Glass serves a critical function in diverse technologies ranging from medicine to telecommunications. Fax machines and text scanners use micro-optic lens arrays made from ion-exchanged, gradedindex glass to image original documents. Data for long-distance communication by telephone or computer are transmitted in the form of modulated laser light through highly transparent glass fibers. These signals are boosted for long-haul transmission by stimulated emission in erbium-doped glass optical amplifiers. Lap-top computers use flat-panel-display glass in their liquid-crystal screens (see “Materials for Flat-Panel Displays,” MRS Bulletin, March 1996). Gall-bladder and hysterectomy surgeries have been made minimally invasive in part because of slender glass imaging endoscopes.

Functionality in glass is expanding rapidly. New properties are often attributable to new defect species created by strong optical or electrical fields. For example rare-earth-doped ZBLAN glass (a commercial fluoride glass), when exposed to high peak-intensity femtosecond laser pulses, becomes phosphorescent. (See the cover of this issue.) The phosphorescence is visible to the naked eye up to 12 hours after illumination. Other compositions show similar effects. Phosphorescent glasses are transparent and may be more easily formed into contiguous films and fibers than powder or microcrystalline phosphors.

Glass is the most important material in optics, with uses in optical fibers, lenses, mirror substrates, and prisms. The silicate family, the prime material in this article, is well-suited for passive optical functions such as light guidance in a fiber. Because of its many desirable properties, one is tempted to use glass in active functions as well—such as in amplification and modulation of light. As early as the beginning of the 1960s, glasses doped with rare-earth ions were used as gain media for lasers. Nd:glass lasers and Erdoped fiber amplifiers are two examples of the success of the use of doped glass to perform an active optical function. The modulation and switching of light proved to be a more difficult task to perform with glass systems. Glass has very low optical nonlinearity, and the application of an electric field does little to the optical wave traveling in the material.

In 1991 Myers, Mukherjee, and Brueck submitted a polymer film on a silica substrate to a poling process, which consists of the application of high voltages at a temperature of ~300°C. After cooling to room temperature with the voltage still applied, the poled sample was illuminated with a strong infrared (ir) laser beam. As expected they found that material frequency-doubled the incoming radiation and some green light could be measured at the output. When a test was carried out to discard any contribution from the substrate to the frequency-doubled light, they discovered that it was the silica glass that was generating the green light and that they had induced a strong optical nonlinearity in silica by thermal poling. Since then poling of glasses in the context of nonlinear optics has been the subject of many publications, including the report that ultraviolet (uv) radiation can advantageously replace thermal excitation.

In 1978 Kenneth Hill of the Communications Research Centre in Ottawa discovered that intense blue light propagating inside the core of a germanium-doped silicate glass fiber modified the core refractive index sufficiently to form a measurable permanent hologram. Because germanium-doped silica is the material of choice for the core of most optical fiber in use today for optical communications, this “photosensitive” phenomenon has been recognized as having tremendous practical importance. Fiber Bragg gratings in particular form excellent narrow-band optical filters with a multitude of applications: sensors, fiber laser mirrors, wavelength multiplexers (the acronym for wavelength multiplexing and demultiplexing in systems is WDM), and dispersion control devices—to name a few that are already commercially available. The importance of this field can be verified in just about any current issue of journals related to lightwave technology and applications of optics. Fiber Bragg gratings are mentioned everywhere. In fact a recent issue of the Journal of Lightwave Technology is entirely devoted to this topic (and to poling of silica, a field reviewed by W. Margulis in this issue).

