1. Introduction

Liquid crystal diffraction gratings have attracted considerable attention due to their considerable potential for application in displays, photonics and optical communications. [1, 2] The high birefringence and sensitivity to an applied field of liquid crystals (LCs) facilitates the development of highly functional optical devices for use in information display. Since LC molecules can be re-oriented by applying a voltage, most existing LC gratings are switchable electrically.

A new technology for fabricating a liquid crystal–polymer device has been developed in recent years. This technology is called Stressed Liquid Crystals (SLCs). [3, 4] The concentration of polymer used in this device is between that in polymer-dispersed liquid crystals (PDLCs) [5, 6] and polymer-stabilized liquid crystals (PSLCs). Typically, PDLCs and PSLCs contain polymer concentrations at ~30–50 wt%, and <10 wt%, respectively. These devices are operated based on the light scattering effect, and required to apply a high electric field. However, an SLC device can be operated without an applied voltage [4], but rather by shearing [7] substrates. Fast response [8] and large-phase retardation can be achieved using an SLC grating. This study presents novel tunable grating based on SLC devices. The grating can be modulated by shearing a distance or applying an AC voltage to tune the intensity and polarization of the diffracted beams.

2. Experiment

The SLC films were prepared by mixing a Norland optical adhesive NOA65 (15 wt%) (as a photo-polymerizable monomer) with a nematic liquid crystal LC (85 wt%). The LC used was K15 (Merck) with an ordinary refractive index of n_o= 1.5309, and a birefringence of Δn = 0.1754. Drops of homogeneously mixed K15-NOA65 were then sandwiched between two indium-tin-oxide (ITO)-coated glass slides separated by a 25 µm-thick plastic spacer to produce a sample. Prior to UV exposure [9], the cell was heated to T=120°C, at which LCs are isotropic. Then, the cell maintained at 120°C, was exposed under an unpolarized UV light (365nm) through a grating mask (spacing was 50µm). Curing intensity before the photo-mask was 12 mW/cm² and the exposure time was 25 minutes [Fig. 1(a)]. Before shearing, the randomly aligned LC domains were dispersed in the polymer network (Fig. 1(b)-upper). Then, one substrate of the SLC device was fixed, and the other substrate was sheared into various lengths (Fig. 1(b)-bottom). This shearing step generates a uniform alignment of the LC domains.

 

Fig. 1. (a). Setup for UV-curing a sample through a grating mask; (b) schematic representation of the cell structure before (upper) and after (bottom) shearing.

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Grating was observed under an optical polarized microscope (OPM). All cells were measured using an He-Ne laser (632 nm) as the probe beam at room temperature. Shearing length (L_shear) of the SLC grating was controlled by a micro-screw (precision is 1µm), and shearing was along the grating vector.

3. Experimental results

Figure 2 shows the images of an SLC grating under OPM before and after shearing. The regions enclosed by red- and green-dotted lines are the regions under the opaque and transparent regions of the photo-mask during UV curing (Fig. 1), respectively. The UV light, which exposed the sample through transparent regions, formed a polymer-rich scattering conformation. The transparent regions stayed at a scattering state after stressing due to the dense polymer network (Fig. 2); this finding is reasonable since monomer concentration is getting lower in transparent regions than that in opaque regions during polymerization. Thus, during polymerization, some monomers in the opaque regions diffused toward adjacent transparent regions to equilibrate the monomer concentration across the sample, and, thus, form dense polymer networks in the transparent regions. Then, UV light was then scattered by the formed polymer networks in the transparent regions into the adjacent opaque regions where light polymer networks formed. The monomer diffusion, thus, formed the center dark line in opaque regions, where are LC-rich regions. The LCs were well aligned along the grating vector, since under a crossed-polarizer OPM, the region with well-aligned LCs appeared dark [Fig. 2(a)]. After stressing, the dark line widened [Fig. 2(b)], indicating that more LC-polymer composites are aligned toward the grating vector during stressing [Fig. 1(b)]. Figures 2(c) and 2(d) show images of the same conformation under a parallel-polarizer OPM. The regions with well-aligned LCs are bright, indicating that the LC alignment in opaque regions can change under stress [Fig. 1(b)], and such an alignment change after stressing can be utilized to control the phase difference of an incident beam through the SLC grating. Like a typical LC device, an SLC grating can be controlled also by applying a voltage. Thus, a grating can be modulated by shearing a distance or applying a voltage to tune diffracted beam intensity and polarization demonstrated below.

 

Fig. 2. Images of an SLC grating under a polarized optical microscope before and after shearing (50 µm); P, polarizer; A, analyzer.

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Figure 3 presents diffraction patterns of an SLC grating before and after shearing (L_shear=50 µm) under an OPM. The polarization of the probe-beam was parallel to the grating. Figure 4 plots the measured intensities of the zero- and first-order diffractions by shearing various lengths under the cross-polarizer condition with the polarizer axis at an angle of ~45° relative to the grating vector. Each of these two orders is seen to be modulated as the phase retardation varied by shearing.

 

Fig. 3. Diffraction patterns of an SLC grating observed under, (a) P//A, (b) P⊥A. upper, before stress; bottom, after shearing with a L_shear =50µm; P, polarizer; A, analyzer.

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Fig. 4. Variation of the zero- and first-order diffraction intensities with shearing length Lshear under a cross-polarizer condition with the polarizer axis at an angle of ~45° relative to the grating vector.

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Diffractions of the SLC grating were then examined by applying AC voltages [10] with the sample sheared lengthwise. The measurement was also performed under a cross-polarizer condition with the polarizer axis making an angle of ~45° with the grating vector. Figure 5 shows the diffraction patterns. Figure 6 depicts the measured relations between the first-order diffraction efficiency and applied voltage under the parallel-polarizer condition as the sample was sheared from 0–60 µm. The diffraction efficiency (η) was defined as η = I_d/I_i, where I_i and I_d are intensities of the probing and diffraction beams. Similar to results obtained with the sheared sample (Fig. 4), the SLC grating can also be modulated electrically. Notably, the modulation effect becomes obvious as shearing length increases.

 

Fig. 5. Diffraction patterns of an SLC grating sheared with a length L_shear of (a) 0 µm, (b) 30 µm, and (c) 60 µm under the application of various AC voltages. Measurements were performed under the cross-polarizer condition with the polarizer axis at an angle of ~45° relative to the grating vector.

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Fig. 6. Variation of the first-order diffraction intensity with the application of AC voltages under a parallel-polarizer condition with the polarizer axis at an angle of ~45° relative to the grating vector.

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Polarization of the first-order diffraction with the sample stressed was measured. The measurement was performed with the polarizer axis parallel to the grating vector, and the analyzer axis was rotated with the polarizer. Figure 7 presents measurement results. Due to the phase variation in the opaque regions under stressing, the polarization of the first-order beam was controlled by shearing the sample.

 

Fig. 7. Variation of the first-order beam with sample under stress. Measurements are performed with the polarizer axis parallel to the grating vector, and rotating the analyzer axis.

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4. Conclusion

In conclusion, this study presents a novel SLC grating that can be modulated by shearing a length or applying an AC voltage. Both the intensity and polarization of diffracted beams from a SLC grating can be tuned. The device capable of tuning the intensity and/or polarization of diffracted beams is highly demanded in various optical systems. Thus, SLC gratings have a good potential for practical applications.