Profs. Dierolf and Cargill use gallium-nitride to develop light-emitting semiconductors.

One of the goals of the optics revolution is to develop all-optical networks that use light waves, not electronic components, to transmit light signals, switch the signals on and off, and amplify them.

An all-optical network, says Volkmar Dierolf, associate professor of physics at Lehigh and a member of the Center for Optical Technologies, would enable scientists and engineers to increase the capacity of optical transmission lines.

Developing all-optical networks is one of the major research thrusts of the COT. Jean Toulouse, professor of physics is heading up this effort, which includes faculty from physics (Toulouse, Ivan Biaggio and Dierolf) and materials science and engineering (Himanshu Jain), as well as several researchers from Penn State University, Lehigh's chief partner in the COT. Jointly, the group is developing nonlinear novel materials and studying nonlinear effects that can be used to manipulate light by using light.

Dierolf studies lithium niobate (LiNbO3), which is used in many nonlinear applications and is desired in integrated optical systems because of its favorable optical properties.

He has developed a method of using a laser to directly write a ferro-electric domain structure into lithium niobate crystals. The method shows promise for optimizing nonlinear phenomena, such as the conversion of an infrared laser into a green laser.

By "flipping" the ferro-electric axis up and down, says Dierolf, this pattern can be written directly using a laser with a confocal microscope. The depth of the writing is 30 microns; the period is 15 microns.

Dierolf's group has also learned to detect directly, with high spectral, spatial and temporal resolution, the moment at which the ferro-electric axis is flipping.

"This enables us to obtain feedback and control the phenomenon," he says.

Dierolf and Slade Cargill, department chair of materials science and engineering at Lehigh and also a COT member, have applied to the U.S. Department of Energy for a research grant. They are proposing to use electron beams, lasers and ion-beams to achieve structures with feature sizes of less than 5 microns.

Earlier this year, Dierolf published two articles on his work in ferroelectric domain imaging and waveguide imaging.

"Ferroelectric domain imaging by defect-luminescence microscopy" appeared in the Journal of Applied Physics and was co-authored by Chris Sandmann, a graduate student in physics at Lehigh; S. Kim and Venkat Gopalan of Penn State's department of materials science and engineering; and K. Polgar of the Institute for Solid State Physics and Optics of the Hungarian Academy of Science.

"Confocal two-photon emission microscopy: A new approach to waveguide imaging," co-authored by Dierolf and Sandmann, was published in the Journal of Luminescence.

In another project, which is supported by the U.S. Army Research Laboratory and the COT, Dierolf and Cargill are developing light-emitting semiconductors based on gallium-nitride. The semiconductors could have applications as sensors, or detectors. One of the researchers' goals is to use gallium-aluminum-nitride-based semiconductor materials, which possess a very wide bandgap and have shorter emission wavelengths that allow a tighter focus than visible light.

The Army is interested in this project because of the semiconductors' potential for detecting biological and chemical warfare agents.

The project is a multidisciplinary endeavor, combining Dierolf's expertise and the physics department's facilities for photo-luminescent analysis with the materials science and engineering department's facilities for cathodoluminescence, in which materials are characterized with electron beams.

Dierolf and Cargill hope to push the luminescence wavelength further into the ultraviolet region. This ability would be of great use to the display industry, particularly high-density information storage on DVDs and CDs. Because the smallest "spot" of storage is limited by the wavelength, light signals with shorter wavelengths afford more density and therefore more storage capacity.

Light-emitting displays (LEDs) have the potential for using less energy than liquid-crystal displays, which are illuminated by an inefficient phenomenon known as backlighting. Ultraviolet LEDs, combined with proper conversion of the wavelength into red, green, and blue, show promise for very bright active displays and thus for brighter computer monitors.

Dierolf has learned to do spatial mapping of luminescent layer displays with photolumescence, which allows him to see property variations in different locations on a device. Scientists seek displays with homogeneous properties everywhere; therefore, it is necessary to understand why and where properties are different, why a region of a display is bright or dull, why it emits or does not emit light.