High-throughput microfabrication of axially tunable helices

Light, as we know it, travels in a straight line. This also applies to a laser beam which is basically a more intense form of light. Lasers have been used in welding, drilling, cutting, forming and other manufacturing processes ever since they were invented more than 60 years ago. A new generation of lasers has been developed in the past few decades. Commonly referred to as "ultrafast lasers" or "ultrashort-pulsed lasers," this type of laser can output consecutive flashes of light ("pulses") each having very short duration (< 1 picosecond = 10-12 s).

 

These pulses are so short that thermal energy, which is the main cause of material damage induced when using other types of lasers, cannot dissipate too far from the laser irradiated region. The result is that the precision of laser processing can be improved to micron or even nanometer scale, enabling new applications including eye surgery, cutting display panels, and the fabrication of medical transplants.

 

Despite the unparalleled precision, so far ultrafast laser processing is largely considered as a slow fabrication method. This is because in most cases the laser beam is focused to a single "spot" ("Gaussian focus") in space, which limits how fast the laser beam can cover a large area or volume. One way to increase processing speed is projecting a 2D image instead of focusing to a single spot. This is a laser beam shaping technique and it can increase the processing speed for 2D patterns that are on the transverse plane (perpendicular to the laser propagation direction). However, for complex 3D structures, it is necessary to extend the technique to the axial direction (parallel to the beam propagation).

 

In the paper mentioned above, a team led by Prof. Xiaoming Yu at the University of Central Florida (UCF) collaborated with Prof. Stephen M. Kuebler's group at UCF and Prof. Meng Zhang's group at Kansas State University to demonstrate for the first time that microfabrication can be significantly accelerated using a three-dimensionally structured, "twisting" beam.

 

They demonstrated that a laser beam can be made to bend and even twist in space, enabling the fabrication of three-dimensional (3D) structures "volumetrically" and greatly increasing fabrication speed. This work has been published in Photonics Research Vol 10, No. 2, 2022 (He Cheng, Pooria Golvari, Chun Xia, Mingman Sun, Meng Zhang, Stephen M. Kuebler, Xiaoming Yu. High-throughput microfabrication of axially tunable helices[J]. Photonics Research, 2022, 10(2): 02000303).

 

Fig. Fabrication of a helix array. Each helix is fabricated volumetrically using a three-dimensionally structured, "twisting" beam.

 

The beam is generated by encoding information ("phase mask") to a Gaussian beam by a liquid-crystal device called a spatial light modulator. The phase mask looks like a swirling vortex which makes the beam "twist" while propagating down range.

 

The team combined this twisting beam with a microfabrication technique called "two-photon polymerization (2PP)", by which a liquid photopolymer ("resin") solidifies when it absorbs laser energy. The unique feature of 2PP is that the chemical reaction that triggers liquid solidification takes place only when the laser intensity reaches its peak, so the other part of the beam is nearly unaffected. The team's idea was that by combining structured light fields with 2PP, 3D structures can be fabricated using a single or a few exposures, so the fabrication time could be a fraction of what it normally takes using the conventional Gaussian beam.

 

In the paper, the team established a mathematical framework to model twisting beams and successfully fabricated micro-helices with tunable axial pitch length. This work is a step towards true-3D microfabrication and brings 2PP to many industrial applications such as the manufacturing of computer chips, medical devices, and microfluidic networks.