1. Introduction

Electrically switchable liquid crystal (LC) spatial light modulators are useful for photonics and optical communications. A range of liquid crystal phase gratings, optical beam deflectors, and switchable microlenses have been reported by taking the advantages of refractive index modulation (magnitude or phase) of liquid crystal over the inorganic based devices in terms of fast response, low power consumption and simplicity of construction [1–6]. The blazed LC gratings have emerged as a promising device suitable for steering the light by deflecting one polarization state of the incoming light and do not respond to the other polarization of the light. The liquid crystal blazed gratings, in general, are prepared by using a laminated prism sheet or a patterned photoresist layer at one of the substrates of an electro-optical cell in which the LC is filled [7].

Among the liquid crystal materials, cholesteric liquid crystal phases present a mesoscopic helical ordering. The long axis alignment of molecules is relatively rotated between successive layers of molecules, creating a helical rotation along the helix direction that is generally transverse to the layers. Helical pitch is defined as the average distance along the helix direction over that the relative rotation of successive layers corresponds to the complete 2π helical rotation of the anisotropic molecules. In a cholesteric liquid crystal the helix direction can be aligned parallel to the confining surfaces (called a fingerprint texture). Typically the fingerprint texture has the helix axes randomly distributed with no long-range ordering in the plane of the confining surface. Applying an electric field to the LC cell having uniaxially rubbed alignment layers and specific boundary conditions can induces uniform stripes, in which the helix axis lies parallel to the confining surfaces along a selected helix alignment direction. A problem arises in that the deformed helix parallel to the substrate surfaces and uniformly induced refractive index modulation are unstable at the zero applied electric field. Polymer stabilization of the uniform fingerprint texture has been employed and led to the development of the switchable polymer-stabilized cholesteric diffraction grating [8]. As one of the configurations described in the previous study to provide effective stabilization of the uniform fingerprint texture, a small concentration of photoreactive monomer dissolved into a cholesteric liquid crystal and optically polymerized with UV light to produce either bulk or surface localized polymer network [9].

In this paper, we report a method of fabricating a blazed grating using the combination of photo-induced localization of polymer on one substrate with a prismatic microstructure in a cholesteric LC. Our approach is a single-step fabrication for achieving an electrically-switchable LC blazed grating.

2. Experimental setup

The electro-optical cell was prepared from glass substrates whose inner surfaces were coated with indium-tin-oxide transparent electrodes and uniaxially rubbed polyimide (DuPont PI-2555) as the alignment layers. The two substrates were assembled with the rubbing direction at 180° fashion and separated by 10 micron spacers. The cholesteric liquid crystal sample was prepared by mixing a nematic liquid crystal BL006 (Δn=0.29, Δε=+17.3 at f=1kHz, Merck), chiral dopant R-1011 (1,2-bis[4-(4-pentyl cyclohexyl)benzoate]-1-(R)-phenyl ethane, Merck), reactive monomer RM 257 (1,4-bis[3-(acryloyloxy)propyloxy]-2-methyl benzene) and photoinitiator Irgacure 651 (2,2-dimethoxy-2-phenly acetophenone, Ciba Additive Corporation) at a ratio of 94.35%, 0.4%, 5%, 0.25% to produce about 10 micron helical pitch for uniform cholesteric stripes (the cholesteric stripe pattern formation strongly depends on the thickness to the pitch (d/p) ratio). The mixtures were filled into cells by capillary action under a slightly heated condition (25~35°C) and then cooled to room temperature.

Using 322nm collimated UV light we successfully developed localized polymer layer on the UV incident side. Figure 1(a) illustrates the cholesteric helix rotation and the stripe direction before and after pattern formation. The optical axis of the cholesteric twists from the bottom to top substrate at the initial planar state. With applied electric field to the substrates, the optical axis of the LC reoriented to the perpendicular with respect to the surface alignment layers. Figure 1(b) depicts the slantwise UV irradiation, in which the incident UV light is 45° tilted from the normal.

 

Fig. 1. (a) The picture depicts the helix rotation with respect to the surface rubbing directions of alignment layers and the formation of one-dimensional pattern from a cholesteric liquid crystal. Black arrows display rubbing direction and Blue lines on the top substrate represent the stripe direction after director reorientation by the applied field. (b) The schematic illustration of the UV polymerization apparatus with a 45-degree slantwise irradiation and the formation polymer grating after UV-induced polymerization.

