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.

Semiconductor “quantum dots” refer to nanometer-sized, giant (103–105 atoms) molecules made from ordinary inorganic semiconductor materials such as Si, InP, CdSe, etc. They are larger than the traditional “molecular clusters” (~1 nanometer containing ≤100 atoms) common in chemistry yet smaller than the structures of the order of a micron, manufactured by current electronic-industry lithographic techniques. Quantum dots can be made by colloidal chemistry techniques (see the articles by Alivisatos and by Nozik and Mićić in this issue), by controlled coarsening during epitaxial growth (see the article by Bimberg et al. in this issue), by size fluctuations in conventional quantum wells (see the article by Gammon in this issue), or via nano-fabrication (see the article by Tarucha in this issue).

Colloidal suspensions have been part of the repertoire of human industry since Paleolithic artists first dispersed pulverized minerals in rendered animal fat to make paint. Remarkably, colloidal suspensions' primary industrial applications have changed little in the 20 or 30 millennia since. People use colloidal suspensions to disperse materials into fluid media without dissolving them. Familiar examples include surface coatings such as paints and inks, and slurries used for paper making and powder transport. Dividing the material finely enough to disperse it sometimes instills the resulting particles with novel properties. Ancient Romans took advantage of quantum confinement effects in metallic and semiconductor nanoclusters to color glass—research on the mechanisms and applications of nanoclusters' properties is ongoing. In all of these cases, the dispersed material is the important agent; colloidal dispersal simply facilitates its exploitation. More recently, colloidal particles have been deliberately used to modify the rheological properties of their carrier fluids. For these applications, the material from which the colloidal particles are made is less important than how their dispersal influences the medium.

This issue of MRS Bulletin focuses on a new class of applications for colloidal suspensions stemming from some particles' innate ability to organize themselves into beautifully ordered arrays. These colloidal crystals acquire interesting and useful properties not only from their constituent materials but also from the spontaneous emergence of mesoscopicorder that characterizes their internal structure.

The enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials—in which light can be generated, guided, modulated, amplified, and detected—need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. Long-distance communications, local-area-networks data transfer, and chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions. Silicon is the material of choice for reliable and low-cost optoelectronic integrated circuits because it is the leading semiconductor in the electronic arena and since a wellestablished processing technology exists for this material. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable for the implementation of fundamental optical functions such as light emission and modulation. At the moment, hybrid integration of compound-semiconductor optical functions with Si electronic functions is providing the gateway from electronic to photonic technology. However several strategies are being considered to engineer the optical functions of Si and to realize fully Si-based or at least Si-compatible optoelectronics.

Silicon-on-lnsulator (SOI) technology has been around since the 1960s when so-called silicon on sapphire (SOS) was first introduced. Silicon on sapphire has been used for many years for the fabrication of spaceborne and high-speed integrated circuits. It is still used in the fabrication of radio-frequency circuits.

More recent SOI materials involve only silicon and silicon dioxide—the two most common materials used in the fabrication of integrated circuits—as opposed to SOS, which requires the use of an alumina substrate.

Silicon-on-insulator technology has been used for a long time in niche applications such as spacecraft electronics and devices operating in a hightemperature or radiative environment. Recently however much attention has been paid to SOI technology because it is extremely suitable for the fabrication of low-voltage integrated circuits. Such circuits are in high demand for all kinds of portable systems, ranging from cellular phones to laptop computers. In August of 1998, IBM, Sharp, and other semiconductor manufacturers announced the development of SOI chips for high-speed computing and telecommunication con-sumer electronics. Most major semiconductor companies are putting considerable effort into SOI-circuit development for mainstream low-power applications.

The diamond films of the early 1980s presented two quite different challenges. First how could this new form of diamond be exploited technically? Second, how could this clearly nonequilibrium generation of diamond be understood and the understanding be used to maximum effect? We shall be discussing the ideas of theory and modeling, and we will show how they have contributed to the interplay of science and technology.

The science of diamond films is the art of beating nature in the use of carbon. Theory gives the understanding to improve this art. One way in which we improve on nature is in new geometries: controlled growth over selected surfaces o surface regions. The coverage, defect density, microstructure, and rate of growth are key issues. Another way to beat nature is controlled doping. Could wmake n-type semiconductors or lasers using diamond films? A third direction might be routes to control interfaces. Grai boundaries and the regions between small, misaligned crystals affect thermal properties and electron emission. Difficulties with electrical contacts may limit the use of diamond films as semiconductors or insulators. Substrate-film adhesion can determine tribological performance.

