A silica-nanoparticle-and-azo-dye-doped liquid crystal (LC) phase grating with multistable and dynamic modes based on photo- and nanoparticle-induced alignments was demonstrated. The photoalignment suppressed the electrophoretic movement of the silica nanoparticles in the hybrid aligned nematic (HAN) LC cell to maintain the vertical orientation of LCs during the electrical operation process by means of the azo dyes adsorbed on the silica networks in the homogeneously aligned side of the sample, and contributed two LC structures to form a grating. Through the excitation of a DC pulse with proper polarity and an AC voltage, the multistable and dynamic diffraction efficiencies of the grating were achieved, respectively, by electrophoretically controlled silica nanoparticles and the electrically controlled birefringence effect of LCs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Liquid crystal (LC) diffraction gratings have attracted considerable interest since their diffraction properties can easily be tuned through the LCs’ large electro-optic effects. LC-based tunable diffraction gratings have great potential for use in light modulators [1,2], 3D displays [3,4], beam steering [5,6], optical computing devices , and optical data storage [8–10]. There are several methods for fabricating LC gratings that possess a periodically changing refractive index. The natural ability of materials such as cholesteric LCs in a fingerprint state can form a periodic bulk structure [11–13]. Striped electrodes can generate a spatially periodic electric field distribution in the LC bulk, thus forming an LC grating [14,15]. The photoalignment method can been used to fabricate holographic gratings including intensity gratings and phase gratings [16–21]. A holographic recording in polymer-dispersed LCs can realize a periodic refractive index profile that includes polymer lamellae and LC droplets [22–24].
Switching diffraction gratings are interesting for several optical and photonic applications. Generally, switching between high and low diffraction efficiency in an LC grating is performed by applying an electric field through the LC cell, altering the LC orientation or texture. The diffraction efficiency of LC gratings that are optically switchable through the trans-cis isomerization of azo dyes has been demonstrated [25–28]. Many scientists have studied such gratings, including azo-dye-doped polymer-dispersed LCs [29–31], azobenzene-polymer-stabilized LCs , and dye-doped LCs with a polymer grating . Note that light scattering occurs easily when using a photosensitive polymer due to the mismatch in the refractive indices between the LC balls and polymer matrix and undesirable LC director distortions caused by polymer sticks. Lee et al. reported an LC grating with multistage optical memory that used a dye-dispersed LC/polymer composite medium . The polarization of the rewriting beam controlled the multistage optical memory that is required for editing the optical information. Wang et al. showed an LC circular Dammann grating that was fabricated by using the substrates coated with photoalignment materials. This grating had optically multistable and electrically tunable characteristics . Lin et al. demonstrated a gelator-doped LC phase grating with multistable and dynamic modes . Thermo-reversible association and dissociation of gelator molecules commanded LCs in the multistable mode. However, the multistable mode of the gelator-doped LC grating required a long time to adjust the temperature in the grating. This work presents an LC grating with multistable and dynamic modes based on azo dye and silica nanoparticles-doped LCs through photo- and nanoparticle-induced alignments. The multistable operation of this grating was easier than that of other gratings. The diffraction efficiencies of the grating in the multistable and dynamic modes could be modulated by applying a DC pulse and an AC voltage, respectively.
2. Materials and sample fabrication
In this study, the LC mixture was comprised of nematic E7 (from Echo Chemical), azo dye methyl red (MR; from Sigma-Aldrich) and silica nanoparticles Aerosil R812 (primary particle size, 7 nm; from Degussa-Huls). R812 is a hydrophobic organosilane-modified silica nanoparticle . Hydrophobizing can shift the triboelectricity of the particles to a negative value. The R812:MR:E7 mixing ratio was 2:1.7:96.3 wt%. The empty cell was constructed using two indium-tin-oxide (ITO)-coated glass slides separated by 5 µm spacer balls. One of the glass substrates was coated with the vertical alignment film n,n-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP, from Aldrich); the other was only rubbed (without the coated alignment layers). The rubbed ITO glass slide was used in the experiments because MR molecules were adsorbed on the rubbed ITO substrate much faster than on the homogenous alignment layer. The surfaces with and without the vertical alignment films were denoted SVA and SR, respectively. The LC molecules near SVA were aligned vertically, while those near SR were aligned homogenously due to the rubbing treatment. Accordingly, the performed cell had a hybrid aligned nematic (HAN) LC structure.
