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This paper presents the optically controllable light scattering based on dye-doped liquid crystals (DDLCs) in a cell, whose substrates are coated with poly(N-vinylcarbazole) (PVK) films. The optical control mechanism is the light-induced dissolution of PVK in DDLCs, which reforms the disordered LC distribution into multiple and micron-sized LC domains. The induced thermal effect on the process is investigated in detail. Scanning electron microscopy images are obtained to show the surface structures of the produced PVK films. The generated scattering can be switched back to the original one by particular thermally induced phase separation. Results indicate that the light-induced thermal effect and photoisomerization lead to the dissolution of PVK in DDLCs. Finally, scattering mode light shutter with different transmission is successfully achieved by illuminating the cell under various light intensities.
©2012 Optical Society of America
1. Introduction
Optically addressed devices based on liquid crystals (LCs) such as photonic crystals [1], displays [2], gratings [3], and micro-lens arrays [4] have been extensively studied. Moreover, LC applications have already expanded to bistable, tristable, and multistable modes to achieve low-power consumption or other potential functions that promote green energy [3–9]. Several methods for stable modes have been developed in recent years, including polymer-stabilized cholesteric textures [5, 6], anti-ferroelectric LCs [7], micropatterned surface alignment [8], and gelator-doped LCs [9]. Based on one of our previous work [10], we demonstrated that particular thermally induced phase separation (TIPS) can be used to re-produce rough PVK layers that realigns LCs into multiple and micron-sized LC domains. Two scattering mechanisms were introduced, namely, surface scattering and volume scattering [11, 12]. The results indicate that the formed domain size determines the scattering performance [13, 14]. Thus, the scattering performances can be easily controlled by modulating the TIPS processes.
In this study, two key materials, were used to optically generate several domains and micron-sized LCs, are PVK and disperse red 1 (DR1). PVK is a widely used polymer for polymer light-emitting diodes (PLEDs), which emit in the near-UV, violet-blue range [15]. Investigations on PVK, such as its optical limiting response [16], photorefractive effect [17], and photoageing mechanism [18], have been continuously developing. Bussiere et al. [18] analyzed the consequences of photoageing on PVK and conducted depth-profiling experiments as well as roughness and stiffness measurements using atomic force microscopy (AFM) in combination with other techniques. The results show that the photo-degradation process is governed by chain-scission and cross-linking reactions. Another key material is the azo dye, DR1, which can induce photo-isomerization under green (from trans to cis-isomers) or red (from cis to trans-isomers) light irradiation. Azo dyes exhibit two molecular configurations, namely, the trans and cis isomers, which are rod-like and bent, respectively. The rod-like trans-isomers are aligned with the LCs by the guest-host effect in the DDLCs cell. Upon green light irradiation, the rod-like trans-isomers are transformed to the bent cis-isomers via the trans-cis isomerization process [19, 20]. Thus, the transformation of the molecular structures of DDLCs between the trans and the cis isomers can result in LC reorientation and affect the LC alignment [21]. Also, azo dyes absorb light energy to facilitate the thermal effect in DDLCs cells [22, 23]. In addition, the photo-induced cis-isomers of the azo dye can reduce the clearing temperature of LCs. Tabiryan et al. [24] demonstrated that azo-LCs can induce an isothermal phase transition from nematic to isotropic states via the photo-isomerization of the trans to cis states by UV illumination. In brief, the clearing temperature of the LCs depends on the population of cis-isomers. Accordingly, heating from the light-induced thermal effect and the reduction of the clearing temperature from the photoisomerization of azo materials modulate the solubility of PVK in DDLCs.
