Holography, coherent light amplification and optical phase conjugation with photorefractive materials

Photorefractive materials are being widely investigated for applications in holographic data storage1. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known2, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed3,4,5,6, but they have practical disadvantages and only laboratory demonstrators have been built7,8,9,10. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate — available both in large size and with excellent homogeneity — that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.

/pdf/non-volatile-holographic-storage-in-doubly-doped-lithium-3y2ugfesqd.pdf

Non-volatile holographic storage in doubly doped lithium niobate crystals

Defects in LiNbO3—I. experimental aspects

This review presents the fundamental theoretical concepts concerning the photogalvanic effect (PGE)—the phenomenon of appearance of a direct current in a homogeneous medium under uniform illumination. This effect can occur in all media lacking a center of symmetry, in particular, in ferroelectrics, piezoelectrics, gyrotropic crystals, and in gases and liquid possessing natural optical activity. The starting point of a systematic microscopic theory is the asymmetry of the elementary electronic processes—their noninvariance with respect to spatial reflection. Within the framework of the theory, we study the most important mechanisms of the PGE in the regions of impurity, interband, and intraband light absorption. Possible observable manifestations of the PGE are discussed. Theoretical results are compared with experimental data.

https://iopscience.iop.org/article/10.1070/PU1980v023n03ABEH004703/pdf

The photogalvanic effect in media lacking a center of symmetry

In LiNbO3 at light intensities above 106 W/m2 much stronger refractive index changes can be induced, as expected from measurements at low intensities. Most experimental data have been published on iron-doped LiNbO3 crystals. We propose a two-center charge-transport model for LiNbO3:Fe that describes most results at low and high intensities quantitatively. It explains the intensity dependence of steady-state refractive index changes and enhanced holographic sensitivities at high light intensities as well as the presence of light-induced absorption changes.

Light-induced charge transport in LiNbO 3 :Fe at high light intensities

The photorefractive properties of LiNbO3∶Fe and LiNbO3∶Cu have been studied in combination with optical absorption-, Mossbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO3∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe2+ impurities and the optical absorption band around 2.6 eV in LiNbO3∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe3+ to Fe2+ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably.

A. Rauber

Papers

Photorefractive centers in LiNbO 3 , studied by optical-, Mössbauer- and EPR-methods

A High Performance Connectorized LED Package for Fiber Optics