This paper presents the electro-optical characteristics of polarization-independent holographic gratings (HGs) recorded in polymer-dispersed liquid crystals (PDLCs) using off-resonant laser beam. The key mechanism is based on the low light absorbance by the materials used, which enables a slow polymerization-induced phase separation. The off-resonant light can penetrate through the cell without much energy loss (absorbance) and can be uniformly absorbed across the LC cell to produce uniform PDLC structures. The intensity-modulated interference field, which is generated by two linearly polarized off-resonant laser beams, is adopted to record the HGs. The fabricated HGs are electrically switchable and polarization independent. Moreover, the diffractions of the HGs fabricated using off-resonant light is better than those produced by resonant light.
© 2015 Optical Society of America
In recent decade, the applications of polymer-dispersed liquid crystals (PDLCs) have been widely developed because of their desirable light-scattering and electro-optical properties [1–5]. The differences between two types of PDLCs, namely, polymer-ball-type (PBT) and LC-droplet-type (LCDT), have been presented in previous studies [3, 6, 7]. The methods to fabricate PDLCs include encapsulation and phase separation. Phase separation methods, which include thermal-induced phase separation (TIPS), solvent-induced phase separation (SIPS), and polymerization-induced phase separation (PIPS), are identified based on which approach can be adopted to achieve phase separation between the LCs and the polymers . We have also proposed a novel phase separation method (particularly, TIPS) recently . In the present study, we focus on a PIPS method to fabricate PDLC films, as well as on the applications of phase/amplitude holographic gratings (HGs). Polymerization in the PIPS process can be initiated by photons (photo-curable), temperature (thermal-curable), and others. The phase separation of the composite of LCs and polymers occurs with the growth of the polymer matrix, and then a polymer/LC matrix containing discrete domains is generated. In the photo-polymerization process, which is adopted in this study, higher (lower) optical irradiation results in the formation of smaller (larger) domains.
In several cases, however, some photo-initiators are necessary to initiate and accelerate the polymerization process. Upon absorbing photons, the photo-initiator becomes a free radical and reacts with the used monomer, thereby triggering polymerization reactions. To reduce the required time for phase separation, resonant light, which can be efficiently absorbed by the used materials, is employed as the light source. From the technological point of view, the monomer and photo-initiator, as well as the wavelength of the light irradiation, are the most important parameters that influence the polymerization rate, PDLC structures, droplet size, and others. As mentioned, the highly efficient absorbance of photons from resonant light results in a high polymerization rate. According to [3, 8–10], monomers diffuse toward the light source during the PIPS process. In the present study, we proposed a photo-polymerization induced phase separation using off-resonant light, in which more uniform structures of the matrices (domains) are formed compared with those generated by resonant light. The polymerized polymers are formed uniformly from one substrate to another, rather than aggregate with each other onto the substrate close to the light source. In addition to the uniform formation, the inexpensive laser light source used to obtain HGs is another advantage of the proposed method. The mechanism is also described.
The optical recordings of holographic gratings (HGs), which are based on PDLCs using two coherent laser beams and derived from CW/pulsed lasers to produce interference fields, have attracted significant attention because of their potential use in optical storage . These reported HGs based on PDLCs including phase/amplitude gratings and polarization/intensity gratings. Studies have demonstrated HGs using the interference field from resonant light because of the high absorbance and fast fabrication. In 2003, Ono et al. reported the stable pure polarization holographic recordings in polymer LCs films with azobenzene side groups using orthogonal linearly polarized He–Ne laser beams (off-resonant light) and subsequent annealing. The resulting gratings converted the polarization of the diffracted beams with high diffraction efficiency . Sio et al. also reported the holographic gratings containing light-responsive liquid crystals . Moreover, regarding the advantage of the use of inexpensive laser, Sharma et al. formed holographic transmission gratings and Bragg reflection gratings in PDLCs cells using He–Ne laser [13, 14].
