Nanosecond Electro-Optic Switching of a Liquid Crystal

Electrically induced reorientation of nematic liquid crystal (NLC) molecules caused by dielectric anisotropy of the material is a fundamental phenomenon widely used in modern technologies. Its Achilles heel is a slow (millisecond) relaxation from the field-on to the field-off state. We present an electro-optic effect in an NLC with a response time of about 30 ns to both the field-on and field-off switching. This effect is caused by the electric field induced modification of the order parameters and does not require reorientation of the optic axis (director).

Figure 1

Ellipsoid of optic tensor. (a) Uniaxial NLC with $\Delta n>0$, $\Delta \epsilon <0$, and $\widehat{\mathbf{n}}$ along the $z$ axis. (b) Frederiks transition: $\widehat{\mathbf{n}}$ reoriented by electric field $\mathbf{E}$ applied along the $z$ direction. (c) Electrically modified order parameter (EMOP) effect: Electric field $\mathbf{E}$ along the $x$ axis increases ${\tilde{\epsilon }}_x$ and ${\tilde{\epsilon }}_z$, while decreasing ${\tilde{\epsilon }}_y$. (d) Field-off and field-on optical ellipsoids are shown together for comparison.

Figure 2

(a) EMOP effect: field-induced changes in birefringence $\delta n$ (blue) follow both the “on” and “off” edges of rectangular voltage pulses $U$ (red) with a characteristic response time of about 30 ns; NLC CCN-47; $T=45°\mathrm{C}$. (b) Experimental setup: NLC cell sandwiched between two 45° prisms.

Figure 3

(a) Birefringence change $\delta n$ (gray dots) in response to a voltage pulse $U$ (red) at $T=43°\mathrm{C}$ is fitted with Eqs. (3, 5), ${\tau }_u=31\mathrm{ns}$, ${\tau }_b=1.76\mathrm{ns}$, ${\alpha }_u=6.5\times {10}^{-20}{\mathrm{m}}^2{\mathrm{V}}^{-2}$, ${\alpha }_b=4.7\times {10}^{-20}{\mathrm{m}}^2{\mathrm{V}}^{-2}$ (solid black line). The blue dashed line shows the biaxial contribution with ${\tau }_b=1.76\mathrm{ns}$ and ${\alpha }_b=4.7\times {10}^{-20}{\mathrm{m}}^2{\mathrm{V}}^{-2}$. Fitting of the voltage profile by Eq. (7) with $U_0=626\mathrm{V}$, ${\tau }_{\mathrm{on}}=3.2\mathrm{ns}$, ${\tau }_{\mathrm{off}}=3.2\mathrm{ns}$, ${\tau }_a=18\mu \mathrm{s}$, $t_{\mathrm{on}}=93\mathrm{ns}$, and $t_{\mathrm{off}}=487\mathrm{ns}$ provides no visible deviations from experimental points. (b) The fitting parameters for uniaxial and biaxial contributions are field independent, as in the theoretical model. (c) Temperature dependences of ${\alpha }_u$, ${\alpha }_b$, and ${\tau }_u$.