1. Introduction

Recently, the polymer/liquid crystal (LC) dispersions have attracted growing interests because of their potential applications in novel electro-optic devices such as light shutter, smart window and privacy glass [1–6]. There are two conventional types of polymer/LC dispersions, the polymer-dispersed LCs (PDLCs) and polymer-stabilized LCs (PSLCs) [7]. The former, which contains a small amount of nonreactive LC molecules (≈30 wt.%), usually form small LC droplets with a diameter of several micrometers in the continuous polymer matrix by encapsulation or phase separation. The latter, which contains large concentration of nonreactive LCs (90-95 wt.%) and a small amount of reactive mesogens (RMs) (5-10 wt.%), can form a polymer network to stabilize the alignment of nonreactive LC molecules by elastic interactions. Besides, a novel class of polymer/LC dispersions with relatively higher concentration of RMs (> 10 wt.%), which can lead to a stable polymer matrix with nanopores, was investigated intensively in the past decade [8–10]. Remarkably, when the unpolymerized components inside pores are washed out by organic solvents, the initial properties of polymer-templated LCs can be preserved after the refilling process without the destruction of LC polymer networks fabricated via photopolymerization.

Therefore, many fancy optical devices have been presented based on the polymer-templated LCs. The super-reflective LC films reflecting both right-circularly polarized (RCP) light and left-circularly polarized (LCP) light have been reported by either refilling the right-handed cholesteric liquid crystals (CLCs) into the left-handed CLC polymer template with the same pitch lengths, or assembling two opposite handedness CLC templates which are refilled with optical adhesive and cured thoroughly [8,11]. Tunable devices with wide photonic bandgaps (PBGs) regulated by solvent, light and electric fields have also been demonstrated by refilling various components into CLC polymer templates [12–15]. Moreover, full-color reflective display, optical sensor and tunable laser were reported using polymer-templated CLCs [16–21]. Other than the CLC polymer templates, blue phase (BP) LC templates with unprecedented thermal stability have been developed [22]. These LC template-based devices have been proven to operate at a broader temperature range, insensitive to mechanical stress and can be configurated dynamically by the refilling process.

However, the polymer-templated LCs are usually fabricated after being cured by the ultraviolet (UV) light directly and patterned by using a photomask. The high-intensity UV irradiation, potentially harmful to the human health, is impractical to holography because single longitudinal mode (SLM) UV lasers are usually expensive. The holography based on biofriendly visible wavelength, e.g. green wavelength, is of great significance considering the cost-efficiency of laser source and high tolerance to environment vibration [23]. Remarkably, in related works by taking the advantages of the green-light induced polymerization, the holographic polymer-dispersed LCs (HPDLCs) based on the rose bengal (RB)/N-phenylglycine (NPG) photoinitiation system have sparked a storm of popular research interests. Advanced optical elements such as highly reflective volume Bragg grating, optical switch, two-dimensional (2D) holographic photonic crystal (PC) and 2D Penrose photonic quasicrystals have been demonstrated previously [24–27]. The storage of naked eye-recognizable colored three-dimensional (3D) images were also demonstrated in the HPDLC gratings [28]. Moreover, by combining HPDLCs with computer-generated holograms (CGHs), square-wave phase grating, electrically switchable reconstructed image and optical vortex have been recorded successfully [29–31]. Recently, the two-photo polymerization (TPP), an advanced bottom-up 3D micro- and nano-fabrication technique to generate desired LC alignment layer or directly write on RMs, has been widely used to fabricate novel LC photonic devices, such as phase modulators and microlens array [32–34].

Herein, mostly inspired by the recent advances of polymer-templated LCs and the photopolymerization based on the RB/NPG photoinitiation system, we fabricated polymerized-templated LC holographic gratings via green-light recording. Different from conventional Raman-Nath or Bragg LC gratings which usually regulate the diffraction efficiency (DE) by electric field, we propose a facile approach to regulate the DE of holographic gratings by changing the refractive index (RI) of refilled fluids and broadened tunable range of DE has been achieved. By optimizing the concentration of RMs in polymer-templated LC gratings, the template after being washed-out can be refilled with suitable fluidic components. And the dependence of the first-order DE on the concentration of RB and NPG molecules was discussed in detail. We also investigated the polarization-dependency of diffraction properties with discrete linear polarization (LP) angles. The reconfigurability of polymer-templated LC gratings were characterized by monitoring the relationship between first-order DE and temperature when organic solutions with different RI n were refilled. Furthermore, we explored the practical potential of polymer-templated LCs for recording holograms using green light.

