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The present study reports that isothermal phase transition induced by photoisomerization of azobenzene liquid crystals (azo-LCs) from trans- to cis-isomers results in the dissolution of poly(N-vinylcarbazole) (PVK) into azo-LCs. Transparent (scattering) states can be demonstrated using uniform (rough) morphologies of PVK generated by slow (rapid) phase separation of PVK and azo-LCs from cis- to trans-isomers. The PVK films were examined in detail using scanning electron microscopy. Scattering performance resulting from the rough PVK surface induced micron-sized LC domains, and transparent performance resulting from the reformed uniform PVK surface can be optically and reversibly switched. Finally, all-optically controllable and highly efficient (contrast ratio of 370:1) scattering mode light modulators based on azo-LCs and PVK films were demonstrated.
©2013 Optical Society of America
1. Introduction
Liquid crystal (LC) scattering devices, such as light shutters [1, 2], light switches [3], wave plates [4], and lenses [5], among others, have been extensively developed. Polymer-dispersed LCs [6–8] and polymer-stabilized cholesteric textures [1–3] have been paid particular attention. Several disadvantages of such devices, such as their high operating voltage [6, 7], low contrast ratio [8], and others, must be improved. Based on previous studies, Chen et al. introduced PVK into scattering mode light shutters and proposed the particular thermally induced phase separation (TIPS) of LCs and PVK films [9, 10]. Particular TIPS can be achieved by increasing the temperature of PVK film-coated LC cells by direct heating [9] and/or by light-induced thermal effect [10]. The former can be used to fabricate scattering mode light shutters with advantages such as low driving voltage, thin cell gap, polarization-independent scattering, fast response, and being polarizer free. However, the latter, based on dye-doped LC in PVK films [10], can be used to demonstrate an optically and thermally control device, but cannot achieve high contrast and transmission, all-optical control and requires high-power lasers.
In the current study, we combine the mechanisms of scattering mode light shutters using PVK morphologies, isothermal phase transition caused by photo-isomerization from trans- to cis-isomers, slow dark-relaxation and rapid photo-isomerization from cis- to trans-isomers to design all-optically controllable, and multi-stable devices. No thermal and/or electric fields are necessary in our work. PVK and azobenzene LCs (azo-LCs) are the key materials used to achieve our demonstration. The properties of PVK are reported in [9, 10]. Azo-LCs, which feature the combined properties of azobenzene dyes [11, 12] and nematic LCs, have two isomers: rod-like trans- and bent cis-isomers. Trans-azo-LCs are similar to common nematic LCs according to elastic continuum theory [13] and have several electro-optical properties, such as dielectric anisotropy and birefringence. Moreover, no anisotropic reorientation or alignment anchoring occurs when trans-azo-LCs are optically transformed into cis-azo-LCs due to the extremely high concentration of photo-responsive units in azo-LCs [11, 12]. In other words, cis-azo-LCs can disturb the LC alignment in situ and induce an isothermal phase transition from nematic to isotropic states via photoisomerization of trans- to cis-azo-LCs by UV illumination. Regarding to the population of trans- and cis-azo-LCs, which determines the order parameter of the azo-LCs and their absorption spectra, the ratio of the trans- to cis-azo-LCs can be controlled by light illumination [14].
Importantly, according to the mechanism of particular TIPS [9, 10], which involves a combination of dissolution process and TIPS, the coated PVK is dissolved into the LCs after heating to temperatures higher than the switching temperature, close to the clearing temperature of the LCs. Experimental examinations indicate that isotropic cis-azo-LCs transformed by UV illumination (photoisomerization) cause the dissolution of PVK and azo-LCs. The larger the amount of cis-azo-LCs produced, the lower the azo-LCs clearing temperature (order parameter), and the higher the solubility of the PVK and azo-LCs solution. After that, the LC cells with isotropic azo-LCs resulting from isothermal phase transition show transparent (bright) states. When the UV light source is turned off, extremely slow dark-relaxation from cis- to trans-azo-LCs and phase separation of PVK and trans-azo-LCs could be observed simultaneously. Phase separation is attributed to the insolubility of PVK and trans-azo-LCs, and occurs very slowly because of the long lifetime of cis-azo-LCs, which is in the order of 10 h [15–17]. The slow rate of phase separation results in a uniformly flat PVK surface, which is very similar to the initial coated PVK surface. On the other hands, rapid phase separation by photoisomerization from cis- to trans-isomers can be employed to generate rough PVK surfaces, which display a scattering (dark) state due to the small size LC domains. Isothermal phase transition (from trans- to cis-isomers), dark relaxation (from cis- to trans-isomers), and photoisomerization (from cis- to trans-isomers) are three key factors considered to all-optically controllable transitions between the transparent state, scattering state, and gray-scales. Therefore, the different roughness of reformed PVK layers determines the final scattering performances.