In this study, the photo-initiator [Rose Bengal (RB)] was doped into the monomer and LC mixture to initiate the polymerization processes. A He–Ne laser (λ = 632.8 nm), with a wavelength that is out of the range of resonant absorbance by RB, was employed as light source for the PIPS processes. The key concept is based on the low absorbance of He–Ne laser by the materials used, indicating that the light source cannot be strongly absorbed to accelerate the polymerization processes. The generated polymer structures by off-resonant light are examined by scanning electron microscopy (SEM). Accordingly, such selected materials and mechanisms are then applied to fabricate HGs with periodic scattering and transparent regions. In addition, the concentration of the monomer used, which is one of the keys to enhance the performance of HGs, was optimized. The diffraction properties of the HGs fabricated through off-resonant polymerization processes were significantly better than those produced by resonant light.
The nematic LC, monomer, and photo-initiator in this study were 5CB (K15, Merck), di-pentaerythritol pentaacrylate (DPPA, Polysciences), and RB (Aldrich), respectively. The optimized weight ratio (K15:DPPA:RB) of the above materials was 80:19:1. The homogeneous mixture was then injected into an empty cell, fabricated by assembling two indium tin oxide (ITO)-coated glass slides, and separated by two 25 μm-thick spacers. After filling, the edges of the cell were sealed with epoxy to produce a LC cell. The absorption spectrum of the mixture shows that the range of the resonant absorption wavelength spans 475 nm to 600 nm [Fig. 1(a)].
In this study, a linearly polarized red light derived from a He–Ne laser (λR = 632.8 nm) and a linearly polarized green light derived from an Argon ion laser (λG = 514.5 nm) were employed as an off-resonant light and a resonant light, respectively. According to the absorption spectrum, the absorbance values of the green (λG) and the red (λR) lights by the 25 μm LC cell are 37.5% and 1.4%, respectively [Fig. 1(a)]. Based on Beer–Lambert law (or Beer’s law) , the relationship between the intensities of incident and output beams, attenuation coefficient (absorbance coefficient), and thickness of the cell can be expressed asFigure 1(b) depicts the theoretical variations of the incident green and red light beam intensities from one substrate to that of the 25 μm LC cell. Clearly, the resonant light can be highly absorbed, unlike the off-resonant light. In other words, as an off-resonant light goes into the RB-doped LC cell, a small part of the light energy can be absorbed, indicating that the optical density of the off-resonant light is almost uniform in the bulk of a LC cell from one substrate to another. Accordingly, such an approach can provide a uniform energy along the propagating direction and can be adopted to fabricate HGs.
3. Results and discussion
Before fabricating HGs using off-resonant light, the structures of the polymer generated by one resonant and one off-resonant lights were examined using SEM. In our previous studies [3, 8–10], the polymer film formed by photo-curing PIPS adhered mainly onto the substrate, facing the incident light because of the diffusion of LCs and monomers during photo-polymerization. This information indicates that the LCs and monomers diffuse outward and toward the light source, respectively. However, these phenomena can also be observed in photo-polymerization by resonant light beam. To verify the difference between the polymer structures formed by resonant and off-resonant light beams, the cross-sections of two 25 µm-thick LC cells were examined by SEM after being separately illuminated with green (resonant light, λ = 514.5 nm) and red (off-resonant light, λ = 632.8 nm) light beams. Figure 2(a) presents the cross-section SEM image of a PDLC cell after green light (intensity 1 W/cm2) irradiation for 20 s. Clearly, the polymers (DPPA) aggregated, and then diffused onto the substrate close to the light source, producing a rough PDLC surface. Moreover, Fig. 2(b) depicts the cross-section SEM image of a PDLC cell after red light (intensity 1 W/cm2) irradiation for 20 min. The polymers (DPPA) were uniformly distributed from one substrate to the other one, rather than diffused to only one substrate. Notably, the reaction rate of photo-curing PIPS by green light illumination is much rapid than that by red light illumination, and thus the polymer structures [Fig. 2(b)] are much uniform, and the tiny branches, crossing through the bulk of the LC cell (from top to bottom substrates), were formed.
