New perspectives paper outlines the transformative potential of Goddard & Braun's SCRIBE approach

10/1/2021 Jenny Applequist

A new paper outlines the potential of SCRIBE -- an innovative lithographic approach for making three-dimensional optical and photonic structures inside a wafer -- relative to ULI, the previous state-of-the-art solution. The paper has been honored with a "Featured Article" designation by the Applied Physics Letters journal.

Written by Jenny Applequist

In late 2020, a team of researchers led by professors Lynford Goddard and Paul Braun unveiled a new technology called SCRIBE, an innovative lithographic approach for making three-dimensional optical and photonic structures inside a wafer. Now, a follow-up paper, which compares SCRIBE to the previous state-of-the-art and outlines a vision for future work, has been judged “one of the journal’s best” by the Applied Physics Letters (APL) journal, and been honored with a “Featured Article” designation.

Lynford Goddard
Lynford Goddard

For years, the leading method for direct-write subsurface fabrication was “ultrafast laser inscription” (ULI), in which a laser is used to permanently change the material properties of glass by heating it locally. However, as noted by Braun, “ULI can only make small changes in the material properties and therefore can offer relatively limited control over the optical properties.” Despite its long reign as the best available solution, “it’s had challenges with getting commercial adoption, because the performance is just not strong enough to enable a large number of convincing applications,” says Goddard.

The SCRIBE approach, instead of altering the properties of glass, uses a laser to selectively polymerize a photoresin within a porous scaffold. Goddard explains, “With our method, we’re able to get a significantly greater change in the material properties, such as the refractive index, which describes how much light is bent by the material.”

Paul V. Braun
Paul V. Braun

The new capability has far-reaching implications. “SCRIBE is much better than ULI when scaling the device size down,” he says. “SCRIBE’s high refractive index contrast leads to more compact waveguide footprints... The high contrast also enables higher numerical aperture lenses, ... which are important for focusing light into and collecting light from smaller volumes. SCRIBE also offers a continuously variable refractive index, which we can use to produce novel graded-index optical and photonic devices not possible with ULI.”

Those capabilities could have a range of important applications. In the new paper, the authors focused on SCRIBE’s potential to enable sending of data within and between computer chips via a dense network of three-dimensional waveguides.

Goddard notes that today, Internet data are sent long distances using optical fiber. If optical connectivity could be pushed from such large-scale applications down to the level of individual devices—say, to establish an optical connection between the hard drive and memory in a computer—we could overcome a range of current limitations in the rate of data transmission. The current optical connection technologies don’t scale down as easily as electrical connections, but SCRIBE may come to the rescue. “We want to make chips that can do that same sort of optical data transfer, but do it in a very compact way, so that we can get data communication between the different parts of the computer, especially the input/output ports.”

                                   An envisioned volumetric photonic integrated circuit (VPIC) optical interposer as enabled by SCRIBE.
                                   An envisioned volumetric photonic integrated circuit optical interposer as enabled by SCRIBE.

The new paper is an “APL Perspectives” article, meaning that the authors were invited to write it in order to suggest how the research community could move forward to further develop and apply the new technology. It is entitled “Toward the Realization of Subsurface Volumetric Integrated Optical Systems” and was authored by Corey Richards and Christian Ocier (current and former graduate students of Braun, respectively) and Jinlong Zhu (formerly a postdoc under Goddard, now at Huazhong University of Science and Technology), in addition to Goddard (who is Director of the Institute for Inclusion, Diversity, Equity, and Access; Associate Dean for Diversity, Equity, and Inclusion in the College of Engineering; and the Edward C. Jordan Professor of Electrical and Computer Engineering) and Braun (who is the Director of the Materials Research Laboratory, the Grainger Distinguished Chair in Engineering, and a Professor of Materials Science & Engineering and Chemistry). It appears in the September 27, 2021 issue of Applied Physics Letters (vol. 119, no. 13), and is available at https://doi.org/10.1063/5.0059354.


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This story was published October 1, 2021.