If theory is to play a role in controlling film deposition, we need to understand the role of theory itself. Theory can add value at several distinct levels. At the highest level, modeling has the potential to provide a substitute for experiment, especially when information is needed about behavior at extreme conditions. When the phenomena are very fast or very complex, theory can be used to interpret limited experiments. At a more modest level, even simple quantitative models can illustrate the many processes occurring during film growth. Atomistic theories of this type can identify the rate-determining steps and point to ways of influencing them. Mesoscopic theories, especially combined with macroscopic approaches like elasticity theory, can identify routes to improved performance.

In order to gain a fundamental understanding of friction, one must understand how the energy associated with work performed to overcome the frictional force is converted into heat at the molecular level. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline layer adsorbed on the surface of an ideal, atomically flat crystal. This system is ideal for studies of “interfacial” friction—that is, friction attributable to atoms and molecules immediately adjacent to the plane along which sliding occurs. Moreover it is directly accessible to experiments with a quartz crystal microbalance (QCM) and to theoretical studies through analytic calculations or molecular-dynamics (MD) simulations. The geometry is vastly simpler than that of contact between macroscopic objects in which the friction necessarily reflects the collective behavior of a multitude of buried contacts. (See the article by T. Baumberger and C. Caroli in this issue.) It is also far simpler than a case in which shear occurs within the bulk of a material rather than being confined to an interface. (See the article by D.A. Rigney and J.E. Hammerberg in this issue.) Nonetheless even in this simple geometry, friction can be far from negligible. A remarkable range of shear stresses, from 10−2 to 1010 N/m2, have been measured by a variety of techniques for wear-free geometries involving contact between crystalline interfaces.

Perhaps the most remarkable observation one can make about colloidal suspensions is that they exist at all. Particles dispersed in a fluid medium have a natural tendency to aggregate under the influence of van der Waals attraction. Yet the fortunes of a great many natural and industrial processes require colloidal particles to remain dispersed or at least to aggregate at a controlled rate. The existence of colloidal suspensions as varied as milk, inks, and metallic sols attests to the efficacy of a variety of stabilizing mechanisms. As early as 1809, Russel realized that many naturally occurring colloidal particles are charged. By the end of the century, Schultz and Hardy demonstrated that the resulting electro-static repulsions were strong enough to stabilize their suspensions against flocculating. This mechanism—arguably the best understood—continues to yield new surprises despite more than a century of analysis. The most recent burst of activity has been driven by the development of new and quite general techniques for measuring colloidal and macromolecular interactions. Its counterintuitive result—that like-charged particles some-times attract each other—may have ramifications in areas as diverse as protein crystallization, self-assembly of nano-structures, and the stabilization of industrial suspensions. This article touches briefly on the well-established theory of electrostatic stabilization in colloidal suspensions. The emphasis here is on the approximations that have provided the community with an analytical theory at the expense of overlooking recently discovered effects.

Scientists and engineers are most familiar with single-crystal or polycrystalline field-responsive or “smart” materials with responses typically occurring while the materials remain in the solid state. This issue of MRS Bulletin focuses on another class of field-responsive materials that exhibits a rapid, reversible, and tunable transition from a liquidlike, free-flowing state to a solidlike state upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior in response to an externally applied electric or magnetic field and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids, respectively. They are often described as Bingham plastics, and exhibit a strong field-dependent shear modulus and a yield stress that must be overcome to initiate gross material deformation or flow. Prototypical ER fluids consist of linear dielectric particles (such as silica, titania, and zeolites) dispersed in nonconductive liquids such as silicone oils. Homogeneous liquid-crystalline (LC) polymerbased ER fluids have also been recently reported. MR fluids are based on ferromagnetic or ferrimagnetic, magnetically nonlinear particles (e.g., iron, nickel, cobalt, and ceramic ferrites) dispersed in organic or “aqueous liquids. Unlike ER and MR fluids, ferrofluids (or magnetic fluids), which are stable dispersions of nanosized superparamagnetic particulates (~5–10 nm) of such materials as iron oxide, do not develop a yield stress on application of a magnetic field. Applications of ferrofluids are primarily in the area of sealing devices (see Rosensweig for more information). Since ferrofluids are well-known and have been extensively discussed elsewhere in the literature, they will not be treated in detail here.