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The formation of polymer network was initiated by illuminating the sample using a collimated and un-polarized UV light from a 150W Xenon lamp (Oriel 6258) (applied for 1hr, selected for 322nm wavelength of UV light at 0.08mW/cm² incident intensity with electric field 0.32 V/µm, 1 kHz square wave) at ambient temperature. The polarizing optical microscopy (POM) was performed with a Nikon OPTIPHOT2-POL, with the transmittance axes of the polarizer and analyzer crossed. POM image was obtained by evacuating the liquid crystal using a 70/30 v/v mixture of hexane and dichloromethane, with the solvent refreshed seven times over seven days. After solvent evaporation, the cells were carefully opened for the SEM study. The SEM image was taken using a Hitachi S-2600N scanning electron microscope operated at 10kV in secondary electron imaging mode and sample was 20 degree tilted up. The diffraction patterns were collected with normal incident He-Ne laser (λ=632.8nm) beam and recorded with a charge-coupled-device (CCD) detector. A He-Ne laser beam was polarized perpendicular to the grating axis for maximum grating contrast. No analyzer was used.

3. Results and analysis

After polymerization the grating patterns were investigated with POM and SEM image. The POM study shows uniform periodically arranged bright and dark regions (Fig. 2(a)). The 45-degree sample gives sharp contrasted stripes. Thus, sample is expected to show large optical beam bending effect. The SEM image of Fig. 2(b) reveals continuous surface with a prismatic polymer structure of the 45-degree UV irradiated sample. By using a short wavelength UV we are able to localize the polymer film near the cell surface. Clearly, the short wavelength UV light accelerates the monomer diffusion to the one side during the polymerization process. This characteristic can be explained as a result of the anisotropic polymer densification. The slantwise UV irradiation induces a gradient of light and brings photopolymerization localized on the substrate nearest to the UV light source. UV polymerized sample makes asymmetric polymer grating which has a large distribution on the one side.

 

Fig. 2. (a) The POM image of the sample formed with UV irradiation at a 45-degree incident angle from normal reveals one-dimensional polymer stripes after the removal of liquid crystal. A gradient of dark and bright pattern is observed. The stripes follow the rubbing direction (the black arrows represent the rubbing directions of the top and bottom alignment layers) and the grating period is 10µm. (b) The SEM image corroborates the prismatic grating polymer microstructure.

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The optical diffraction of the LC grating can be switched at different applied voltages to display the difference in effective refractive indices between the valley and hill regions. This is a signature of liquid crystal blazed gratings with the prismatic polymer microstructure. Figure 3 presents the images (recorded on the CCD camera) of optical diffraction of the blazed grating under different applied field conditions. The optical image of the diffraction pattern of the sample at the zero voltage condition reveals a light diffraction with an asymetrical light intensity between among the -1, 0, and +1 orders. The intensity ratio of the three diffraction orders is 2:1.4:1. The grating is switched to a first-order diffraction by applying an electric field of 0.9 V/µm, where the intensity ratio of the -1 and +1 order diffraction is 1:0.9 respectively. Further increase the electric field to 2 volts per micron, the optical diffraction was swtiched off with only the strong zero order. The grating efficiency is strongly dependent on the wavelength-to-period ratios. In practical use, a considerable enhancement of one of the first-order diffraction at zero voltage can be achieved by further reducing the grating period.

 

Fig. 3. Optical diffraction patterns of the blazed-grating sample under different magnitude of applied field.

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The angular dependence of the -1st order diffraction angle of the sample was further measured by rotating the sample along the grating axis (horizontal) with respect to the normal incident of He-Ne laser beam. It can be seen that the diffraction angles of the -1st order increase as the increase in incident angle regardless the direction of sample rotation. Furthermore, an asymmetrical pattern was observed when the sample was rotated along the horizontal direction. The maximum difference (at the ±53 degree from 0 degree) in the -1st order diffraction angles with respect to the horizontal sample rotation directions is 32.8% (Fig. 4). This difference is much larger than that of a symmetric grating sample, which has about 20 % angle deviance.

 

Fig. 4. The beam deflection angle of the -1st order as a function of incident angle for the 45-degree blazed grating sample. The maximum difference of diffraction angles from the ±53° incident directions of the He-Ne laser beam is 32.8%.

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4. Summary

In summary, we demonstrated a single-step method for fabricating liquid crystal blazed grating by using a photo-induced localization of polymer with a prismatic microstructure. The SEM study reveals that prismatic nature of polymer microstructure on one of the substrate. Optical diffraction studies display an asymmetric first-order diffraction pattern at the absence of electric field. The grating can be switched between the grating-on and grating-off states at a small applied electric field. A high performance optical beam deflector based on this concept can be achieved by further optimization of the grating period, depth and angle as well as the incident wavelength and polarization independence.