Effective conductivity due to continuous polarization reorientation in fluid ferroelectrics

A smectic-$A$ (Sm$A$) liquid crystal phase of fluid layers with in-plane polarization $\text{P}$ is shown to exhibit effective conductivity in the semiconducting range during electric-field-induced polarization reorientation, but becomes insulating once the polarization is aligned with the field. Such fluid ferroelectrics sandwiched between highly insulating layers enable electro-optic devices with long-term dc electrostatic control of polarization and optic axis orientation.

Figure 1

Block polarization reorientation in a fluid, polar liquid crystal. (a) Chemical structure and phase sequence on cooling of W623. (b) Electro-optic geometry, showing a bent-core molecule in the Sm${\mathit{AP}}_F$ phase in a bookshelf cell with ITO electrodes and rubbed polyimide insulating layers. The director n is along the layer normal z, independent of applied field E. The polarization P is normal to the director and makes an angle $\varphi$ relative to the cell normal x. An analog electro-optic response is observed with normally incident light and the cell between the crossed polarizer and analyzer. (c) Model of electrostatically controlled block polarization reorientation, showing liquid crystal and polyimide insulating layers, with thickness $d$ and dielectric constant $\epsilon$, and ITO electrodes. When the applied voltage $\left|V\right|<V_{\text{sat}}=2d_{I}P/{\epsilon }_I$, the polarization P reorients as a homogeneous block to exclude the electric field from the liquid crystal. (d) Equivalent circuit of the cell under time-varying applied voltage $V(t)$.

Figure 2

Experimental measurements of electro-optic and current response of W623 in a 4.8-$\mu$m-thick cell with ITO electrodes and rubbed polyimide alignment layers. (a) Electro-optic response with the layer normal oriented at ${45}^{°}$ to crossed polarizers ($T={158}^{°}$C). The current through the cell $i$ in the analog reorienting regime has a constant magnitude $I$, i.e., the cell is capacitive, with the liquid crystal polarization acting effectively as a local short circuit. $I^{\prime }$ is the magnitude of the current step when the slope of the applied voltage changes. The background slope of the total current arises from the finite cell resistance. The triangular applied voltage V varies between ±40 V. (b) Current $i$ at different temperatures with a triangular voltage applied to the cell. The current peaks due to ions are circled in (a) and (b). (c) Saturation applied voltage measured as a function of spontaneous polarization. The red line is a fit. (d) Current $i$ vs time for triangular applied voltages with different driving frequencies ($T={145}^{°}$C). In all cases, polarization current flows when $\left|V\right|<V_{\text{sat}}$, as illustrated by the box highlighting the 10-Hz data. (e) $I$ and $I^{\prime }$ measured as a function of the frequency of the applied triangular voltage ($T={145}^{°}$C). The lines are best fits. (f) Optical transmission in ac (50-Hz triangular voltage) (solid line) and dc (symbols) applied fields ($T={145}^{°}$C). The transmitted light intensity level with an applied dc voltage remains constant for at least one hour.

Figure 3

Simulated electro-optic response of a polar Sm$A$ bent-core liquid crystal cell driven by a 10-Hz triangular voltage with $V_{\text{max}}=2V_{\text{sat}}$. The cell parameters are given in the text. (a) Applied voltage. (b) Polarization orientation and optical transmission. (c) Polarization current. (d) Electric field in the liquid crystal. In the analog regime, the internal field is small but nonzero for finite driving frequencies.