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The Next Big Thing:
Nanoscale Optical Structures Light the Way

The wavelength of light is measured in nanometers, so it's not surprising that optical scientists have a strong interest in a new generation of nanoscale devices for directing, amplifying, switching, processing, sensing and otherwise manipulating light.

photo by Gary Meek Nanoscale structures offer new applications for photonic band gap structures, optical waveguides, mirrors and other devices. Research scientist Jung Park uses a confocal microscope to study a self-assembled polymer structure that has potential photonic applications. (300-dpi JPEG version - 1.24Mb)

In April, Georgia Tech researchers announced a new self-assembly technique for producing complex three-dimensional polymer structures with potential applications as photonic bandgap materials, optical waveguides, laser arrays and beam steering systems. Reported in the journal Science, the simple technique involves directing moist air across a polymer material dissolved in a fast-evaporating solvent. In its simplest form, the process requires only that researchers exhale across the polymer solution.

"This represents an easy way of making materials with the regular structure needed for optical and photonic applications," says Mohan Srinivasarao, a polymer chemist in the School of Textile and Fiber Engineering and School of Chemistry and Biochemistry. "This is completely a self-assembly process. We have shown that with very little work you can form nicely ordered structures."

Supported by the National Science Foundation, Srinivasarao and colleagues produce interconnected arrays of spherical air bubbles in uniform sizes from 0.2 microns (200 nanometers) up to 20 microns in diameter.

The process begins with dissolving a polymer such as polystyrene in a fast-evaporating solvent such as toluene or benzene. The solution containing the dilute polymer is placed on a glass slide, and moist air is directed across it as the solvent evaporates rapidly, dropping the solution's temperature by as much as 25 degrees Celsius. Moisture from the warm air condenses on the surface of the solution, forming a layer of uniform-size droplets packed tightly together like billiard balls. Because the water is denser than the solvent, the layer of droplets sinks into the polymer, allowing another layer to quickly form on top of it.

The process repeats itself for one to two minutes until all of the solvent is evaporated. The water then evaporates layer by layer, leaving a highly ordered and interconnected network of air bubbles.

The hole diameter in this polymer-structure sample is 3.5 microns. (300-dpi JPEG version - 266k)

"We have focused on how to modify the refractive index so we can use these as a photonic bandgap material, which is the first application we will go after," Srinivasarao adds. "But what we will be able to do is limited only by the imagination."

In the nearby School of Materials Science and Engineering, Chris Summers leads a group investigating emissive and active photonic crystal structures for the near-infrared to visible spectral region. The group is investigating the properties of photonic crystals fabricated into thin films, as well as those formed by self-assembled opal and inverse opals, to tune and increase the intensity of luminescence in phosphors. They are also developing high-dispersion materials, and comparing the properties of two-dimensional thin-film photonic crystals with three-dimensional opal structures.

Developed over the past decade, photonic crystals are created by the self assembly of nanoparticles and the nanopatterning of optical materials to create a periodic dielectric constant that creates an optical band gap in the material. The crystals can act as optical cavities, waveguides or mirrors, offering researchers new mechanisms for controlling light.

"Photonic crystals offer new ways of adding to the functionality of optical materials and are expected to have a revolutionary impact on optical signal processing and data transmission systems for communications and displays," explains Summers, who is director of the Phosphor Technology Center of Excellence at Georgia Tech.

Fabricated from luminescent and elctro-optic materials, the crystals offer better control over the emission, spectral purity and dispersion properties of light. Ultimately, this will allow development of a range of new devices, such as displays with more highly saturated colors, faster response times and lower thresholds, and fast all-optical switches for optical communication and computing systems.

For more information, contact Mohan Srinivasarao, School of Textile and Fiber Engineering, Georgia Tech, Atlanta, GA 30332-0295. (Telephone: 404-894-9348) (E-mail: mohan.srinivasarao@textiles.gatech.edu);    or Chris Summers, School of Materials Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245. (Telephone: 404-385-0697) (E-mail: chris.summers@mse.gatech.edu)