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Optically switchable and axially symmetric half-wave plate based on photoaligned liquid crystal films
Optical Materials 57 (2016) 23e27
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Optically switchable and axially symmetric half-wave plate based on photoaligned liquid crystal films C.-C. Lin a, T.-C. Huang a, C.-C. Chu b, **, Vincent K.S. Hsiao a, * a b
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou, 54561, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung, 40201, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 23 March 2016 Accepted 6 April 2016 Available online 14 April 2016
We demonstrate an optically switchable half-wave plate (HWP) composed of a photoaligned and axially symmetric liquid crystal (ASLC) film containing two azobenzene derivatives, methyl red (MR) and 4butyl-40 -methoxyazobenzene (BMAB). MR is responsible for photoalignment, and BMAB is used for optical tuning and switching the state of polarization (SOP) of probe beam (633 nm HeeNe laser) passing through the MR/BMAB doped ASLC film. The photoaligned ASLC film is first fabricated using a lineshaped laser beam (532 nm) exposure applied on a rotating LC sample. The fabricated ASLC film can passively change the linearly polarized light. Under UV light exposure, the formation of cis-BMAB (bendlike shape) within the film disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation back and simultaneously generates cis-MR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP under alternating UV and visible light exposure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Liquid crystals Half-wave plate Azobenzene
1. Introduction Polarization beam splitters (PBSs) and half-wave plates (HWPs) play crucial roles in optical systems that require passive and active tuning of polarization. In active tuning of linear polarized light, a mechanical force, such as rotation [1,2], must be applied on an HWP composed of a rounded and thin-film coated glass to achieve different linear state of polarization (SOP) between s-polarization and p-polarization. To precisely and actively tune and switch polarization, a liquid crystal (LC)-based variable retarder [3e13] layered with an LC film of a twisted structure can change the SOP of incident laser light under an applied electrical field. In general, LC twisting is completed by passive LC alignment in which two or more rubbed glass substrates with parallel or perpendicular orientation are used. However, applying LC alignment by using this rubbing technique is difficult in LC-based micro- or nano-devices. Recent advances in the photoalignment of LC by using an azobenzene (Azo) derivative as a photoaligned agent enable
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (V.K.S. Hsiao).
(C.-C.
http://dx.doi.org/10.1016/j.optmat.2016.04.006 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Chu),
[email protected]
noncontact LC alignment without the use of rubbed glass substrates [14e22]. The deformation of Azo under light exposure aligns and anchors the LC orientation. Compared with the rubbed substrate technique, photoalignment can more efficiently achieve the submicron-scale resolution for LC alignment. The Azo derivatives not only can perform the passive photoalignment of LC, but also can actively tune and switch the LC orientation under UV and visible light illumination [23e29]. Under UV light illumination, the generation of cis-Azo disrupts the LC orientation and changes the LC phase from nematic to isotropic. Under visible light illumination, the LC reverts back due to the generation of trans-Azo. The optical properties from photoresponsive LC can be modulated under alternative UVevisible illumination. In this study, we demonstrate an optically switchable HWP (linear retarder) composed of a photoaligned and axially symmetric LC (ASLC) film containing two Azo derivatives, methyl red (MR) and 4butyl-40 -methoxyazobenzene (BMAB). MR is responsible for photoalignment [19], whereas BMAB [30] is used for optical tuning and switching the SOP of light passing through the ASLC film. MR is an Azo dye and commonly used as a pH indicator. Laser exposure at a wavelength of 532 nm generates the cis-form of MR (bend-like shape) and anchors the LC, resulting in LC photoalignment. BMAB is an LC-like Azo and undergoes reversible trans-cis photoisomerization
24
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under alternating UV and visible light irradiation [31]. In our MR/ BMAB doped LC system, the photoaligned ASLC film is fabricated using a line-shaped laser beam (532 nm) exposure applied on a rotating LC sample. Under UV light exposure, the formation of cisBMAB (bend-like shape) disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation and simultaneously generates cisMR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP between s-polarization and p-polarization under alternating UV and visible light exposure.
