Light-driven mass transport

Mass transport that requires thermal treatment of the samples. The SRGs cannot be erased, even after heating to temperatures close to Tg in some cases. 

Mass transport in liquid-crystalline azopolymers. Here two important effects should be considered thermal effects and the cooperative behavior of the mesogenic unities. The SRGs formed are in phase with the interference pattern, and they also show strong polarization dependence of the writing 

The azochromophore must be covalently attached to the polymer chain (either as a side chain or in the main chain). Attempts to produce SRGs with guest-host systems led to very small surface modulations about 15 nm or less). Though photoisomerization can be induced in these systems, efficient SRG formation is not possible, probably because the polymer chains are not induced to move unless covalently attached. However, SRG formation of a doped system in which the polymer also contained an azobenzene unit has also been studied (see Section 14.4.4). The much less efficient inscription 

6 Diffraction efficiency of the SRG as a function of recording power and energy, with the laser polarization set at 45° with respect to the s-polarization. From reference 40. 

One important question lies in whether there is any difference in orientation of the chromophores between the peaks and valleys of SRGs at the film surface. A conclusive answer would probably bring important information on the mechanisms of mass transport. Let us first consider what should one 

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In some of these works, the mechanisms responsible for the SRGs might be completely different from the light-driven mass transport. Indeed, the following mechanisms have been suggested ... 

Geue et al. investigated the thermal erasure of SRGs obtained via light-driven mass transport, at various temperatures, which allowed them to estimate the activation energy for erasure to be 2.6 eV. For erasing at T < Tg, the SRG was erased by flow of polymer material perpendicular to the initial surface, accompanied by the formation of an intrinsic density grating, as commented on in the work of reference 95. At T > Tg, the lateral density modulation was equalized by a lateral flow of material. 

One can note some discrepancies in the literature. Some researchers have reported no dependence on polarization for ablation or photodegradation, but for ELBL films there was some dependence, though not as strong as in SRGs originating from light-driven mass transport. In addition, the phase of the SRG appears to be different in distinct papers. Egami et and Che et... 

In a later work, Frey et al used the time-resolved grating translation technique to investigate the SRG and anisotropy gratings (index and absorption). Particular attention was paid to the phase shift between the SRG and the interference pattern. They showed that a Tr-shift exists, thus confirming previous results of SRGs obtained by light-driven mass transport. It was also shown that only weak SRGs are formed with s-polarization, in contrast to the more efficient p-polarization. 

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