Open Access
Add to BibSonomyAbstract
Optically-induced grating in ZnTPP doped chiral nematic liquid crystals (CLC) is demonstrated by dual-mode operations, where the dynamic or storage functionality is selected by the amplitude of dc voltage. While the storage can be erased and rewritten by a high dc pulse, it can persist without an electric field, which is the result when the CLC undergoes a switching between a planar and a focal conic state. This tunable dual-functionality grating combines the advantages of bistable storage of CLC and high photosensitivity of dopant, and thus can be potentially exploited in photonic devices to implement different optical information processing in situ.
© 2017 Optical Society of America
1. Introduction
Nematic liquid crystals (LCs) are important electro-optic materials, which by now are widely used as display and information processing components. Due to the large anisotropies and their sensitivity to external fields, LCs can undergo various deformations and convections, resulting in a strong optical nonlinearity. This controllable optical effect can be exploited to modulate light, which is of interest in technologies including optical information storage and processing, beam shaping or steering, light communications, switchable holograms and gratings [1–4].
Among photonic devices, optically-induced gratings based on LCs have drawn much attentions due to the flexibility: it is convenient to realize specific spatial and temporal distribution of optical field, thus optically-induced gratings with customized patterns and desired switching are easily acquired.
Meanwhile, optical materials with different host LCs and dopants are considered to enhance the grating performances, which usually belong to the following categories:
- ① materials containing photosensitive dopants. Doped LCs can work at a very weak light intensity and give rise to a super large optical nonlinearity, where the LC molecules are reoriented by a non-uniform space-charge field created by light-induced modulation of charges from dopants. The most important functionality of these materials is the optical storage, where the spatial information carried by optical field can be stored even when the optical field has been canceled [5–8];
- ② materials with azo-dye in the surface. A layer of azo-dye acts as LCs substrate, whose trans-cis conformation change under polarized UV light alters the alignment of LCs, thus modulates the morphologies of gratings. Furthermore, by varying the polarization of UV light, photo-alignments can be reversibly switched [9-10];
- ③ materials composed of azo-dye in the bulk nematic LC film. The azo molecules undergo photoisomerization by patterned absorbing light, then reorient the host LC molecules via intermolecular coupling, resulting in a periodic deformation of LC film [11–13]. In some cases, the photoisomerization of azo molecules may even trigger isothermal nematic–isotropic phase transition and modulate the refractive index of host LCs [14,15];
- ④ materials composed of azo-dye in the bulk chiral nematic LC (CLC). CLC is a special kind of LCs possessing a helical super-structure. Photoirradiation of the CLC containing azo-dye results in a change in helical pitch, thus optically switches the samples between different electro-optic modes [16–18].
The above systems can implement only one of dynamic or storage functionality. However, for some applications, LCs possess both functionalities with switchable in situ are highly desirable. In the current work, we report a doped CLC system, whose optically-induced grating possesses both dynamic and storage functionalities. This dual-mode operations can be switched by dc field, and the storage operation can be erased by high dc pulse thus a rewritable grating can be obtained. Due to our newly-designed material composition and structure, this device combined the advantages of bistable storage of CLC and high photosensitivity of dopant, thus can be potentially used in data storage and holographic recording.
2. Experimental arrangement
A CLC material was prepared by mixing nematic 5CB with left-handed chiral agent S811 in a weight ratio of 18:7. Zinc porphyrin (ZnTPP, 5, 10, 15, 20-tetraphenylporphrinatocopper(II)) [19] was doped into CLC host at a concentration of 1.25wt%. Being a bio-molecule, ZnTPP possesses excellent photosensitivity as it is an active ingredient in chlorophyll. Furthermore, ZnTPP and host CLC can form a uniform solution, where a strong coordination bonding is formed between the -CN group in 5CB and Zn atom in ZnTPP [20]. This molecular level coupling gives rise to a high solubility of ZnTPP in CLC host, thus makes the CLC sample very stable. Even after several months of work, the samples didn't show any detectable degradation in performance.
The doped CLC was filled into standard planar cells with thickness of d = 9μm, where two ITO glass substrates were coated with polyimide (PI) and rubbed unidirectionally to obtain planar alignment , where is the orientation of CLC director in the surface. The cells filled with the CLC mixture showed a typical planar texture with reflection band between 500 and 550nm, indicating that the helical axis was perpendicular to the substrates.
