1. Introduction

Liquid crystal displays (LCDs) have become the leading technology for information display. They are used everywhere from smart phone, e-reader, digital camera to computer monitor and TV, because of their advantages of high resolution, high brightness, flat paneled, lightweight and low manufacturing cost [1,2]. In a LCD, the liquid crystal does not emit light but modulates the intensity of the light produced by edgelight or backlight [1–3]. Edgelit LCDs have the advantages of compact and light weight. In an edgelit LCD, a point light source (such as LED) is placed at the edge of the display. The light source produces an unpolarized light which is coupled into a light waveguide plate (typically plastic or glass plate) with a refractive index higher than the surrounding air. When the light hits the interface between the waveguide and air (with incident angles larger than a critical angle ${\varnothing }_c$), it is totally reflected by the interface and continues to propagate in the waveguide. In order to lead light out of the waveguide, scattering particles are dispersed in the waveguide [3–8]. When light hits a scattering particle, it will change propagation direction and hits the interface with an angle smaller than the critical angle and thus comes out of the waveguide. Thus the waveguide produces a 2-dimensional illumination for the LCD. For current light waveguide plates, there is, however, a problem that the light coming out of the waveguide plate is unpolarized. A polarizer must be used to convert the unpolarized light into polarized light such that the LC can modulate the intensity. In this process, more than half of the light is absorbed by the polarizer and thus is wasted.

There have been efforts to develop light waveguide plates which can produce polarized light, aiming at improving light efficiency of LCDs. For example, H. J. Cornelissen et al. developed a polarizing light waveguide plate with a saw teeth microstructure [9,10]. It selectively refracts light with a particular polarization and produces a polarized light. The technology, however, has not been commercialized ten years after its invention, probably due to the manufacturing cost.

In this paper we report a novel light waveguide plate made from polymer stabilized nematic liquid crystal film [11–14]. The liquid crystal has a poly-domain microstructure with domain size comparable to the wavelength of visible light. The orientation of the liquid crystal is more or less uniform in each domain. The orientation of the liquid crystal is confined in a vertical plane perpendicular to the plate. The azimuthal angle of the liquid crystal orientation remains the same, but the polar angle (defined with respect to the normal of the plate) varies from domain to domain [15,16]. It scatters the light with polarization in the plane but not the light with polarization perpendicular to the plane. The incident light produced by the edgelight is unpolarized and can be decomposed into two components with orthogonal linear polarizations. Only the component with polarization in the liquid crystal plane is scattered out. Thus the waveguide plate converts the unpolarized light produced by the edgelight into linearly polarized light by selective scattering.

Experiment and result

In our experiment a mixture was constructed from 95.8% nematic liquid crystal (LC) E44 (from Merck), 3.9% bifunctional monomer HCM-009 (from HCCH, China) and 0.3% photo-initiator BME (from Polyscience). The mixture had a positive dielectric anisotropy (_{$\Delta \epsilon >0$}) and was filled into a cell consisting of two parallel glass substrates with transparent ITO (indium tin oxide) electrode. On top of the ITO, a layer of polyimide was spin coated, baked and rubbed for the homogeneous alignment of the LC. The cell thickness was controlled by 10 μm spacer. The cell was then irradiated by UV light to photo-polymerize the monomers in the absence of applied voltage. The formed polymer network was anisotropic and parallel to the LC, as shown in Fig. 1.

 

Fig. 1 Schematic diagram of the active polarizing light waveguide plate. (a) voltage off state, (b) voltage on state.

