This work first reports wide-band tunable photonic bandgap (PBG) devices based on nematic-refilling cholesteric liquid crystal polymer template samples. By changing the type of refilling nematic liquid crystal (NLC) and sample cell gap, the PBG features of the template sample can be crucially influenced. A physical model related with the NLC infiltration into the template nanopores based on capillary action is used to qualitatively explain the tunable PBG features of the refilling template samples. In addition, a nearly full white (480 nm − 720 nm) spatially tunable PBG device based on a NLC-refilling template wedge cell is demonstrated.
© 2015 Optical Society of America
Cholesteric liquid crystal (CLC), or chiral nematic liquid crystal, is a nematic liquid crystal (NLC) with a helical axis along the twisting direction of LC molecules. The CLC helical structure induces some interesting optical properties, such as, optical activity and selective Bragg reflection . The helical pitch of the CLC can be selected to be on the hundred nanometer scale such that CLC can selectively reflects the circularly polarized light with a handedness similar to the helix of the CLC in visible region. Thus, CLC can be regarded as a one-dimensional photonic crystal (1D PC) for specific circular polarization of light and is suitable for some photonic applications such as color filters , reflective displays [3, 4], and low threshold laser devices [5–9].
Developing advanced techniques to improve CLC optical properties has been an important and emphasized topic in the LC field in recent years, to obtain better properties or performances in CLC-based devices [10, 11]. LC photonic template cells through washing-out/refilling, have large advantages such as increase in stability, simultaneous reflection of left- and right-handed polarized light with reflectivity enhancement over 50%, multiple reflection bands, and flexibly changeable photonic bandgap (PBG) feature by refilling material replacements [12, 13]. Choi et al. reported that the PBG of a refilling CLC template cell can be electrically tuned and the response time can be significantly shortened to 43 μs . Guo et al. used a right-handed CLC template cell with a left-handed refilling CLC to successfully obtain a CLC reflector which can reflect both left and right circularly polarized lights at the same time . By adjusting properly the pitch of the refilling CLC, multiple reflection bands can be obtained in a single sample. These multiple reflection bands can be externally tuned by thermal and optical methods [16–18]. Furthermore, Castles et al. used the same technique to develop a refilling blue phase (BP) template cell. This template may dramatically extend the BP temperature range from 40 °C to 250 °C . Although the above-mentioned investigations indicated that the LC photonic template is useful in advanced photonic applications, few researchers have proposed the influences of inherent property of refilling LC material and of the template morphology on the PBG feature of the template sample during and at the end of the infiltration process. This study is first to demonstrate experimentally that by changing NLC type and cell thickness, two key factors, refilling NLC viscosity and polymer template nanopore density, both determine the filling ratio of LCs into the template nanopores; hence, in the pitch and PBG feature of the NLC-refilling CLC polymer template sample during and at the end of the infiltration.
In the past, proposed tunable PBG devices based on refilling CLC templates exhibited some excellent properties. For example, the response time of the electrically tunable CLC template cell was short (~43 μs) and the tuning range of the optically or thermally tunable CLC template was wide (>100 nm) [14, 16–18]. However, the driving electric field of the electrically tunable template was very high (31 V/μm). The response for either optically or thermally tunable template was very slow. Additionally, in the LC molecular reorientation in the nanpored polymer template during electrical, thermal or optical tunings, the hysteresis resulting from LC anchoring from the nanopored polymer walls and incremental scattering resulting from the inconsistent reorientation of LCs in the nanopores, are detrimental to the reliability of such tunable PBG devices. Spatial tuning method is superior to the above-mentioned methods in some aspects because of its multiple advantages, such as, high reliability from the orientation of LC invariance in the nanopores of the template during spatial tuning process, time saving, and wide spectral tuning ability. Therefore, the present study also demonstrates a wide-band (480 nm–720 nm) spatially tunable PBG device based on a NLC-refilling CLC template wedge cell. Such PBG device has high potential for developing a wide-band tunable reflector/filter and laser working in the nearly full-white region.