A holographic optic setup was used to investigate the induced gratings under the combined application of writing beams and a dc electric field, where dc field is perpendicular to the substrates, as shown in Fig. 1. A cw linearly polarized laser beam at 457nm (MSL-FN-457, Changchun New Industries Optoelectronics Technology Co., Ltd.), with polarization parallel to the alignment , was split into two writing beams (I1 = I2 = I) with equal intensities, and then overlap on the CLC sample with a spot diameter of 2 mm. With cross angle α, I1 and I2 produced a light intensity grating with spatial periodicity Λ0(α) = λ/sinα and wave vector q0(α) = 2π/Λ0(α), and the bisector of the two beams was normal to the cell surface. A weak probe beam I0 at 633 nm was incident normally to the sample and diffracted by induced gratings, where diffraction efficiency η is defined as a ratio of intensity of 1st order diffraction to the intensity of incident probe beam. Moreover, in order to track the helical pitch change in doped CLC under the effects of dc- and optical-field, a white light from a Xenon lamb and a spectrophotometer were used to determine the transmission spectra of CLC. In addition, the temperature of the samples was controlled at room temperature (26°C).
Besides the diffraction technique, a polarizing microscope (POM) with CCD camera was also used to capture the textures in sample.
3. Experimental results
We observed optically-induced gratings and examined their characteristics by changing the amplitude of dc voltage, where dual-mode operations were demonstrated and compared according to different voltage ranges:
3.1 The low dc range of Vc1 <Vdc< Vc2
With a combined application of Vdc and writing beams, a strong diffraction can be easily and quickly produced. The wave vector of grating is accordance with of light intensity pattern. After withdraw the Vdc and writing beams, the diffraction will quickly disappear within 1 second, thus the grating operates at dynamic mode.
The time evolution of 1st order diffraction is demonstrated in Fig. 2, where the higher the light intensity I, the faster the formation process. In addition, the threshold voltage Vc1 decreases as the I becomes stronger. For example, Vc1 is 15, 12 and 10V for I = 5, 10 and 20mW/mm2, respectively.
Fig. 2 Temporal evolution of diffraction in dynamic operation, where Vdc = 20 and Λ0≈10.0μm (α≈2.6°).
3.2 The medium dc range of Vc2 <Vdc< Vc3
When dc field is increased beyond threshold Vc2, diffraction efficiency η becomes much stronger. The time evolutions of η under different I are demonstrated in Fig. 3, showing that η still persists even after dc and writing beams are both turned off, thus the grating operates at storage mode. The storage result is verified by examining the η after several weeks, where η is almost unchanged.
Fig. 3 Temporal evolution of diffraction in storage operation, where Vdc = 40 and Λ0≈10.0μm (α≈2.6°).
In addition, an optical field-induced lowering of threshold Vc2 was also observed in this dc range, where Vc2 ≈35, 28 and 25V for I = 5, 10 and 20mW/mm2, respectively.
3.3 The high dc range of Vc3 <Vdc
The aforementioned persistent grating can be erased by a dc pulse, providing that whose strength is sufficient higher than threshold Vdc≥Vc3≈90, as shown in Fig. 4. During this erasing operation, η will increase abruptly to a high level as the application of dc pulse, and then decay to zero after the cancellation of dc pulse.
Fig. 4 The erasing operation under dc pulses with different amplitude and width, where the previously established grating is recorded by Vdc = 45 and I = 10mW/mm2.
It is found that when a higher dc pulse is applied, the corresponding duration (width) needed for erasing the torage effect will be shorter. Furthermore, if the amplitude of dc pulse is fixed, the erasing result will depend on the duration, as shown in curves 1 and 2. However, if Vdc<Vc3, no matter how long dc pulse is applied, the storage effect will still sustain without any impairment, as shown in curves 3.
Finally, it should be noted that under such dc range, even if writing beams are used, the aforementioned dynamic or storage grating cannot be created, which means only erasing operation is available.
Besides the above temporal behaviors, the static behaviors of grating are also demonstrated in Fig. 5, which indicates that η strongly depends on the dc ranges. The abrupt enhancement of η when dc is increased from low range to medium range reveals that an underlying mechanism change has taken place during this process, namely, the mechanisms responsible for the dynamic operation or storage operation are different.