3. Results and discussion
Figures 1(a)~1(d) present the process of fabricating a LC grating, and Figs. 1(e)~1(h) illustrate the schematic diagrams of this sample. Figure 1(a) shows the polarizing optical microscopy (POM) image of the silica-nanoparticle-and-azo-dye-doped LC cell. The image in Fig. 1(a) is bright because of the phase shift obtained from the HAN LC structure. Different LC structures were achieved for a grating by utilizing the photoalignment associated with a regularly striped mask. Before the photoalignment, a positive DC pulse excitation was applied to the sample. In this study, the positive and negative polarities of a DC voltage source were connected to the SR and SVA, respectively, and this configuration was termed positive DC voltage excitation. Figure 1(b) presents the POM image of the sample after applying the positive DC pulse voltage of 50 V for 1 s. When the positive DC voltage was applied to the sample, the LC molecules reoriented perpendicular to the substrates. The negatively charged silica nanoparticles moved electrophoretically toward the rubbing side of the sample and accumulated on the SR, forming aggregate networks [38–40]. The silica networks screened the influence of the planar alignment and supported the homeotropic orientation of LCs, which was retained after the DC voltage was turned off as shown in Fig. 1(f) and memory state was realized. Consequently, the image in Fig. 1(b) is dark due to the silica nanoparticle-induced vertical orientation of the LCs. To achieve a multistructure via photoalignment, a diode-pumped solid-state laser of wavelength (λ) 532 nm was used as the laser source. The sample was irradiated by the green laser with intensity (I) 12.58 mW/cm2 through a chromium photomask with 100 µm-wide striped apertures. The polarization direction of the laser beam was parallel to the rubbing direction of the sample, as shown in Fig. 1(g). Figure 1(c) presents the result of the sample after irradiation with the green laser for 5 h. When the sample was stimulated with the green laser, the azo dye molecules underwent trans-cis photoisomerization, molecular reorientation, diffusion, and finally adsorption onto the SR, with the long axis perpendicular to the polarization of the green laser beam [41,42]. However, the absorption of azo dyes was weak because the polarization of pumping beam was perpendicular to the long axis of azo dyes. The amount of adsorbed azo dyes was not enough to affect the vertical orientation of the LC molecules. Accordingly, the color of the pumped region in Fig. 1(c) was still dark. Then, a negative DC pulse voltage of 50 V was applied to the sample for 1 s. Figure 1(d) shows the result. In the non-pumped region, the image became bright because the negatively charged silica nanoparticles moved and accumulated on the SVA electrophoretically. These nanoparticles on the SVA did not affect the primary hybrid alignment of the sample. In the pumped region, the color remained dark because the azo dye molecules were adsorbed on to the silica networks and thus the electrophoretic movement of silica nanoparticles was suppressed. The adsorption phenomenon of azo dyes relates to the dipole–dipole interaction between the azo dye molecules and the ITO surface of substrate . The silica nanoparticles that underwent a force from adsorbed azo dyes could not move away when the negative DC pulse was applied. These silica networks on the SR still stabilized vertical orientation of LC molecules as depicted in Fig. 1(h). Accordingly, two different LC structures were established in the sample by the photo- and nanoparticle-induced alignments.