This investigation demonstrates scattering mode light shutter with different transmission (scattering) by light-induced dissolution of PVK based on DDLCs in a PVK films-coated LC cell. The stably multiple scattering performances, or the so-called multistable scattering modes, generated by reformed PVK layers with several domain sizes can be applied for achieving several scattering modes. Scattering is induced by the disordered LC distribution of optically treated double-sided PVK films. The light-induced dissolution of PVK based on DDLCs includes two main mechanisms, namely, the light-induced thermal effect and the photoisomerization effect [19, 20, 22, 23]. The solubility of PVK in DDLCs directly depends on the temperature of the DDLCs and the population of the azo dye cis-isomers, resulting in low order parameter and reduction of clearing temperature. The temperature of the DDLCs results from the light-induced thermal effect, whereas the population of the cis-isomers is from the photoisomerization of azo dyes (DR1). These details of the light-induced dissolution and morphologies of the PVK layers are also examined. Additionally, the optically switched scattering state can be completely switched back to the original scattering state by treating with the particular TIPS process [10]. Restated, the stable scattering performance can be optically controlled, and the multistability of scattering light shutter can be successfully achieved. This multistable light scattering can be used for display devices and other potential applications.
2. Experiments
The materials used in the current work are E7 (nematic LC, clearing temperature TC = ~61 °C, Fusol-material), DR1 (dichoric azo-dye, Aldrich), and PVK (polymer, Sigma-Aldrich). Given the presence of powdered PVK, a chlorobenzene solution containing PVK at a weight ratio of 98.36:1.64 was prepared and spin-coated onto indium-tin-oxide (ITO)-coated glass slides. The substrates were then prebaked in an oven at 80 °C for 20 min and subsequently post-baked at 120 °C for 120 min. The thickness of the fabricated PVK film without mechanical rubbing was determined as ~0.2 μm. Two non-rubbed PVK-coated glass substrates were combined to fabricate an empty cell with a cell gap of 6 μm. The azo dye, DR1, was doped into the LC host at a concentration of ~1.5 wt%. The DR1-doped LC was homogeneously filled into the empty cell, and the edges of the cell were then sealed with epoxy to yield the cell. According to our previous work, the transparent cell will become scattering (become opaque) one after thermal treatment via particular TIPS processes [10]. High scattering can provide an efficient dark state that can be used to fabricate light-modulation devices. Thus, the originally fabricated cell was treated with a particular TIPS process. Finally, the high-transmittance cell was gradually heated from 25 °C to 80 °C at a heating rate of ~30 °C/min. The DDLCs cell temperature was maintained at 80 °C for 5 min to dissolve PVK homogeneously. Subsequently, the DDLCs cell was cooled to 25 °C at a cooling rate of ~30 °C/min. The transmittance abruptly dropped to 0.4% at approximately 54 °C during cooling.
3. Results and discussion
Figure 1(a) shows the experiment setup for the optically switched scattering state in this system. A diode-pumped solid-state (DPSS) laser (λ = 532 nm) was used to illuminate the scattering DDLCs cell. To produce a light beam with a uniform intensity, the laser beam from the laser head was passed through an expander and an aperture, and finally illuminated onto the DDLCs cell. The diameter of the illuminated spot is approximately 3 mm. The duration of the illumination is 10 min, and the selected intensities of green light are 0.92, 0.97, 1.02, 1.07, 1.12, 1.17, and 1.22 W/cm2. Notably, after we obtained one transmission of the stable scattering, the DDLC cell was then treated with particular TIPS to switch the transmission back to the original scattering state (original PVK morphology) before another intensity green light was illuminated onto the DDLC cell. To monitor the scattering performances, a red probe beam from an unpolarized He-Ne laser (λ = 632.8 nm) was normally incident onto the illuminated spot of the DDLCs cell. The transmitted red light was received by a photodetector placed behind the DDLCs cell. The various transmittances of the stable scattering modes probed by the He-Ne laser were obtained under different green light intensities [Fig. 1(b)]. Therefore, the transmittance depends on the green light intensity. Notably, 100% transmittance was defined as the transmittance of the transparent cell after being filled with DDLCs without thermal treatment (particular TIPS processes). Experimentally, the stable transmittance was optically modulated ranging from 0.1% and 41.2%. The optically and thermally controllable scattering mode light shutter with different transmission is produced. The green light illumination process can modulate the original scattering state to other scattering one with transmission of 0.1%-41.2%. The transmission of the optically controllable scattering mode light shutter after blocking the green light is stable. Accordingly, the multistable scattering LC light shutter is demonstrated. The minimum transmittance (0.1%) of the optically reproduced DDLCs cell is lower than that of a scattering DDLCs cell after thermal treatment via particular TIPS processes [10]. The reason for this mechanism is discussed in later sections.