In this study, all-optically controllable and highly efficient scattering mode light modulators based on azo-LCs and PVK films are reported. The main mechanism involved includes the dissolution of PVK and azo-LCs resulting from isothermal phase transition, reformation of PVK films via slow/rapid phase separation by slow/rapid isomerization of azo-LCs. In addition to the described properties, such optical azo-LCs devices present high contrast ratio, polarization-independent scattering, multi-stability (or so-called gray-scales), and being polarizer free. The transmittance of the scattering mode light modulator can be optically controlled between transparency and opacity. Total gray-scale control is a remarkable improvement compared with a previous work [10].
2. Experiments
The materials used in this study included azo-LCs (nematic phase from 8 °C to 59 °C, BEAM Co.) and PVK (polymer, Sigma-Aldrich). PVK was spin-coated onto indium-tin-oxide-coated glass substrates. The detailed processes involved can be found in [9, 10]. Two non-rubbed PVK-coated glass slides were combined to fabricate an empty cell with a cell gap of 6 µm. Finally, azo-LCs were filled into the empty cell and the edges of the LC cell were sealed with epoxy. The fresh filled cell presented clear transparency because of the uniformity of the PVK films. To transform the transparent LC cell into a scattering one as the initial dark state, treatment by particular TIPS [9] may be performed. However, in this study, an optical method was used instead of particular TIPS to achieve an all-optical control light modulator. According to [18], trans-azo-LCs have maximum absorption near 350 nm and can be transformed into cis-azo-LCs with an absorption spectrum red-shifted toward the blue-green region. However, the spectrum obtained revealed that the absorbance of trans-azo-LCs at 532 nm is low; such low absorbance could lead to isomerization of the LCs into cis-azo-LCs as well. Simultaneously, the cis-azo-LCs then absorb green light to isomerize into trans-azo-LCs again.
Consequently, excitation by green light will induce rapid phase separation of PVK and azo-LCs, resulting in rough PVK surfaces and small-sized LC domains that highly scatter incident (visible) light. Here, high scattering can provide an efficient initial dark state. The fresh LC cell (trans-azo-LCs dominant) was illuminated with green light (λ = 532 nm) at an intensity of 252 mW/cm2 for 120 s to obtain the initial scattering state [Figs. 1(a) and 1(b)]. The all-optically controllable light modulator between scattering, transparent, and gray-scale modes will be shown. The UV and visible light sources used to photoisomerize the azo-LCs included an Ar-ion laser (λ = 365 nm) and a diode-pumped solid-state (DPSS) laser (λ = 532 nm), respectively. However, for practical application, light sources from lamps or bulbs can be used to optically control the transmission of LC devices.
Fig. 1 Images of the LC cell observed under a crossed-polarized optical microscope and photographed by a camera; (a)-(b) Scattering state obtained by green light illumination (252 mW/cm2 for 120 s). (c)-(d) Transparent isotropic state achieved by UV light illumination (7.4 mW/cm2 for 120 s; photo-induced isothermal phase transition). (e)-(f) The isotropic phase LC cell after dark relaxation for 24 h, transferred to nematic phase (transparent state). The distance between the LC cell and background paper in (b), (d), (f) is about 8 mm.
