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Stephen R. Forrest

My research group focuses on fundamental issues surrounding photonic materials, devices, and systems. Our areas of emphasis can be divided into two general topics: group III-V semiconductor optoelectronic integrated devices, and organic thin film optical devices. In addition, there are projects that integrate the advantageous properties of both of these materials systems.

The projects currently being pursued include integrated transmitters consisting of lasers, waveguides, and semiconductor optical amplifiers. Here we are exploring new methods of integrating diverse optical components on a single III-V epitaxial wafer, finding means to reconcile the divergent materials requirements of different devices in a simple and high-performance manner. One way of achieving this is the "twin waveguide" approach, where optically active layers (for example, lasers) are positioned on top of waveguiding layers that interconnect different devices on the chip, with optical mode coupling between the layers providing the necessary interactions between the devices. In addition, we are exploring very high bandwidth, high-sensitivity avalanche photodetector optical receivers targeted for lightwave communications systems. Many of these optical receivers and transmitters require development of new materials systems to access different parts of the optical spectrum. Hence, we also have projects in the new materials system consisting of InGaAsN(P) alloys. Recently, we have demonstrated lasers for optical communications based on these semiconductors.

In organic thin film materials, we are also developing a range of integrated organic devices, which are perhaps best exemplified by the vertically stacked, full-color organic light-emitting devices for flat panel displays. This device, shown right, emits the red (R), green (G), and blue (B) primary colors through intervening transparent contacts and organic films developed specifically for this purpose. Each of the R, G, and B subpixels is independently energized by passing current through the appropriate layers. Electrically excited molecules, called excitons, emit light of a particular color, depending on the molecular structure and composition of that layer. Our work extends from this practical realization all the way to study of the fundamental properties of organic materials: how they grow in ordered layers, the nature of their excited states, and the limitations to charge transport through those layers. After more than fifty years of research in organic semiconductors, there is still much that is unknown about their basic properties. Our work is dedicated to understanding and using many of these properties to create an entirely new generation of optoelectronic devices.

Fabricating and growing materials for optoelectronic devices presents enormous challenges to materials and device scientists, and thus is a particular focus of our activities. We have constructed an ultrahigh vacuum gas source molecular beam epitaxial (MBE) growth system in which we can manipulate and control growth of semiconductor layers at the atomic level. The system is unique in that it is capable of growing structures based on GaAs and InP of the highest quality and level of control, in addition to crystalline organic thin films with an equal degree of precision. Our group pioneered the field of van der Waals epitaxy, or "quasi-epitaxy" now being vigorously pursued in many laboratories worldwide.