Fig. 1 (a) Experiment setup for the optical modulation of the scattering performance. The diameter of the illuminated spot is ~3 mm. (b) Transmittances of the stable scattering modes, which were achieved under different green light intensities. The probed beam is a He-Ne laser beam.
The following experiments were conducted to verify the mechanism of the light-induced dissolution of PVK into DDLCs for the modification of the scattering performance of DDLCs cells. These experiments were performed to study the light-induced thermal effect, which plays a key role in the process of scattering modulation. In brief, a thermal imager (IRI 4040, Irisys) was used to measure the DDLCs cell temperature under green light illumination. Figure 2 shows the temperature variations as a function of the duration of green light illumination onto the scattering DDLCs cell. Initially, the clearing temperature of the DDLCs sample was approximately 54 °C. The experimental results show that the temperature of the scattering DDLCs sample rapidly exceeded 34 °C under green light illumination for approximately 60 s. A higher green light intensity can exert a stronger light-induced thermal effect on the DDLCs cell. Moreover, the DDLCs cell temperature slightly decreased after 300 s. The reasons for this finding are as follows: first, the scattering performance decreases with increasing duration of green light illumination. Therefore, the reduction in light-scattering mean free path results in temperature reduction after 300 s [25]. Second, as described above, DR1 dye is a kind of dichroic dyes; the molecular reorientation of the DR1 molecules induced by the photoisomerization process leads to an anisotropic absorption of light [26–28]. Thus, the absorbance of the polarized green light by the DDLCs is reduced, so that its temperature slightly decreases. Thirdly, the adsorption of 532 nm light by cis isomers is lower than that by trans isomers [29]. Accordingly, the absorption of green light decreases, resulting in slight reduction of temperature. After that, the green light is turned off after 600s, and the DDLC temperature will decrease to room temperature naturally. The cooling time is about 40s, depending on the green light intensity. Therefore, the increase and recovery time of the multistable scattering LC light shutter are about 640s and 300s, respectively. The switching time can be improved by properly selected LCs, azo materials, green light intensity and others.
Fig. 2 Temperature variations as a function of the duration of green light illumination onto the scattering DDLCs cell. The cell was fabricated using two non-rubbing PVK-coated glass substrates after thermal treatment via particular TIPS processes.
Based on our previous study [10], the coated PVK can be dissolved into the LCs after heating under temperatures higher than the switching temperature (TS). TS is defined as the temperature needed to change the mode of the LC cell from transparent to scattering. Moreover, TS depends on the selected LCs. The separate experiments reveal that the TS obtained without green light illumination is approximately 54 °C, which is close to the clearing temperature of the selected LCs. Thus, the solubility of PVK in DDLCs can be enhanced because of the low-order parameter of the photoisomerized cis-isomers in the DDLCs cell, and Ts is reduced to approximately ~35 °C. In addition, the structures of the PVK that reformed on the substrates and were fabricated via different dissolution processes were observed using scanning electron microscopy (SEM). The SEM images are shown below.
