Polarimetric-Based Analysis and Manufacturing of Dye-Doped Liquid Crystal Photoaligned Cells for the Visible Range

Abstract

The accurate and controlled alignment of liquid crystals (LCs) in modern optical devices is of great importance. Photoalignment is one of the most appealing approaches for achieving more versatile alignment in designs. One of the most important parameters of these devices is the thickness and the homogeneity in the photoaligned area, especially in devices that introduce retardance. In this work, we propose a novel polarimetric-based method for the measurement of thickness of homogeneous liquid crystal cells that considers diattenuation effects and how they affect the retardance generated by a liquid crystal variable retarder (LCVR). We experimentally demonstrate the production of dye-doped liquid crystal (DDLC) devices, photoaligned in the visible range with a 532 nm laser light, of two different thicknesses with a very high spatial homogeneity. Thinner devices can be used across the whole visible spectrum despite the residual diattenuation at shorter wavelengths, whereas thicker ones achieve the best degree of polarization (DOP) in the transmitted wavefronts, close to 100%, at longer wavelengths.

Keywords: diattenuation; liquid crystal; methyl red; photoalignment; retardance; thickness.

Figure 1

Set-up for photoaligning MR cells (L1: Lens; LP: Linear Polarizer; D: Diaphragm).

Figure 2

Transmission spectrum of empty cell of (a) 5.5 and (b) 10 $\mathrm{\mu }\mathrm{m}$ in its central point. Two wavelengths used in Equation (17) are marked.

Figure 3

Relation between ellipticity and retardance for thickness cells of (a) 5.5 and (b) 10 $\mathrm{\mu }\mathrm{m}$.

Figure 4

The 8.4 $\mathrm{\mu }\mathrm{m}$ thickness device voltage study at the point in the centre: (a) 1.1 V; (b) 2.1 V; (c) 2.6 V; (d) 3.0 V; (e) 4.3 V; (f) 10.0 V.

Figure 5

The 12.0 $\mathrm{\mu }\mathrm{m}$ thickness device voltage study at the point in the centre: (a) 1.1 V; (b) 1.5 V; (c) 1.7 V; (d) 3.1 V; (e) 6.5 V; (f) 16.0 V.

Figure 6

Colour plots of the retardance vs. tilt angle for the visible spectrum for the (a) 8.4 $\mathrm{\mu }$m and (b) 12.0 $\mathrm{\mu }$m devices, where each overlapped white line is a π rad multiple of the retardance. Axial cut for the three wavelengths used in the paper to sample the visible spectrum for the (c) 8.4 $\mathrm{\mu }$m and (d) 12.0 $\mathrm{\mu }$m devices.