“With a lighter but stiffer shaft and heavier clubhead the ball goes farther. Easier,” claims a 1975 advertisement for Shakespeare graphite irons. New materials such as graphite, boron, and titanium, have made sports equipment stronger yet lighter and thus more powerful. For athletes, sports have become, as the ad stated, easier. Serves over one hundred miles per hour are commonplace on the professional tennis tour, and athletes such as Tiger Woods are making par five golf holes obsolete. Sports organizations do not, however, always embrace these innovations that facilitate play. Major League Baseball retains its traditional mandate requiring only wood bats, the International Tennis Federation prohibited double strung tennis rackets, and the United States Golf Association banned asymmetrically dimpled golf balls. These technology regulations emerged to prevent the sport from becoming “easier,” protecting sport integrity.

Silver gallium sulfide (AgGaS2), silver gallium selenide (AgGaSe2), and silver gallium indium selenide (AgGaxIn(1 − x)Se2) are unique nonlinear crystals suitable for a wide range of three-wave mixing applications. They combine strong nonlinear coupling with phase matching across a wide (0.5 − 12.5 μm) transmission range. These properties provide the basis for CO2 laser harmonic-generator (HG), visible (VIS), and near-infrared (NIR) pumped opticalparametric-oscillator (OPO) and sumand difference-frequency-generator (SFG/DFG) systems for producing tunable laser radiation from approximately 0.65 to 12 μm. A wide range of pump lasers is now available, including diode lasers capable of direct pumping of AgGaS2 and efficient diode-pumped lasers for pumping any of these crystals. Significant progress in material characteristics and nonlinear optical (NLO) performance has been accomplished. Bulk absorption for production-quality AgGaS2 is ~0.01 − 0.02 cm across the 0.8−9-μm region. Exceptional crystals of AgGaS2 have absorption <0.0005 cm−1 at 1.064 μm, and even at 0.633 μm, absorption is ~0.015 cm−1. Figure 1 illustrates the late 1980s vintage and present transmission of AgGaS2. For AgGaSe2, absorption for production material ranges from 0.010 cm to 0.018 cm−1 from 1 μm to 11 μm. Certain crystals can have 0.007−0.008 cm−1 absorption at 10.6 μm. The NIR transmission curves for AgGaSe2 and AgGaxIn(1 − x)Se2 appear in Figure 2. This combination of optical properties, along with their good physical characteristics, make these two crystals versatile for NLO applications spanning the VIS to the infrared (ir).

A central challenge to modern materials science is the rational design and synthesis of new materials possessing exceptional properties. Recent advances in first-principles modeling methods and the availability of increasingly powerful computational resources make this goal increasingly achievable. The strength of these modeling methods lies in their predictive ability. They are able to reproduce the crystal structures and elastic properties of a large class of materials to within 2–3% of experimental values and have predicted a number of phase transitions that have been verified experimentally.

Despite the power of these methods, the process of designing materials from first principles is not usually a straight-forward or simple one. It requires overcoming a number of obstacles, some of them quite formidable. First a calculable figure of merit that correlates well with the desired property must be identified. While this may be straightforward in some cases, in others—such as predicting the ability of a material to isolate radionuclides over million-year time scales—the process of reducing complex properties to a few calculable variables can be rather difficult. Next a promising chemical system and a realistic set of crystal structures must be selected. This is not trivial because predicting the structures that can crystallize in a given system can be exceedingly challenging. However a wide variety of methods are available to aid in the generation of promising structures — comparative crystallography, algorithms based upon the concepts of crystalline nets and close packing, modern alloy theory methods, and simulated annealing strategies being some examples.

The following article, originally published in Materials Research Society Proceedings Volume 482, is based on the address that Shuji Nakamura, recipient of an MRS Medal Award, presented during Symposium X of the 1997 MRS Fall Meeting. Nakamura was recognized for “the development of lattice-mismatched GaN based heteroepitaxy and its application to the creation of blue and green light-emitting diodes and short wave-length laser diodes.”

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).

The study of nanometer-sized semiconductor crystals has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical and chemical properties can be systematically tuned by variation of the size, according to increasingly well-established scaling laws. This article describes colloidal semiconductor nanocrystals belonging to the II-VI and III-V families, and outlines strategies for obtaining electrical access to such dots.