Figure 3 shows the SEM images of the DDLCs cell substrates before and after treatment with light-induced PVK dissolution. The intensities of the green light irradiated onto the scattering DDLCs cell are 0.92, 0.97, 1.02, 1.07, 1.12, 1.17, and 1.22 W/cm2. The SEM images reveal that the dimensions and roughness of the branch-like structures are determined by the different green light intensities. The initial thermally treated PVK structures are shown in Fig. 3(a). Figures 3(b)-3(e) show that the original branch-like PVK structures were dissolved by DDLCs under illumination of green light with intensities of 0.92, 0.97, 1.02, and 1.07 W/cm2, respectively. The phase separation of the dissolved PVK and DDLCs occurred after switching off the green light, which facilitated the reformation of the PVK surfaces. Under these four intensities, the surfaces of the reformed PVK structures became increasingly more uniform and smoother as the green light intensity increased. In other words, the branch-like PVK morphologies were destroyed. Thus, the transmission of the DDLCs cell increased with increasing green light intensity (at intensities below 1.07 W/cm2). This finding indicates that the solubility of PVK in these four cases remains low, and that only the branch structures of PVK on substrates close to the bulk can be dissolved by DDLCs under green light illumination. Moreover, the cis-isomers in these four cases are too few to sufficiently reduce the order parameter (clearing temperature) of the DDLCs because of the low green light intensity. The phase separation rates in Figs. 3(b)-3(e) should therefore be low. In consideration of this point as well as the amount of dissolved PVK, the surfaces of the slowly produced, reformed PVK tend to be uniform [Figs. 3(b)-3(e)]. In other words, the few dissolved PVK molecules in DDLCs cannot sufficiently reform the rough surface structures to provide high scattering after green light blocking. Figures 3(f)-3(h) show the SEM images of the DDLCs cell substrates after treatment with green light illumination at 1.12, 1.17, and 1.22 W/cm2 intensities, respectively. Figure 1(b) shows that all three DDLCs cells exhibit high scattering performances. Moreover, the reformed PVK are mesh-like structures whose density increased as the green light intensity increased. Briefly, experimentally, when the illumination green light exceeds 1.12 W/cm2, the solubility of the PVK film by DDLCs is high enough to dissolve not only the branch structures of PVK on substrates close to the bulk but also the PVK film close to substrate. After switching off the green light, phase separation occurs, and mesh-like structures are reproduced. In our previous study [10], the cooling rates (phase separation rates) of the particular TIPS processes determine the surface roughness of the PVK structures. A rougher PVK structure is associated with higher scattering. A comparison of Figs. 3(e) to 3(h) reveals that the solubility significantly increased under green light illumination at intensities above 1.12 W/cm2. This finding indicates that the solubilities of PVK in DDLCs significantly increased. The solubility of PVK in isotropic LC is higher than that in nematic LC [10]. Thus, the cis-isomers of the azo dye clearly lowered the clearing temperature of the DDLCs, which resulted in the increased solubility shown in Figs. 3(f)-3(h).
Fig. 3 SEM images of DDLCs cells before and after treatment with light-induced PVK dissolution based on DDLCs scattering as a result of particular TIPS processes: (a) morphologies of the initial thermally treated PVK film (no light illumination). The intensities of the green light irradiated onto the scattering DDLCs cell are (b) 0.92, (c) 0.97, (d) 1.02, (e) 1.07, (f) 1.12, (g) 1.17, and (h) 1.22 W/cm2.
Figure 4 shows the images of the optically controllable scattering light shutter based on the DDLCs cell. The green light intensities are 0.97, 1.07, and 1.22 W/cm2, and the corresponding transmittances of the DDLCs cell are 10.2%, 41.2%, and 0.1%, respectively. The diameter of the illuminated area is approximately 3 mm. The illuminated areas represent variable scattering performances, which are determined by the different sizes of the LC domains. The scattering performance can be easily modulated by green light illumination at different intensities. Moreover, the transmittance/scattering of the optically controllable DDLCs light shutter can be completely switched back to the original scattering state via treatment with the particular TIPS.
Fig. 4 Images of the optically controllable multistable light shutter based on the DDLCs sample. The intensities of the green light are (a) 0.97, (b) 1.07, and (c) 1.22 W/cm2. The areas marked by the white dotted lines are the illuminated areas with diameters of ~3 mm.
Three scattering DDLCs cells treated with light-induced PVK dissolution by green light at 0.97, 1.07, and 1.22 W/cm2 intensities were observed under a crossed-polarized optical microscope (POM); the results are shown in Figs. 5(a) -5(c), respectively. The sizes (diameters) of the DDLCs domains in the optically treated cells range from a few submicrometers to 10 μm. Macroscopically, the light-induced PVK dissolution based on the DDLCs cell under different green light intensities produces LC domains with different sizes.