3. Results and discussion
Figures 1(a) and 1(b) respectively show a photograph of the LC cell as well as its image obtained by crossed-polarized optical microscopy (POM) at the initial scattering state at room temperature (~25 °C). The optical treatment is described in the Experiment section. The initial high-scattering state was achieved by green light illumination. Compared with other kinds of LCs scattering devices with the same cell gap, the scattering performance of the LC cell in this study is extremely high (the distance between the LC cell and the background paper is about 8 mm). The LC cell was then illuminated with UV light derived from an Ar + laser to transform trans-azo-LCs into isotropic cis-azo-LCs (isothermal phase transition). The temperature of the LC cell did not exceed the clearing temperature of the LCs in the trans-state (data not shown); thus, isothermal phase transition may be inferred to occur after UV illumination (photoisomerization). The coated PVK partly dissolved into isotropic cis-azo-LCs. Figures 1(c) and 1(d) respectively show a photograph of the LC cell as well as its image obtained by crossed-POM after treated of the cell with UV illumination (intensity: 7.4 mW/cm2; illumination duration: 120 s). Importantly, no significant thermal effect was observed during UV light illumination. The mixture of PVK and cis-azo-LCs presented an isotropic state, as evidenced by the dark image under the crossed-polarizers shown in Fig. 1(c). The isotropic LCs with an uniform refraction index show clear transparency [Fig. 1(d)], and no light scattering resulting from the morphology of PVK surface can be observed. After the UV light was turned off, cis-azo-LCs began to slowly isomerize into trans-azo-LCs via dark relaxation [16], resulting in phase transition from isotropic to LC states as well as the slow phase separation of the PVK and azo-LCs because of the long lifetime of the cis-azo-LCs [15–17]. The details of the phase separation, named isomerization-induced phase separation (IIPS), can be read in [19]. Based on the mechanism of phase separation and [9], the dimensions and roughness of the branch-like structures of reformed PVK increase with the increase of the cooling rate. Here the “cooling rates” can be considered the isomerization rates from cis- to trans-azo-LCs. Hence, long dark relaxation time caused the extremely low phase separation rates, so that the surface morphology of PVK was uniform and regular, associated with high transparency. Figures 1(e) and 1(f) respectively show a photograph of the LC cell as well as its image obtained by crossed-POM after UV-illumination of the cell (7.4 mW/cm2 for 120 s) after dark relaxation for 24 h. The LC phase appeared and generated large multi-domains on the non-alignment treated PVK surfaces, as shown in Fig. 1(e). The transparent state (no scattering of visible light) was permanently stable. Also, a color shift from orange [Fig. 1(d)] to yellow [Fig. 1(f)], which indicates dark relaxation (spontaneously reverse isomerization) from cis- to trans-azo-LCs because of the difference in absorption spectra [18, 19], can be observed. Clearly, Fig. 1 shows all-optical switching between bright (transparent) and dark (scattering) states. The stabilities of optically switchable gray-scales and morphologies of each reformed PVK are shown below.
Figure 2 presents the configurations of the all-optically controllable and highly efficient scattering mode light modulator based on azo-LCs and PVK films. Figure 2(a) shows the fresh filled LC cell, revealing a transparent state. The transparently isotropic state, shown in Fig. 2(b), is obtained by shinning with UV light via isothermal phase transition. The solubility of azo-LCs and PVK increases at that time. The next step can be divided into two parts, one is rapid phase separation by illuminating with green light [Fig. 2(c)]; the other one the slow phase separation via dark relaxation [Fig. 2(d)]. The former gets a stable scattering state, and the latter stable transparent state [19]. The details can be read in the previous paragraph.
Fig. 2 Configuration of the all-optically controllable scattering mode light modulator of (a) fresh transparent LC cell; (b) transparently isotropic state obtained by shinning with UV light; (c) stable scattering state obtained by rapid phase separation by green light illumination; (d) stably transparent state achieved by slow phase separation via dark relaxation.
