Time Resolved Experimental Analysis of the Electric Field Induced Biaxial Order Reconstruction in Nematics

We present the time resolved experimental characterization of the biaxial switching between two topologically distinct textures of a nematic liquid crystal cell submitted to a strong electric field. This fast electro-optical effect is governed by the electric induced order reconstruction in the nematic bulk, which is an actual challenge for a complete theoretical description of the nematodynamics. The electric measurements across the cell are suitable to this purpose since they well discriminate among the dielectric, the ionic, and the order reconstruction contributions. A phenomenological model describes the experimental data, allowing the measurements of the order reconstruction characteristic time.

Figure 1

Transition cascade from the quasiplanar $H$ state to the bent $V$ texture induced by a vertical electric field, via the intermediate $I$ configuration. The $I$ state is characterized by a thin wall, in the middle of the cell, where the order reconstruction occurs.

Figure 2

Typical dynamical response of a thin cell very close to the nematic-isotropic transition temperature, subject to a rectangular electric pulse of $1\mathrm{m}\mathrm{s}$ duration and of amplitude above the threshold for the $H\mathrm{\text{−}}V$ transition. The upper curve $1$ is the light intensity transmitted by the sample and the bottom curve $2$ is the electric current flowing across the sample. The acquisition starts when the electric field is switched on. The electric pulse duration ${\tau }_p=1\mathrm{m}\mathrm{s}$ is long enough to allow the full evolution of all transient dynamical phenomena.

Figure 3

Electric current response of a $2.40\mu \mathrm{m}$ thick cell at $T=25°\mathrm{C}$, submitted to a rectangular electric pulse of $1\mathrm{m}\mathrm{s}$ duration and of amplitude $E=8.9\times {10}^6\mathrm{V}/\mathrm{m}$, above the threshold $E_{\mathrm{t}\mathrm{h}}\simeq 6.9\times {10}^6\mathrm{V}/\mathrm{m}$ for the $H\mathrm{\text{−}}V$ transition. The large peak due to the dielectric molecular reorientation is visible in the main frame. The two peaks due to the nematic order reconstruction $a$ and to the ion current $b$ are well resolved in the smaller box, where the signal is 10 times magnified.

Figure 4

Plot of the temporal peaks positions in the electric signal after the dielectric reorientation. The linear family is related to the ion current (triangular points); the other set is connected to the nematic order reconstruction (circular points). The highlighted points correspond to the two peaks $a$ and $b$ in the small box of Fig. 3. The continuous lines are best fits of the experimental data.