2. Experimental Fig. 1 (a) shows the schematic of our optical setup used to achieve the photoalignment in ASLC films. The LCs used were a nematic LC (NLC, MDA 3461, Merck Taiwan), a blue-phase LC (BPLC, LCM- RTBP.1328VIS, LC MATTER Corp.), and a cholesteric LC (CLC) formed by a 15 wt% chiral dopant (ZLI-811, Merck) and 85% NLC. The LC samples were fabricated as followed: first, an epoxy-sealed sandwiched open cell was made by two glass substrate (rubbed or unrubbed) within a 12 mm plastic spacer. Second, a mixed LC containing Azo derivatives was mixed using mixer and sonicator and injected into the sandwiched open cell. Finally, the LC sample was kept vertical to make sure the LC diffused in the sandwiched for 10 min before the photoalignment process. A 532-nm diodepumped solid state laser was used as the light source for photoalignment because the peak absorption of MR is nearly 532 nm. The laser beam was passed through a line mask of 200 mm wide and 8 mm long. A cylindrical lens then focused the line-shaped laser beam onto the rotating sample (4 Hz) for 30 min. The SOP of the LC sample was measured without post-treatment at room temperature. Fig. 1 (b) shows the optical setup used to measure the SOP using a HeeNe laser as probe beam. A rounded and rotational glass HWP was placed directly in front of the probe beam to ensure that the s-polarization to p-polarization was 50:50. After passing
Fig. 1. Schematic of (a) fabricating a photoaligned ASLC film and (b) measuring the SOP from the LC sample under light exposure.
through the LC sample, the probe beam was separated by a PBS and the transmitted (p-polarization) and reflected (s-polarization) intensities were recorded by a photodetector, separately. Two irradiated beams, UV (peak wavelength of 365 nm) and visible (532nm wavelength laser), alternatively illuminated the LC sample. All the measurements were performed at room temperature. 3. Results and discussion The electrically controllable LC HWP is normally fabricated using two rubbed substrates that anchor and align the LC molecules. Here, we used two rubbed substrates of parallel orientation filled with BMAB-doped BPLC to create an optically switchable HWP. Fig. 2 shows transmittance changes based on different switching states of BMAB-doped BPLC and the corresponding switching mechanism from the LC sample. Before light irradiation (light-off state), the SOP of the sample showed 100% p-polarization, as shown in Fig. 2a (black circle). Because the SOP of the probe laser before passing through the BPLC was adjusted to have an s-polarization to p-polarization ratio of 50: 50, the LC sample served as a passive HWP and passively changed the linear SOP of the probe beam to 100% p-polarization. Under 365-nm UV light exposure (2 min), the LC sample changed the SOP of the probe laser causing it to become 50% p-polarized, as shown in Fig. 2a (blue square in the web version), and 40% s-polarized, as shown in Fig. 2b (blue square in the web version). Under 532-nm laser exposure (2 min), the LC sample changed the SOP of the probe laser to become 25% ppolarized, as shown in Fig. 2a (green circle), and 65% s-polarized, as shown in Fig. 2b (green circle). Almost 20% of the linear SOP of a probed laser passing through BMAB-doped BPLC film can be controlled and switched by alternating UV and visible light irradiation. Fig. 2c shows the POM images of the BMAB-doped BPLC film under UV and visible light exposure. Before UV light exposure, the BPLC showed a cholesteric phase of planar texture. Under UV light exposure, the bent-like cis-BMAB disturbed the cholesteric phase and changed the phase to be isotropic. Visible light irradiation generated a rod-like trans-BMAB and reverted the LC to a cholesteric phase. The phase transition between cholesteric and isotropic was reversible by alternating UV and visible light irradiation. Fig. 3 shows the linear SOP switching mechanism inside the LC film that involves using two rubbed substrates. Before light exposure, the BPLC was aligned by the parallel oriented substrate and the SOP of the probe beam showed 100% p-polarization. Under UV light exposure, the cis-BMAB disrupted the BPLC and changed the SOP of the probe beam. Because the BMAB can serve as the photoaligning agent, the visible light (532-nm laser) irradiation, which serves as the driving force of generating trans-BMAB, cannot revert the LC orientation to the original alignment. Therefore, the original SOP of 100% p-polarization dropped to 25%, as shown in Fig. 2a (green circle). The effect of photoalignment from trans-cis photoisomerization of BMAB under UV and visible light exposure caused the SOP to become 65% s-polarized, as shown in Fig. 2b (green circle). The optically switchable HWP was achieved by the BMABdoped BPLC under alternating UV and visible light irradiation conditions. A controlled experiment without the use of rubbed substrates showed no reversible switching of SOP in the same BMAB-doped BPLC film, indicating that a stable photoaligned Azo derivative is necessary if rubbed substrates are not used. We further demonstrated an optically switchable LC HWP without the use of rubbed substrates. The axially symmetric, passive polarization convertor was created using photoalignment techniques, as shown in Fig. 1a, using an NLC containing two Azo derivatives, MR and BMAB. Fig. 4 depicts the transmittance (T) of the probe beam dependent on the b angle (T vs. b), which is the angle between the polarization axis and the LC director [19]. The
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Fig. 2. Transmittance changes based on the state of external light exposure (2 min between each UV and visible light illumination) using (a) p-polarized and (b) s-polarized light as the probe beam; (c) POM images (picture area: 270 mm 320 mm) of BMAB-doped BPLC film under alternating UV and visible light conditions.