An estimated 11 million people in the United States have at least one medical-device implant. Orthopedic implants account for 51.3% of all implants. They include fixation devices (usually fracture fixation) and artificial joints, used in 77% and 23% of the cases, respectively. Among the joint-replacement procedures, hip and knee surgeries represent 90% of the total and in 1988 were performed 310,000 times in this country. Currently more than half the joint-reconstruction devices are used with bone cement, which is a polymer grout that keeps the prosthesis components in place in the bone. The fixation in the other cases depends on the bone's ability to grow in contact with the device. This can be achieved by making the prosthesis surface porous such that bone grows into interstices or by making the surface chemically reactive with bone tissue such that a continuous, uninterrupted transition is formed from tissue to device. Bioactive glasses (BGs) and ceramics are the materials of choice to achieve this effect on bone-tissue bonding. Bone-growth stimulation is also sought in the treatment of difficult fractures. In the United States alone, there are 1.23 million fractures that require a bone plate. Of that total, approximately 1 million require between 10 cm3 and 100 cm3 of graft material to stimulate bone repair. At this time, autogenous bone graft represents the gold standard: This graft is typically bone tissue taken from the patient's own pelvic bone. Given the morbidity associated with this procedure and the frequently insufficient quantities available, extensive efforts for suitable alternatives are currently under way. Calcium phosphate ceramics and glasses, either by themselves or as carriers for bone (or “osteogenic”) cells or various bone-growth factors, are also prime candidates for these applications.

The range of applications for ionically conducting materials in technology is very broad and includes batteries; fuel cells; sensors; electrochromic displays; catalysts, oxygen, and hydrogen pumps; and bionics. In most of these applications, the mobility of the ions is a determining factor for optimum performance. Examples are liquid-electrolyte and plastic Li+ ion batteries for portable electronics, solid oxide fuel cells such as the oxygen ion-conducting stabilized zirconia electrolyte for energy conversion, and exchange of different ions in oxide glasses for the fabrication of micro-optic lenses. Therefore research on the dynamics of ionic transport that clarifies the mechanisms limiting the mobility of the ions is useful. These dynamic processes are also fascinating from a basic research point of view and qualitatively follow general patterns independent of the chemical and physical structures of the materials, suggesting that some fundamental physical mechanism is at work. On a quantitative level, the ionic-transport properties depend on the structures of the materials seemingly in well-defined patterns, which can be used to advantage in choosing materials for specific applications.

Low-phonon energy glasses are desirable hosts for rare-earth (RE) ions because they enable emission from RE energy levels that would otherwise be quenched in high-phonon energy glasses. Such emissions are of interest for fiber amplifiers operating at telecommunications wavelength band s of 1.31, 1.46, and 1.55 μm, and for up-conversion lasers and three-dimensional displays.

Phonons are optical-frequency molecular vibrations in a material. If the RE energy level of interest lies only a few phonons in energy above the next lower lying level such as the 1G4 level of Pr3+, which is only 3,000 cm −1 above the 3F4, only three Si—O vibrational phonons (1,100 cm−1) are required to bridge the gap as shown in Figure 1. Thus any electrons excited to the 1G4 level via an external pump source will be deexcited to the 3F4 on down to the 3H4 ground state via phonons, and no radiation of usable light will be produced. This is why emission from the 1G4 level of Pr3+ is absent in silicates and why researchers have gone to great lengths to make low-phonon energy glasses.

Phosphate glasses generally have lower glass-transition temperatures and greater thermal-expansion coefficients than silicate and borate glasses, and so are candidate materials for specialty glass-metal sealing applications. Phosphate glasses also have desirable optical properties (rare-earth stimulated-emission cross sections, low thermo-optical coefficients, ultraviolet transparency, etc.) and so are used as laser glasses. Recent developments of novel compositions for biomedical applications and as fast-ion conductors have also revived interest in the study of phosphate glass structures.

The structure/property relationships in simple phosphate glasses have not received nearly the attention paid to silicate and borate-based systems. This is partially due to the difficulty in preparing anhydrous compositions with more than 50-mol% P2O5. Ultraphosphate compositions (where the [O]/[P] ratio is less than 3) are highly reactive to ambient water. In addition because P2O5 sublimes at about 300°C, ultraphosphate glasses have not been studied to the extent that metaphosphate ([O]/[P] = 3) and polyphosphate ([O]/[P] > 3) compositions have.