probe light. By varying the polarization of probe light, different azobenzene groups can be accessed and aligned, as seen in Figure 4. This affects the growth rate and the final density of the new state.
If this mechanism is correct, it would imply that organized phases (liquid-crystalline phases) may be induced not only thermally or in solution (as with the classic thermotropic and lyotropic liquid-crystal polymers), but also by using light under certain circumstances. While the case of photoinduced liquid crystalยฑisotropic transitions is well documented, [12] this would be the first instanceรto our knowledgeรwhen a liquid-crystalline phase has been photoinduced in a polymer material with no known thermotropic behavior.
The proposed scenario is most probable and seems to be supported by the polarization dependence documented in Fig- ure 4. Nevertheless, we have no experimental evidence for the existence of the liquid-crystalline phase at this time. That the liquid-crystalline phase has the tendency to improve the grating formation process was reported by Stracke et al. recently. [13] They found an amplification of the modulation depth in holographic surface relief gratings of a liquid-crystalline material containing azobenzene moieties after annealing the sample above T g . This was explained by the thermally induced reorientation of mesogenic chromophores. We are planning new experiments with slightly modified polymers as well as different optical and X-ray probes.