transmittance of the LC samples was measured between two polarizers in parallel or perpendicular directions. The maxima T at b ¼ 0 and b ¼ 90 and minimum T at b ¼ 45 , as shown in Fig. 4a, measured when the two polarizers were perpendicular, proved the successful fabrication of an ASLC polarization convertor. The results of T versus b when the two polarizers were parallel also proved that the photoalignment technique used herein can be a useful tool for fabricating an ASLC polarization convertor by using MR as photoaligning agent. The measurement of T versus b, as shown in Fig. 4b, was also applied in the control experiment without the addition of MR. The results revealed no changes in transmittance, because changing the b angle value indicated a strong anchoring effect by MR on LC photoalignment. A passive LC HWP is achieved by a MR/
Fig. 3. The switching mechanism of Azo-doped LC film using two rubbed substrates and the SOP of the probe beam under alternating UV and visible light, where UV light exposure increases the percentage of p-polarization and visible light exposure increases the percentage of s-polarization.
BMAB-doped NLC by using line-masking, sample rotating, and a laser exposure photoalignment technique. The active HWP was demonstrated with photoaligned MR/ BMAB-doped NLC by recording the intensity of the polarized probe beam under UV (blue in the web version) and visible (green) light exposure, as shown in Fig. 5. For the optically switchable LC that entails using BMAB as a photochromic dye, cis-BMAB was generated and the LC orientation was changed under UV light exposure. The SOP of the BMAB-doped LC was changed because of the bent shape of cis-BMAB. However, the SOP of the probe beam appeared to stay the same when the MR/BMAB-doped NLC film was illuminated with UV light for 10 min. Applying exposure to visible light immediately afterward, which can generate trans-BMAB, also caused no change in the SOP of the probe beam. After alternating UV and visible light illumination (cycle 3), the p-polarization of the probe beam started increasing and the s-polarization of the probe beam started decreasing accordingly. Applying visible light illumination immediately afterward returns the linear SOP of the probe beam (s- and p-polarization) to its original intensity. We observed a gradually increasing percentage of p-polarization (with a corresponding decrease in s-polarization) of the probe beam in cycles 1 and 2 because of the deformation competition between MR and BMAB under both UV and visible light. Before being subjected to UV light illumination, the NLC was photoaligned by MR; however, the first UV illumination generated cis-BMAB, thus decreasing the anchoring efficient of MR. The first cycle of visible light illumination generated trans-BMAB, thereby increasing the anchoring efficiency. After two cycles of alternating UV and visible light illumination, the anchoring effect from MR and BMAB reached a new equilibrium, and a linear SOP from a MR/BMAB-doped NLC film could be switched under UV and visible light illumination. Since the behavior of trans-cis photoisomerization of MR and Azo derivative is different, the first two cycle of UVevisible light illumination is necessary. Further investigation of adjusting the concentration of MR and Azo derivative in the LC system and replacing MR as anchoring molecules may reduce the time of cycle 1 and 2.
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Fig. 6. POM images (picture area: 270 mm 320 mm) of MR/BMAB-doped NLC film.
Fig. 4. Transmittance versus the b angle of (a) MR/BMAB-doped NLC and (b) BMABdoped NLC.
