Light-Induced Depoling

The modulation of the susceptibility, which is monitored by the variation of SHG intensity, is associated with the change of the medium refractive index An given by  

What is also very interesting from the practical point of view is that such an alternate illumination of the zwitterionic chromophore during the corona poling leads to a significant increase of x susceptibility. The observed increase of 12 intensity by a factor of three (Fig. 42) corresponds to an improvement of x susceptibility (and correspondingly of the E-O coefficient r) by 70%. Also, the temporal stability of the induced noncentrosymmetric order in alternately illuminated thin films is significantly better than in untreated films (an increase of the decay time constant by a factor of 5 [174]). 

---

Fig. 43. Proposed mechanism of light-induced depoling process with photoisomerizable molecules...

FIG. 8.5 Photo-induced depoling of the DRl-PMMA copol er blended with PMMA wiidi l( w/w DRl-PMMA in PMMA, with p- and s-polarized blue light irradiation 120 mW/cm from a 488 nm Ar laser). The second harmonic probe was s-polarized. The irradiating pump being turned on and off is indicated. Adapted from reference 8, and redrawn by permission of ACS. 

Similar PID results have been reported in very high Tg (up to SSO C) azo-polyimides. It has been shown previously that irradiation of the donor-embedded polyimide derivatives with polarized light alone at room temperature induces a quasi-permanent nonpolar orientation, which can be thermally erased only by heating the polymer above Tg. While, the lifetimes of the polar order generated by thermal poling of the donor-embedded polyimides were found to be on the order of tens of years to centuries at room temperature, photoisomerization can efficiently depole these polymer films in a matter of minutes at room temperature. Indeed, Figure 8.6 shows the effect of p-polarized irradiation on the SH signal of PI-2, which had been previously thermally... 

---