4. Discussion
We explain the mechanism of our device according to the dynamic mode or storage mode, respectively.
4.1 Dynamic mode
- ① Firstly, we mention that if the doped CLC is under the effect of an ac, the grating cannot be induced under whatever ac voltage and writing beams intensity used.
- ② Secondly, the formation of dynamic mode needs joint application of dc- and patterned light-field, which implies that an internal space-charge field Es is formed due to the drift of light-induced charges under dc field Ed [21–23]. However, if the doped CLC is under ac driving, the space-charges will move back and forth according to the ac frequency, consequently the positive and negative space-charges will quickly recombine and the Es will be quenched accordingly, which results in disappearing of gratings. In order to further support this charge drift driven effect, a photo-current measurement is carried out, where strong photo-currents are shown in Fig. 6 depending on the dc voltage and light intensity.
Fig. 6 photo-induced currents as a function of light intensity and dc voltage. Dark currents (black curve), photo-currents (red and blue curves) are measured when sample is illuminated uniformly by 457nm laser beam with different intensity.
- ③ Furthermore, due to the particularity of ZnTPP dopant, the drift of charges under dc gives rise to surface effect [24-25], whose mechanism has been mentioned in the system of nematic doped with CuTPP [26]. The illumination of writing beams results in the presence of charge carriers in bright regions whereas absence in dark regions. The photoinduced charges from dopant ZnTPP drift to the surface of CLC film under Ed, and then distribute spatially on the surface in accordance with light intensity pattern, resulting in a periodic Es to reorient CLC molecules to form a phase grating, as shown in Fig. 8(b). After the dc and the writing beams are turned off, space-charges will diffuse and recombine, resulting in the decay of Es. Thus this operation is dynamic.
- ④ Specifically, the observation of a lowering of threshold voltage with increasing light intensity, further supports our assumption of surface effect induced gratings, which is attributed to the optically induced surface electric field in the LC-alignment layer interface and has been explained in Ref [27].
- ⑤ In a separate measurement, we also monitor the reflection band of sample by spectrometer, to demonstrate the effect of Es on the reorientation of CLC, as shown in Fig. 7. It was found that with only dc, or only writing beams, the reflection band will remain the same. However, if both dc (Vc1<Vdc<Vc2) and writing beams are used, the reflection band will change and shift to blue side. Afterward, the reflection band can restore if dc and writing beams are turned off. It was assumed that the formation of Es modulates the helical structure of CLC. This conclusion can be further supported by replacing dc with ac field, where the reflection band will be unperturbed with or without writing beams, revealing Es cannot be established. Please note that further increasing the dc up to Vc2<Vdc<Vc3 will quench the reflection band, and even withdraw the dc and writing beams cannot recover the reflection band. This fact will be explained later as a focal conic state occurs in CLC sample and destroys the helical structures.
Fig. 8 Schematic diagrams: (a)initial P state; (b) grating formation: under the joint application of dc and writing beams, modulated surface charges will form, producing a periodic Es to buildup gratings; (c) grating storage: after turning off dc and writing beams, coexistence of P and FC states appears, whose distribution corresponds to the light intensity pattern.
4.2 Storage mode
- ① When Ed is increased to medium range of Vc2<Vdc<Vc3, the induced Es will be enhanced accordingly, thus produces a larger change of refractive index and results in a much stronger diffraction efficiency η, which can be confirmed by comparison of Fig. 2 and Fig. 3. Meanwhile, in the dark regions, the strong Ed will turn CLC molecules from planar (P) state into focal conic (FC) state (random distribution of the helical axes) [28]; whereas in the bright regions, the effective electric field Ed-Es is much less than Ed, thus the CLC molecules would rather stay in P state (uniform helical axes). Consequently, two states (FC state and P state) corresponding to the light intensity pattern coexist spatially, as shown in Fig. 8(c), which persist even after canceling dc and writing beams, result in storage operation. The storage grating can be demonstrated in Fig. 9(c) under POM, whose periodic microstructure coincides with light intensity pattern. On the contrary, in the case of low dc range of Vc1 <Vdc< Vc2, no such grating structure remains, as shown in Fig. 9(b). These two contrast evidences are consistent with the results of Fig. 2 and Fig. 3.
