Editorial overview: Optical Materials

Hewak, D.W. (2002) Editorial overview: Optical Materials. Current Opinion in Solid State and Materials Science, 5 (6), 469-470. (doi:10.1016/S1359-0286(02)00013-X).

Record type: Article

Abstract

Optical materials are now well established as key contributors to the technological advances of the past century. Without the optical transparency of silica glass, the ability to generate and manipulate light in semiconductors, crystals and glass, or create planar optical waveguides in analogy with advances in microelectronics, the speed and complexity of modern telecommunications networks could not have been realized.

The earth is now covered in optical fibre, hair thin threads of silica glass, carrying light pulses at speeds now approaching 40 Gigabits per second. It is estimated that hundreds of millions of kilometers of fibre has been laid under the sea and buried in the earth. Silica may be the ultimate optical material, however, for the most part its very inertness and stability, the properties that make it an ideal passive waveguide, limits its active applications. In general, the active characteristics of glass, its acousto-optic, magneto-optic and electro-optic properties, are weak. In addition, the ability to optically switch light, through non-linear effects, or to amplify light directly in a fibre, are for the most part difficult.

Nature has been kind in providing an amplification scheme in which rare earth doped silica glass provides gain and amplification around the key telecommunications window at 1.55 microns, the wavelength at which fibre loss is a minimum. The erbium doped fibre amplifier however is an exception rather than the rule and increasing data traffic is demanding optical amplifiers at other wavelengths emerging for use in telecommunications. To amplify light at wavelengths outside the current standard of 1.55 microns, for example around 1.3 microns, another widely used transmission window in telecom fibre, or at wavelengths near 1.4 and 1.6 microns, silica glass fibre is not practical. While optical amplification at 1.55 microns enjoys efficiencies of almost 100%, other dopants and amplification schemes in silica glass fibre are problematic. To actively manipulate light, to amplify and to develop all-optical networks, we must turn to new optical materials.

The contributions to this section begin a series of articles that will provide an overall review of the role of optical materials in today's technology, and provide a glimpse into the future of where these advances will lead. We begin in this issue with four contributions to the field, which cover diverse applications, but with a common theme, the active manipulation, and application of light.

The key technology enabling the rapid increase in capacity and speed in today's optical networks is wavelength division multiplexing. A series of discrete wavelengths, each carrying individual data streams are combined and transmitted through a single fibre. Each wavelength however requires amplification and here new optical fibre materials are being developed for this application. In his review paper, optical fibre materials, Sugimoto describes new glass technologies that are providing more efficient amplification at new wavelengths, as well as broadening and flattening the bandwidth of the 1.55 micron amplifier. Glasses based on heavy metal oxides, phosphates are considered, and the spectroscopy of new dopants to provide the gain medium required are discussed.

With the optical fibre amplifier replacing the bottleneck of electronic amplification and providing uninterrupted flow of light through a system, interest has been spurred in the area of all optical switching. The refractive index of a fibre varies nonlinearly with the intensity of incident light through a variety of electromagnetic phenomenon. Jha et al. reviews several important examples of nonlinear optical effects, the techniques used to characterize them, and the application of optical nonlinearity. In addition, the emerging fields of surface plasmon effects, microstructured fibre and photonic band gap structures and their applications are introduced.

Turning to organic materials, two papers look at emerging applications of the next generation of polymer materials. Curry and Gillin discuss recent advances of light emitting diodes based on organic polymers. In analogy with rare earth doped glass, key results are being obtained through the application of organolanthanide molecules which provide visible and infrared emission with high efficiencies. Advances in the field are leading to a clearer understanding of the mechanisms which quench radiative emission and reduce efficiency. Clearly polymers, with

their low cost and relatively simple manufacturing methods easily scaled to mass production, hold considerable promise. Yager and Barrett describe the exploitation of an unexpected property of polymers containing optical chromophores. Thin films of these materials respond mechanically to low power light, resulting in a transfer of an arbitrary optical pattern to the surface of the film, in a reversible process. They describe a number of proposed mechanisms for this effect, a topic that is under intense study. The ability to directly alter thin films in this way has attracted much interest for practical applications. Optical elements such as polarizers, couplers, filters have been produced. The reversible nature of the process has been exploited in the gratings used to tune laser emission wavelengths. As a direct patterning technique, features on a nanometer scale can be achieved.

In these four publications we see emerging themes and connections between the technologies being developed. New optical materials are required for the next generation of optoelectronic devices while a better understanding of the chemistry and physics of these materials is critical to fully exploit the technology.

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