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1. Femtosecond 3D photolithography through a digital micromirror device and a microlens array

   https://doi.org/10.1364/AO.457847  

   Jakkinapalli, Aravind; Baskar, Balaji; Wen, Sy-Bor (May 2022, Applied Optics)

   Based on the microscale 3D point cloud projection with a digital micromirror device (DMD) and a microlens array (MLA) developed recently, we explore the capabilities of this specific type of 3D projection in 3D lithography with femtosecond light in this study. Unlike 3D point cloud projection with UV continuous light demonstrated before, high accuracy positioning between the DMD and the MLA is required to have rays simultaneously arrive at the designed voxel positions to induce two-photon absorption with femtosecond light. Because of this additional requirement, a new positioning method through direct microscope inspection of the relative positions of the DMD and the MLA is developed in this study. Because of the usage of a rectangular MLA, around four rays can arrive at each projecting voxel at the same time. Thus, to the best of our knowledge, a new algorithm for determining the pixel map on the DMD to the 3D point cloud projection with a femtosecond laser is also developed. It is observed that a very long exposure time is required to generate 3D patterns with the new 3D projection scheme because of the very limited number of rays used for projecting each voxel with the new algorithm. It is also found that 3D structures with desired shapes should be projected far away from the MLA ( $\sim <\#comment/>15f$to $30f$, with $f$being the focal distance of the MLA) in the 3D lithography with this femtosecond 3D point cloud projection. For patterns projected closer than $10f$, shapes are distorted because of unwanted voxels cured with the 3D projection technique using a DMD and MLA. 

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2. 3D photolithography through light field projections

   https://doi.org/10.1364/AO.399508  

   Zhang, Hongjie; Wen, Sy-Bor (September 2020, Applied Optics)

   A methodology of 3D photolithography through light field projections with a microlens array (MLA) is proposed and demonstrated. With the MLA, light from a spatial light modulator (SLM) can be delivered to arbitrary positions, i.e., voxels, in a 3D space with a focusing scheme we developed. A mapping function between the voxel locations and the SLM pixel locations can be one-to-one determined by ray tracing. Based on a correct mapping function, computer-designed 3D virtual objects can be reconstructed in a 3D space through a SLM and a MLA. The projected 3D virtual object can then be optically compressed and delivered to a photoresist layer for 3D photolithography. With appropriate near-UV light, 3D microstructures can be constructed at different depths inside the photoresist layer. This 3D photolithography method can be useful in high-speed 3D patterning at arbitrary positions. We expect high-precision 3D patterning can also be achieved when a femtosecond light source and the associated multi-photon curing process is adopted in the proposed light field 3D projection/photolithography scheme. Multi-photon polymerization can prevent the unwilling patterning of regions along the optical path before arriving to the designed focal voxels as observed in our single photon demonstrations. 

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3. Two-color 3D printing for reduction in femtosecond laser printing power

   https://doi.org/10.1364/OE.525826  

   Akash, Anwarul_Islam; Johnson, Jason_E; Arentz, Fredrik_C; Xu, Xianfan (July 2024, Optics Express)

   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. 

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4. 3D-printing nanocrystals with light

   https://doi.org/10.1126/science.add8382  

   Pan, Jia-Ahn; Talapin, Dmitri V. (September 2022, Science)

   The ability to fabricate custom three-dimensional (3D) objects on demand has revolutionized prototyping and small-scale manufacturing processes. From low-cost filament extruders that a hobbyist can use to replace a plastic battery cover, to laser sintering machines for metal spacecraft parts, the reach of 3D printing technologies in low- and high-end markets continues to broaden. A crucial part of this progress has been the expansion of the library of materials that can be 3D-printed. Nanocrystals have many functional properties, but their integration with 3D printing has been limited, mostly relying on the use of polymer material as a scaffolding. On page 1112 of this issue, Liu et al. ( 1 ) demonstrate the 3D printing of nanocrystals using a method known as two-photon lithography. The intense beam of an infrared femtosecond laser induces simultaneous absorption of two photons in a very small volume, triggering photochemical reactions at nanocrystal surfaces. 

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5. Investigation of Best Practices in Volumetric Reconstruction for Plenoptic PIV

   https://doi.org/10.18409/ispiv.v1i1.151  

   Sapkota, Bibek; Kelly, Dustin; Tan, Zu Puayen; Thurow, Brian S. (September 2021, 14th International Symposium on Particle Image Velocimetry)

   This paper investigates the effect of smoothing operation in 3D reconstruction using a plenoptic camera. A plenoptic camera - also known as light field camera - features a commercial off the shelf camera with added microlens array (MLA) behind the imaging lens, directly in front of the sensor. The main lens focuses the light to the MLA plane, where each microlens then re-directs the light to small regions of pixels behind, each pixel corresponding to different angle of incident (T. Fahringer (2015)) (Adelson and Wang (1992)). Thus, MLA encodes angular information of incident light rays into the recorded image that assist to acquire 4D information (u,v,s,t) of light-field including both position and angular information of light rays captured by the camera (Ng et al. (2005)) (Adelson and Wang (1992)). 

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