2. Experiments

The polymer/LC dispersions were prepared by a mixture of NLC E7 (n_e = 1.74, n_o = 1.52, Xianhua, China), RMs (${\bar{n}_{RMs}}$ = 1.56, NJSJ, China), photoinitiator RB (Aladdin), coinitiator NPG (Aladdin) and a small amount of cross-linking monomer N-vinylpyrrolidinone (NVP, n_NVP = 1.51, from Aladdin). Five different RMs, RM257, RM82, RM006, RM021 and RM010, were mixed with 30:20:20:20:10 ratio. Among the five different RMs, RM257 and RM82 are bifunctional monomers with two reactive acrylate groups on both ends of the molecules that form the polymer network. While the others, RM006, RM021 and RM010, are monofunctional monomers with only one acrylate group that modify and stabilize the polymer structure. Mixing five RMs was clarified to have a better performance than using one individual RM [20,21]. Although the RI of prepolymer syrup changes from liquid to solid after polymerization, the average RI of RMs could be measured by Abbe refractometer or m-line method [35,36]. A higher DE would be achieved when the nematic LC E7 with a relatively large birefringence is chosen to enhance the RI contrast. NVP not only facilitates the cross-linked polymer structure during the photopolymerization process, but also helps to dissolve the RB and NPG in LC mixtures [24]. The material compositions for each sample are displayed in Table 1, where A1∼A6, B1∼B6 and C1∼C5 represents LC mixtures with different concentration of RMs, RB and NPG, respectively. Since the concentrations of RB and NPG in the mixtures were relatively low and the slight variation would dramatically influence the diffractive behaviors, the samples B1∼B5 and C1∼C5 were prepared by mixing RB and NPG with varied weights when the amounts of other components was fixed. To standardize the data, the compositions are presented by weight concentration and the calculated values remains two decimal places in Table 1. The mixtures were ultrasonicated for 20 min, stirred magnetically at 1200 rpm for 16 hours to ensure homogeneous, and filled into the commercial LC glass cells with a cell gap of 5 μm through capillary forces above the clearing point (cp) temperature. The indium-tin-oxide (ITO) and polyimide (PI) were coated in the inner side of the cell substrates. The PI layers were rubbed in the anti-parallel directions.

Table 1. The material compositions of samples.

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The optical setup to fabricate the holographic gratings is shown in Fig. 1(a). A beam from SLM laser (Coherent Compass 315M) operating at λ_w= 532 nm was expanded, collimated, divided by a polarizing beam splitter (PBS) cube, and simultaneously irradiated the sample at an interference angle θ_w ≈ 6.10° for 20 min. Each beam provided an equal intensity of ∼20 mW/cm² with the same LP angle which was vertical to the grating vector. The LC cell filled with sample were clamped with the alignment direction vertical to the incident plane. The theoretical period of the holographic gratings, Λ, was calculated to be ∼5 μm according to the Bragg’s Law, $\Lambda = \textrm{\; }{\lambda _w}/(2\sin ({{\theta_w}/2} ))$. A 633 nm LP beam emitted from the He-Ne laser (Melles Griot 05-LHP-991) was used as the probe beam. A quarter-wave plate (QWP) was placed before the sample to generate LCP light in the default situation and could be replaced by a half-wave plate (HWP) to alter the LP angles. Then, the probe beam irradiated on the LC cell by a slightly oblique angle to detect the variation of diffraction intensity during the photopolymerization process. When the photopolymerization was accomplished, the position of LC cells was rotated to be perpendicular to the probe beam. Two silicon photodetectors (Thorlabs PDA36A) connected to the oscilloscope (TDS 1001B, Tektronix) were used to monitor the intensity of different diffraction orders. The measurements were conducted by normal incidence if not stated specially. The first-order DE was calculated from the intensity of first-order diffraction divided by the sum total intensities of transmission and diffraction. For in-situ monitoring, the relative DE was defined since the calculated results were not accurate by oblique incidence.

