Spatially twisted liquid-crystal devices

TL;DR

The paper addresses non-symmetric patterned twisted nematic liquid-crystal devices, where front and back alignment layers carry different patterns to enable direction-dependent polarization transformations. It develops a generalized Jones-matrix framework for dual-plates and shows that external electric fields can reconfigure the polarization topology, including transitions among effective charges $q ~ 1/2$, $q ~ 1$, and $q ~ 0$. To explain these effects, the authors couple Frank–Oseen elasticity with a genetic-algorithm energy minimization to obtain spatial tilt theta(z) and twist distributions, yielding a multi-slice Jones matrix J_phi_f(alpha,V) that agrees with Stokes-polarimetry. The work introduces a practical framework for voltage-controlled, non-symmetric LC devices and points to future multi-q-plate and dual-pattern implementations for reconfigurable polarization structuring.

Abstract

Nematic liquid-crystal devices are a powerful tool to structure light in different degrees of freedom, both in classical and quantum regimes. Most of these devices exploit either the possibility of introducing a position-dependent phase retardation with a homogeneous alignment of the optic axis -- e.g., liquid-crystal-based spatial light modulators -- or conversely, with a uniform but tunable retardation and patterned optic axis, e.g., $q$-plates. The pattern is the same in the latter case on the two alignment layers. Here, a more general case is considered, wherein the front and back alignment layers are patterned differently. This creates a non-symmetric device which can exhibit different behaviours depending on the direction of beam propagation and effective phase retardation. In particular, we fabricate multi-$q$-plates by setting different topological charges on the two alignment layers. The devices have been characterized by spatially resolved Stokes polarimetry, with and without applied electric voltage, demonstrating new functionalities.

Figures (6)

• Figure 1: Twisted nematic liquid-crystal cells.a. Illustration of liquid crystals twisting between two glass plates, uniformly aligned at $0^{\circ}$ and $\alpha$ for the front and back layers, respectively, spaced apart by a distance of $L$. The action of this configuration is shown on the Poincaré sphere on a horizontally polarized input state (black dot) for varying twist angles $\alpha$ between $-\pi/2$ and $+\pi/2$, with a birefringence of b. $\Gamma = \pi$, and c. $\Gamma = 1001\pi$ in the adiabatic following regime.
• Figure 2: Fabricated samples. False colour images of a. discretized DP(0,1/2), and b. DP(1,2) between crossed polarizers under a microscope illuminated with white light. The topological pattern on each glass plate is also shown. Note that the $q=1/2$ pattern is discretized into 16 slices for the patterning process.
• Figure 3: Stokes vectors reconstruction. Reconstructed average Stokes vectors in each of the 16 slices (coloured points) for the cardinal input states (black points), and theoretical fit (line) using the TNLC Jones matrix with $\Gamma_{\text{fit}}=51.7$ for a. DP(0,1/2), and b. DP(1/2,0). The experimental Stokes vectors plotted are the average values in each slice of the discretized sample.
• Figure 4: Stokes vectors reconstruction. For horizontally polarized input light, the theoretical and experimentally reconstructed local Stokes vectors (arrows) for a. DP(1,2) and b. DP(2,1). The arrow color is a measure of the local polarization's ellipticity, with left-hand circular as red, right-hand circular as blue, and linear as green.
• Figure 5: Externally applied voltage on DP(0,1/2).a. Reconstructed local Stokes vectors (arrow) from a horizontally polarized input for voltage: $V_{pp}=3.00$, 6.00, 8.00, and 12.00 V. The color corresponds to the polarization ellipse angle $\psi$ with respect to the horizontal. b. The experimental data (dots) are the output Stokes vectors.