There is a growing demand for nonlinear optical materials for a variety of applications—lasers and coherent sources, electrooptic devices, communication technologies, and optical processors and computers. Nonlinear optics is a vast field requiring materials with diverse performance features. Photorefractive (PR) materials, which experience a change in the refractive index under the effect of inhomogeneous illumination, constitute a relevant branch of the field. They behave as third-order nonlinear materials, which can be considered, in general, as photorefractive. However, the materials more commonly designated as photorefractives involve a charge-transport-induced nonlinearity, and it is these materials which are the object of this issue of the MRS Bulletin.

At variance with conventional (often designated as Kerr) nonlinear materials, photorefractives are sensitive not to the local light intensity but to its spatial variation; i.e., they are nonlocal materials. This feature makes them more complicated to deal with than their conventional counterparts, since a χ(3) susceptibility cannot be properly defined (except as a k-dependent function). On the other hand, this sensitivity gives them some unique and interesting features. In particular, an interference light pattern illuminating the crystal and the generated index grating are phase-shifted, leading to remarkable beam coupling and amplification effects. The coupling gain can be markedly enhanced by applying alternating electric fields or by oscillating the interference fringes with a piezoelectric mirror. Efficient image amplifiers have been made using this effect.

Lithium niobate (LiNbO3) is a paradigmatic photorefractive (PR) material. It provided the first experimental evidence of the PR effect and still maintains a key position in the field. This position is fostered by the fact that large congruent single crystals with good optical quality are routinely grown, enabling technological applications. A remarkable example, a PR narrow-band interference filter has recently been developed and put on the market.

A relevant property of LiNbO3, the high electrooptic figure of merit, n3r, assures efficient PR performance. Another unique feature is the occurrence of a bulk photovoltaic (PV) effect, i.e., the generation of a voltage (in an open circuit) or a current (in a short circuit) as a consequence of homogeneous illumination. The bulk PV effect acts, in a way, like an internal electric field, enhancing the PR effect. On the other hand, thermal fixing or stabilization of PR gratings has been successfully accomplished through an interplay between proton and electron dynamics. LiNbO3 is also the choice substrate for the commercial fabrication of waveguide devices such as modulators, wavelength filters, multiplexers, and demultiplexers. Moreover, lasing action as well as nonlinear effects have been achieved due to the marked inhibition of the PR effect caused by heavy Mg doping. This illustrates the intimate connection between PR behavior and defect structure, i.e., between optics and materials science.

The photorefractive (PR) effect has been studied for more than 25 years and many applications for optical signal processing such as correlation, real-time holography, dynamic interconnections, and optical memories have been developed. The main focus of study for the PR effect has been oxides (ferroelectrics and sillenites) in which the useful spectral range lies in the visible. Applications for telecommunication systems and eye-safe devices have required extending the spectral range into the near infrared (1.0 to 1.5 μm), and so the exploration of different materials. It has been shown that the bulk semi-insulating III-V semiconductors GaAs and InP, and more recently the II-VI compound CdTe, were efficient materials for this spectral range. III-V materials offer the advantage of availability as bulk semiinsulating materials of high crystalline perfection and homogeneity regarding their electrical properties due to their importance as substrate materials in micro and optoelectronic technology. However, these materials have not been optimized for PR applications, so quantitative analyses of PR experiments related to the specific material defect properties are necessary for further developments. It has equally been shown that the PR effect can be used as an efficient tool for materials characterization.

Bandgap engineering of thin semiconductor layers and defect engineering combine to form photorefractive (PR) quantum well structures. PR quantum wells are semi-insulating thin films useful for dynamic holography and other coherent and incoherent optical applications. As materials for thin-film dynamic holography, they have high nonlinear-optical sensitivity and high speed.

The PR effect translates a spatially varying irradiance, from the interference of two or more coherent light beams, into a refractive index grating. The multiple-step PR process begins with photoexcitation of charge carriers, followed by transport and trapping of charge at deep defects. The trapped space-charge generates electric fields that alter the refractive index of the material through the electrooptic effect. The same laser beams that generate the gratings diffract from the gratings, leading to a rich variety of multiple-beam effects, such as two-wave and four-wave mixing.

Because the PR process involves several distinct physical parameters, such as carrier mobility and electrooptic coefficients, optimized performance requires a coincidence of favorable properties in a single material. Rather than relying on coincidence, bandgap engineering of multiple layers of semiconductors provides a way to individually tune the separate material pa rameters. Likewise, defect engineering in semiconductors provides flexibility in the choice of defects, their concentrations, and degree of compensation. Bandgap and defect engineering combined make custom designed PR materials possible.

“Optical computing” is a term which has been used to describe many types of optical systems. This article concentrates on one optical computer design which appears to be particularly promising and competitive with digital electronic systems and which can use photorefractive (PR) crystals. We begin by discussing the motivation for constructing optical and optoelectronic computing systems and go on to describe two approaches to constructing optical computers. The first does not use PR crystals as yet but serves to indicate other approaches. The second approach involves the operation of one type of optical neural network. We discuss in detail the recording of holographic memories in a PR crystal for inclusion in the optical neural network described here.

I presume that many of you will be surprised to learn that John von Neumann, a great mathematician, and a remarkable man, had even a passing interest in materials science since his name is usually associated with work in function theory, mathematical logic, the mathematical foundations of quantum mechanics, game theory, and of course with the development of computer logic. Actually, he was deeply interested in the evolution of materials science and technology, and he played an important role in giving prominence to materials science at a critical time in its evolution. But first let me tell you a little about his life.

John von Neumann was born in Budapest, Hungary, in 1903 into a prominent business family. His mathematical genius was recognized early by his high school teacher, Lázsló Ratz, who insisted that he receive special tutoring since mathematical geniuses tend to flower early. He became a close and, indeed, a lifetime friend of a slightly older fellow student, Eugene Wigner, who was inspired by the same mathematics teacher. The two of them frequently wandered home together after school, with von Neumann providing a tutorial on some aspect of mathematics while Wigner, who had a comparably brilliant mind, absorbed everything. The two students had different personalities, but shared a great love of mathematics.

Von Neumann was never a narrow genius. He soaked up knowledge of all kinds rapidly and was exposed to much because the von Neumann family dinner gatherings were devoted to discussions of technical, historical and cultural affairs, as well as business. This great versatility in interests was a characteristic trademark of von Neumann‘s entire life.