2. Sample preparation and experimental setups
The blended CLC-monomer materials used to fabricate the CLC polymer template (“template” for short in later content) in this study include 66.8 wt% E7 (NLC, from Merck), 15 wt% R811 (right-handed chiral dopant, from Fusol-Material), 15.5 wt% RMM691 (chiral monomer, from Merck), 2.5 wt% RMM257 (achiral diacrylate monomer, from Merck), and 0.2 wt% Irg184 (photoinitiator, from Pufeng). In E7 host, the HTP values of R811 and RMM691 are about 11.24 and 3.73 μm−1, respectively. The photoinitiators can absorb UV light to generate free radicals which may trigger the chain polymerization process of RMM691 and RMM257, where the function of RMM257 is to strengthen the crosslinking polymerization. Each empty cell is pre-fabricated by assembling two cleaning glass slides which are pre-coated with anti-parallel alignment layers. Two narrow plastic spacers with variable thicknesses are placed between the two glass slides to control the cell gap (d). Four stages in fabricating each refilling CLC template sample are summarized briefly as follows:
- Before-curing stage. The uniformly blended CLC-monomer materials are injected into empty cells, and then diffuse throughout the cells via capillary effect to form the CLC-monomer composite cells.
- After-curing stage. Each CLC-monomer composite cell is cured with one UV beam of 1.1 mW/cm2 for 40 minutes to complete the photopolymerization of the cell.
- After-washing stage. The UV-cured cell is then immersed in cyclohexane in dark for about seven days for washing out the residual fluids of CLC (E7 and R811) and nonreactive monomers in the cell. In this stage, the transparent template cell can be obtained after completely removing the residual fluids and drying out in an oven for 3 hrs by vaporizing the remnant cyclohexane.
- After-refilling stage. A NLC is then refilling into the template cell to form a NLC-refilling template sample.
In the present work, four types of refilling NLCs, represented by HTW-114200 (from Fusol Materials), E7, MDA 03-3970, and MDA 04-1602 (all from Merck), are available. Their properties, including refractive indices (at 589 nm and 20 ○C) and viscosities (μ) (at 20 ○C), are listed in Table 1. ne, no, and na are the ordinary, extraordinary, and average refractive indices of the LCs, respectively, where na is defined as (ne + no)/2.
To develop a spatially tunable PBG device, this study also fabricates a refilling template wedge cell. The fabrication procedure of the wedge cell is same as that shown previously. The thickness gradient of the wedge cell is obtained by only placing a spacer of 125-μm-thickness at the thickest side between the two glass substrate of the cell. No any spacer is put on the thinnest side of the cell. The NLC used to refill into the formed template wedge cell is E7.
3. Results and discussion
3.1 PBG features of the sample measured at the four stages for the fabrication of E7-refilling template sample
Figure 1 indicates the reflection spectra of the sample with a 30 μm thickness measured at before-curing, after-curing, after-washing-out, and after-refilling stages for a NLC(E7)-refilling template sample fabrication, presented by black, red, green, and blue curves, respectively. The spectral positions of the PBG of the sample and thus its central wavelength (λc), are distinct at different stages. λc of the sample’s PBG shifts slightly from 720 nm to 695 nm after UV-curing stage. The result is attributable to the slight decrease in helical pitch of the chiral polymer following the volume shrinkage through polymerization and crosslinking reactions . At after-washing-out and after-refilling stages, the sample’s PBG disappears and reforms, respectively. These results reflect that a spiral structure inside the sample is just temporally hidden at the after-washing-out stage and reappears at the after-refilling stage. The sample observed through SEM shows a polymer template adhered on the inner surface of the glass substrate near the UV irradiated side at after-curing stage. Top view and side view SEM images of the template with an average thickness of 6.3 μm attaching on the substrate are shown in Figs. 2(a) and 2(b), respectively. The template itself collapses [Fig. 2(b)], which is responsible for the concealment of the spiral structure and thus the disappearance of the sample’s PBG at after-washing-out stage [green curve in Fig. 1]. In addition, the template is porous with numerous nanopores (<100 nm) inside [Fig. 2(a) inset]. The template collapse at the after-washing-out stage is primarily attributed to the hydraulic stress elimination of residual fluids in the nanopores acting on the polymer wall of the template after drawing out its fluids.
