Fabrication of Microscale Optical Components Using Multiphoton Lithography

SCRIBE, which stands for “Scanning and Controlled Light-induced Birefringence,” is a form of multiphoton lithography used for high-resolution 3D printing and microfabrication. It is a powerful technique that enables the fabrication of complex, three-dimensional structures at the micron and sub-micron scale with high precision and resolution. SCRIBE utilizes the phenomenon of two-photon polymerization to create intricate structures. Two-photon polymerization is a nonlinear optical process in which the absorption of two photons occurs simultaneously, leading to the excitation of a photoactive molecule. By concentrating the laser light in a small focal volume, high-intensity conditions are achieved, allowing the polymerization of a resin. SCRIBE finds applications in various fields, including microfluidics, tissue engineering, photonics, micro-optics, and microelectromechanical systems (MEMS). It enables the fabrication of intricate microstructures, such as microchannels, microlenses, microscaffolds, and microdevices with high precision and complexity. SCRIBE, as a form of multiphoton lithography, combines the advantages of high-resolution 3D printing with the ability to fabricate intricate structures. SCRIBE  depends on a mechanism called two-photon absorption. The researchers use silicon that has been etched to have microscopic pores and oxidized into transparent silica. They then fill it with a material called photoresist, which can undergo a chemical process by which it changes its optical properties only when it absorbs two photons simultaneously a process that is quite rare unless very intense light is used.

In a new study published in the peer-reviewed journal, ACS Photonics, Professor Paul Braun and colleagues at the University of Illinois Urbana-Champaign used the technique of multiphoton lithography to print inside an existing porous material with high intensity laser light. This allowed the researchers to selectively modify regions of the interior and manufacture custom small-scale optical devices in a procedure called subsurface controllable refractive index via beam exposure, or SCRIBE.

The research team were able to show an improvement from a baseline of 36% to a new value of 49% in the efficiency of fabricated lenses and a clear improvement in the color uniformity resulting from the 2D line gratings. The researchers take advantage of this by focusing laser light to create high intensities only in specific regions. This allows them to create custom designs for the material’s optical properties in three dimensions to “write” optical components.

The authors used a two-photon fluorescence imaging system to map the photoresist’s density and correct the laser power needed for the desired result. Second, they smoothed out errors that are especially prominent near the writing boundary by modulating the material’s position as the laser writes. Finally, they introduced a time delay between laser exposures to minimize time-dependent effects in the photoresist interaction. By incorporating these three improvements, the researchers achieved tighter control over their patterned devices, achieving more precisely fabricated components that are much more effective. To demonstrate the versatility of their method, they fabricated a 100-by-100-micrometer optical device that alters light to form specific color patterns, a line grating, that reproduces the shape and colors of the UIUC logo.

Multiphoton lithography offers a powerful and versatile approach for fabricating microscale optical components with high resolution, three-dimensional capabilities, material versatility, and the potential for integration into complex optical systems. Its wide range of applications spans fields such as telecommunications, imaging, sensing, biotechnology, and micro-optics, opening up new possibilities for advanced optical devices and systems. The new study showed that multiphoton lithography can now accurately fabricate microscale optical components with new capabilities that do not yet exist for other fabrication methods.