Fig. 6 shows the POM images of a MR/BMAB-doped NLC film under different treatments. Before photoalignment, the film showed aggregation from Azo derivatives, as shown in Fig. 6a. The
Fig. 5. Changes of SOP from MR/BMAB-doped NLC film under UV (blue) and visible (green) light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
aggregation disappeared after the LC sample was photoaligned under 532-nm laser illumination, as shown in Fig. 6b, and an ASLC film was generated. UV light illumination generated cis-BMAB and changed the nematic phase to the isotropic phase. The isotropic phase reverted to the nematic phase after the sample was illuminated with 532-nm laser light. Heating and right after cooling the sample directly after illumination with UV light returned the MR/ BMAB-doped NLC film to its original phase. The same LC sample preparation and optical measurement (SOP of probe beam and POM images) were performed on the MR/ BMAB-doped CLC sample, as shown in Figs. 7 and 8. Compared with the MR/BMAB-doped NLC sample, the optical switchable efficiency of s-polarization in the CLC sample (approximately 20%) was twice that of the NLC sample (approximately 10%). Also the required UVevisible illumination before the performance of optically switching polarization is avoided. Furthermore, in contrast to
Fig. 7. Changes in SOP from photoaligned MR/BMAB-doped CLC film under UV (blue) and visible (green) light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Acknowledgments This work is supported by the Ministry of Science and Technology (MOST) of Taiwan under project MOST 103-2221-E-260016-MY3 and MOST 103-2113-M-040-003, and Chung Shan Medical University.
References
Fig. 8. POM images (picture area: 270 mm 320 mm) of MR-BMAB-doped CLC film.
the NLC sample, UV light illumination instantly decreased the ppolarization of the probe beam and increased s-polarization in the CLC sample. The optically switchable SOP became stable after 3 cycles of alternating UV and visible light illumination. 4. Conclusion We demonstrate an optically switchable LC-based HWP by doping NLC and CLC with two Azo derivatives, MR and BMAB, without the use of rubbed substrates. The MR served as photoalignment agent, whereas BMAB served as a photoswitching agent. In our MR/BMAB-doped LC system, an axially symmetric and photoaligned LC film is made applying a line-shaped laser beam (532 nm) onto a rotating LC sample. Under UV light illumination, the formation of cis-BMAB disrupts the LC molecules, switches the LC orientation and further changes the SOP of the probe beam. Under visible light illumination (532-nm wavelength laser), the formation of trans-BMAB reverts the LC orientation, and this illumination generates the cis-MR and further helps anchoring the LC molecules in the previous photoaligned orientation. The photoaligned MR/BMAB-doped LC films can switch over 10% p-polarized light to s-polarized light under UV and visible light illumination.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optically switchable and axially symmetric half-wave plate based on photoaligned liquid crystal films C.-C. Lin a, T.-C. Huang a, C.-C. Chu b, **, Vincent K.S. Hsiao a, * a b
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou, 54561, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung, 40201, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 23 March 2016 Accepted 6 April 2016 Available online 14 April 2016
We demonstrate an optically switchable half-wave plate (HWP) composed of a photoaligned and axially symmetric liquid crystal (ASLC) film containing two azobenzene derivatives, methyl red (MR) and 4butyl-40 -methoxyazobenzene (BMAB). MR is responsible for photoalignment, and BMAB is used for optical tuning and switching the state of polarization (SOP) of probe beam (633 nm HeeNe laser) passing through the MR/BMAB doped ASLC film. The photoaligned ASLC film is first fabricated using a lineshaped laser beam (532 nm) exposure applied on a rotating LC sample. The fabricated ASLC film can passively change the linearly polarized light. Under UV light exposure, the formation of cis-BMAB (bendlike shape) within the film disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation back and simultaneously generates cis-MR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP under alternating UV and visible light exposure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Liquid crystals Half-wave plate Azobenzene
1. Introduction Polarization beam splitters (PBSs) and half-wave plates (HWPs) play crucial roles in optical systems that require passive and active tuning of polarization. In active tuning of linear polarized light, a mechanical force, such as rotation [1,2], must be applied on an HWP composed of a rounded and thin-film coated glass to achieve different linear state of polarization (SOP) between s-polarization and p-polarization. To precisely and actively tune and switch polarization, a liquid crystal (LC)-based variable retarder [3e13] layered with an LC film of a twisted structure can change the SOP of incident laser light under an applied electrical field. In general, LC twisting is completed by passive LC alignment in which two or more rubbed glass substrates with parallel or perpendicular orientation are used. However, applying LC alignment by using this rubbing technique is difficult in LC-based micro- or nano-devices. Recent advances in the photoalignment of LC by using an azobenzene (Azo) derivative as a photoaligned agent enable
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (V.K.S. Hsiao).