Additional experiments were implemented to understand the detailed mechanism underlying the reappearance of the PBG structure of the sample in the after-refilling stage. Figures 3(a) and 3(b) show the dynamic evolutions of both the PBG of the sample and its λc after E7 is refilled into the template cell for two days, respectively. The refilling template sample’s PBG was absent initially, but gradually grew and became complete with concomitant increase both in reflectivity and red-shift as the refilling time increased. λc of the reappeared PBG of the sample red-shifts with a decreasing rate of up to a terminal value of approximately 600 nm within a day. Experimental result indicates that the thickness of the template expands from approximately 6.3 μm to 10.2 μm after refilling E7 into the template cell from 0 h to 4 h, as shown in Fig. 4. The expansion is responsible for recovery of the spiral structure of the template and of the sample’s PBG structure in after-refilling stage. The gradual swelling of the template in the refilling process can be attributed to the well-known capillary action mechanism which usually occurs when fluid meets a solid structure with small vacancies or narrow channels . When NLC is injected into the washed-out template, part of the LC molecules, mostly near the nanopore polymer walls, first enter into the nanopores along its wall surfaces because surface adhesion between the heteromolecules of LC and nanopore polymer wall is stronger than cohesion between the LC homo-molecules. Following the infiltration of LC molecules near the nanopore polymer wall, LC molecules further away from the polymer wall flow into the nanopores due to cohesive attraction from those already infiltrated LC molecules near the nanopore polymer wall. Moreover, the experimental results shown in Fig. 4 indicate that the micrometer-order swelling of the refilling template is considerably thicker than the average pore size of the nanopores (<100 nm) in the template. This finding indicates that the swelling of the refilling template must not be contributed just from the refilling of the shallowest nanopores. The nanopores or parts of them in various depths of the template must be intercommunicated through nano-sized or finer channels such that the LCs can infiltrate from the shallow to deep nanopores in the template, resulting in the swelling of the entire refilling template. This infiltration process is successive such that the template gradually swells and the pitch gradually elongates. Furthermore, the increase in ratio of the refilling NLC in the nanopores may result in the increase of the average refractive index of the refilling template because the refractive index of LC is higher than the template polymer. These phenomena may cause a gradual increase in reflectivity, PBG completeness, and the concomitant λc red-shift. As demonstrated in the previous report , the viscous drag of a fluid primarily originated from the cohesion between the molecules of the fluid. The viscous drag of LCs increases because of the increasing length along the LC fluid infiltrating path when it infiltrates into deeper nanopores of the template . This will cause the infiltration rate to decrease upon refilling time increase, resulting in the decrease of the speed of the red-shift for the PBG and thus for λc (Fig. 3).
Notably, the NLCs in the nanopores are the same (E7) at the after-curing and after-refilling stages. The NLCs can occupy the nanopores at the after-curing stage because of phase separation after UV photopolymerization and at the after-refilling stage because of the infiltration process after the LC is refilled into the washed-out template. The NLC must naturally occupy 100% of the space in each nanopore of the template after the phase separation at the after-curing stage but probably not at the after-refilling stage. The NLC infiltration process into the template is affected by many factors such as the viscosity of the refilling LC and the nanopore morphology of the template. The high viscosity of the LC and low density of the nanopore may prevent the refilling LC from reoccupying 100% of the space of each nanopore of the washed-out template. This phenomenon may cause the PBG and thus the λc that appeared at the after-refilling stage cannot red-shift back to the original spectral position at the after-curing stage. The associated influences of the viscosity of the refilling NLC and the nanopore morphology of the template on the filling ratio of the LC in the nanopores will be discussed in the next two sections.