Figures 3 shows the dynamic variations in the transmittance of a fabricated LC cell in transparent state illuminated with various green laser intensities. A DPSS (pumping) laser illuminated the transparent LC cell, and red light derived from a He–Ne laser (λ = 632.8 nm, which is out of the range of the absorption band of the azo-LCs) was adopted as a probe beam. The transmitted probe beam was received by a photo-detector placed behind the LC cell. The distance between the photo-detector and the LC cell was about 25 cm. The initially transparent state of the LC cell (cis-azo-LCs dominant), which indicates that most of the azo-LCs are cis-isomers, was achieved by illumination with UV light (~7.4 mW/cm2, λ = 365 nm for 120 s). A transmission of 100% was defined as the transmission of the LC cell when the filled LCs were at isotropic state and clearly transparent. The various transmittances of the LC cell (cis-azo-LCs dominant) illuminated by the DPSS laser show that the transmittance decreases with increasing illumination duration and DPSS intensity. Moreover, the switching times from transparent to scattering states decreased with increasing DPSS intensity, which indicates that the transformation rate of isomers from cis- to trans-state increased with the DPSS intensity. Hence, the transparent LC cell can be optically switched into a scattering one. As well, trans-azo-LCs are stable so that the scattering LC cell is permanently stable.
Fig. 3 Dynamic transmittances of a transparent LC cell (cis-azo-LCs dominant, prepared by illumination with UV light at an intensity of 7.4 mW/cm2 for 120 s) with the illumination duration and various green laser intensities of (I) 320, (II) 252, (III) 184, (IV) 130, and (V) 98 mW/cm2.
The scattering LC cell can be also switched back into a transparent one via UV illumination. The experimental setup applied in this experiment is similar to that for getting Fig. 3, except that a pumping laser was used (Ar+ laser, λ = 365 nm). Figure 4 shows the dynamic transmittances of the LC cell at the initial scattering state illuminated with various UV intensities. Here, the extremely low transmittances were prepared by illumination of fresh filled LC cell (trans-azo-LCs dominant) with DPSS laser (~252 mW/cm2, λ = 532 nm) for 120 s. Notably, such low transmittances can also be prepared by illumination of UV-illuminated LC cell (cis-azo-LCs) with DPSS laser (~98 mW/cm2, λ = 532 nm) for 60 s. Experimentally, the longer the UV irradiation duration, the higher the transmittance obtained. High UV intensity can also reduce the switching time from scattering to transparency. The mechanism of optical switching from scattering to transparency is described as follows. UV illumination results in photoisomerization of trans- to cis-azo-LCs, where the order parameter of LCs is reduced. Optically controlled reduction of order parameter indicates the appearance of isothermal phase transition. Eventually, the scattering LC cell with trans-azo-LCs is optically switched into a highly transparent one with isotropic cis-azo-LCs. The isotropic phase can also induce the dissolution of PVK and cis-azo-LCs, as described in Introduction section. The dissolution of PVK and cis-azo-LCs, slow dark-relaxation from cis- to trans-azo-LCs, and slow phase separation are the key factors considered to obtain a stable transparent state [19].
Fig. 4 Dynamic transmittances of the scattering LC cell (prepared by illumination with a DPSS laser at an intensity of 98 mW/cm2 for 60 s) with the illumination duration and various UV intensities of (I) 37, (II) 32, (III) 19, (IV) 15, (V) 11 mW/cm2.
Experimentally, in this study, LC cells (PVK) filled with azo-LCs presented high scattering and transparent states by illumination with green and UV light, respectively. This result shows that the transmittances of the states remain stable transparency, scattering or gray-scales at temperatures lower than the clearing temperature of the azo-LCs. Concerning the stably transparent state, as shown in Figs. 1(d) and 1(f), the transmittance versus time (dark relaxation) was obtained, as depicted in Fig. 5. Experimentally, the transmittance was almost invariable after UV irradiation. During dark relaxation, the isotropic cis-azo-LCs, resulting in isothermal phase transition from LC phase to isotropic phase, and the dissolution of PVK into cis-azo-LCs, gradually “self-isomerized” into trans-azo-LCs. Due to the long lifetime of cis-azo-LCs, slow phase separation of the PVK and trans-azo-LCs occurred, followed by generation of large-sized LC domains (uniform surfaces) that cannot scatter visible light. Hence, the transparent states, which are controlled by UV illumination, are permanently stable.