If an inorganic cluster exceeds a certain size—generally in the 10s of unit cells—then it will likely possess a bonding geometry characteristic of a bulk phase. Above this critical size, the nature of the chemical bonds in the cluster remains fixed as a function of the size, but the total number of atoms—or the surface to volume ratio—changes smoothly. This leads to a slow extrapolation of the properties for ideal nanocrystals toward bulk values with increasing size, according to the scaling laws. The ability to control systematically the properties of inorganic materials by variation of size and shape is an important development with many implications for how materials should be processed and assembled.

Many scaling laws have been investigated, including the size variation of bandgap, charging energy, magnetization reversal, and melting. Study of the scaling laws reveals lessons for how to make nanocrystals. This article focuses on the properties of colloidal semiconductor nanocrystals, how to make them, and ways of gaining electrical access to them. Advances in metal, magnetic, and structural nanomaterials are also occurring. Semiconductor dots produced by other processing techniques in the articles by Bimberg, Gammon, and Tarucha in this issue.

This article focuses on the synthesis and processing of novel magnetorheological (MR) fluids. The process for preparing MR fluids typically involves introducing magnetic particles into base liquid under low shear conditions. This is followed by ball milling in the fluid with zirconia (ZrO2) grinding media for about 24 h. High-purity carbonyi iron (Fe) powders have been used for the synthesis of ironbased MR fluids while the ferrite-based MR fluids used magnetic manganesezinc ferrite and nickel-zinc ferrite powders.

Typical volume fractions of the magnetic phase that lead to MR fluids with respectable yield stresses tend to be about 0.3–0.5. Higher volume fractions, in principle, can lead to higher strength MR fluids. However, higher volume fractions tend to cause a significant, and often undesirable, increase in the “off-state” viscosity of the MR fluids. The rationale for selection and the role of different components of MR fluids are briefly discussed in the following sections.

The basic requirement golfers have about clubs is simple: “Help me to lower my score.” To the materials scientist, this means designs using the best materials. Golf clubs can be separated into three categories—woods (for long-distance hitting), irons (medium distance and accuracy), and putters (to get the ball into the hole on the green). In this article, we will discuss how material developments—in association with changes in design—have contributed to lower scores. It is very difficult to compare past achievements with those of today, perhaps with the exception of driving distances. However clubs have evolved tremendously, and it is difficult to imagine that Bobby Jones using hickory shafts could compete—at least in distance—with John Daly, Freddy Couples, or Tiger Woods, the present-day warriors armed with equipment often constructed from highstrength steel or graphite-epoxy shafts and an oversized hollow titanium head. Today professional golfers are driving farther, hitting greens with greater regularity, and sinking longer putts with the equipment they now have available attesting that the clubs constructed from advanced materials are contributing to better performance.

Silicon-based optoelectronics is a diversified technology that has grown steadily but not exponentially over the past decade. Some applications—such as smart-pixel signal processing and chip-to-chip optical interconnects—have enjoyed impressive growth, whereas other applications have remained quiescent. A few important applications such as optical diagnosis of leaky metal-oxide-semiconductor-field-effect-transistor circuits, have appeared suddenly. Over the years, research and development has unveiled some unique and significant aspects of Si-based optoelectronics. The main limitation of this technology is the lack of practical silicon light sources—Si lasers and efficient Si light-emitting devices (LEDs)—though investigators are “getting close” to the LED.

Silicon-based optoelectronics refers to the integration of photonic and electronic components on a Si chip or wafer. The photonics adds value to the electronics, and the electronics offers low-cost mass-production benefits. The electronics includes complementary-metal-oxide semiconductors (CMOS), very large-scale integration (VLSI), bipolar CMOS, SiGe/Si heterojunction bipolar transistors, and heterostructure field-effect transistors. In this discussion, we will use a loose definition of optoelectronics that includes photonic and optoelectronic integrated circuits (PICs and OEICs), Si optical benches, and micro-optoelectromechanical (MOEM) platforms. Optoelectronic chips and platforms are subsystems of computer systems, communication networks, etc. Silicon substrates feature a superior native oxide, in addition to excellent thermal, mechanical, and economic properties. Silicon wafers “shine” as substrates for PICs and OEICs.

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.