The direct optical switching of bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis- isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.
© 2013 Optical Society of America
In the recent decade, cholesteric liquid crystals (CLCs), also known as chiral nematic liquid crystals (LCs), have been studied by many scientists [1–5]. CLCs usually consist of nematics and a chiral dopant, as well as have two stable textures, planar and focal conic textures. The former presents colorless transparency when the value of pitch length (p) multiplied by the average refractive index of LC  is considerably longer or shorter than the wavelength of visible light; the latter shows strong light scattering because of the randomly arranged helical axes. Such various potential bistable (reflective planar and scattering focal conic states) LC display (LCD) technologies have been developed because they present low power consumption [5–7]. Watson et al. reported the mechanism of transformation between the transitent planar and equilibrium planar states of CLCs . As for planar textures, circularly polarized light with the same handedness as that of the planar textures of CLCs is selectively reflected. Briefly, the central reflection wavelength (λc) of the selective reflection band (Bragg reflection) by planar textures can be described by p and, and can be expressed as λcp for normally incident light. Pitch length (p) is determined by the concentration (c) of the chiral dopant and the relative helical twisting power (HTP) according to their relationship HTP = 1/pc. Positive (negative) values of HTP represent right- (left-) handed helix. Moreover, the related HTP values depend on the temperature of a mixture. In summary, the central reflection wavelength and pitch length of CLCs can be thermally tuned.
In addition to the thermally tuned pitch length of CLCs, the photo-tunable pitch length of CLCs, consisting of photosensitive chiral dopant and nematic LCs, has also been a subject of interest, especially for azobenzene materials [7, 9–16]. Given the properties of photo-isomerization between trans- and cis-isomers, the formation of azobenzenes can be transformed through light illumination, resulting in various HTP values, pitch lengths, and central reflection wavelengths, among other properties. Controlling the pitch length by changing the HTP of a chiral dopant via trans-cis photoisomerization was first reported by Vinvogradov et al. . Moreover, photoactive LC polymers, or the so-called cholesteric LC polymers, were developed and used to demonstrate many applications [12, 13]. White et al. reported that photo-tunable azobenzene LCs with a 2000 nm tuning range of reflection are notched by UV illumination . The materials were developed using an axially chiral bis(azo) molecule, QL76, with a high HTP in the trans/trans state (60 μm−1) and a relative low HTP in the cis/cis state (~27 μm−1). The changes in HTP are partially reversible, indicating that reduced HTP values cannot be optically tuned back to initial values. However, HTP values can be thermally or spontaneously reverted to their initial values if azobenzene isomers are completely converted back into the trans state through heating and then cooling, or dark relaxation (spontaneous reverse isomerization). Additionally, the effective optically controllable chirality of azobenzene chiral-doped CLCs is also adopted to demonstrate the switching of blue phase LCs .
In [17, 18], it has been shown that without any external fields, the planar and focal conic textures of CLCs are stable at room temperature. The comparison of the free energy of these two textures indicates that an energy barrier exists between them, and that the free energy of focal conic textures is higher than that of planar ones because of defects in the former. Briefly, the height of the energy barrier depends on pitch length, and increases with decreasing pitch length. Accordingly, without any external fields, if the pitch length of CLCs (energy barrier between planar and focal conic textures) is short (high) enough, the CLCs present bistable, that is, stable planar and focal conic textures. Moreover, the energies of defects increase with increasing pitch length. As pitch length is elongated (shortened), the free energy of the defects in the focal conic texture increases (decreases) and the CLCs tend to remain stable at planar (focal conic) textures.
As described above, bistable LCD technologies have attracted considerable interest in the past decade. Approaches to switching textures between reflective planar and scattering focal conic states of CLCs are continually developed; such approaches include voltage application and temperature control. In this paper, we report a direct optical switching method for CLCs, in which the two textures are switched by optically tuning the pitch length of CLCs in chiral azobenzene-doped LCs. The pitch length can be increased and decreased as the azobenzene material is irradiated with UV and green light, respectively. In brief, if the pitch length of the CLCs is longer than a critical value then the CLCs tend to be stabilized in planar textures; nonetheless, the bistable properties of CLCs form. Moreover, transformation from planar to focal conic textures can be optically realized via cis to trans photo-isomerization, as well as by decreasing pitch length. Conversely, long pitch length as a result of trans to cis photo-isomerization causes the CLCs to be stable in planar textures, indicating that the textures of CLCs can be optically switched from focal conic to planar textures. Thus, optically addressable, erasable, and re-addressable LCDs in azobenzene chiral-doped LCs can be fabricated.
