Photonics Goes Flexible



New Nanofabrication Method Could Enable New Generation of Optical Devices
By Mark Dwortzan

Integrating electronic and photonic components into curved, flexible, biocompatible surfaces could usher in a new generation of technology, from implantable medical monitoring devices to materials that shield people from radiation to invisibility cloaks. At the moment, however, we’re only halfway there: While the past decade has seen a dramatic increase in flexible electronics applications such as electronic paper-like display devices, researchers have made only limited progress in adapting photonics technologies to non-rigid materials—particularly those with features sized at the nanoscale.

Despite recent innovations in nanofabrication techniques, producing photonics devices on flexible surfaces has proven to be a complex, laborious process requiring multiple steps and a restricted range of material choices. But a new method developed by Assistant Professor Hatice Altug (ECE, MSE)—with her PhD students Serap Aksu (MSE), Min Huang (ECE) and Alp Artar (ECE) and two researchers from Northeastern University’s ECE Department—enables the patterning of nanoscale features on a wide range of flexible surfaces in a single fabrication step. The method, which is based on a lithography technique that uses stencils to pattern surfaces, is described in the frontispiece cover story of the October 11 edition of Advanced Materials.

“We demonstrated single-step fabrication of plasmonics (metallic structures that confine and manipulate light at the nanoscale) on a variety of flexible, stretchable and unconventional substrates using stencils,” said Altug. Aksu added, “These substrates included PDMS, a widely used polymer in micro and nanofluidics; parylene C, a biocompatible polymer; and even a plastic food storage roll film that the researchers purchased from a convenience store.”

In the Advanced Materials paper, the researchers showed that their new low-cost, high-throughput method quickly and accurately produces plasmonic features on stretchable, polymer film surfaces. The metallic, bow-tie-shaped features they fabricated on a PDMS surface were sized below 100 nanometers and spaced below 50 nanometers apart, all within a 10-nanometer error tolerance. The researchers also showed that their method is highly advantageous for transferring nanostructures onto highly curved surfaces such as small radius optical fibers, which could function as optical sensor probes to monitor processes or detect substances in hard-to-access places.

The method’s enabling technology, nanostencil lithography (NSL), applies the same principle as stenciling in arts and crafts, but at the nanoscale. After fabricating a silicon nitride stencil with a desired pattern of apertures, the researchers placed the stencil on a polymeric substrate. To create plasmonic structures, they deposited gold at the apertures. When they removed the reusable stencil, they obtained a nearly perfect transfer of the nanoparticle pattern with geometries complementing the stencil apertures.

The researchers thus demonstrated for the first time that NSL can enable single-step, high-throughput, large-area fabrication of nanostructures on flexible materials at the nanoscale.

“Existing approaches for device fabrication on polymeric surfaces are either complex, specific to certain materials, involve multistep fabrication, or are more suitable to process structures with micron-scale dimensions as oppose to nanoscale,” Huang explained. “In contrast,” said Artar, “our fabrication method is much simpler and involves only a single fabrication step, and, as a result, provides better resolution. In addition, unlike any other method, we can easily clean and reuse our mechanically robust nanostencils many times, which is essential for high-throughput fabrication.”

The researchers next aim to optimize the NSL fabrication technique further on flexible substrates for novel applications including biosensing.

This research is supported by the National Science Foundation, Office of Naval Research, Massachusetts Life Science Center, BU Photonics Center and Army Research Laboratory.