Fig. 5 POM images of DDLCs cells after treatment with light-induced dissolution of PVK based on DDLCs cell as a result of particular TIPS processes. The green light intensities are (a) 0.97, (b) 1.07, and (c) 1.22 W/cm2.
Figure 6 shows the plot of the scattering performances against the polarization of the incident light of the optically treated DDLCs cell [blue squares: Fig. 3(e); red circles: Fig. 3(h)]. An unpolarized laser beam from a He-Ne laser (λ = 632.8 nm) passed through a rotatable polarizer and was normally incident onto the illuminated area of the DDLCs cell. The 0° polarization is defined as the transmission direction perpendicular to the green light polarization. The insets (a) [(c)] and (b) [(d)] of Fig. 6 show the scattering patterns, when the polarized laser beam with 0° and 90° polarization (normally incident), respectively, penetrates through the DDLC cell corresponding to Figs. 3(e) and 3(h). We setup a screen behind the DDLC cell, and the distance between cell and screen is 24 cm. According to the observations, the spatial anisotropy of light scattering cannot be observed. Additionally, it’s clear to see that the scattering performance of the DDLC cell, illuminated with green light intensity ~1.07 W/cm2, is polarization-dependent, while that with green light intensity ~1.22 W/cm2, is polarization-independent. Moreover, the blue squares in Fig. 6 show that different polarizations of the incident light lead to different scattering performances. The ratio of the scattering performances with the polarizations, defined as T90/T0, is approximately 0.65. The polarization-dependent scattering is caused by the molecular reorientation of the LCs in the domains as induced by the photoisomerization process [21]. This effect causes the LCs to reorient along the direction perpendicular to the polarization of the green light [26]. The uniformly reformed surface cannot easily realign the LCs and destroy the LC order. Thus, the scattering performances are related to the polarization state of the probe beam. The birefringence of the LC domains results in the various refractive index mismatch, as probed by light beams with different polarization states. The refractive index mismatch of the LCs and PVK, as detected using a linearly polarized light with its polarization along the direction parallel to the long axis (ne) of the LCs, is smaller than that of the short axis (no) of the LCs. Hence, T90/T0 is lower than 1. Moreover, the red circles indicate that the scattering performances are polarization-independent because of the rough PVK surface, which realigns the DDLCs to obtain high scattering. Finally, the low polarization-dependency can be improved by using other light-induced thermal materials that prevent LC reorientation or by changing the polarized green light to an unpolarized one.
Fig. 6 The blue squares and red circles show the scattering performances as a function of the polarization state of the probe beam based on the optically treated DDLCs cell under 1.07 and 1.22 W/cm2 intensities of green light illuminated onto the DDLCs cell, respectively. The insets show the scattering patterns, when the polarized laser beam with (a) [(c)] 0° and (b) [(d)] 90° polarization (normally incident), penetrates through the DDLC cell corresponding to Figs. 3(e) and 3(h). The red and green arrows present the polarizations of red and green light, respectively.
4. Conclusion
In conclusion, a multistable, scattering-mode, LC light shutter was presented in this study. The scattering performances resulting from the multidomains of disordered LCs can be easily controlled by green light illumination. In brief, PVK dissolution into DDLCs and phase separation are the keys to the generation of optically controllable scattering light shutter. Moreover, heating by light-induced thermal effect and the reduction of the DDLCs clearing temperature (isothermal phase transition) are the major effects of PVK dissolution, which result in optically and thermally controllable light scattering. In addition, the advantages of the reported light scattering shutter include the reduction in power consumption, multistability, optical control, and others. The optically controllable and multistable light scattering shutter can be used in several LC applications, such as energy-efficient smart windows, scattering mode LC displays, and others.
Acknowledgment
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Grant No. NSC 101-2112-M-006-011-MY3 and NSC 99-2112-M-006-002-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center as well.
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