Light Bent in 3-D Crystals

Optical waveguiding of near-infrared light has been achieved through features embedded in self-assembled, three-dimensional photonic crystals, a University of Illinois team reported.

The researchers said they are the first to produce the optically active crystals, which have applications including low-loss waveguides, low-threshold lasers and on-chip optical circuitry.

Key to the fabrication technique -- which uses multiphoton polymerization and a laser scanning confocal microscope -- is a self-assembled, colloidal material that exhibits a photonic bandgap, said the head of the team, Paul Braun, PhD, a professor of materials science and engineering at the university and head of the Braun Research Group there.

Paul Braun, a professor of materials science and engineering at the University of Illinois, leads a research group that has achieved optical waveguiding of near-infrared light through features embedded in self-assembled, 3-D photonic crystals. (Photo: L. Brian Stauffer; courtesy University of Illinois at Urbana-Champaign)

In previous work, reported in 2002, Braun’s research group was the first to show that through multiphoton polymerization they could embed a polymer feature inside a silicon dioxide, self-assembled colloidal crystal.

Now, in a paper accepted for publication in Nature Photonics and posted on the journal’s Web site, Braun and his team demonstrate actual optical activity in waveguides and cavities created in their colloidal crystals.

“Taking our earlier work as a starting point, we built upon recent advances in theory and computation, improvements in materials growth techniques and better colloidal crystallization capabilities to produce this new photonic material,” said Braun, who also is affiliated with the university’s Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory and Micro and Nanotechnology Laboratory.

To make their optically active devices, the researchers begin by assembling a colloidal crystal of uniform silica spheres that are 900 nms in diameter. After removing the solvent, the researchers fill the spaces between the spheres with a photoactive monomer. Then they shine laser light through a microscope and into the crystal, polymerizing the monomer at the desired locations.

Next, they remove the unpolymerized liquid and then fill the structure with silicon. Finally, they etch away the silica spheres, leaving the desired optical features embedded in a 3-D photonic crystal.

“Using spheres 900 nms in diameter creates a bandgap at 1.5 µms, which is the wavelength used by the telecommunications industry for transmissions through fiber-optical cables,” Braun said. “Creating these waveguides by coupling colloidal assembly and multiphoton polymerization is simpler and less expensive than conventional fabrication techniques, especially for large-area photonic crystals.”

With Braun, co-authors of the paper are Stephanie A. Rinne, a postdoctoral fellow at the Beckman Institute, and Florencio García-Santamaría, a postdoctoral research associate in the department of materials science and engineering.

The work was funded by the US Army Research Office, the National Science Foundation and the US Department of Energy.

For more information, visit: www.uiuc.edu