To confirm the LC structures in the pumped region of the grating as described in Fig. 1(d), experiments were performed that exploited the same setup but without a mask. Figure 2 presents the results. The POM results showed that the color of the pumped region under the crossed polarizers remained dark regardless of the rubbing direction. The transmission–voltage (T–V) curves in the pumped and in the non-pumped regions of the sample in Fig. 2 were measured and shown as the red square and blue diamond curves in Fig. 3, respectively. Experiments with varied illumination duration were also performed and shown in Fig. 3. A He–Ne laser (λ = 632.8 nm) was utilized to measure the T–V curves. The red laser beam passed through the sample placed between two crossed polarizers, with the polarizer axes at ± 45° from the rubbing direction of the sample. These transmittances declined to zero as the AC voltage (f = 1 kHz) increased because the LC molecules were orientated parallel to the electric field. The non-pumped region showed the standard T–V curve of a HAN cell. The phase of the non-pumped region without applications of voltages was calculated as ~1.6π from the blue diamond curve in Fig. 3 according to T = sin2(δ/2), where T and δ are the normalized transmittance and the phase of LC cell, respectively. This value was close to the phase of a HAN cell that was filled with LC E7 only. The T–V curves of samples with illumination did not have a peak and their initial transmittances were much lower than those in the non-pumped region. These initial transmittance values decreased to near zero and then increased when the illumination duration increased. When the illumination duration increased to 5 h, the curve approached a straight line because the silica networks on the SR with 5 h illumination were covered with enough azo dye molecules and were unlikely to be affected by the application of the negative DC pulse. These silica networks under azo dyes still stabilized the vertical orientation of the LCs. When the illumination duration was short, only parts of silica nanoparticles were coved by the adsorbed azo dyes. Accordingly, the LCs near the SR were tilted and the initial transmittance was higher than that with illumination duration of 5 h. When the illumination duration reached 7 h, the azo dye molecules that were adsorbed on the SR with the long axis perpendicular to the polarization of the pumping beam were sufficient to change the alignment of the LCs. Adsorbed azo dyes that provided a homogeneous alignment caused the LCs on the SR to be tilted, and the initial transmittance increased again. The director of LCs was estimated to re-orient at ~22° from the rubbing direction.
To verify the reason for the induced vertical orientation of the LCs after illumination, atomic force microscope (AFM) was employed to observe the surface topography of the substrate with a rubbing treatment. Figure 4 displays the AFM images of the rubbing substrate of the sample that was first applied with a 50 V positive DC pulse for 1 s, illuminated with I = 125.8 mW/cm2 for 5 h, and then excited by a 50 V negative DC pulse for 1 s. To obtain the substrate for the AFM measurement, the epoxy seals on the edges of the sample were removed. Then the two substrates were separated carefully. The rubbed substrate was sluiced with the solvent, n-hexane, for a few seconds to remove LCs on the substrate. After the solvent evaporated in the room temperature, the substrate was ready for the AFM measurement. The results show that many of the silica nanoparticles still aggregated on the SR in the pumped region after the negative DC pulse voltage. The AFM results provide evidence that the homeotropic orientation of the LC sample in the pumped region was due to the aggregated silica nanoparticles that were prevented from moving electrophoretically by the adsorbed azo dyes.
The multistable property of the non-pumped region shown in Fig. 2 was also investigated. The utilized setup was the same as that for measuring T–V curves; Fig. 5 illustrates the results. When a positive DC pulse of 1 s duration was applied to the sample, the optical transmittance decreased to zero during the pulse duration because the LC molecules were oriented parallel to the electric field. After pulse excitation, the transmittance increased to a stable value after a few seconds (Fig. 5(a)). Various magnitudes of positive DC pulse caused various stable transmittances. These stable transmittances increased to their maximum and then decreased to zero as the positive DC pulse increased. The variation in the stable transmittances with the positive DC pulse was similar to that of the dynamic curve (the blue diamond curve in Fig. 3). The electrophoretic movement of silica nanoparticles between two substrates can be described as tdr = d2/μV, where tdr, d, μ, V are the drifting time of silica nanoparticles, the distance between the two electrodes, the mobility of silica nanoparticles, and the applied voltage, respectively . When the applied pulse duration was too short, no memorized state could be obtained. In the experiments, the pulse duration of 1 s was used to realize the memorized state under a low voltage.
