Highly efficient optical director reorientation of liquid-crystalline polymer induced by dye additives

Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The materials under study were composed of a nematic liquid-crystalline polymer (NLCP) and a small amount (0.05–0.5 wt.%) of conformationally active (azobenzene) or stable (anthraquinone) dye impurity. Light action on the homogeneously aligned polymer films above glass transition temperature leads to the director reorientation and, consequently, to a change in the extraordinary refractive index. The effect is associated with the dye molecule excitation and related change of intermolecular forces. In the case of NLCP with conformationally active dye dopant, an extremely high orientational optical response was detected (nonlinear coefficient is $n_2\sim 0.1{\mathrm{cm}}^2/\mathrm{W}$). In contrast, the efficiency of orientational light action on NLCP with conformationally stable dye dopant is of the same order of magnitude as that of dye-doped low-molar-mass liquid crystals. At the normal light incidence on the NLCP doped with azo-dye, the threshold director orientation is observed which is similar to the Fréedericksz transition under the action of magnetic and electric fields. The obtained high-orientational optical response of NLCP caused by azo-dye dopant in combination with the possibility of the recording of deformed structure in the glassy state, typical for polymer compounds, reveals new opportunities in photonics applications.

Figure 1

Structural formulas of polymer PAA, azobenzene dye KD-1, and anthraquinone dye D-16.

Figure 2

(a) Schematic of the experimental setup: (1) cw neodymium laser ($\lambda =473$ or 532 nm), (2) neutral-density filter, (3) double Fresnel rhomb, (4) lens, (5) NLCP, (6) polarizer, (7) screen; $\alpha$ is the angle of the light incidence on the sample. (b) ${\mathbf{E}}_e$ and ${\mathbf{E}}_o$ are electric fields of extraordinary and ordinary light waves, $\phi$ is the angle of the polarization plane rotation, and $\mathbf{E}$ is the light field incident on NLCP.

Figure 3

(a) The aberrational pattern developed during 40 min under illumination of the PAA+0.05% KD-1 cell at $110{}^{\circ }\mathrm{C}$ with the normally incident light beam of the extraordinary polarization ($\phi =0^{\circ }$). The light wavelength is 473 nm; the light intensity on the beam axis is $I_e=0.8{\mathrm{W}/\mathrm{cm}}^2$. (b) The pattern formed upon rapid upward shift of the cell with respect to the light beam. (c) The pattern formed upon rapid rotation of the polarization plane from $\phi =0^{\circ }$ to $\phi ={90}^{\circ }$. The angular size of the photographs is $\sim 0.2$ rad.

Figure 4

Experimental dependencies of the steady-state magnitude of the light-induced refractive index $\left|\mathrm{\Delta }n\right|$ on the intensity $I_e$ of the extraordinary component of the light field at ($\circ , \times , +$) normal and ($▵$) oblique incidence ($\alpha$ = ${40}^{\circ }$) of the light beam on the PAA+0.05% KD-1 cell at $110{}^{\circ }\mathrm{C}$. The angle of the polarization plane rotation at the normal incidence $\phi$ is ($\circ , ▵$) $0^{\circ }$, ($\times$) ${30}^{\circ }$, ($+$) ${60}^{\circ }$. Curves (1) and (2) for the normal and oblique incidence are constructed by formulas (5) and (8).

Figure 5

Dependencies of steady-state magnitude of the light-induced refractive index $\left|\mathrm{\Delta }n\right|$ and the number of aberrational rings $N$ of the self-defocusing pattern on the intensity $I$ ($\phi =0^{\circ }$) at the oblique incidence of the light beam ($\lambda =532\mathrm{nm}$) on the PAA+0.5% D-16 cell at $110{}^{\circ }\mathrm{C}$; (1) the total number of rings $N$ and corresponding $\left|\mathrm{\Delta }n\right|$, (2) $N$ and $\left|\mathrm{\Delta }n\right|$ due to thermal effect, and (3) $N$ and $\left|\mathrm{\Delta }n\right|$ due to the orientational contribution to the self-defocusing.