Download Full Size | PDF

After the polymerization, both the LC and polymer network in the LWGP were in the homogeneous state along the alignment layer rubbing direction in the absence of applied voltage, as shown in Fig. 1(a). The polymer network had a strong aligning effect on the LC and tended to keep the LC in the homogenous state [12,13,17–19]. The ordinary and extraordinary refractive indices of the LC and polymer network were close to each other, respectively. The LWGP was a uniform optical medium and did not scatter light no matter where light was incident and what the polarization was. Light propagated through the LWGP without scattering. When a voltage was applied across the cell, an electric field perpendicular to the plate surface was generated. The electric field tended to align the LC in the vertical direction (the z direction), because of the positive dielectric anisotropy of the LC, while the polymer network did not reorient and tended to keep the LC in the horizontal direction (the x direction). Because the positions of the polymer fibers were random, the LC was switched into a polydomain structure as shown in Fig. 1(b). Because the aligning forces of the electric field and polymer network were in the xz plane, the LC was confined to orient in the same plane. In different domains, the tilt of the LC was different. The LWGP became a non-uniform optical medium. The LC might be scattering dependent on the polarization of the incident light. If the incident light had polarization in the y direction [Ray 2 shown in Fig. 1(b)], it encountered the same refractive index (the ordinary refractive index of the LC) in different domains and thus would not be scattered. When it hit the interface of the LWGP with a large angle (defined with respect to the normal of the LWGP), the light would be bounced between the two interfaces of the LWGP due to total internal reflection and thus propagated through the LWGP. If the incident light had polarization in the xz plane [Ray 1 as shown in Fig. 1(b)], it encountered different refractive indices in different domains and thus would be scattered. The propagation direction changed. When it hit the interface of the LWGP with a small angle, it would be refracted out of the plate. Thus the light coming out of the LWGP is linearly polarized along a direction in the x-z plane. For the unscattered light with polarization in the y direction, it can be reflected backward with its polarization rotated to the xz plane when it hit the combination of a quarter waveplate and a reflector at the other end of the plate. On its way back, it would be scattered and came out of the LWGP with polarization in the x direction.

We first characterized the polarization-dependence of the scattering of the polarizing LWGP. In the experiment an unpolarized laser beam was shined on the LWGP at normal angle. A polarizer and then a photo-detector were placed directly behind the LWGP as shown by the inset in Fig. 2. The collection angle of the detector was 4°. The polarizer was rotated to check the polarization of the light after the LWGP. In the absence of applied voltage, the LC was in the uniform homogeneous state and was not the scattering. The unpolarized laser light passed through the LWGP and its polarization remained unpolarized. The intensity of the light was high and varied slightly when the polarizer was rotated as shown in Fig. 2(a). The variation was probably due to the fact that the ordinary refractive indices of the LC and polymer network were not exactly matched. When an AC voltage of 20 V was applied, the LC was switched to the inhomogeneous state and became scattering. The unpolarized incident light can be considered as combination of two linearly polarized light in two orthogonal directions. For the component with the polarization perpendicular to the xz (LC) plane, the refractive index was the ordinary refractive of the LC independent of the orientation of the LC in the xz plane and therefore passed through the LWGP. For the component with the polarization parallel to the xz plane, the refractive index varied from domain to domain, dependent on the orientation of the LC in the xz plane, and therefore it was scattered away from the original propagation direction. Therefore the unscattered light was linearly polarized in the direction perpendicular to the rubbing direction. When the polarizer was rotated, the transmission oscillated between 0% and 30% as shown in Fig. 2(b), indicating the transmitted light is linearly polarized.

 

Fig. 2 Transmission vs. polarizer angle of the active polarizing LWGP. (a) V = 0 V, (b) = 20 V

Download Full Size | PDF

We then studied the optical property of the polarizing LWGP with edgelight. In the experiment, three point white LEDs were installed on the edge of the LWGP. The light was couple into the LWGP. When light propagated through the LWGP, some light was scattered out of the plate. A polarizer and a photo-detector were placed on top of the LWGP as shown by the inset in Fig. 3. The distance between the LWGP and the detector was about 1 cm. The light scattered out of the LWGP was polarized as shown in Fig. 3 where the detector was placed at the position 1 cm from the LED edgelight. The scattered light intensity depended on where the edgelight was installed and the voltage applied. When the LED edgelight was installed at the edge perpendicular and parallel to the alignment layer rubbing direction, the results are shown in Fig. 3(a) and 3(b), respectively. In the absence of applied voltage, the LC was in the homogeneous state and the scattered light intensity was low. The weak scattering was probably due to incomplete relaxation of the liquid crystal. When the voltage was applied, the liquid crystal was switched to the multi-domain structure. When the voltage was turned off, the liquid crystal tended to relax back to the initial homogeneous state. The relaxation was incomplete that the liquid crystal cannot fully relax back the initial homogeneous state, and therefore there was a weak scattering even the voltage is turned off.

