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Chemistry of Materials for Photonics
It is important for ONR to support basic research on materials for photonic devices. Communication based on photonic transmission is expanding rapidly, and the medium offers greatly increased bandwidth. Unfortunately, the cost of conversion (modulation) of photonic data to electronic form is very high, and the systems tend to be complex. Operation of systems at video rates and above, for example, requires electro-optic modulators that depend on nonlinear optical materials. These materials are also critical to the design of optical processors and high-density optical storage systems. Continued and focused support of basic work on nonlinear optical materials will speed the availability of the inevitable “10-decibel (dB)” improvement in both data storage and computing speed that is certain to emerge from research in this area.
Second-order nonlinear optical materials have two principal applications. The first is electro-optic modulation, and it is suggested that ONR focus its support on chemistry related to the design of materials for this application. The second application is frequency doubling, and support in this area should emphasize novel techniques and approaches to phase matching as this is the gate to progress in the frequency doubling arena. Work on third-order materials should have a much lower priority, particularly work associated with systems based on organics with extended π-systems.
There has been considerable progress in the design of organic polymers for nonlinear optics. Further increases in the electro-optic coefficients are important, but achieving such increases is not the central problem. Systems that exhibit temporal stability of non-centrosymmetric alignment are required for practical applications. Generation of temporally stable alignment by means other than high-field poling would also be of interest.
Photorefractive materials represent a particularly interesting subset of nonlinear optical materials. These materials may be the enablers for practical, high-speed holographic information storage. Holographic storage would allow a huge improvement in data management, since data could be processed in page form (or image form) rather than bit by bit. Virtually every aspect of the technology required to implement high-speed holographic information storage is available, except the storage medium itself. The photorefractives offer a real prospect for meeting the requirements for this application. Recent work has demonstrated the photorefractive effect in organic materials, but basic research directed toward understanding the issues of temporal stability and separation of the chemistry underlying information retrieval and information storage remains to be done. Work on organic second-order nonlinear optical materials is directly pertinent to this effort, as well.
Pockels effect, second-order, nonlinear optical materials provide the basis for beam splitters and logic gates for photonic processors, and photorefractives offer promise for advanced photonic storage devices. However, the design of photonic processors is currently limited by the fact that there exists no analogue of the energy storage devices, capacitors, and batteries that are critical components of electronic processor technology. There is a need for research directed toward development of materials that would provide this function. Essentially, these materials can be viewed as substances that store photon energy, perhaps in
excited (metastable) electronic states that can be made to re-emit on command. Examples of photonic energy storage exist in the form of chemiluminescent materials, but the systems currently available are not applicable for a variety of reasons. This area of materials research requires a breakthrough that will emerge only as a result of basic research.
Displays represent another important area of organic materials research for information processing. Flat panel displays are currently based on twisted nematic liquid crystals and require at least one substrate transistor to drive each pixel. Use of surface stabilized ferroelectric liquid crystals would provide greatly increased switching speed and a wider viewing angle and would, in principle, simplify the display design. The address of each pixel would require only an orthogonal grid array, which is less demanding (and cheaper) to produce than the current method. The introduction of ferroelectric displays is limited by the instability of the surface alignment, leading to mechanical and thermal shock instability. The alignment techniques that have evolved empirically suffice for the twisted nematic liquid crystals, but they are inadequate for ferroelectrics. The alignment process, in which interactions of individual molecules with the surface give rise to bulk alignment in layers that are microns thick, is not understood. There are theories that describe the forces controlling alignment, but there is no reasonable explanation for the generation of tilt from this interaction. Yet, the stability of alignment and control of tilt are critical to a device's performance and stability. Basic research directed toward development of a molecular-level understanding of the interaction of mesogens and alignment surfaces is essential to further development of this important area.
There have been recent discoveries of anti-ferroelectric phases for display applications that are worthy of support. These phases have hysteresis that is accessible under an applied dc offset voltage that appears to provide some of the best characteristics of the ferroelectrics. The advantage of this approach is that removal of the bias voltage reportedly results in reversion to a state that is susceptible to alignment from classical surface preparation. Hence, in principle, if the display loses alignment, repair can be achieved by inducing changes in the dc potential across the cell.
Organic photo-emitting diodes also offer great promise for display applications. Major advances have been made in the use of semiconducting, polymer-based light-emitting junctions. A barrier to progress at this time is the need for reliable electrode structures that provide efficient electrical contact with the polymeric materials. Research is also needed on the nature of the properties of organic materials that render them useful as efficient electron transport media. Hole transport appears to be easily achieved in organics, but the number of materials that function as electron transport media is very limited, and these materials have been discovered largely by accident. Basic research to develop an understanding of the structural aspects of organics that allow them to mediate electron transport efficiently could have an important impact on this field.
Another area involving active organic photonic materials is that of resists for microelectronic device fabrication. Feature sizes are now in the range of a few tenths of a micron and will probably reach about two-tenths of a micron before the end of the century. This implies a need for image edge placement and control of about two-hundredths of a micron, or 200 angstroms. At these dimensions there are energy and molecular diffusion processes that can blur the placement and size of a device's features. For example, triplet
energy migration in polymers has been demonstrated over dimensions approaching one-tenth of a micron. In chemically amplified resists, diffusion of the photogenerated catalysts may contribute to blurring. Fundamental studies of these and other aspects of resist materials are essential to the continued evolution of integrated circuit dimensions and the computing power gain that flows therefrom. An area of special opportunity is basic studies of the phenomena that underlie resist function. Opportunities lie in studies of diffusion, energy transfer, dissolution kinetics, photochemistry, and other processes that are responsible for resist function.
