Kerr effect at high electric field in the isotropic phase of mesogenic materials

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Figure 1

(a) Experimental incidence geometry, in which the cell is sandwiched between two right angle prisms for oblique light incidence at 45°; the electric field is applied along the $x$ axis. (b) Dynamics of the effective field-induced birefringence (circles) response to the applied voltage pulse (squares) at 30 °C above $T_{NI}$.

Figure 2

(a) Temperature dependence of $E^2/\delta n_{\mathrm{eff}}$ for applied electric field $E=29\mathrm{V}/\mu \mathrm{m}$ (triangles), $58\mathrm{V}/\mu \mathrm{m}$ (pluses), $88\mathrm{V}/\mu \mathrm{m}$ (circles), and $115\mathrm{V}/\mu \mathrm{m}$ (saltires). (b) Plot of $T^{\prime }-T_{NI}$ vs $E^2$. All dashed lines show the corresponding results of the linear fitting performed for high temperatures $T>T_{NI}+25{}^{\circ }\mathrm{C}$.

Figure 3

(a) Plot of $E^2/S$ vs $E^2$ at $T_{NI}+30{}^{\circ }\mathrm{C}$ (squares), $T_{NI}+42{}^{\circ }\mathrm{C}$ (triangles), and $T_{NI}+51{}^{\circ }\mathrm{C}$ (circles). (b) Temperature dependence of ${\varepsilon }_2/{\varepsilon }_1$ (squares) and $a(T-T^{*})/{\varepsilon }_1$ (circles).

Figure 4

Dynamics of the switching in the isotropic phase at temperatures well above $T_{NI}$. (a) Experimental optic response at $T_{NI}+30{}^{\circ }\mathrm{C}$ (circles) and the respective linear fitting (solid line) for two different applied electric fields; note the slower switching on at higher fields. (b) Electric-field dependence of ${\tau }_{\mathrm{on}}$ (open symbols) and ${\tau }_{\mathrm{off}}$ (closed symbols) at $T_{NI}+30{}^{\circ }\mathrm{C}$ (circles) and $T_{NI}+42{}^{\circ }\mathrm{C}$ (squares).