(C.-C.
http://dx.doi.org/10.1016/j.optmat.2016.04.006 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Chu),
[email protected]
noncontact LC alignment without the use of rubbed glass substrates [14e22]. The deformation of Azo under light exposure aligns and anchors the LC orientation. Compared with the rubbed substrate technique, photoalignment can more efficiently achieve the submicron-scale resolution for LC alignment. The Azo derivatives not only can perform the passive photoalignment of LC, but also can actively tune and switch the LC orientation under UV and visible light illumination [23e29]. Under UV light illumination, the generation of cis-Azo disrupts the LC orientation and changes the LC phase from nematic to isotropic. Under visible light illumination, the LC reverts back due to the generation of trans-Azo. The optical properties from photoresponsive LC can be modulated under alternative UVevisible illumination. In this study, we demonstrate an optically switchable HWP (linear retarder) composed of a photoaligned and axially symmetric LC (ASLC) film containing two Azo derivatives, methyl red (MR) and 4butyl-40 -methoxyazobenzene (BMAB). MR is responsible for photoalignment [19], whereas BMAB [30] is used for optical tuning and switching the SOP of light passing through the ASLC film. MR is an Azo dye and commonly used as a pH indicator. Laser exposure at a wavelength of 532 nm generates the cis-form of MR (bend-like shape) and anchors the LC, resulting in LC photoalignment. BMAB is an LC-like Azo and undergoes reversible trans-cis photoisomerization
24
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under alternating UV and visible light irradiation [31]. In our MR/ BMAB doped LC system, the photoaligned ASLC film is fabricated using a line-shaped laser beam (532 nm) exposure applied on a rotating LC sample. Under UV light exposure, the formation of cisBMAB (bend-like shape) disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation and simultaneously generates cisMR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP between s-polarization and p-polarization under alternating UV and visible light exposure.
2. Experimental Fig. 1 (a) shows the schematic of our optical setup used to achieve the photoalignment in ASLC films. The LCs used were a nematic LC (NLC, MDA 3461, Merck Taiwan), a blue-phase LC (BPLC, LCM- RTBP.1328VIS, LC MATTER Corp.), and a cholesteric LC (CLC) formed by a 15 wt% chiral dopant (ZLI-811, Merck) and 85% NLC. The LC samples were fabricated as followed: first, an epoxy-sealed sandwiched open cell was made by two glass substrate (rubbed or unrubbed) within a 12 mm plastic spacer. Second, a mixed LC containing Azo derivatives was mixed using mixer and sonicator and injected into the sandwiched open cell. Finally, the LC sample was kept vertical to make sure the LC diffused in the sandwiched for 10 min before the photoalignment process. A 532-nm diodepumped solid state laser was used as the light source for photoalignment because the peak absorption of MR is nearly 532 nm. The laser beam was passed through a line mask of 200 mm wide and 8 mm long. A cylindrical lens then focused the line-shaped laser beam onto the rotating sample (4 Hz) for 30 min. The SOP of the LC sample was measured without post-treatment at room temperature. Fig. 1 (b) shows the optical setup used to measure the SOP using a HeeNe laser as probe beam. A rounded and rotational glass HWP was placed directly in front of the probe beam to ensure that the s-polarization to p-polarization was 50:50. After passing
Fig. 1. Schematic of (a) fabricating a photoaligned ASLC film and (b) measuring the SOP from the LC sample under light exposure.
