1. Introduction

The fingerprint structure of cholesteric liquid crystal (CLC), which is usually used in diffraction gratings, is formed by imposing specific boundary conditions on substrates under the application of a suitable voltage. The diffraction angle and efficiency of such a grating can be controlled by varying the period of the grating under the electrical field, photo-isomerization, and temperature. The self-organizing and stimuli-responsive characteristics of CLC gratings enable high tunability and simple fabrication, which makes them promising to be applied in 1D and 2D nonmechanical beam-steering constructs, spectrum scanning, etc [1–7].

Like its tunable diffraction angle, the rotation of a CLC grating is an important topic of relevance to 2D beam steering. Wu et al. proposed that the direction of CLC gratings can be controlled by the thickness-to-pitch ratio (d/p) and the alignment layers [8]. The orientation of the liquid-crystal (LC) director in the “middle layer” of the cell determines the stripe direction of the grating. Using this principle, Jau et al. and Zhang et al. demonstrate orthogonal switching of the CLC grating vector by electric field, light, and temperature, respectively [1,4]. In addition, rotatable CLC gratings were then proposed by tuning the helical pitch in a hybrid-alignment LC cell or a semi-free film [5–7]. However, since the rotation of the grating vector is a consequence of a change in pitch, the diffraction angle of the $m^{th}\text{order},{\theta }_m={\text{ sin}}^{–1}\left(2m\lambda /p\right)$, is inevitably changed, which seriously limits the usefulness of the CLC gratings.

Ma et al. reported on photoalignment-based 1D and 2D CLC gratings, demonstrating how such techniques are useful for making a variety of large-area CLC gratings [9]. Utilizing the repeatability of photoalignment, this work demonstrates two photoalignment-based approaches to control the orientation of CLC grating diffractions. In the first section, we show how grating vector can be reoriented by conducting double-sided photoalignment with a different polarization angle of the pump laser. The relationship among pump polarization, alignment axis, and resulting grating vector is discussed. In the second section, we show the fabrication of (axially symmetric) radial and concentric CLC gratings and how the 1D grating diffraction can be rotated by varying the probe position with respect to the grating. The electrical tuning of the diffraction angle and potential applications are also discussed.

2. Orientational control of CLC gratings with double-sided photoalignment

The CLC is composed of nematic liquid crystal E7 and chiral agent S811 (from Merck). In E7, a helical twist power of ~11 μm^–1 at 25 °C is measured. The pitch is adjusted to ~4 μm. The CLC mixture is then doped with 1 wt% azo dye, Methyl Red (MR) (2-[4-(dimethylamino)phenylazo] benzoic acid (C₁₅H₁₅N₃O₂), from Aldrich), for photoalignment. After homogeneous mixing, the MR-doped CLC is injected into a sandwiched cell made from two ITO-coated glass substrates without additional surface treatment. The cell gap was ~4.8 μm, yielding a d/p ratio of about 1.2.

Photoalignment can be induced by irradiating a MR-doped LC cell with green light. In LC, the MR molecules experience Brownian diffusion in the cell and adsorbed on the substrates due to electrostatic forces and dipole-dipole interactions at the LC–substrate interface [10,11]. Polarized optical pumping induces an easy axis of the MR adsorbed on the substrates with its orientation determined by the competition between the anisotropic desorption and adsorption [10,12,13]. Finally, the MR adsorption to the substrate surface, ultimately leading a re-alignment of the LC molecules [14]. When the process is complete, the long axis of the dye will be perpendicular to the pump polarization. There are two LC/ITO interfaces in the cell. Operating in the LC phase, the polarization of the pump light continues to vary as it propagates in the unaligned LC, owing to the strong birefringence. Therefore, while the photoalignment at the front interface can be easily induced, the alignment at the rear interface from the light source usually remains unchanged. To resolve this issue, Lin et al. developed a technique called double-sided photoalignment [15], in which the photoalignment process is conducted above the clearing temperature of the LC. In the isotropic phase, the pump light sees a uniform index, (n_e + 2n_o)/3, and so the polarization state will not be affected by the LC. Unidirectional photoalignment on both substrates can thereby be achieved. The alignment axis is perpendicular to the pump polarization.

In our experiments, a green diode-pumped solid-state (DPSS) laser at 532 nm is used as the pump source. A quarter wave plate and a rotatable linear polarizer are traversed between the pump source and sample to control the polarization angle of the pump light, because the pump light from the DPSS laser is polarized at a fixed angle [Fig. 1(a)]. The double-sided photoalignment of the MR-CLC is conducted at a sample temperature of ~60°C (above the clear point) and a pump intensity of ~100 mW/cm² for 10 min. According to Cheng et al. [16], keeping the sample at a high temperature in the dark (say, ~80°C) may lead to noticeable desorption of the dye from the substrates. The illumination and temperature conditions used in this study is carefully controlled so that, during the photoalignment, the MR molecules are well adsorbed on both the substrates [17]. In addition, the pump intensity determines the rate of photoalignment [18]. Due to dye absorption, the pump intensity exponentially decays as the light propagates in the cell, hence the photoalignment at the front substrate is faster than that at the rear substrate. The double-sided photoalignment processes in this work are conducted with sufficiently-strong and -long exposure to ensure the alignment quality of the rear substrate. Both substrates provide sufficiently strong and uniform aligning forces of the same direction. Upon removal of the pump, a 1 kHz AC field of ~1.5 V is applied to the MR-CLC to generate the fingerprint grating. Figure 1(b) depicts that the pump polarization is perpendicular to the alignment axis and parallel to resultant grating vector. By repeating the photoalignment process with different polarization angles, the grating vector can be effectively “rotated” [Fig. 2]. The orientational control of the CLC grating by pump polarization is further confirmed by grating diffractions [Fig. 2(b)]. Since the diffraction efficiency of the CLC grating depends on the angle between the polarization and grating vector, an unpolarized He-Ne laser at 633 nm is used as the probe to prevent polarization-selective effects at different rotation angles. Unlike other rotation mechanisms, such a rotation of the grating vector does not involve a change of the pitch, and hence the diffraction angles (θ_m) are preserved. As will be demonstrated in the next section that changing the pitch will not cause the grating to rotate upon complete formation.