To record and elucidate the performances of HGs, two linearly polarized red light beams from a high-power He–Ne laser (λR) with an intersection angle of 1° were setup to incident onto the LC cell filled with the described mixtures, and their incident bisector was normal to the surface of the LC cell. To yield an intensity-modulated interference field on the LC cell, the two red beams were set to have the same polarization and intensity, ~280 mW/cm2. Theoretically, the grating spacing (Λ) is approximately 36.26 µm according to the equation Λ = λ/2sin(θ/2) [where λ and θ are the wavelength (632.8 nm) and the intersection angle (1°) of the pumping beam]. During the processes of photo-curing PIPS, the self-diffraction [16, 17] from the pumping beams (He-Ne laser) can be observed. To prevent the pumping beams from disturbing the measurement of diffraction signal of probing beam, one of the pumping beams was blocked for a second to measure the diffraction efficiency of the other pumping beam. Diffraction efficiency is defined as the ratio of the intensity of the diffracted beam to that of the incident beam. To optimize the concentration of the used polymer DPPA, four LC cells filled with four different concentrations of DPPA were prepared to record the HGs. The four different weight ratios of the materials (K15:DPPA:RB) were (a) 50:49:1, (b) 65:34:1, (c) 80:19:1, and (d) 84:15:1. Figures 3(a) and 3(b) indicate the measured first- and second-order diffraction efficiencies of the recorded HGs. Obviously, the diffraction efficiencies of HGs using different compositions increased initially, and then decreased with the increase in the duration of illumination, indicating that the mixtures in the high-intensity regions of the intensity-modulated interference field experienced PIPS to produce HGs. Consequently, the widths of the dark stripes [see Fig. 4] increased with the illumination time (photo-curing PIPS) due to the diffusion and aggregation of monomers. The overexposure (too long illumination time) resulted in the undesired photo-curing PIPS and diffusion of monomers from bright to dark stripes, which reduced the diffraction efficiency of the generated gratings. In addition, the optimized concentration of DPPA to achieve the highest diffraction efficiency is 19 wt%. If the DPPA concentration is too high, the reaction rate of the photo-curing PIPS would be relatively too fast, and thus the diffraction efficiency at the end of the recording time is relative low. Inversely, if the concentration of DPPA is too low, then DPPA would be insufficient to generate uniform HGs with high diffraction efficiency. According to the experimental results, the optimized concentration of the DPPA was experimentally proven to be 19 wt%, in which the highest (~14.2% and ~1.96% for first- and second-order diffraction efficiencies, respectively) and the diffraction efficiencies at the end of the recording time (~8.0% and ~1.0% for first- and second-order diffraction efficiencies, respectively) can be obtained. Accordingly, the optimized concentration of the mixture (K15:DPPA:RB = 80:19:1) was selected in the following experiments.
Figures 4(a)–4(d) present the HG patterns observed under a parallel-polarizer polarized optical microscope (POM), recorded under the illumination of two red beams (~280 mW/cm2) for 5, 10, 20, and 30 min. The grating spacing (Λ) was measured to be around 36 µm, consistent with the theoretical value. Clearly, the dark (bright) stripes depict the high (low) intensity regions of the intensity-modulated red interference field. The periodic structures of HGs were transparently isotropic and randomly scattered. The photo-curing PIPS with off-resonant light (dark stripes) illumination resulted in uniform and tiny branches, crossing through the bulk of the LC cell from the top to bottom substrates, and generated PDLCs. In addition, the grating patterns (stripes) were formed gradually. Moreover, the width of the dark stripes became wider with the increase in illumination time. However, regarding their diffraction efficiencies, the HGs formed by 10-minute illumination [Fig. 4(b)] exhibited the highest diffraction efficiency because of their uniform periodic structures.
