**6. Conclusion**

The chapter reviewed recent progress of two-wave mixing and beam amplification in novel type of hybrid structures that combine photoconductive and photorefractive properties of inorganic crystals together with the high birefringence and anisotropy of LC (or PDLC) layers. When inorganic crystal acts as photoconductive substrate, the photo-generated charge carriers control the LC molecules alignment and subsequent light modulation. As a result, the two-wave mixing happens in a liquid crystal layer (the active layer) with high amplification values; however, the fringe period of the recorded gratings is limited to few micrometers. When inorganic crystal acts as photorefractive substrate, the two-beam coupling happens at both the photorefractive substrate and LC layer where the space charge field is the driving force for LC molecules reorientation and refractive index modulation. In this configuration, all the processes are controlled by light-permitting submicron resolution.

Depending on the designed structure assembly and operation principle, dynamic holographic gratings at Raman-Nath or Bragg match regimes of diffraction can be recorded. The large trap density and small grating spacing typical for photorefractive materials allow to reach the Bragg matched conditions in all optically controlled structure in contract to the large grating spacing and small trap density typical for LC-based electro-optically controlled structures, which support the Raman-Nath regime of diffraction. Furthermore, by selecting the photosensitivity of inorganic substrate, the spectral interval can be easily adjusted in a regions, where the LC molecules (or PDLC) are not enough sensitive.

The proposed organic-inorganic hybrid structures can control transmission, reflection and scattering of light and are considered in playing an essential role in 3D holographic display technologies and providing sub-micron spatial resolution, large viewing angle and low driving voltage. The reviewed examples can be classified as novel type of non-linear optical components, which exhibit attractive capabilities for light manipulation, coherent image amplification, to control the group velocity of modulated signal beam, in video display technologies. The advantage of simple fabrication and compactness open the way to design varieties of new structures and elements that meet the up-to-date requirements of 3D display technologies and optical information processing.