The photo- and nanoparticle-induced alignments established two different LC structures in the sample. The pumped region was switched from a hybrid to a homeotropic alignment and the non-pumped region retained its hybrid alignment. As described in Fig. 1, a silica-nanoparticle-and-azo-dye-doped LC grating was fabricated. The electro-optical properties of the grating shown in Fig. 1(d) were measured. This LC grating possessed dual operation modes that were multi-stable and dynamic modes. Figure 6 presents the variation in the POM images for the grating with the positive DC pulse in the multistable mode. The images were obtained after applying various magnitudes of positive DC voltage with 1 s duration. The results show that the transmittance of the non-pumped region in the grating decreased as the amplitude of the positive DC pulse increased, while that of the pumped region remained dark. The reason was that the LC molecules in the pumped region were aligned vertically, and the alignment of the LCs in the non-pumped region changed from a hybrid alignment to a vertical alignment as the positive DC pulse increased. As mentioned above, when a positive DC voltage was applied to the sample, the negatively charged silica nanoparticles in the sample moved electrophoretically toward the rubbing substrate and accumulated on the SR. The higher positive DC voltage accumulated a higher density of nanoparticle networks which screened the influence of the planar alignment and supported the homeotropic alignment of the LCs. The LC grating could be reversed back into its original state (Fig. 6(a)) by applying a negative DC voltage excitation of 50 V for 1 s to accomplish the multistable mode operation.
Figure 7(a) shows the variation of diffraction efficiencies of the silica-nanoparticle-and-azo-dye-doped LC grating in the multistable mode. The red laser (λ = 632 nm) was used to measure the diffraction efficiencies of the LC grating. The laser light was polarized parallel (s-polarization) or perpendicular (p-polarization) to the rubbing direction of the sample and was typically incident on the LC grating where there was no analyzer behind the sample. Each measurement in Fig. 7(a) was performed after applying various magnitudes of the positive DC pulse for 1 s. The first-order diffraction efficiency of a phase grating was determined by the relative phase difference between the non-pumped (hybrid alignment) and pumped (vertical alignment) regions. Consequently, the maximum first-order diffraction efficiency was detected when the s-polarized light was incident on the grating. The first-order diffraction efficiency of the LC grating under s-polarization light was ~7% at V = 0 and decreased gradually to zero as the positive DC pulse increased because the LCs in the non-pumped region changed from hybrid to vertical alignment and that in pumped region maintained its alignment vertically. To increase the diffraction efficiency at V = 0, a large cell gap could be used in the experiments. Figure 7(b) presents the diffraction patterns of the LC grating in the multistable mode under s-polarization light.
Figure 8 shows the POM images of the silica-nanoparticle-and-azo-dye-doped LC grating in the dynamic mode. When an AC voltage (f = 1k Hz) was applied to the grating, only the LCs in the non-pumped region re-oriented due to the vertical alignment of the LCs in the pumped region. The LCs in the non-pumped region were re-oriented from the HAN structure to vertical orientation as the AC voltage increased because the LC tended to be parallel to the direction of the electric field. Figure 9 presents the variation of the diffraction efficiencies and the diffraction patterns of the LC grating under s-polarized light in the dynamic mode. These results were similar to those in the multistable mode shown in Fig. 7. The diffraction efficiency of the grating could be modulated by a low AC voltage. The LC molecules returned to their original orientation, and the diffraction reverted to the maximum when the AC voltage was turned off. The fabricated grating functioned as a conventional LC grating. The diffraction efficiency variations in the silica-nanoparticle-and-azo-dye-doped LC gratings were similar to those in the gratings that were based on pure LCs. The durability of the silica-nanoparticle-and-azo-dye-doped LC grating was tested. The diffraction efficiency was almost unchanged after an AC voltage of 6V was switched on and off 100 times.
A polarization-dependent LC phase grating with electrically tunable multistable and dynamic modes based on photo- and nanoparticle-induced alignments was demonstrated. The grating consisting of hybrid and vertical LC structures was accomplished through the electrophoretic movement of silica nanoparticles and the photo-alignment of azo dyes. The photoalignment suppressed the electrophoretic movement of silica nanoparticles from the adsorption of azo dyes on silica networks, causing vertically aligned LCs during the electrical operation process in a HAN cell. Applying a positive DC pulse voltage, the silica nanoparticles moved and accumulated on the SR to stabilize the homeotropically orientation of LCs, giving multistable diffraction efficiencies for the grating. In the dynamic mode, a low AC voltage modulated the diffraction efficiency of the grating.
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