 

Fig. 1. (a) The optical setup for holographic photopolymerization and DE measurement. (b) Schematic illustration of the fabrication process of a reconfigurable holographic grating. HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarizing beam splitter.

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Figure 1(b) schemes the fabrication process of the polymer-templated LC holographic gratings. The glass cells filled with samples were exposed under the illumination of interference patterns. A periodically alternated 3D polymer network with nanopores containing nonreactive E7 and unpolymerized monomers was fabricated under the visible-light induced photopolymerization of RMs and NVP. The RI contrasts induced between bright and dark regions were principle to compose the holographic gratings. Then, the samples were immersed in organic solvent, e.g. ethanol etc., for five days to wash out the unpolymerized components completely and dried to obtain the residual LC polymer template. Different organic solvents or LCs could be refilled into the template by capillary forces. The optical particulars of the samples were observed by a crossed polarized optical microscope (POM, PM6000, Nanjing Jiangnan Novel Optics, China).

3. Results and discussion

In order to reveal the relationship between the material composition and the diffractive behaviors of polymer-templated LC gratings, the samples with different concentration of RMs ranged from 15 to 65 wt.% (A1∼A6) were prepared and cured under the same condition. As shown in Fig. 2(a), the variation of first-order DE was measured for different samples after three stages of (i) being polymerized (denoted by “▪”), (ii) washed-out/dried (●) and (iii) refilled with E7 (▴). The highest DE was achieved in the sample A2 with 25 wt.% RMs, while gradually decreased as the concentration of RMs was further increased in varied processes. The variation of DE in different samples can be explained by the RI variation in either bright regions or dark regions. In the sample A1 with a lower RM concentration, the RI in bright regions will be relatively closer to that of E7, resulting in the decrease of RI contrast and DE consequently. On the contrary, the higher RM concentrations (A3∼A6) will enlarge the volume ratio of network in dark regions and reduce the RI in dark regions, which finally decrease the RI contrast between two regions. The increase of DE after being washed-out is mainly caused by the increscent differences of RI when the nanopores in the LC polymer network are full of air after the drying process. Similarly, the average RI of pure E7 is larger than that of the impure E7 mixed with unreactive monomers, etc., thus contributing to a higher RI contrast between bright and dark regions. As a result, the DE of sample after being refilled with E7 (stage iii) was higher compared with that of which were as-polymerized (stage i). However, we failed to obtain the first-order DE of samples with 55 wt.% and 65 wt.% RMs after being refilled with E7 (stage iii). Generally, the average pore size in the template are usually less than 100 nanometers [10,13,18,19]. More RMs available in photopolymerization process would form a more sturdily polymerized template and lead to the decrease of the size and density of nanopores [13]. The nonreactive LC and unpolymerized components inside the nanopores are more difficult to be washed-out and refilled with higher RM concentrations, which possibly explains the lack of the first-order DE of samples with 55 wt.% and 65 wt.% RMs in stage iii. Specially, the measured DE of samples after being washed-out/dried (stage ii) might be higher than the maximum first-order DE for a sinusoidal modulation. We suspected that the higher DE might be related to the relatively low RI of air and the collapse of “sponge-like” polymer networks, resulting in broader peaks and steeper slopes than sine wave and would be more similar to square wave.

 

Fig. 2. (a) The first-order DE of holographic gratings versus RM concentration after being polymerized (stage i), washed-out/dried (stage ii) and refilled with E7 (stage iii). POM images of (b) samples A1∼A6 containing variable RM concentration after being polymerized (stage i) and (c) sample A2 containing 25 wt.% RMs in three different stages. The two red arrows denote the polarizer and analyzer. The yellow arrow denotes the alignment direction. The black dots in images are spacers. The scale bar is 20 μm.

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The overall transmittance of LC cell, which is calculated as a ratio of the sum total intensities of transmission and diffraction to the incident one, is about 75%∼80%, and the scattering losses and absorption are about 25%∼20% with slight differences between different samples. Normally, the light scattering in HPDLC could be reduced by minimizing the droplet size into the subwavelength range. However, the smaller pore size will make it more difficult to refill organic solvents and LCs into the polymer scaffold. In fact, large scattering losses in holographic gratings might also be caused by other reasons such as the mismatch of RI at the interfaces and imperfection of polymer network, which are large fabrication challenges that exist for high quality holographic gratings [37,38]. It is worth noting that the scattering losses in stage ii would increase dramatically and the overall transmittance would decrease because of the collapse of polymer network in the absence of fluid inside the polymer network, further changing the overall morphologies of the polymer network [21]. The phenomenon might be disadvantage for some practical applications, but it can still be used to verify the characteristic properties of the polymer network. Particularly, in the presence of fluid, the network could be swollen to reduce the scattering losses and increase the overall transmittance.