through the LC sample, the probe beam was separated by a PBS and the transmitted (p-polarization) and reflected (s-polarization) intensities were recorded by a photodetector, separately. Two irradiated beams, UV (peak wavelength of 365 nm) and visible (532nm wavelength laser), alternatively illuminated the LC sample. All the measurements were performed at room temperature. 3. Results and discussion The electrically controllable LC HWP is normally fabricated using two rubbed substrates that anchor and align the LC molecules. Here, we used two rubbed substrates of parallel orientation filled with BMAB-doped BPLC to create an optically switchable HWP. Fig. 2 shows transmittance changes based on different switching states of BMAB-doped BPLC and the corresponding switching mechanism from the LC sample. Before light irradiation (light-off state), the SOP of the sample showed 100% p-polarization, as shown in Fig. 2a (black circle). Because the SOP of the probe laser before passing through the BPLC was adjusted to have an s-polarization to p-polarization ratio of 50: 50, the LC sample served as a passive HWP and passively changed the linear SOP of the probe beam to 100% p-polarization. Under 365-nm UV light exposure (2 min), the LC sample changed the SOP of the probe laser causing it to become 50% p-polarized, as shown in Fig. 2a (blue square in the web version), and 40% s-polarized, as shown in Fig. 2b (blue square in the web version). Under 532-nm laser exposure (2 min), the LC sample changed the SOP of the probe laser to become 25% ppolarized, as shown in Fig. 2a (green circle), and 65% s-polarized, as shown in Fig. 2b (green circle). Almost 20% of the linear SOP of a probed laser passing through BMAB-doped BPLC film can be controlled and switched by alternating UV and visible light irradiation. Fig. 2c shows the POM images of the BMAB-doped BPLC film under UV and visible light exposure. Before UV light exposure, the BPLC showed a cholesteric phase of planar texture. Under UV light exposure, the bent-like cis-BMAB disturbed the cholesteric phase and changed the phase to be isotropic. Visible light irradiation generated a rod-like trans-BMAB and reverted the LC to a cholesteric phase. The phase transition between cholesteric and isotropic was reversible by alternating UV and visible light irradiation. Fig. 3 shows the linear SOP switching mechanism inside the LC film that involves using two rubbed substrates. Before light exposure, the BPLC was aligned by the parallel oriented substrate and the SOP of the probe beam showed 100% p-polarization. Under UV light exposure, the cis-BMAB disrupted the BPLC and changed the SOP of the probe beam. Because the BMAB can serve as the photoaligning agent, the visible light (532-nm laser) irradiation, which serves as the driving force of generating trans-BMAB, cannot revert the LC orientation to the original alignment. Therefore, the original SOP of 100% p-polarization dropped to 25%, as shown in Fig. 2a (green circle). The effect of photoalignment from trans-cis photoisomerization of BMAB under UV and visible light exposure caused the SOP to become 65% s-polarized, as shown in Fig. 2b (green circle). The optically switchable HWP was achieved by the BMABdoped BPLC under alternating UV and visible light irradiation conditions. A controlled experiment without the use of rubbed substrates showed no reversible switching of SOP in the same BMAB-doped BPLC film, indicating that a stable photoaligned Azo derivative is necessary if rubbed substrates are not used. We further demonstrated an optically switchable LC HWP without the use of rubbed substrates. The axially symmetric, passive polarization convertor was created using photoalignment techniques, as shown in Fig. 1a, using an NLC containing two Azo derivatives, MR and BMAB. Fig. 4 depicts the transmittance (T) of the probe beam dependent on the b angle (T vs. b), which is the angle between the polarization axis and the LC director [19]. The
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Fig. 2. Transmittance changes based on the state of external light exposure (2 min between each UV and visible light illumination) using (a) p-polarized and (b) s-polarized light as the probe beam; (c) POM images (picture area: 270 mm 320 mm) of BMAB-doped BPLC film under alternating UV and visible light conditions.
transmittance of the LC samples was measured between two polarizers in parallel or perpendicular directions. The maxima T at b ¼ 0 and b ¼ 90 and minimum T at b ¼ 45 , as shown in Fig. 4a, measured when the two polarizers were perpendicular, proved the successful fabrication of an ASLC polarization convertor. The results of T versus b when the two polarizers were parallel also proved that the photoalignment technique used herein can be a useful tool for fabricating an ASLC polarization convertor by using MR as photoaligning agent. The measurement of T versus b, as shown in Fig. 4b, was also applied in the control experiment without the addition of MR. The results revealed no changes in transmittance, because changing the b angle value indicated a strong anchoring effect by MR on LC photoalignment. A passive LC HWP is achieved by a MR/
Fig. 3. The switching mechanism of Azo-doped LC film using two rubbed substrates and the SOP of the probe beam under alternating UV and visible light, where UV light exposure increases the percentage of p-polarization and visible light exposure increases the percentage of s-polarization.
