Photoinduced birefringence

A PAP may be either amorphous or liquid crystalline and usually comprises suitably functionalized photochromic azobenzene and mesogenic moieties as side groups. The latter play an important cooperative or synergistic role with the photo-chromic group enhancing the overall birefringence (photoinduced anisotropy). 

C Wang, H Fei, Y Qiu, Y Yang, Z Wei, Y Tian, Y Chen, and Y Zhao, Photoinduced birefringence and reversible optical storage in liquid-crystalline azobenzene side-chain polymers, Appl. Phys. Lett., 74 19-21, 1999. 

X-ray measurements also indicate photocrystallization at exposure values E > Typical results, i.e., X-ray diffraction patterns of photocrystallized samples (this stage refers to curve 5 of Fig. 7.11), show four crystalline peaks located at 29 = 24°, 30°, 41°, and 45°. These can be indexed as 100, 101, 110, and 111 of the hexagonal Se crystal, respectively. Similar X-ray diffraction patterns were obtained by Ishida and Tanaka [26] examining photoinduced birefringence in a-Se thin films. 

The experimental value reported for PDN6S in Table V is very preliminary and is based on only three experimental points derived from prism-coupled waveguide reflectivity measurements. At this point, spurious effects due to heating, photoinduced birefringence etc. can not be conclusively ruled out. This preliminary data is presented only to provide a comparison with the calculated value. 

The dependence of photoinduced birefringence on light intensity, as presented in Figure 5.12, shows saturation at 150 to 200 mW/cm. It should be noted, however, that the values saturate versus the irradiation power bur not versus the illumination time (Figure 5.10A). 

The kinetically determined Iso-TGB phase transition in copolymers of the KM series (shown in Figure 5.7) allows also for isothermal nonequilibrium photorecording in the copolymers. Figure 5.15 shows the evolution of photoinduced birefringence in a film just cooled from 100°C, where the... 

We should emphasize that by using thick but still transparent IsoSm films, we can obtain huge phase delay even for moderate values. For instance, the maximum value of the photoinduced birefringence shown in Figure 5.15 (curve 1) was measured for a 24-pm-thick film and corresponds to the phase delay value AO = -245°, i.e., close to the three-quarter-wave plate. 

FIG. 5.15 Growth of the photoinduced birefringence in (f) just-cooled and [2) I-day-old copoiy-mer films (KM2S, 23 jlm) under light illumination (nonfiltered white light, 0.3 W/cm ). 

Todorov, T., Nikolova, L., and Tomova, N. Polarization holography. 1 A new high-efficiency organic material with reversible photoinduced birefringence. Appl. Opt. 23, 4309 (1984). 

Kozlovsky, M. V., and Lazarev, V. V. The photoinduced birefringence in thick films of chiral azodye copolymers (submitted to Macromol. Chem. Phys.). 

FIG. 11.19 Erasure of the photoinduced polar orientation with a monochromatic light at 2co frequency. The sample was a 0.1. m thick film of the DRI-MMA 35/65 copolymer. After seeding, the sample was continuously irradiated with a 2e> beam (fluence used was a few MW/cm, which corresponds to the 2cn seeding beam intensity). The curve in the insert presents the evolution of the photoinduced birefringence in the same conditions. 

Another mechanism responsible for the optically induced anisotropy is angular redistribution (AR) of molecules. This mechanism has been widely developed to explain photoinduced birefringence and dichroism. In most experimental cases, there is evidence of some rotation of molecules during the photoisomerization cycle (see Reference 2, for example). This rotation results in AR, because the molecules remain longer in states with lower excitation probability, and so more molecules are accumulated perpendicular to the pump polarization. The AR process is initiated by the AHB, and these two processes should be studied simultaneously in the framework of general... 

The first question that had to be answered was. What were the optimal experimental conditions to photoinduce and photoeliminate birefringence. A typical experimental setup is shown in Figure 13.3. Since the polymer film is absorbing the pump light, we could calculate a maximum film thickness that would not completely absorb the pump beam. This is approximately 4 divided by the initial film absorptivity thus, in the case of pDRlA and pDRlM homopolymers, the useful thickness cannot exceed 300 nm. Obviously, polymers with lower absorptivities at the laser wavelength or with lower chromophore concentration can afford thicker useful films. 

FIG. 13.3 Experimental setup for measuring photoinduced and phott eliminated birefringence. S, Ht = Sample on heating stage, Sh = Shutter, P = Polarizer, A = Analyzer. 

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