- ② Afterward, if a high dc (Vc3<Vdc) pulse is applied, the distributed FC states and P states are forced to switch to a uniform homeotropic state under such a high electric field, and after the cancellation of dc pulse, CLC molecules return back to the initial uniform P state, where the helical structure and reflection band recover. In order to verify this conclusion, we carried out another experiment, where the recorded sample was placed between crossed polarizer and analyzer, and the changes of textures inside sample were tracked by POM. It was clearly shown that during the period of high dc pulse, the existing grating texture turned into uniform dark state, demonstrating a uniform homeotropic reorientation. After the finish of dc pulse, the uniform dark state returned back to the texture of uniform P state, as shown in Fig. 8(a). After this erasing operation, the system is ready to the next dynamic or storage operation, thus this storage grating is rewritable.
Fig. 9 (a) texture of initial P state; (b) texture after combined application of low dc (20V) and writing beams, and then cancellation them, showing no grating structure remained; (c) texture after combined application of medium dc (40V) and writing beams, and then cancellation them, showing an ordered grating is formed [200μm*200μm].
In the end, we reiterate that compared with conventional systems possessing storage functionality, our system works on peculiar mechanism as follows:
- a) Our system differs from conventional system in storage process: in photorefractive system, an additional external dc field is needed to maintain the internal Es to sustain the grating, and the storage quality will degrade gradually due to the decay of Es. Instead, in here, an inherent bistable characteristics of CLC, without the help of additional electric field, is exploited to store optical information, resulting in a long duration memory without weakening. Besides, our system works on symmetric configuration, where the bisector of the two writing beams was normal to the cell surface, thus the photoelectrons will drift along the longitudinal direction to the surface of CLC film under the external dc field, and then distribute spatially on the surface in accordance with light intensity pattern. Namely, the space-charge field is local and optically-induced, thus no two beam-coupling effect will present in our experiment. In contrast, in conventional photorefractive system, due to the asymmetric configuration where the bisector of the two writing beams was tilted with respect to the normal of cell surface, the space-charge field is π/2 phase shifted from the incident light intensity pattern, thus the two beam-coupling effect is easily observed.
- b) In addition, our system differs from popular pure CLC system of storage display in record process: in the latter, switching between P state and FC state is exploited to record electric signal [29,30]. However, due to pure CLC without absorbing dopant, each state cannot respond to the light field, thus the system is uniform without light patterns and it is impossible to record optical signal. In our system, due to the high photosensitivity of dopant, the light intensity pattern will modulate the spatial distribution of P and FC states, which carry the optical signals.
5. Conclusion
In conclusion, a dual-mode optically-induced grating is examined in doped CLC system, based on our newly-designed composite material. We conclude that the grating formation is due to the surface effect, while the grating storage is ascribed to the peculiar bistable state of CLC. By tuning the amplitude of applied voltage, both dynamic and storage functionalities can be realized, which can be potentially exploited to make certain photonic devices to implement different optical information processing in situ.
Funding
This work was supported by the National Natural Science Foundation of China (Grant No. 11374067), the Guangdong Provincial Science and Technology Plan (Grant No. 2014A050503064, 2016A050502055 and 2016A030313698).