3.2 PBG features of the template samples refilling with NLCs of various viscosity coefficients
This work studies PBG features of template sample refilling with various NLC materials. Four samples with same thickness of 30 μm are prepared by refilling four various NLC materials into four identical washed-out template samples. Figure 5 shows the associated experimental results. Refractive indices and viscosity coefficients for the four NLC types are tabulated in Table 1. The spectral position of PBG and thus of its central wavelength for the refilling template sample is significantly dependent on refilling NLC type. The Bragg reflection of a CLC for normal incidence is characterized by the following formula :Eq. (1), the central wavelength of PBG for the NLC-refilling template sample depends on the value of na or p. λc (~505 nm) of the PBG measured based on MDA-1602-refilling template sample (blue curve) significantly deviates from that based on the HTW-114200-refilling template sample (~600 nm, red curve) for around 95 nm. The large deviation of the two central wavelengths must not be induced by the very small difference (in order of 10−3) between the nearly equivalent values of na for the two refilling NLC materials (Table 1). Therefore, the pitch is dominative in determining λc for the NLC-refilling template samples. The pitch recoveries of the refilling template samples are different for those samples with various types of refilling NLC. Based on the positive correlation between λc and μ (i.e., a lower value of μ results in longer λc) clearly shown in Fig. 5, an idea that the pitch of the NLC-refilling template sample is directly dependent on the refilling NLC viscosity is addressed. As addressed in the previous paragraph, the E7-refilling template sample can gradually swell and the pitch can gradually elongate with increased refilling time before ceasing of the NLC infiltration into the template nanopores. The capillary action is attributed to the surface adhesion forces between the heteromolecules of LC and nanopored polymer wall competing to the cohesion forces between NLC homo-molecules. Because the production of viscosity for a fluid is mainly attributed to the cohesion force between the liquid molecules , a stronger viscosity of refilling NLC indicates a stronger cohesive intermolecular force of refilling NLC molecules. When viscosity coefficient of the refilling NLC is higher, cohesion is stronger. The increase in viscosity of the refilling NLC may weaken its capillary action as well as decrease the rate of infiltration and ratio of the NLC refilling volume in the template nanopores. These changes result in a short pitch elongation and a low average refractive index, resulting in a small PBG red-shift of the refilling template sample. As presented in Table 1, the viscosity coefficients of the refilling NLCs for HTW-114200, E7, MDA-3970, and MDA-1602 are 85, 90, 142, and 203 cP, respectively. Notably, the two former NLCs with approximate viscosity coefficients refilling into the identical template cell leads to a similar amount for the PBG and its λc red-shift in the spectrum. The idea mentioned above is consistent with the experimental results displayed in Fig. 5. The dynamic evolution of the PBG for the template sample after the highly viscous MDA-1602 is refilled into the sample for 72 h is shown in Fig. 6. Compared to results shown in Fig. 3, the lowly viscous E7 refilling into the identical template sample, both rates of the completeness and red-shift of PBG are lower, and the refilling time for the PBG red-shift termination is longer when the viscosity of the refilling NLC is higher.
3.3 PBG features of the refilling template samples with various cell gaps
This work also investigates PBG features of the refilling template samples with various cell gaps. In this experiment, five empty cells with various cell gaps of 12, 30, 50, 100, and 125 μm are prepared to fabricate five template samples refilling with the identical E7 via the same fabrication procedure. PBG features for all five samples show little discrepancy in either before-curing or after-curing stage, indicating less cell gap influence on the pitch and therefore on the PBG features of the sample before or after UV irradiation, as shown in Fig. 7. Figure 8(a) presents the PBG features of the template samples with various cell gaps in the end of the after-refilling stages. The spectral PBG position and λc of the refilling template sample decrease with the increase in cell gap, thereby implying that a refilling template sample with a thicker cell gap has a shorter pitch and lower average refractive index in the spiral structure. Figure 8(b) shows the corresponding orange, yellow-green, and green reflection patterns from the refilling template samples with cell gaps of 30, 50, and 100 μm, respectively, under a reflection-mode polarizing optical microscope with crossed polarizers.
