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.
Optoelectronic tweezers based on photorefractive space charge fields: recent achievements and challenges