BMAB-doped NLC by using line-masking, sample rotating, and a laser exposure photoalignment technique. The active HWP was demonstrated with photoaligned MR/ BMAB-doped NLC by recording the intensity of the polarized probe beam under UV (blue in the web version) and visible (green) light exposure, as shown in Fig. 5. For the optically switchable LC that entails using BMAB as a photochromic dye, cis-BMAB was generated and the LC orientation was changed under UV light exposure. The SOP of the BMAB-doped LC was changed because of the bent shape of cis-BMAB. However, the SOP of the probe beam appeared to stay the same when the MR/BMAB-doped NLC film was illuminated with UV light for 10 min. Applying exposure to visible light immediately afterward, which can generate trans-BMAB, also caused no change in the SOP of the probe beam. After alternating UV and visible light illumination (cycle 3), the p-polarization of the probe beam started increasing and the s-polarization of the probe beam started decreasing accordingly. Applying visible light illumination immediately afterward returns the linear SOP of the probe beam (s- and p-polarization) to its original intensity. We observed a gradually increasing percentage of p-polarization (with a corresponding decrease in s-polarization) of the probe beam in cycles 1 and 2 because of the deformation competition between MR and BMAB under both UV and visible light. Before being subjected to UV light illumination, the NLC was photoaligned by MR; however, the first UV illumination generated cis-BMAB, thus decreasing the anchoring efficient of MR. The first cycle of visible light illumination generated trans-BMAB, thereby increasing the anchoring efficiency. After two cycles of alternating UV and visible light illumination, the anchoring effect from MR and BMAB reached a new equilibrium, and a linear SOP from a MR/BMAB-doped NLC film could be switched under UV and visible light illumination. Since the behavior of trans-cis photoisomerization of MR and Azo derivative is different, the first two cycle of UVevisible light illumination is necessary. Further investigation of adjusting the concentration of MR and Azo derivative in the LC system and replacing MR as anchoring molecules may reduce the time of cycle 1 and 2.
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Fig. 6. POM images (picture area: 270 mm 320 mm) of MR/BMAB-doped NLC film.
Fig. 4. Transmittance versus the b angle of (a) MR/BMAB-doped NLC and (b) BMABdoped NLC.
Fig. 6 shows the POM images of a MR/BMAB-doped NLC film under different treatments. Before photoalignment, the film showed aggregation from Azo derivatives, as shown in Fig. 6a. The
Fig. 5. Changes of SOP from MR/BMAB-doped NLC film under UV (blue) and visible (green) light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
aggregation disappeared after the LC sample was photoaligned under 532-nm laser illumination, as shown in Fig. 6b, and an ASLC film was generated. UV light illumination generated cis-BMAB and changed the nematic phase to the isotropic phase. The isotropic phase reverted to the nematic phase after the sample was illuminated with 532-nm laser light. Heating and right after cooling the sample directly after illumination with UV light returned the MR/ BMAB-doped NLC film to its original phase. The same LC sample preparation and optical measurement (SOP of probe beam and POM images) were performed on the MR/ BMAB-doped CLC sample, as shown in Figs. 7 and 8. Compared with the MR/BMAB-doped NLC sample, the optical switchable efficiency of s-polarization in the CLC sample (approximately 20%) was twice that of the NLC sample (approximately 10%). Also the required UVevisible illumination before the performance of optically switching polarization is avoided. Furthermore, in contrast to
Fig. 7. Changes in SOP from photoaligned MR/BMAB-doped CLC film under UV (blue) and visible (green) light illumination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Acknowledgments This work is supported by the Ministry of Science and Technology (MOST) of Taiwan under project MOST 103-2221-E-260016-MY3 and MOST 103-2113-M-040-003, and Chung Shan Medical University.
References
Fig. 8. POM images (picture area: 270 mm 320 mm) of MR-BMAB-doped CLC film.
the NLC sample, UV light illumination instantly decreased the ppolarization of the probe beam and increased s-polarization in the CLC sample. The optically switchable SOP became stable after 3 cycles of alternating UV and visible light illumination. 4. Conclusion We demonstrate an optically switchable LC-based HWP by doping NLC and CLC with two Azo derivatives, MR and BMAB, without the use of rubbed substrates. The MR served as photoalignment agent, whereas BMAB served as a photoswitching agent. In our MR/BMAB-doped LC system, an axially symmetric and photoaligned LC film is made applying a line-shaped laser beam (532 nm) onto a rotating LC sample. Under UV light illumination, the formation of cis-BMAB disrupts the LC molecules, switches the LC orientation and further changes the SOP of the probe beam. Under visible light illumination (532-nm wavelength laser), the formation of trans-BMAB reverts the LC orientation, and this illumination generates the cis-MR and further helps anchoring the LC molecules in the previous photoaligned orientation. The photoaligned MR/BMAB-doped LC films can switch over 10% p-polarized light to s-polarized light under UV and visible light illumination.
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