**Acknowledgments**

As discussed earlier, the Debye screening length in photorefractive material need to be very small to support high concentration of effective trap density. For BSO:Ru crystal, the Debye screening length of 0.08 μm has been calculated from Eq. (7) and materials parameters in **Table 1.** Moreover, as shown in **Figure 10**, the *Esc* decreases with increasing the grating spacing from 1 to 2 μm. As the gain amplification is controlled by the space charge field, it follows the behaviour of the *Esc* on the way to decrease with increasing the grating spacing Λ. In that aspect, multi-layer structure may increase the effective interaction length and optimize the *Esc*

The prime significance of the reviewed hybrid structure is all optically controlled processes. For example, the switching ability BSO:Ru/PDLC structure is demonstrated at **Figure 11**. An image pattern (rectangular mask) is placed into the input plane of 4-f optical system, and the structure is illuminated with 1064-nm Gaussian beam. When the pump light illuminates the device, the PDLC layer transparency is changed due to the induced space charge field in the photorefractive substrate. Therefore, by controlling the droplet size and consequently the driving voltage of the LC molecules from one side and optimizing the charge carriers' concentration in crystal matrix (providing high enough density for high resolution space charge field), the proposed structure can be further optimized. Moreover, the beam coupling can be significantly improved by addition of nanoparticles in LC layer, which affects the dielectric anisotropy and decreases the driving (threshold) voltage.

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

**Figure 11.** Gaussian laser beam propagating through BSO/PDLC hybrid structure and image mask (rectangular shape) evolution when the light is at "on" and "off" position (right side). All processes are controlled by near infrared

penetration depth (however limited by scattering losses).

**5.3. Applications**

498 Holographic Materials and Optical Systems

**6. Conclusion**

light only.

Financial support by the Ministry of Science and Technology, Taiwan under the contracts: MOST 105-2221-E-009-110; 104-2221-E-009-164; 104-2221-E-009-151 and Bulgarian Science Fund under the projects DFNI T-02/26 and H-08/9 are gratefully acknowledged.
