**6. Conclusion**

Indeed, we have here a very unique combination of photosensitzers, coinitiators and third components showing very different mechanisms of radical photogeneration. With the three PIS presented here, 16 different combinations of PS‐Co, PS‐additive and PS‐Co‐additive were measured both in real‐time FTIR (RT‐FTIR) and holographic recording. The results in term of both FRP and holographic recording of these combinations are summarized in **Table 1**. The performance of the PIS toward homogeneous‐free radical photopolymerization are given by

and maximum rate of conversion *R*c (s‐1), while the gratings are charac‐

and maximum rate of grating formation *R*η (s‐1).

as a function of final monomer conversion *C*<sup>f</sup>

is plotted as a function of the final monomer

The existence of a relationship between the evolution of monomer conversion under uniform irradiation (FRP) and that of diffraction efficiency under holographic exposure is not straight‐

**Figure 12** shows that no clear correlation exists between the final conversion *C*<sup>f</sup>

.

The picture is completely different when the maximum rate of grating formation *R*η are plotted as a function of the corresponding maximum rate of monomer conversion *R*c. As can be seen in **Figure 13**, a monotonic curve is obtained despite the various photochemical reactions and

.

achieved in

the final conversion *C*<sup>f</sup>

392 Holographic Materials and Optical Systems

terized by their final diffraction yield ηf

**Figure 12.** Final diffraction efficiency of grating recording ηf

forward. In **Figure 11**, the final diffraction yield ηf

photopolymerization kinetics of the 16 PS‐Co combinations.

homogeneous FRP and grating efficiency ηf

conversion *C*<sup>f</sup>

.

In this chapter, it was shown that many photoinitiating systems are usable for holographic polymerization. Type I and type II are widely exploited even if they are not the most efficient in term of radical quantum yields (especially, visible type I PIS). If in the UV‐vis curing and photopolymerization field, three‐components systems are widely described and used, because they proved high reactivity in photopolymerization reactions, their application is not so much developed for holographic recording through polymerization. This can be due to higher complexity of the photochemistry and choice of components, as described in this chapter. However, it was shown that three‐component photoinitiating systems can be great choice for application in holographic recording: high diffraction grating building rates with high final diffraction yields were obtained paving the way toward highly sensitive holographic materials. Even if photopolymerization and holographic recording is not straightforward, the challenge is worth and the need to improve both photosensitivity and diffraction efficiency of a photo‐ polymerizable recording medium is certainly the driving force to pay more attention on the development of three‐components and photocyclic initiating systems specifically designed for such application. As many different physical and chemical processes (photochemistry of the PIS, monomer, mass transport and gelation of the polymer matrix) are taking place in the medium to give rise to index modulation. Optimizing the material requires a fair knowledge of all these processes, which is tricky as many parameters are involved. This insight is needed to tailor the material combinations to meet the specifications required by the user.