 

Fig. 1 (a) Experimental setup for double-sided photoalignment. (b) Relationship among pump polarization, alignment axis, and grating vector.

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Fig. 2 (a) Optical micrographs and (b) far-field diffraction patterns of CLC gratings with different grating vectors.

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3. Radial and concentric CLC gratings

In this section, we show that photoalignment can also be used to fabricate radial and concentric CLC gratings. With such gratings, a 1D diffraction pattern can be produced by operating off-axis and is rotatable by changing the probe position. Figure 3 illustrates a technique called axially-symmetric double-sided photoalignment. In the fabrication process, the green pump laser of 300 mW/cm² is expanded to form a collimated beam with a diameter of 21 mm and then line-focused by a cylindrical lens. The sample is set at ~65°C and mechanically rotated at ~140 rpm. Upon ~60 min exposure, the alignment axis becomes parallel (tangential) to the radius if the pump beam is polarized along (perpendicular to) the long axis of the line shape, ultimately forming a radial (concentric) CLC grating by applying ~1.5 V (AC 1 kHz) in the absence of the pump light [Figs. 4(a) and 5(a)].

 

Fig. 3 Experimental setup for producing (a) radial or (b) concentric CLC gratings.

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Fig. 4 (a) Optical micrograph of radial CLC grating under a cross-polarizing optical microscope. (b) Diffraction patterns obtained by probing different positions of the grating.

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Fig. 5 (a) Optical micrograph of concentric CLC grating under a cross-polarizing optical microscope. (b) Diffraction patterns obtained by probing different positions of the grating.

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By positioning the probe beam at the center of an axially-symmetric CLC grating, a concentric diffraction pattern is formed in the far field [Figs. 4(b) and 5(b), far left panels]. Moving the probe beam away from the center, the concentric pattern transforms into 1D with the diffraction direction dependent on the type of grating (radial or concentric) and the probe position. For instance, if probing the right side of the center at the 3 o'clock position, a vertical (horizontal) diffraction pattern can be obtained for the radial (concentric) CLC grating [Figs. 4(b) and 5(b), second panels from the left]. Rotation of the 1D diffraction pattern can be achieved by varying the probe position. We emphasize again here that the diffraction angles remain unchanged during the rotation. Moreover, an axially-symmetric CLC grating can also be used to detect vibrations by monitoring the rotation of the diffraction pattern.

In Fig. 6, we demonstrate that diffraction angles are reversibly tunable by varying the applied voltage. The first-order diffraction angle is reduced with increasing voltage from 1.5 V (the threshold voltage for inducing the fingerprint texture) [19]. Figure 6(b) depicts that the first-order diffraction angle can be tuned between 34.9° and 41.5° by varying the applied voltage between 1.5 V to 6.15 V. Note also that, such tuning is not only reversible but also continuous. Above 6.15 V, the grating diffraction vanishes because the lying cholesteric helix is completely unwound, transitioning into the homeotropic state. Unlike the CLC grating devices reported in [5–7], a change in pitch does not lead to a reorientation of the grating upon complete formation. Nonetheless, switching between the radial and concentric gratings could be achieved by using the electrical drive scheme proposed in [1] or photoisomerization to induce a temporary change of the pitch during the grating formation.

 

Fig. 6 (a) Diffraction pattern of concentric CLC grating captured at various voltages. (b) First-order diffraction angle as a function of applied voltage.

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Since the photoalignment is repeatable, it may be affected by the light of the wavelength within the absorption spectrum of MR (mainly in the blue–green regime). Therefore, the 633 nm He-Ne laser was used as the probe in this study to achieve good stability of the grating throughout the operation. There are a variety of photoalignment materials available for substitution if a green or blue probe beam is required for operation. The repeatability of the photoalignment has also been examined. We found that MR-doped LC cells can withstand realignment for at least 40 times. The quality of alignment, such as uniformity and speed, begins to decrease after several tens of rotation. It is worth noting that, with optimized photoalignment materials, repeated alignment without degradation at thousands of cycles has been demonstrated in [20] The grating rotation requires temperature control and optical illumination to achieve double-sided photoalignment and an electric field for the subsequent grating formation and period tuning. To reduce the overall size and operation complexity of the system for the practical use, electrically induced dielectric heating [21] can be considered for temperature control in the photoalignment process.

4. Conclusion

This work demonstrated rotatable one-dimensional CLC gratings and axially symmetric gratings with controlled direction and distribution of the photoalignment on the substrates. Photoalignment with CLC in fingerprint texture is used to fabricate simply a functional diffraction phase grating. The optical direction-tuning of the CLC grating is achieved by temperature-controlled double-sided photoalignment. The strips of the CLC are neatly arranged in the direction of photoalignment. The CLC grating thus can be easily rotated in any direction by changing the polarization of the pumping beam. This rotation can be operated multiple times at different angles and reversible because the material has photoalignment repeatability. Additionally, the position-dependent rotatable CLC gratings are achieved by radial and azimuthal circular CLC gratings. These not only diffract light in arbitrary directions but also have electrically tunable diffraction angles. The diffraction angle can be reversibly controlled over a range of ~6.6° by changing the voltage. With broad tuning range, low-power operation, and compact structure compared to typical electro-optic crystal-based devices, the CLC gratings demonstrated herein are potentially applicable for 2D beam steering and related applications.