**1. Introduction**

The demand to develop advanced functional devices with fast operation speed, high memory capability, submicron spatial resolution and low energy consumption continuously tends to increase. For example, to reach a wide viewing angle, the pixel pitch of the recording media

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

needs to be comparable or smaller than the wavelength of the light. Another approach is to design all optically controlled devices for light control and manipulation. All these features make the combination between outstanding properties as large anisotropy and strong birefringence typical for organics (offering easy processing, large structural flexibility and low cost) with the excellent photosensitivity and photoconductivity of inorganic materials (providing mechanical stability and energy gap manipulation) attractive to design single, compact structures with enhanced functionality. Moreover, an emergent need of devices sensitive at the near-infrared spectral range (the required illumination for biological and medical samples) is of particular importance for biomedical sensing and with significant impact for the society. In that aspect, the holography, due to its unique nature, is expected to play an essential role.

The following chapter is focused on the two-wave mixing in organic-inorganic hybrid structures and dynamic holography recording resulting to formation of light-induced gratings. The two-wave mixing (denoted also as a two-beam coupling) takes place in a variety of nonlinear media as photorefractive materials (the photorefractive effect refers to refractive index modulation in response to light), third-order non-linear media (as Kerr media) or semiconductor amplifiers. The two-wave mixing is expressed by two beam interactions, forming an interference pattern, which is characterized by periodic spatial variation of the light-intensity distribution. Its main significance is the ability of unidirectional optical energy exchange between the two beams (gain amplification) allowing weak beam to grow exponentially with the distance, which opens various opportunities for designing a new structures and elements for practical realizations.

Depending of the organic-inorganic hybrid structure design and in more particular how the inorganic material control the dynamic grating formation, they can operate at Raman-Nath or Bragg match regime of diffraction:


carrier migration, high-trap density and space-charge field come from the inorganic substrate, whereas the beam amplification is provided by the LC layer. As a result, the proposed hybrid structures act as dynamic holographic grating at Bragg match regime allowing sub-micron spatial resolution. Moreover, the above configuration is easy and simple to fabricate (no need of conductive layer deposition [and in a case of PDLC, no need of alignment layers and polarizers]), and all the processes are controlled only by the action of light. Therefore, they are noted as all optically controlled hybrid devices.

The chapter is organized as following: at the beginning, a brief introduction of two-wave mixing and gain amplification phenomena as a consequence from the non-linearity of photorefractive effect will be reviewed. The main significances of the optical energy exchange and phase shift between the interference pattern and refractive index grating are discussed. Next, the two-wave mixing in two types of organic-inorganic hybrid structures (electro-optically controlled and all optically controlled) will be presented and compared. Examples as optically addressed spatial light modulation devices; in dynamic holography and image processing are demonstrated and discussed. In concluding remarks, further prospective to design varieties of novel all optically controlled hybrid devices will be discussed.