A photo-sensitive CLC mixture was formulated by homogeneously mixing 88.7 wt% of nematic LCs (MDA-00-3461, clearing temperature ~92 °C, Merck) with 12.3 wt% of left-handed chiral azobenzene dopant (Ql-3c-S, HTP~60 μm−1, BEAM Corp.). Figure 1 plots the chemical structure of Ql-3c-S. These materials are commercially available. An empty 12 μm-thick cell fabricated using two glass slides without any alignment treatment was filled with the homogeneous CLC mixture through capillary action. The original (i.e., all trans-isomers) central reflection wavelength and the clearing temperature of the CLC mixture at planar textures were 250 nm (pitch length ~150 nm) and 80 °C, respectively. The properties of Ql-3c-S can be read and are discussed in [15, 16]. The most important properties are the optical reduction of HTPs by UV exposure and partial recovery by visible light illumination. That is, the pitch length of the CLC mixture can be optically and repeatedly elongated and shortened. In this study, UV light from a diode laser (λ = 405 nm) was used to elongate the pitch length of the CLC mixture. Conversely, green light from a diode-pumped solid state laser (λ = 532 nm) was used to shorten the pitch length of CLC mixture.
3. Results and discussion
Figure 2(a) shows a freshly filled LC cell at room temperature (~25 °C). Because the substrates were not treated with any alignment layers, the LC cell clearly depicted slight scattering, which can be attributed to multi-domain helical structures. The helical axes of the focal conic textures were not located along the same direction. The LC cell was then illuminated with UV light at an intensity of 8.3 mW/cm2 for 35 s to obtain planar textures. The clearly transparent LC cell is shown in Fig. 2(b). The HTP of the chiral azobenzene decreases with the duration of UV light illumination. As indicated in [17, 18], CLCs tend to be stable at planar textures at a sufficiently long pitch length. The transformation shown in Fig. 2 supports the aforementioned finding. The dynamics of optically tunable pitch length is discussed later in the paper. The central reflection wavelength of the chiral azobenzene-doped LC in planar textures red shifts to the infrared region. Notably, the yellow color results from azobenzene absorbance [15, 16]. Moreover, the planar texture is permanently stable, but the pitch length (central reflection wavelength) gradually decreases or blue shifts when UV light is turned off. This reduction or blue shift is expected because cis-isomers are converted back into trans-isomers because of dark relaxation (spontaneous reverse isomerization from cis- to trans-isomers). The required duration for completing dark relaxation depends on the lift time of cis-isomers.
Prior to demonstrating the direct optical switching of bistable cholesteric textures, observations on the optically induced red and blue shifting of the central reflection wavelength (pitch length) are presented. One LC cell, filled with chiral azobenzene-doped LCs and illuminated with UV light (intensity of 8.3 mW/cm2) for 35 s, was prepared and stored in the dark for longer than 24 h to ensure that all the azobenzene chiral molecules are spontaneously inverse isomerized to trans-isomers. The blue curve in Fig. 3(a) depicts the initial planar textures of the CLCs with a pitch length of about 150 nm. Notably, the optical energy at a wavelength below ~550 nm is absorbed by the chiral azobenzene Ql-3c-S; therefore, the decline in transmission near a short wavelength can be observed. Figure 3(a) shows the transmission spectra of the CLC materials during illumination with UV light (8.3 mW/cm2) at various durations. The central reflection wavelength gradually red shifts from the UV to the IR region. This shift is attributed to the chirality of the used chiral azobenzene progressively decreasing with exposure to UV light, indicating that pitch length increases. The blue-green absorbance by the chiral azobenzene also decreases because of photo-isomerization from trans- to cis-isomers and the difference in absorbance between trans- and cis-isomers. Therefore, transmission in the blue-green region increases. The UV tunable range of the central reflection wavelength (pitch length) ranges from ~250 (150) nm to longer than 850 (510) nm.
The difference in absorbance between trans- and cis-isomers also results in the partial optical tuning of the red-shifted central reflection wavelength (elongated pitch length) to a short wavelength (pitch length). Figure 3(b) presents the central reflection wavelength recoverable through green light illumination at various durations onto the UV-illuminated CLC cell. The initial 0s state indicates the CLC cell after being illuminated with UV light (8.3 mW/cm2) for 35 s. To demonstrate the blue shifting of the central reflection wavelength and to prevent the chiral azobenzene from being converted into scattering focal conic textures because of disturbance, the selective intensity of the green light should not be excessive. Thus, the intensity selected was 5.3 mW/cm2. In the experiment, the central reflection wavelength was blue shifted from IR to visible wavelength, and stopped at about 590 nm because the cis- to trans- photo-isomerization process reached its dynamic equilibrium. In other words, the chirality of Ql-3c-S partially increases with green light illumination. The tunable range (blue shifting) of the central reflection wavelength (pitch length) ranges from longer than 850 (510) nm to 590 (350) nm. Theoretically, the stopped wavelength depends on the incident light wavelength and absorbed optical energy because of the absorption spectra of cis- and trans-isomers [15, 16]. Additionally, the duration required for the red or blue shifting of the central reflection wavelength decreases with increasing intensity of exposed light. Nevertheless, under excessive incident light intensity, the reorientation of LCs is disturbed and CLC textures tend to be converted and stabilized at focal conic textures. As indicated by these results, central reflection wavelength, pitch length, and chirality (HTP) can be optically switched by two different lights with different wavelengths.