POM images of sample A1∼A6 in stage i with rubbing direction oriented at 45° with respect to the analyzer are shown in Fig. 2(b). After the green-light interference, the photopolymerization preferentially occurred in bright regions to form a dense polymer network, while a sparse polymer network was generated in dark regions. As a consequence, the RIs in bright and dark regions will be different and the periodic grating patterns with alternating green and yellow stripes can be observed. Since the increase of RM concentration would enlarge the volume ratio of dense network and reduce the density fluctuation between bright and dark regions, which can be verified by scanning electron microscopy (SEM) in Fig. 3, the polymer-rich (bright) region will be expanded and the LC-rich (dark) region will be reduced. Accordingly, the green and yellow stripes in POM images in Fig. 2(b) refer to the bright and dark regions, respectively. POM images of sample A2 in stage i, stage ii and stage iii are compared in Fig. 2(c). As shown in the middle of Fig. 2(c), the dark regions were replaced with black stripes in POM images, while the bright regions were almost consistent with that in stage i. The differences might relate to the density contrast of the polymer network. Because of the dense network formed in bright regions, the anisotropic properties of LCs were maintained in bright regions, although most of the unpolymerized components were washed out before the drying process. The degeneration of anisotropy in dark regions caused by replacing the dominant unpolymerized components with air in the sparser networks results in the color vanishment. After being refilled with E7 (stage iii), brighter yellow stripes appeared again in the bright regions, possibly caused by the higher RI of E7 compared with the initial impure E7 mixed with unreactive monomers, etc. If not stated specifically, the samples containing a fixed RM concentration of 25 wt.% in accordance with sample A2 with a higher DE will be discussed below to reveal the flexibility of wash-out/refill process in the implementation of reconfigurable polymer-templated LC holographic gratings.

 

Fig. 3. SEM images of samples A1∼A6 after being washed-out/dried (stage ii).

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To understand the effect of RB as photoinitiator, samples B1∼B6 (summarized in Table 1) were investigated by varying the concentration of RB from 0.1 wt.% to 0.9 wt.% with a fixed amount of other components. As shown in Fig. 4(a), the relationship between relative DE and irradiation time for the samples containing different weight concentration of RB was measured by oblique incidence of probe beam. The relative DE was enhanced dramatically by increasing the weight concentration of RB up to 0.7 wt.%. However, as shown in Fig. 4(b), the relative DE declined slightly after reaching a steady plateau by loading more RB molecules (>0.7 wt.%). Furthermore, we studied the influence of NPG as co-initiator on the performance of gratings after photopolymerization. Samples C1∼C5 with varied concentration of NPG from 0.2 wt.% to 1.4 wt.% are also cataloged in Table 1. The relative DE and DE of samples C1∼C5 were also measured by using an oblique-incident and a normal-incident probe beam, respectively [Figs. 4(c)–4(d)]. To note, the concentration of RB in current experiments was fixed to be 0.7 wt.% to acquire the highest DE as discussed above. As shown in Fig. 4(c), the dependence of DE on the concentration of NPG was similar to that on the weight of RB. It is evident that 10 mins is adequate in the holographic fabrication under a total light density of 40 mW/cm², although further extending the exposure time is favorable of maintaining the DE with a maximum value. By increasing the weight of NPG above 0.6 wt.%, the relative DE was stable and declined gradually [Fig. 4(d)]. We suspect that the decrease of DE in the samples with higher concentration of RB and NPG might be possibly related to the weak solubility of molecules. Considering the findings in Fig. 4(b) and Fig. 4(d), an appropriate condition of 0.7 wt.% RB and 1.0 wt.% NPG is suggested to achieve a better diffractive performance in sample C4.

 

Fig. 4. The relative DE of polymer-templated LC gratings versus exposure time of (a) samples B1∼B6 containing variable RB concentration and (c) samples C1∼C5 containing variable NPG concentration. The DE versus (b) RB concentration and (d) NPG concentration.

