University of Illinois researchers develop refinement to 3D printing technique for subsurface optical devices

3D printers are commonly used to fabricate objects by layering melted plastic or metal, but this method is not suitable for microdevices or situations where layering is not feasible. However, researchers at the University of Illinois Urbana-Champaign, specifically the groups led by Lynford Goddard and Paul Braun, have been working on a solution to this challenge. They have developed a process that allows for printing directly into the bulk of an existing three-dimensional material using a technique called multiphoton lithography.

In multiphoton lithography, high-intensity laser light is employed to print inside a porous material. The material used by the researchers is silicon that has been etched to have microscopic pores and then oxidized into transparent silica. These pores are filled with a special material called photoresist, which undergoes a chemical process to change its optical properties when it absorbs two photons simultaneously.

By focusing the laser light to create high intensities only in specific regions, the researchers can selectively modify the material’s optical properties in three dimensions. This process, known as subsurface controllable refractive index via beam exposure (SCRIBE), allows for the fabrication of custom small-scale optical devices. The researchers recently refined the procedure, resulting in significantly tighter control over the fabricated devices.

The improvements described in their article include enhanced efficiency of fabricated lenses, with an improvement from a baseline of 36% to a new value of 49%. They also achieved better color uniformity through the creation of 2D line gratings. The researchers believe that this new technique will enable a wide range of optical element designs.

Initially, a two-photon fluorescence imaging system is employed to create a comprehensive map of the photoresist’s density and accurately determine the optimal laser power required for achieving the desired outcome. Subsequently, they address noticeable errors near the writing boundary by adjusting the material’s position while the laser is in operation, thereby minimizing imperfections. Lastly, a time delay is introduced between consecutive laser exposures to mitigate any detrimental time-related influences on the interaction between the photoresist and the laser.

A line grating fabricated with SCRIBE to form the UIUC “I” logo with accurate colors. The left grating was fabricated without the improvements, and the right grating incorporates the improvements. Credit: The Grainger College of Engineering at the University of Illinois Urbana-Champaign

Through the implementation of three enhancements, the scientists successfully attained enhanced regulation over their patterned devices, resulting in the production of meticulously crafted components that exhibit significantly improved efficacy. In a remarkable display of the adaptability of their technique, they manufactured a 100-by-100-micrometer optical apparatus capable of manipulating light to create precise color patterns resembling the UIUC logo in both shape and hues.

“Our research serves as a testament to the remarkable potential of multiphoton lithography in the accurate fabrication of microscale optical elements, offering novel functionalities that remain unmatched by alternative manufacturing methods,” remarked Goddard.

Source: University of Illinois Grainger College of Engineering

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