Figure 4 shows the transmission spectra variations resulting from an LC cell filled with chiral azobenzene-doped LCs being illuminated with UV and green light. Curve (a) in Fig. 4 depicts the transmission spectrum of a freshly filled LC cell. Given the slight scattering of focal conic textures, transmission reaches only about 65%. Curve (b) in Fig. 4 plots the transmission spectrum of the LC cell after it was illuminated with UV light (8.3 mW/cm2) for 35 s. The CLC has a planar texture, with its central reflection wavelength located in the IR region. Curve (c) in Fig. 4 shows low transmission (high scattering) of the LC cell when the chiral azobenzene-doped LCs were irradiated with green light (28.8 mW/cm2) for 20 s. Notably, the selective intensity of the green light is much higher than that used in demonstrating the results shown in Fig. 3(b). Because the textures of the initially stable state are planar, the disturbance caused by rapid and violent trans-cis isomerization is required in the destruction of planar texture stability; this destruction enables the realization of scattering focal conic textures. Additionally, the relatively low transmission at a wavelength shorter than 600 nm is caused by cis-azobenzene absorbance. The transparently planar textures can therefore be directly optically switched to scattering focal conic textures. Finally, the scattering focal conic textures were switched back to planar textures through irradiation with UV light, presented as curve (d) in Fig. 4 [8.3 mW/cm2 for 35 s]. As indicated by the transmission spectrum, the central reflection wavelength should be located at the IR region. In the experiment, the transmission spectrum spontaneously reverts to its initial state, depicted as the blue curve in Fig. 3(a). The required duration depends on the lifetime of cis-azobenzene.
Gray scales can be optically achieved (UV illumination) by using the area ratio of focal conic textures to that of planar textures. Figure 5 shows the dynamic measurement of the transmission of a He-Ne laser beam through a chiral azobenzene-doped LC cell when a UV light (8.3 mW/cm2) was turned on and turned off. The initial scattering LC cell was prepared by irradiation with green light (28.8 mW/cm2) for 20 s. The UV light for the first step was turned on at time t = ~30 s, and UV irradiation was maintained for 23 seconds. Initially, the transmission exhibits instability because pitch length increases and the helical axes are not located along the same direction. After this (t = 23 s), the UV light was turned off, and transmission stabilizes at 25% because the pitch lengths of CLCs in some regions were more elongated than the threshold value, indicating that the CLC textures there were optically switched from focal conic to planar textures. The CLCs in other regions remained focal conic textures due to the short pitch length. At t = 142 s, the UV light was turned on again and the LC cell was irradiated with UV for 4 s, resulting in 40% transmission. Experimentally, transmission increases with increasing illumination duration. Finally, transmission reached about 82%. Insets (a) - (c) of Fig. 5 display the images of the LC cell observed under an optical microscope with a transmission of 12% (scattering focal conic textures), 40% (coexistence of scattering focal conic and reflective planar textures), and 82% (reflective planar textures), respectively. The textures of CLCs clearly changed from focal conic to planar after being illuminated with UV light.
Figure 6 presents the optically patternable, erasable, and rewritable cholesteric LC display device photographed with a digital camera. The device was illuminated with UV or green light as a demonstration. Figure 6(a) shows the LC cell with a transparent planar texture, achieved by UV light irradiation at an intensity of 8.3 mW/cm2 for 35 s. Then, a homemade photo-mask with four transparent letters (“NCKU”) was then placed in contact with the chiral azobenzene-doped LC cell. A green light beam with an intensity of 28.8 mW/cm2 was used to illuminate the cell through the photo-mask for 20 s. The scattering patterns of the letters were imprinted onto the LC cell as shown in Fig. 6(b). The letters, at focal conic textures, were displayed in the transparent background (planar textures). In addition, Fig. 6(c) presents the erasure of the addressed patterns on the LC cell; the patterns were erased by UV light illumination at 8.3 mW/cm2 for 35 s. As described above, the pitch length was elongated to switch the focal conic textures back to planar textures. Finally, it was experimentally demonstrated that the optically erasable LC cell is optically rewritable. The LC cell was illuminated with a green light beam through another homemade photo-mask to write another pattern onto the LC cell. Figure 6(d) shows the re-addressed pattern “LC” on the LC cell. In the experiment, the addressed patterns were at stable states at focal conic and planar textures.
In conclusion, this study demonstrates that the direct optical switching of bistable cholesteric textures in chiral azobenzene-doped LCs can be achieved. In accordance with free energy, pitch length can be elongated (shortened) to obtain transparently planar (scattering focal conic) textures by illuminating UV (green) light onto LC cells. The repeatable and reversible processes can be adopted to fabricate an optically addressable, erasable, and re-addressable LCD in azobenzene chiral-doped LCs.
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 102-2112-M-008-016. This work is also partly supported by the Advanced Optoelectronic Technology Center at National Cheng Kung University.
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