Photorefractive Materials
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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 \® 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 phaseshifted, leading to remarkable beam coupling and amplification effects.2 The coupling gain can be markedly enhanced by applying alternating electric fields or by oscillating the interference fringes with a piezoelectric mirror.3 Efficient image amplifiers have been made using this effect. The main characteristic of PR materials is their very high sensitivity; i.e., they show appreciable changes in refractive index for rather low power densities (in the range of mW/cm2). This is a consequence of the mechanism responsible for the nonlinearity: A light-induced charge transport generates a charge redistribution and an associated space-charge field. This causes a correlated change in refractive index via MRS BULLETIN/MARCH 1994
the electrooptic response (Pockels or even Kerr) of the material. This cumulative process is the basis of the high nonlinear sensitivity. Obviously, the price is a slower response time, related to carrier mobility. However, very fast PR responses can be obtained with high intensity pulses and so nanosecond and even picosecond responses have been observed in oxide4"6 and semiconductor7 materials. A typical PR experiment involves recording an index pattern correlated with the interference or mixing of two coherent light beams. This real-time holographic recording process represents a definite advantage over conventional twostep holography. In addition to those interference experiments, four waves can be mixed in PR materials.8'9 A particularly useful configuration, degenerated fourwave mixing (DFWM), generates the phase-conjugated or time-reversed signal of an incident signal beam. This allows making phase-conjugated mirrors presenting gain. A self-pumped device, (one not requiring any pump beam) has also been demonstrated.10 Interesting applications to laser devices and adaptative optics, among others, have already been considered. The PR effect was initially observed in LiNbO3 and much work has been carried out on this material11 and other oxides, such as BaTiO312 and sillenites (bismuth silicate, bismuth germanate, bismuth titanate).13 Extensive work has also been performed on doped semiconductors,14"15 including GaAs, CdTe, and InP. More recently, preliminary experiments1617 have been done on multiple quantum well (MQW) structures. MQW structures feature a high electrooptic response mediated by the quantum-confined Stark effect (QCSE) when an electric field is applied perpendicularly to the MQW layers. In fact, very high d
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