules must be reduced from more than tens of thousands of dollars to less than a dollar. Our work aims to transform IFE target manufacturing from a low-volume, expensive effort into a cost-effective, mass-production enterprise. We are achieving this by generating the processing science for a scalable additive manufacturing approach for producing foam targets using the two-photon polymerization (2PP) technique.
Publication: Saha, Societal Impacts, 2024. https://doi.org/10.1016/j.socimp.2023.100029
Inverse design for manufacturing scalability, enabled by rapid and accurate process models
Over the past decade of research in nanoscale additive manufacturing (AM), there has been significant progress in improving the physical hardware to achieve high-speed printing. However, there has been a lag in improving the understanding of the underlying processing mechanisms, which has led to a lack of predictive models. In the absence of such models, process planning is performed through iterative trial-and-error runs, which are slow, resource intensive, and require highly skilled labor. It is therefore very challenging to perform inverse design, i.e., to select the input processing parameter values that will produce the desired 3D structures that meet design specifications. We are developing rapid yet accurate models of nanoscale AM processes that can enable this kind of inverse design and eliminate this barrier to scalability. For example, our models of photo-polymerization in projection two-photon lithography are 100× faster than commercial finite element packages, and have enabled simulating the printing of multi-layered 3D structures for the first time.
Publication: Pingali and Saha, ASME J. Micro-Nano Manuf., 2024. https://doi.org/10.1115/1.4065706