Parallel Two-Photon Lithography Using a Metalens Array

Project Overview

This project validated the feasibility of using an array of metalenses to parallelize two-photon lithography (TPL) for high-resolution, large-scale additive manufacturing. TPL is a direct laser writing three-dimensional (3D) printing method with the highest resolution to date capable of producing submicron features. Being able to engineer material architecture at the nanoscale has enabled many breakthroughs in mechanical metamaterials, photonic crystals, and lithium batteries, but TPL suffers from extremely slow printing speed as a result of such high resolution. Because most applications for TPL are based on periodic structures like lattices, this feasibility study investigated the viability of using a prefabricated metalens array to enable the simultaneous TPL printing of identical unit cells within a periodic structure in a massively parallel fashion. This metalens TPL approach takes advantage of an array of focusing metalenses to split an incoming laser beam into multiple focal points with designed spacing inside a photoresist. The high laser intensity at each focal point triggers a two-photon polymerization process that locally crosslinks the photoresist. By moving the sample substrate with respect to the metalens array, identical and arbitrary 3D structures can be printed at each focal point by direct laser writing, which produces a connected or disconnected periodic array of repeating structures. We carefully analyzed the relevant photochemical reactions, the required energy output and pulse profile of the femtosecond laser, and the design and fabrication methods of metalenses, and we came to the conclusion that metalens TPL is highly scalable using existing technologies with a projected printing speed enhancement of more than 1,000 times compared to the state-of-the-art commercial two-photon 3D printers.

A key challenge that emerged during our investigation is the fundamental trade-off between having high numerical aperture (NA) lenses, which improves printing resolution, and having a sufficiently large focal distance, which allows for more robust motion control during printing. To overcome this limitation, we designed and simulated a novel crossover focusing scheme that improves the printing resolution beyond the diffraction limit without reducing the focal distance. On the experimental side, we built a proof-of-concept printing system with a commercial microlens array and demonstrated parallel printing of more than 1,000 two-dimensional patterns simultaneously with a diffraction-limited minimum feature size of 2 micrometers. This is the largest degree of multi-focal parallelization over a millimeter-scale area for two-photon printing to the best of our knowledge, which validated the feasibility of large-scale parallelization of TPL in a lens array configuration. These microlenses have low NA and are not index-matched with liquid photoresists so they cannot be used for 3D printing. In collaboration with Stanford University, we also designed and fabricated individual cylindrical metalenses that have high NA and can be directly interfaced with liquid photoresists. With continued research efforts, we will be able to demonstrate parallel 3D printing with custom-made metalens arrays in the near future and eventually scale up high resolution two-photon additive manufacturing to the wafer scale.

Mission Impact

This project continues Lawrence Livermore National Laboratory's research efforts of developing disruptive additive manufacturing technologies for the advanced materials and manufacturing core competency. The new capability of large scale two-photon 3D printing could enable development of architected materials with breakthrough performance and novel properties for energy storage and climate change mitigation (battery and CO₂ electrolyzer electrodes), high-energy-density science (composites and foams for dynamic compression and plasma pulse shaping), quantum science and technology (miniaturized ion traps), and other mission research challenges.

Publications, Presentations, and Patents

Xia, Xiaoxing, and Feigenbaum, Eyal. System and Method for Parallel Two-Photon Lithography Using a Metalens Array. U.S. Pat. App. No. 17/168,743, filed February 5, 2021.