To realize the relation between the PBG feature of the refilling template samples and the cell gap, the SEM images of the template layers for these samples obtained after the washing-out stage are observed. Figures 9(a)-9(e) show the side-view SEM morphologies of the formed template layers for these samples and indicate that the formed template is thicker when the sample cell gap is larger. This is reasonable because, when the cell gap of the sample is thicker, more monomers are available for the long-termed UV-irradiation-induced polymerization to form a thicker template layer. To explain the relation between the template thickness and the PBG feature of the refilling template sample, the top-view morphologies of the template layer are further examined and shown in Fig. 10. Apparently, the density of the nanopores in the template layer is higher if its thickness is thinner. More monomers available in the photo-polymerization process may lead to a more sturdily polymerized template such that the corresponding nanopore density is lower. Moreover, more interconnections between adjacent nanopores may present when the density of the nanopores is higher. For these reasons, a higher NLC filling ratio in the entire template nanopore volume can be obtained for a refilling template sample with a thinner cell gap. Consequently, a longer pitch and a higher average refractive index in the restored spiral structure of the refilling template sample can be obtained, resulting in a longer λc of PBG [Fig. 9(f)].
To confirm if the interfacial tension of LCs on the polymer template (associated with the adhesion between the LCs and template polymer) is one of the major factors that affect the infiltration rate and filling ratio of LCs in the nanopores of the template, we measure the contact angle of the LC droplet on the template polymer. The measured contact angles in average for HTW114200, E7, MDA-3970, and MDA-1602 are 31°, 35°, 27°, and 29°, respectively. The small differences among these contact angles indicate that the discrepancies in wettability and thus in the interfacial tensions for the four LCs on the identical template polymer are small. Therefore, the interfacial tension of LCs on the polymer wall of the nanopores of the template is not crucially in determining the infiltration rate and filling ratio of the LCs into the template, as well as the shifting of the PBG at the after-refilling stage.
3.4 Wide-band spatially-tunable PBG of an E7-refilling template wedge cell
The cell gap-dependent PBG property of NLC-refilling CLC template allows exploiting to the further applications as a spatially-tunable PBG reflector or filter based on a NLC-refilling CLC template wedge cell. Figure 11(a) presents a rainbow-like reflection image of an E7-refilling CLC template wedge cell which has minimum and maximum cell gaps of zero and 125 μm at left and most right edges, respectively, with corresponding positions of x = 0 and 2.0 cm, respectively, along the cell gap gradient of the wedge cell. Figure 11(b) further shows the reflection spectra of the refilling template wedge cell measured at various detectable positions of x = 0.1 – 1.8 cm. Figure 11(c), a summary of experimental results depicted in Fig. 11(b), shows that λc is simply linearly dependent on the measured position of the wedge cell. Figure 11(d) shows the reflection spectral curves (from left to right) in the long-wavelength edges of the reflection band at x = 5200 µm to x = 4600 µm with a measured step of 100 µm. The curve reveals a quasi-continuous spatial tunability in the PBG of the wedge cell. Therefore, the λc of the reflected light from the wedge cell can be spatially tuned quasi-continuously from 495 nm to 695 nm and the spatially tunable PBG covers from deep red region (~720 nm) to blue region (~480 nm). This nearly full white quasi-continuously PBG-tunable feature of the refilling template wedge cell is attributed to the continuously varying thickness of the formed template after UV-curing on the wedge cell through x = 0.1 cm to x = 1.8 cm. Such a device has high potential in fabricating a wide-band tunable reflector, filter, or laser with high reliability and fast and wide-band tuning ability in the visible region.
In conclusion, this study is the first to demonstrate wide-band tunable PBG devices based on NLC-refilling CLC template samples. Experimental results indicate that both the refilling NLC viscosity and the template nanopore density are key factors in determination of the PBG feature of refilling template samples. A physical model associated with the infiltration of NLC into the nanopores of the CLC template based on capillary action is exploited to qualitatively explain the tunable PBG features of template samples. In addition, a NLC-refilling CLC template wedge device with a spatially tunable PBG feature in a wide spectral range in nearly full white region (480 nm − 720 nm) is developed as a highly desirable candidate for use in display and tunable photonic applications.
The authors would like to thank the Ministry of Science and Technology of Taiwan (Contract number: MOST 103-2112-M-006-012-MY3) and the Advanced Optoelectronic Technology Center, National Cheng Kung University, under the Top University Project from the Ministry of Education, for financially supporting this research. The authors are also grateful to Dr. Shie-Chang Jeng for his assistance in measuring the contact angle.
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