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To investigate the polarization dependence of polymer-templated LC gratings, an HWP was inserted into the probe path to alter the LP state of incident He-Ne laser beam [Fig. 1(a)]. To clarify the polarization-dependence of diffraction properties clearly while neglecting the small differences between different samples, the normalized DE was calculated by the formula, $\eta ^{\prime} = ({\eta - {\eta_{min}}} )/({{\eta_{max}} - {\eta_{min}}} )$, where the η_max and η_min are the maximum and minimum diffraction efficiencies among discrete LP angles, respectively. It’s worth noting that the minimum DE would not be zero although the normalized DE is zero. The influence of LP state on the normalized DE after being washed-out/dried (stage ii) is plotted in Fig. 5(a), where x-axis refers to the intersection angle (θ) between the PL direction of probe beam and grating vector. Here, the direction of LC alignment was vertical to the grating vector. The normalized DE changed continuously and reached a maximum value with an intersection angle θ = 90˚ (or -90˚), validating the polarization-dependent properties of polymer-templated LCs. The maximum contrast before being normalized between two orthogonal LP directions of the sample is about 9.8%. As shown in Fig. 5(b), similar variations of normalized DE could be also observed after being polymerized (stage i) in the samples denoted by “Vertical” and “Parallel”, which corresponds to different alignment directions with respect to the grating vector, respectively. Although the minimum DE of both “Vertical” and “Parallel” samples are very low, the minimum DE before being normalized can still reach about 1.5% and 2.0%, respectively. It might be related to the small RI contrast between the bright and dark regions in the corresponding LP angles. The contrast between two orthogonal LP directions are about 9.3% and 10.0%, respectively. It is identified that the best diffractive performance can be achieved when the LP angle is parallel to the alignment direction of LCs (the optical axis of LCs), while almost independent of the directions of grating vector. Therefore, it indicates that the DE of gratings can be dynamically regulated by changing θ.

 

Fig. 5. The influence of discrete LP angles on the normalized DE after being (a) washed-out/dried and (b) polymerized for two samples with different alignment directions.

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The RI contrast that occurs between bright regions and dark regions is crucial to the reconfiguration of polymer-templated LC holographic gratings with tunable diffractive behaviors. The relevance between the first-order DE and the operating temperature of samples refilled with different components is illustrated in Fig. 6(a). Obviously, the changes of RI result in the variations of DE overall. For the polymer-templated LC gratings after being washed-out/dried (●), the slight fluctuations of the DE imply that the average RI is almost temperature independent. A similar trend was identified in the sample after being refilled with ethanol solvent (▴). Since ethanol molecules occupies the nanopores in the LC polymer network, the DE decreases due to reduced RI contrast when compared with the sample with air pores (●). However, for the samples after being polymerized (▪) and refilled with E7 (▾), the distinct variation of DE value and an exceptional inflection point were both observed in the curves, respectively. The occurrence of intriguing phenomena could be ascribed to the intrinsic physical properties of LCs. It is known that the RI of E7 would decrease linearly with the increase of temperature, even in the isotropic phase [39]. Therefore, from room temperature to a certain temperature at the inflection point, the RI of dark regions is higher than that of bright regions, resulting in a positive RI contrast. At the interval between the two individual processes separated by the inflection point which might be related to the cp temperature of E7, the RI contrast goes to zero, diminishing the diffractive behaviors. Then, the further decrease of RI causes a negative RI contrast and the consequent recovery of diffraction by increasing the temperature. Additionally, the critical temperatures, defined as the minimum value at descent-ascent DE curves, differed from the samples after being polymerized (▪) and refilled with E7 (▾). While the RI of E7 is higher than that of the initial unreactive mixture which has been washed out from the as-polymerized sample, an anticipated increase of the critical temperature was confirmed in this work when refilled with E7 (▾). Besides, not any observable deviation of DEs was recognized during the heating and cooling processes, which means the LC polymer template is stable upon the thermal variation and is believed to operate at an elevated temperature higher than 80 ˚C, even than 100 ˚C [16,21]. Expect that, the UV might be another influence factor. The DE of sample after being polymerized (stage i) might slightly increase because of the reaction of the residual monomers [40]. We believe that the template might be insensitive to UV irradiation and stable since the unpolymerized reactive components, such as photoinitiator and residual monomers, are washed out almost completely in stage ii and stage iii.