 

Fig. 3 Scattered light intensity as a function of the polarizer angle defined with respect to the alignment layer rubbing direction. (a) edgelight installed on the edge perpendicular to the rubbing direction. (b) edgelight installed on the edge parallel to the rubbing direction.

Download Full Size | PDF

When 20 V was applied, the LC was switching to the poly-domain state and the scattered light intensity became strong. The measured scattered light varied when the polarizer was rotated, indicating that the scattered light was partially polarized. When the edgelight was installed at the edge perpendicular to the rubbing direction and the applied voltage was 20 V, the maximum scattered light intensity was 120 units, and the contrast ratio between the intensities of the two components of the scattered light with polarization parallel and perpendicular to the rubbing direction was 1.76:1. When the edgelight was installed at the edge parallel to the rubbing direction and the applied voltage was 20 V, the maximum scattered light intensity was increased to 250 units, and the contrast ratio was 2.5:1. The different light outputs in these two geometries were due to the different encountered refractive indices. The liquid crystal was confined in the plane (the liquid crystal plane) predetermined by the alignment layer rubbing and the plate normal. When the LED was installed on the edge parallel to the rubbing, the light propagation direction was perpendicular to the liquid crystal plane. When the LED was installed on the edge perpendicular to the rubbing, the light propagation direction was parallel to the liquid crystal plane.

The intensity of the light scattered out of the LWGP also depended on the distance from the LED edgelight. The scattered light intensity was measured at various positions on top of the LWGP and is shown in Fig. 4. In the experiment, three LEDs were installed, evenly distributed, on the edge. The light intensity first increased slightly with the distance from the edgelight. It peaked at the position 2.5 cm away from the edge. Then the light intensity decreased with the distance. When the distance was short (close the edge where the LEDs were installed), the light was mainly from the central LED (the other two LEDs did not contribute much), and therefore the scattered light intensity was low. As the light propagated through the waveguide plate, more and more light was scattered out, and thus its intensity decreased with the distance. Therefore light coming out of the plate also decreased with the distance. Due to the aforementioned two factors, as the distance increased, the scattered light intensity increased first and then decreased.

 

Fig. 4 Scattered light intensity as a function of the distance from the edgelight. (a) edgelight installed on the edge perpendicular to the rubbing, (b) edgelight installed on the edge parallel to the rubbing. //: polarizer parallel to the rubbing, _┴: polarizer perpendicular to the rubbing.

Download Full Size | PDF

Discussion and conclusion

In the polarizing LWGP, one component of the incident light is strongly scattered out of the plate with polarization parallel to the alignment layer rubbing direction. The other component with polarization perpendicular to the rubbing is weakly scattered and most of it is waveguided to the other edge of the plate. This light can be recycled by placing a quarter waveplate and then a reflector at the other edge. When the light is reflected back, its polarization is rotated 90° and then is scattered out of the plate.

The achieved contrast ratio between the intensities of the two components of the light coming out of the LWGP with the two orthogonal polarizations is about 2.5:1, which is decent but not the best. The contrast ratio can be improved by reducing the scattering of the light with polarization perpendicular to the rubbing direction. This scattering is mainly caused by the imperfect orientation of the polymer network and imperfect matching between the ordinary refractive indices of the LC and polymer network. Although the contrast ratio is not very high, the polarizing LWGP still can significantly improve the light efficiency of edgelight LCDs. The partially polarized light out coming from the plate can be changed into completely polarized light by placing a linear polarizer on top of the LWGP. Assuming the polarizer is perfect, when a regular LWGP is used, the light efficiency of LCD is 50%. When the polarizing LWGP with contrast ratio of 2.5:1 is used, the light efficiency is increased to _{$2.5/(2.5+1)=71\%$}.

The LWGP is switchable and suitable for transparent LCDs which can be operated in transparent mode or display mode. When no voltage is applied to the active LWGP, it is transparent and scenes behind the display can be seen. When a voltage is applied, it becomes milky and blocks the scenes behind.

In summary, we developed a novel polarizing light waveguide plate for edgelit LCDs. It can convert unpolarized light into polarized light by selective scattering. It can significantly improve light efficiency of LCDs.