Photonic Crystals Grow Themselves

Barbara Grant

Researchers have experimented with photonic crystals for more than a decade, seeking to develop materials that can be used as building blocks for photonic devices. An innovation from the University of Rochester may radically change the way these crystals are grown.

A new polymer known as poly (phenylquinoline)-block-polystyrene can literally "grow itself" by organizing into complex mesostructures while still in solution, according to Samson A. Jenekhe, professor of chemistry, chemical engineering and materials science.

"We've designed its molecular structure so that it has the capability to self-organize," he said. The process, called hierarchical self-assembly, is similar to that exhibited by protein molecules, which organize themselves naturally into complex structures. Once in solution, the polymers grow into photonic crystals in a few hours. The approach, developed by Jenekhe and former graduate student X. Linda Chen, saves time and money over other photonic crystal growth techniques, which require human intervention and "a great deal of technological hand-holding," Jenekhe said.

A fluorescent micrograph shows self-organizing molecules beginning to assemble. Courtesy of the University of Rochester.

Deposited as a thin film onto a glass or silicon wafer substrate, the polymer reflects light similarly to the holographic structures seen on many credit cards. The material's periodic, spatial variation in refractive index modulates incident light to produce the iridescence.

Optical materials providing higher speed and efficiency than their electronic counterparts could dramatically improve telecommunications and computing. "What we're able to do now with electrons led to the microelectronics revolution," Jenekhe said. "We'd like to do the same with photons." By enhancing the crystal's emission properties, more efficient lasers and light-emitting diodes can be produced, he said.

Aiming for the visible

In its current configuration, the polymer modulates near-IR light, but Jenekhe's team is manipulating the relative amounts of poly(phenylquinoline) and polystyrene in the molecules to reduce mesostructure size to a few tenths of a micron. This reduction in scale would enable the polymer to modulate visible wavelengths, he noted. Researchers are also developing polymers suitable for visible- wavelength modulation that can self-assemble from water.

"Developing smaller-scale structures using unconventional fabrication techniques will result in a substantial savings of time and money," he predicted.