Femtosecond lasers, capable of 3D printing complex nano-sized objects, are expensive and consume a lot of power. Purdue University researchers, including (left to right) Anwarul Akash and Jason Johnson, have discovered a technique using two lasers, which results in faster prints and less power consumption. (Purdue University photo/Jared Pike)

Two-photon polymerization is an advanced additive manufacturing technique that traditionally uses femtosecond lasers to polymerize materials in a precise, 3D manner. Although this process works well for making high-resolution microstructures, it isn't widely used in manufacturing because femtosecond lasers are expensive and increase the cost of printing parts.

"We combined a relatively low-cost laser emitting visible light with a femtosecond laser emitting infrared pulses to reduce the power requirement of the femtosecond laser," said Xianfan Xu, James J. and Carol L. Shuttleworth Professor of Mechanical Engineering. "In this way, with a given femtosecond laser power, the printing throughput can be increased, leading to a lower cost for printing individual parts."

In Xu's lab, they work with nanoscale optics and laser-based nano-optical engineering. They previously demonstrated a laser-based 3D printing technique that rapidly constructs complex nanoscale 3D objects with smooth features.

In new research just published in the journal Optics Express, the researchers show that this new two-laser approach reduces the femtosecond laser 3D printing power needed by as much as 50% compared to using a femtosecond laser alone.

"3D printing with high resolution has many applications, including 3D electronics devices, micro-robots for the biomedical field and 3D structures or scaffolds for tissue engineering," said Xu. "Our novel, 3D printing approach can be readily implemented in many existing femtosecond laser 3D printing systems."

In this experiment, a femtosecond laser at 50% power barely produces anything. But when combined with a relatively low-power nanosecond laser, it produces a complex 3D buckyball, just 15 microns in diameter.

Finding the right laser balance

The new work is part of the research team's effort to continuously improve the printing speed and reduce the printing cost for two-photon polymerization, which uses the phenomenon of two-photo absorption to precisely cure, or solidify, a photosensitive material.

"In a conventional two-photon polymerization printing process, the femtosecond laser is first used to initiate a photochemical process that reduces the inhibition species in the material before printing starts," said Xu. "We used a low-cost laser for this purpose instead."

The new approach combines single-photon absorption from a 532 nm nanosecond laser with two-photon absorption from an 800-nm femtosecond laser. For this to work, the researchers had to find the right balance between the printing and inhibition caused by the two lasers. They did this by creating a new mathematical model to help them understand the photochemical processes involved and to compute the combined effect of two-photon and single-photon excitation processes. They also used the model to identify the dominant processes controlling how much the femtosecond laser's power could be reduced while still achieving desirable printing results.

Printing detailed structures

After finetuning the new approach, they used it to print various 2D and 3D structures using reduced femtosecond laser power. These included detailed woodpiles measuring just 25 × 25 × 10 μm as well as a micron-scale buckyball, chiral structure and trefoil knot. Experimental results showed that the new method reduced the power required from the femtosecond laser by up to 80 percent for 2D structures and up to around 50 percent for 3D structures.

"This new printing approach could impact manufacturing technologies, influencing the development of devices across consumer electronics and healthcare sectors both now and in the future," said Xu. The researchers are now working to further improve the printing speed and reduce the cost of 3D printing.

The authors would like to thank the U.S. National Science Foundation for its support under the grant CMMI-2135585. Jason Johnson acknowledges the U.S. National Science Foundation for support under the Graduate Research Fellowship Program (GRFP) under grant number DGE-1842166.

Source: Xianfan Xu, xxu@purdue.edu, 765-494-5639

Two-color 3D printing for reduction in femtosecond laser printing power
Anwarul Islam Akash, Jason E. Johnson, Fredrik C. Arentz, and Xianfan Xu
https://doi.org/10.1364/OE.525826
ABSTRACT: 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 800nm femtosecond laser and single photon absorption from a 532nm 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.