Examples of high-performance photorefractive polymer composites include PVK(49.5%)/AODCST(35%)/BBP(15%)/C60(0.5%)D. Wright et al., Appl. Phys. Lett. 73, 1490 (1998) and PVK(49.5%)/DCDHF-6(30%)/BBP(20%)/C60(0.5%).D. Wright et al., Appl. Phys. Lett. 79, 4274 (2001)
The advantage of polymer composites is the ability to tune their PR properties by varying the concentration and type of constituents. However, because many components are combined in the composite, there is an issue of phase separation and crystallization that reduce shelf life of the device. Also, an addition of a plasticizer to lower Tg and therefore enhance chromophore orientation also increases the inert volume, which worsens photoconductive and electro-optic properties. Both of these issues can be resolved by utilizing organic glasses. However, flexibility to tune properties of the glasses is diminished, as compared to polymer composites.
Design of a high-performance photorefractive (PR) polymer composite relies on combining various components to perform certain functionality in the composite. Below are some of these components, their functions, and an example of such a compound.
Sensitizer (S) - A sensitizer is typically added at concentrations of 0.1-1 wt% to promote charge carrier photogeneration. The choice of the sensitizer is determined by the wavelength of photorefractive grating recording. (Examples: C60 and TNF)
Photoconductive Polymer - Provides transport sites for efficient charge (typically hole) transport. (Examples: PVK and PPV derivatives)
Nonlinear Optical Chromophore (NLO) - Needed for an electric field dependent refractive index via birefringence (due to reorientation of polar molecules in the electric field) and electro-optic effect. Typical concentrations of NLO chromophores in the composite are 25-35 wt%. (Examples: MNPAA and AODCST)
Plasticizer (PL) - Needed to lower the glass transition temperature (Tg) of the composite and promote chromophore orientation in the electric field in order to take advantage of the orientational enhancement effect. (Examples: BBP and ECZ)
Some molecules can serve as photoconductors and NLO chromophores at the same time, as well as exhibit glass-forming properties with Tg around room temperature. The only dopant which is needed in this case is a sensitizer. Such organic amorphous glasses are among best-performing organic PR materials. (Examples: DCDHF-6 and ATOP-4)
