06-02 SCRIBE gradient refractive index

[Image above] A line grating fabricated with the SCRIBE method shows the University of Illinois Urbana-Champaign’s “I” logo with accurate colors. The grating on the left was fabricated without improvements, while the right grating used the refined procedure. Credit: Littlefield et al., ACS Photonics (CC BY 4.0)

As someone with suboptimal eyesight since seventh grade, I personally am very familiar with the world of glasses and contact lenses. In my case, either lens type can be used to correct my vision, but I have friends who require such extreme corrections that glasses are the only option. That is because corrective lenses lack something the eye’s natural lens has—a gradient refractive index (GRIN).

The refractive index is the measure of how much a light ray bends when passing from one medium to another. Currently, most engineered lenses have a uniform refractive index, meaning light only bends in a single, fixed way. To fine-tune how light is focused, multiple lenses with different refractive indices are required.

Minimizing the size of multistacked lenses is challenging, however. That is why the extreme corrections my friends require for their eyesight are more easily handled with glasses, which sit away from the eye, than contact lenses, which sit directly on the eye.

In contrast to engineered lenses, the eye’s natural lens has a spatially varying mass density that gives rise to a nonuniform (gradient) refractive index throughout the lens. It ranges from about 1.386 on the outer edges to about 1.406 in the center. This GRIN profile allows the eye to achieve good resolution in a compact format, without the need for multistacked layers.

Researchers have made significant progress over the past two decades in learning how to make bioinspired GRIN optics, as detailed in the October/November 2021 Bulletin. One key to this progress is the technique called direct laser writing, or multiphoton lithography.

This 3D-printing method uses a laser beam to “draw” (solidify) structures within a photoresist (light-sensitive) material. It can achieve structures with feature sizes on the nanometer scale.

In 2020, researchers led by University of Illinois Urbana-Champaign professors Lynford Goddard and Paul Braun proposed a unique way to use direct laser writing to achieve even greater control over the refractive index. Their method, called subsurface controllable refractive index via beam exposure (SCRIBE), involves performing direct laser writing inside photoresist-filled nanoporous silicon and silica scaffolds.

“The mesoporous hosts suspend the 3D structures and stabilize the variable fill fraction of the cross-linked photoresist, enabling refractive index control over a broad range (Δn > 0.3 at visible wavelengths),” they explain in the 2020 open-access paper.

Of course, as with any new technique, the researchers identified several shortcomings that could be addressed in future research. Their follow-up open-access paper published this March addresses some of these concerns.

“We 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 we made,” says Alexander Littlefield, lead author and graduate student in Goddard’s group, in a University of Illinois Urbana-Champaign press release.

They achieved this improvement by making three refinements to the SCRIBE procedure.

  1. Using a two-photon fluorescence imaging system to map the photoresist’s density. This information allowed the researchers to correctly calibrate the laser power to achieve the desired result.
  2. Modulating the material’s position as the laser writes to smooth out errors near the writing boundary.
  3. Introducing a time delay between laser exposures to minimize time-dependent effects in the photoresist interaction.

These refinements allowed them to increase the reliable refractive index range from 0.12 to 0.37, plus decrease the standard deviation in the refractive index by up to a factor of 60.

Though the procedures were developed using the photoresist IP-Dip and the direct laser writing instrument Photonic Professional GT2 by German electronics manufacturer Nanoscribe GmbH, “the general method could be applied to other resists and other serial DLW [direct laser writing] tools,” the researchers write.

The full data for this paper, including the codes used for calibration and fringe analysis, are available at this link.

The 2020 open-access paper, published in Light: Science & Applications, is “Direct laser writing of volumetric gradient index lenses and waveguides” (DOI: 10.1038/s41377-020-00431-3).

The 2023 open-access paper, published in ACS Photonics, is “Enabling high precision gradient index control in subsurface multiphoton lithography” (DOI: 10.1021/acsphotonics.2c01950).