Self-Enhancement of Scalar and Vector Holographic Gratings in Azobenzene Molecular Glassy Films

Abstract

Self-enhancement (SE) effect of scalar and vector holographic gratings (HG) recorded in three different azobenzene molecular glassy films is experimentally studied in both transmission and reflection modes at 532 and 633 nm. The maximal SE factor (the ratio of diffraction efficiency to its initial value) SEF = 42 has been achieved. It is shown that the model of complementary HG can be applied also in scalar transmission thin HG case to explain coherent SE. The possibility of vector HG coherent SE in transmission mode is experimentally demonstrated for the first time (SEF = 4.3). The possibility of coherent HG SE in reflection mode is also established for the first time (SEF = 21). HG recording processes as well as coherent SE processes are found to be independent in transmission and reflection modes being determined by volume and surface relief HG, respectively. The permittivity gradient mechanism is proposed to explain the coherent SE of surface relief HG. Both HG recording and coherent SE efficiencies strongly decrease when HG period is decreased from 2 µm to 0.5 µm. No relaxational SE effect is found. Coherent SE effect in molecular glasses is found to be weaker than in inorganic materials.

Recording and self-enhancement of dynamic polarization gratings in degenerate four-wave-mixing geometry

“…16 . The visibility of the main transmission HG in CSE process is much lower than in normal recording and DFWM cases because the diffracted P 2 wave (Fig.2) substitutes the P 1 wave, and the former is much stronger than the latter .…”

“…First of all, the recording mechanisms are different. Previously 16 we have come to the conclusion that HG recording in our organic molecular glasses at 532 nm is due to the azochromophore photoorientation in the process of trans-cis photoisomerization with a possible contribution of azochromophore photodegradation via polarization selective photoinduced N-N bond breaking. These processes are accompanied by mass transfer enabled by athermal fluidization.…”

“…First, to evaluate the recording efficiency at each polarization pair SDE time dependence was measured in normal holographic recording (two-wave-mixing) geometry. SDE is a ratio of the first-order diffracted beam power to the power of the more distant recording beam when both recording beams (P 1 and P 2 as in Fig.2) are present 16 Table 1 and Table 2, respectively. As in previous case, using REF SDE criterion one can see from Table 2 that the most efficient polarization pair is scalar ss.…”

“…SE can also be regarded as a two-stage hologram recording method, which is profitable when the recording energy or exposure time is limited at the first stage. Recently we have experimentally investigated SE of scalar and vector HG in azobenzene molecular glassy films 16 . However, to our best knowledge, nobody has explored SE in DFWM geometry.…”

“…The second part of the motivation behind this paper was to study the coherent self-enhancement (CSE) of dynamic HG in scalar and vectorial DFWM geometry. Self-enhancement ( SE) of a nonstationary hologram is the increase in its diffraction efficiency (DE) over time under the stimulus of a single beam light irradiation or simply in the dark 16 . Three types of SE can be distinguished: i) CSE due to the holographic recording by diffracted waves; ii) incoherent SE due to the contrast and/or transmission increase of a hologram by incoherent light; iii) relaxational (or dark) SE due to the contrast and/or transmission increase of a hologram by thermostimulated relaxation processes.…”