Local photo-reorientation of liquid crystal using a laser focused on an azo dye-based monolayer

The control of liquid crystal (LC) alignment is very important for applications and has been investigated extensively. The most common alignment method is mechanical rubbing of polymer layers coating the substrates bounding the liquid crystal. Photoaligning mechanisms have also been investigated, with the trans-cis-trans photoisomerization cycle of azobenzene (Fig. 1) and its derivatives widely used.1 Previous research has focused on the global realignment of LCs on a large scale without much attention being paid to details of the local response of the director field. In this study, we investigate the local photo-reorientation of LC by azobenzene-based self-assembled monolayers (azo-SAMs). The azobenzene derivative used to form azo-SAMs in the experiment is the methyl red derivative dMR (Fig. 2).2 When the actinic light is linearly polarized, the isomerization process is characterized by angular-dependent excitation and results in the photoselection of a preferred azobenzene dye orientation perpendicular to the polarization of the light. The LC is aligned in turn with the director along the azobenzene dye.3

cistrans schematic

Figure 1. Photoisomerization of azobenzene.

dMR structure

Figure 2. Chemical structure of dMR.

To study the local photo-reorientation of the LC, a hybrid cell is made with a nematic sandwiched between the azo-SAM and a homeotropic surface. The linearly polarized actinic laser is focused onto a small spot (about 4.3 µm in diameter) on the azo-SAM, and the cell is observed under crossed polarizers (Fig. 3).

Figure 3. Adjusting the optical path of the green laser used to induce photo-isomerization of azo-SAMs. The sample is observed on the microscope using red light illumination with crossed polarizers.

A variety of interesting reorientation phenomena are observed when the plane of polarization of the actinic laser is rotated. For example, Fig. 4 shows the generation and winding up of ring-shaped extinction brushes around the illuminated spot.

Figure 4. Reorientation dynamics of LC on an azo-SAM. Rings in the director field are generated and wound up as the actinic laser polarization is rotated. Each image corresponds to a 90° advance in the orientation of the polarization of the laser (green spot in the center of each image).

References

[1] Z. Sekkat and W. Knoll, Photoreactive Organic Thin Films, Academic Press (2002).
[2] Y. W. Yi, T. E. Furtak, M. J. Farrow, and D. M. Walba, Journal of Vacuum Science and Technology A 21, 1770 (2003).
[3] Y. Yi, M. J. Farrow, E. Korblova, D. M. Walba, and T. E. Furtak, Langmuir 25 997 (2009).

Text and images contributed by Yue Shi.