Fig. 5 Transmission variations of the transparent LC cell (prepared by illumination with UV light at an intensity of 7.4 mW/cm2 for 120 s) kept indoors at room temperature after the UV light is switched off (dark relaxation).
Notably, for the case of the stably transparent LC cell after the processes of cis to trans isomerization are completed (dark relaxation), the restoration behavior from the transparent state (trans-azo-LCs dominant) back to the scattering one, by illuminating with green light (low absorption) can also be achieved, and is explained as follows. The required optical energy (intensity and illumination duration) provided by DPSS laser for switching the LC cell (trans-azo-LCs dominant) is much higher than that for switching the LC cell (cis-azo-LCs dominant) from transparent to scattering state. Because trans-azo-LCs have relative low absorption near 532 nm (comparing with cis-azo-LCs), the green light absorbed by azo-LCs results in rapid and simultaneous photoisomerization of trans- to cis- as well as cis- to trans-azo-LCs in this case. However, eventually, the population of trans-azo-LCs is higher than that of cis-azo-LCs because of the different absorbance at 532 nm. The time required to optically (DPSS laser with intensity of 252 mW/cm2) switch the LC device from the transparent state (trans-azo-LCs dominant) to a scattering one was 120 s. As the result, the required illumination duration (120 s), for switching the LC cell from transparent to scattering states, in this case is much longer than that (25 s) shown in curve (II) of Fig. 3. In other word, importantly, the transparent LC cells can be switched to scattering states no matter which kinds of isomers (trans- or cis-azo-LCs) are at their initial states. Additionally, rapid dissolution of PVK into azo-LCs (photoisomerization from trans- to cis-azo-LCs) and rapid phase separation (photo-isomerization from cis- to trans-azo-LCs), which result in rough PVK surfaces, produce scattering states during illumination with green light.
Optical control of gray-scales can be achieved by UV illumination with different intensities and/or durations. Figures 6(a)–6(d) show the LC cells with four different gray-scales produced by illumination with a UV lamp through a photo-mask at an intensity of 7.4 mW/cm2 for 15, 30, 60, and 90 s, respectively. The corresponding stable transmittances obtained were 4%, 38%, 68%, and 85%, respectively. Stable transmittance increased with increasing UV illumination duration and can reach 100% (isotropic state) by further increasing the UV illumination or intensity. The contrast ratio of our LC device was measured and calculated to be about 370, which is higher than those demonstrated in previous studies [9, 10]. Thus, the morphologies of the optically reformed PVK layers for achieving scattering modes are much denser, rougher, and more disordered than those of thermally reformed PVK layers [9].
Fig. 6 Gray-scale images of the LC cell obtained using a digital camera after UV irradiation (7.4 mW/cm2) of the scattering mode LC cell (prepared by illumination with a DPSS laser at an intensity of 98 mW/cm2 for 60 s) for (a) 15, (b) 30, (c) 60, and (d) 90 s.