The diffraction efficiencies of the recorded HGs fabricated by off-resonant laser light (red light) were independent of polarization. To examine the properties of polarization independence, the variations of the first-order diffraction efficiency of the HGs were measured at various polarizations of the probing beam (He–Ne laser). The diffraction efficiencies were probed by a linearly polarized He–Ne laser, and the angle of the incident probing beam (degree of polarizer) was defined as the angle between the transmissive axis of polarizer and the direction of the grating stripes. Figure 5 shows the plots of the measured first-order diffraction efficiency of HGs [as shown in Fig. 4(b)] as a function of the incident probing beam polarization. The results showed that the diffraction efficiency is polarization independent, and the average of the measured diffraction efficiency is approximately 14.16%. The property of polarization independence can be understood because the generated HGs were the combination of phase and amplitude gratings with transparently isotropic and randomly scattered stripes. Probing beam going through the PDLC stripes can be scattered (low transmission, no birefringence); while through isotropic mixture (no birefringence) will be transmitted. Therefore, the polarization independent phase/amplitude grating can be obtained. Thus, the incident probing beam with various polarizations exhibited the same diffraction phenomenon. Inset of Fig. 5(a) shows the diffraction pattern of the probing beam (He–Ne laser, polarization was parallel to the HGs stripes) diffracted from the generated HGs using off-resonant light. The diffraction signal can also be switched when an AC voltage (1 kHz) was applied, as shown in Fig. 5(b). This event happened because all of the LCs in the LC cell were aligned along the direction of the applied field and the refractive indices of K15 (no = 1.531) and DPPA (np = 1.49) were very close (but not equal); thus, the diffraction signal from the HGs can be partially eliminated. Moreover, the diffraction signals reappeared after the applied voltage was switched off, indicating that the LCs returned to their original state.
However, regarding the HGs recorded by resonant light beams, the employed materials (K15:DPPA:RB = 80:19:1) and LC cells (25 µm thickness) were consistent with those used in previous experiments. The diffraction efficiencies were probed by a linearly polarized He–Ne laser with polarization parallel to that of the pumping beams (green laser). Experimentally, the diffraction performance recorded by resonant light beams (green laser) was lower than that by off-resonant light beams. It is also demonstrated that higher intensity of resonant light beam requires shorter illumination time to achieve the highest diffraction efficiency. Based on the SEM images in Fig. 2, the polymers aggregated and diffused onto one substrate, facing the incident light (resonant light), thus, uniform scattering was difficult to achieve through the LC cell even though the intensity of resonant light beam was reduced. Therefore, although the required illumination time to produce HGs by resonant light is relatively short, the diffraction efficiency of the recorded HGs is low due to the non-uniform structures.
Figures 6(a)–6(d) depict the observations (parallel-polarizer POM) of HG patterns, which were recorded using illumination with four different intensity-modulated interference fields for respective duration to achieve their highest first-order diffraction efficiencies, including the intensities of green laser (illumination time), which are (a) 8.8 mW/cm2 (45 s), (b) 37.6 mW/cm2 (20 s), (c) 284 mW/cm2 (6 s), and (d) 995 mW/cm2 (5 s). The grating spacing (Λ) was also measured to be around 29 µm, which is consistent with the theoretical value. The highest diffraction efficiency of the cell was exposed by the resonant light (green laser) was lower than 9%. Comparing these four images with those shown in Fig. 4, the contrast between the dark and bright stripes of the HGs recorded by off-resonant light is found to be much higher than that by resonant light. Accordingly, highly efficient HGs can be achieved using off-resonant light.
In summary, the effect of the use of off-resonant light on polymerization-induced phase separation was examined in this study. The main mechanism is the low absorbance of the incident light, which enables the formation of uniform polymer structures across the LC cells by slow polymerization processes. This method was also adopted to record HGs by intensity-modulated interference field (off-resonant light) illumination. The fabricated HGs were polarization independent and electrically switchable. In addition, the edges and contrasts of the HGs between the scattered (high-intensity) and transparent (low-intensity) regions were clear and sharp. We also demonstrated the comparison of the performances of the HGs recorded by off-resonant light and the resonant light beams. Accordingly, it is believed that two-dimension HGs, reflection HGs, photonic crystal based on PDLCs, and others, can be achieved with high performance using the proposed method. Some experiments are underway to verify this approach.
The authors would like to thank the Ministry of Science and Technology (MOST) of Taiwan for financially supporting this research under Grant Nos. NSC 101-2112-M-006-011-MY3 and MOST 103-2112-M-008-018-MY3.
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