 

Fig. 6. The relevance between the first-order DE and (a) the temperature of samples with variable components and (d) different organic solvent refilled. Inset: the DE versus RI of refilled components.

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Besides, it is highly flexible to regulate the diffractive behavior dynamically by refilling the polymer-template LC gratings with different organic solvents, eg. methanol (n = 1.3284), ethanol (n = 1.3611), toluene (n = 1.4967) and benzyl acrylate (n = 1.5108), with RI lower than that of the residual polymer network. As illustrated in Fig. 6(b), the first-order DE decreased as the RI of organic solvents increased. The relationship between the DE and the RI of refilling component are plotted in the inset of Fig. 6(b), which can be described by a simple theoretical model. Here, the types of optical diffraction can be distinguished by calculating the dimensionless Cook-Klein parameter Q [37]

According to Eq. (4), the first-order DE decreases nonlinearly as n_r increases, matching well with the experimental observations. Along with the modulation of RI, the volume fractions of polymer network in both bright and dark regions might also be varied as well because of different swelling abilities of organic solvents. All these factors could be the reasons behind the nonlinear variation of DE. Since it is highly flexible to refill fluidic components to achieve a broader RI contrast, the tuning range of DE value would be increased as well.

Furthermore, the distinguishing characteristics of recording holograms was implemented in polymer-templated LCs with mixture same as sample C4 (plotted in Table 1). The optical setup used in the holographic recording is depicted in Fig. 7(a). The reflection light with a diameter of ∼ 1 cm from the Chinese character [marked by a red circle in Fig. 7(b)] on a coin, placed close to the LC cell (∼1.5 cm), interfered with the reference light emitting directly from the SLM laser. As exhibited in Figs. 7(c)–7(d) and Visualization 1, the flash lamp in cellphone was turned on as the white light source to reconstruct the hologram recorded in sample after being polymerized (stage i). A digital camera was placed on the same side of white light source to capture the photographs, presenting obvious offsets of the Chinese character by varying angles of view. Limited by the diameter of the reference and reflection light (∼1 cm), which is smaller than the sizes of the cell, the recording area is relatively transparent as shown in Fig. 7(c) and Fig. 7(d). However, the unexpected photopolymerization in the non-recording area would also generate irregular polymer network with apparent scattering effects. Based on the above discussion, it is understood that the diffractive behaviors of the polymer-templated LCs can be facilely reconfigured by being refilled organic solvents dynamically. However, as shown in Fig. 7(e), the hologram could not be reconstructed using the flash lamp owing to the intrinsic scattering behaviors mainly caused by the collapse of polymer network and increase the mismatch of RI at the interfaces after being washed-out/dried (stage ii). Undoubtedly, if the LC polymer templates are refilled and swollen by organic solvents with a relatively low RI, the recorded holograms can be reconstructed successfully (stage iii). The intriguing properties provide a great potential for advanced applications for anticounterfeiting.

 

Fig. 7. (a) The optical setup for recording hologram. (b) The photograph of the coin used for holographic recording. The recording object was marked by red circle. The photographs reconstructed under white light for samples after being (c-d) polymerized (see Visualization 1) and (e) washed-out/dried. The bright spots in the photos are the reflection of light from flash lamp in cellphone.

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4. Conclusions

In conclusion, we fabricated polymer-templated LC holographic gratings via green-light recording in the presence of RB/NPG photoinitiation system. Either LCs or organic solvents could be refilled into the polymer-templated LC gratings by optimizing the concentration of RMs. And the relevance between the first-order DE and the concentration of RB and NPG molecules was discussed in detail. The polarization-dependency of diffraction properties with discrete LP angles were also investigated. By varying temperature or refilling organic solutions with different RI, the diffraction properties of polymer-templated LC gratings could be dynamically reconfigured. Furthermore, the potential of recording holograms using green light was explored in the polymer-templated LCs. We expect that the reconfigurable polymer-templated LC gratings fabricated by visible-light interference would provide a facile approach to regulate the diffraction properties of holographic gratings apart from electric field, thus paving a way towards a class of novel anti-counterfeiting devices.