Figure 7(a) shows a top-view Scanning electron microscope (SEM) image of the initially coated PVK layer without any treatment. The morphology of the PVK surface is uniform. Figure 7(b) shows a top-view SEM image of the reformed PVK surface fabricated by thermal treatment, or the so-called particular TIPS. The thermal treatment is described in [9]. However, Fig. 7(c) shows an SEM image of the LC cell after being treatment with green light illumination (98 mW/cm2 for 60 s, from isotropic cis-azo-LCs to nematic trans-azo-LCs). Comparison of Figs. 7(b) and 7(c) reveals that the dimensions and morphologies of the PVK surface produced optically [Fig. 7(c)] are much denser, larger, and rougher than that made thermally. It can be understood since the rate of phase separation directly influences the PVK surface morphology, and the rate of photo-IIPS is higher than that of particular TIPS [9, 19]. Rapid phase separation results in roughly reformed PVK layers onto the substrate. The rough PVK surfaces disturb the LC alignment, and generate multi-domains of LCs with differently effective index of refraction to scatter incident (visible) light. Regarding the quantitative measurement, the average width of the reformed PVK branches in Fig. 7(b) and Fig. 7(c) are about 1 and 2.5 μm, respectively. It is also clearly to see that the number of PVK branch-like layers in Fig. 7(c) is larger than that in Fig. 7(b). Furthermore, the scattering performance (contrast ratio) of the optically treated LC cell is higher than that of the thermally treated one. As a result, the phase separation rate obtained by optical treatment is higher than that produced by natural cooling, and thus provides an optical field with higher uniformity to produce reformed PVK layers. Figure 7(d) depicts the top-view SEM image of the reformed PVK surface of LC cell treated with green light illumination (98 mW/cm2) for 30 s. Such PVK morphology with a few branch-like structure provides weak scattering, about 73% transmittance. About the stably bright state (transparent state), Fig. 7(e) displays the SEM image of the LC cell after completion of slow phase separation. The LC cell was treated with UV illumination (7.4 mW/cm2 for 120s, and dissolution of PVK and azo-LCs) and dark relaxation (slow phase separation, 24 h). The SEM image reveals an extremely flat and uniform surface on the substrate and no branch-like structures. Comparison of Figs. 7(e) and 7(a) (the initially coated PVK layer without any treatment) reveals structures similar to each other, which indicates that the PVK structure can be reformed into a flat one very similar to that of the initial coated PVK surface. Hence, the transmittance of the UV-illuminated LC cell is stable and high. Regarding the PVK morphologies of the LC cell with different transmittances (gray-scales), the initial high-scattering LC cells [Fig. 1(a)] were illuminated with green light, and the reformed PVK surfaces were observed by SEM. SEM images of the reformed PVK surfaces illuminated by UV light (7.4 mW/cm2) of different durations (72, 60 and 54 s) are shown in Figs. 7(f)–7(h). The PVK morphologies clearly became increasingly uniform with increasing UV illumination duration. Thus, the quantity of dissolved PVK increases with increasing UV illumination duration.
Fig. 7 (a) Top-view SEM image of initially coated PVK surface. Top-view SEM images of the reformed PVK surfaces of LC cells treated with (b) natural cooling (particular TIPS); green light illumination 98 mW/cm2 for (c) 60 s and (d) 30 s; (e) UV illumination (7.4 mW/cm2 for 120 s) following by dark relaxation (24 h); (f)–(h) UV illumination (7.4 mW/cm2) of the scattering LC cells for 72, 60, and 54 s (gray-scales).
4. Conclusion
In conclusion, we demonstrated an all-optically controllable and highly efficient scattering mode LC light modulator based on azo-LCs and coated PVK films. The device is obtained by photoisomerization (UV illumination) induced isothermal phase transition of LCs from a scattering device to a transparently isotropic one, during which the dissolution of PVK and cis-azo-LCs occurs. The long lifetime of cis-azo-LCs and slow dark relaxation of azo-LCs from cis- to trans-isomers (slow phase separation of PVK and azo-LCs) generates large multi-domains of stable LCs, which cannot scatter visible light. Green light illumination can be used to switch the LC cell from a transparent state to a scattering one, as well as the gray-scales. Most importantly, the PVK structures with various roughnesses, ranging from completely flat surface to the small multi-domains scraggy one, can be generated repeatedly. The precise control of PVK structures has potential applications in other fields, such as surface alignment, pre-tilt angle control, and others.
Acknowledgment
The authors would like to thank the National Science Council (NSC) of Taiwan for financially supporting this research under Grant Nos. NSC 101-2112-M-006-011-MY3 and NSC 99-2112-M-008-021-MY3. This work is also partly supported by the Advanced Optoelectronic Technology Center. Correspondences about this paper can be addressed to Prof. Andy Ying-Guey Fuh at andyfuh@mail.ncku.edu.tw or Prof. Ko-Ting Cheng at chengkt@dop.ncu.edu.tw.
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