References and links
1. I. C. Khoo, “Nonlinear optics, active plasmonics and metamaterials with liquid crystals,” Prog. Quantum Electron. 38(2), 77–117 (2014). [CrossRef]
2. W. Lee and S. T. Wu, “Focus issue introduction: liquid crystal materials for photonic applications,” Opt. Mater. Express 1(8), 1585–1587 (2011). [CrossRef]
3. J. Kim, J. H. Suh, B. Y. Lee, S. U. Kim, and S. D. Lee, “Optically switchable grating based on dye-doped ferroelectric liquid crystal with high efficiency,” Opt. Express 23(10), 12619–12627 (2015). [CrossRef] [PubMed]
4. Z. G. Zheng, Y. Li, H. K. Bisoyi, L. Wang, T. J. Bunning, and Q. Li, “Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light,” Nature 531(7594), 352–356 (2016). [CrossRef] [PubMed]
5. I. C. Khoo, H. Li, and Y. Liang, “Observation of orientational photorefractive effects in nematic liquid crystals,” Opt. Lett. 19(21), 1723–1725 (1994). [CrossRef] [PubMed]
6. P. Pagliusi and G. Cipparrone, “Photorefractive effect due to a photoinduced surface-charge modulation in undoped liquid crystals,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 69(6 Pt 1), 061708 (2004). [CrossRef] [PubMed]
7. W. Lee and S. L. Yeh, “Optical amplification in nematics doped with carbon nanotubes,” Appl. Phys. Lett. 79(27), 4488–4490 (2001). [CrossRef]
8. V. Yu. Reshetnyak, I. P. Pinkevych, T. J. Sluckin, G. Cook, and D. R. Evans, “Beam coupling in hybrid photorefractive inorganic-cholesteric liquid crystal cells: Impact of optical rotation,” J. Appl. Phys. 115(10), 103103 (2014). [CrossRef]
9. W. Hu, A. Srivastava, F. Xu, J. T. Sun, X. W. Lin, H. Q. Cui, V. Chigrinov, and Y. Q. Lu, “Liquid crystal gratings based on alternate TN and PA photoalignment,” Opt. Express 20(5), 5384–5391 (2012). [CrossRef] [PubMed]
10. L. Lucchetti, M. Gentili, F. Simoni, S. Pavliuchenko, S. Subota, and V. Reshetnyak, “Surface-induced nonlinearities of liquid crystals driven by an electric field,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 78(6 Pt 1), 061706 (2008). [CrossRef] [PubMed]
11. X. Li, C. P. Chen, Y. Li, X. H. Jiang, H. J. Li, W. Hu, G. F. He, J. G. Lu, and Y. K. S. Chin, “Color holographic display based on azo-dye-doped liquid crystal,” Opt. Lett. 12, 060003 (2014). [CrossRef]
12. T. Li, Y. Xiang, Y. K. Liu, J. Wang, and S. L. Yang, “Transient reorientation of a doped liquid crystal system under a short laser pulse,” Phys. Lett. 26, 086108 (2009).
13. H. F. Yu, “Recent advances in photoresponsive liquid-crystalline polymers containing azobenzene chromophores,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2(17), 3047–3054 (2014). [CrossRef]
14. Y. J. Liu, Y. C. Su, Y. J. Hsu, and V. K. S. Hsiao, “Light-induced spectral shifting generated from azo-dye doped holographic 2D gratings,” J. Mater. Chem. 22(28), 14191–14195 (2012). [CrossRef]
15. Y. Wen, Z. Zheng, H. Wang, and D. Shen, “Photoinduced phase transition behaviours of the liquid crystal blue phase doped with azobenzene bent-shaped molecules,” Liq. Cryst. 39(4), 509–514 (2012). [CrossRef]
16. Z. Zheng, B. Liu, L. Zhou, W. Wang, W. Hu, and D. Shen, “Wide tunable lasing in photoresponsive chiral liquid crystal emulsion,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(11), 2462–2470 (2015). [CrossRef]
17. J. Guo, H. Wu, F. J. Chen, L. P. Zhang, W. L. He, H. Yang, and J. Wei, “Fabrication of multi-pitched photonic structure in cholesteric liquid crystals based on a polymer template with helical structure,” J. Mater. Chem. 20(20), 4094–4102 (2010). [CrossRef]
18. H. C. Jau, T. H. Lin, Y. Y. Chen, C. W. Chen, J. H. Liu, and A. Y. G. Fuh, “Direction switching and beam steering of cholesteric liquid crystal gratings,” Appl. Phys. Lett. 100(13), 131909 (2012). [CrossRef]
19. E. J. Kim, H. R. Yang, S. J. Lee, G. Y. Kim, and C. H. Kwak, “Orientational photorefractive holograms in porphyrin:Zn-doped nematic liquid crystals,” Opt. Express 16(22), 17329–17341 (2008). [CrossRef] [PubMed]
20. Y. Xiang, T. Li, L. Jie, M. Li, and L. P. Shi, “Optical-induced Freedericksz transition of nematic liquid crystal doped with porphyrin (ZnTPP),” Phys. Lett. A 357(2), 159–162 (2006). [CrossRef]
21. V. Yu. Reshestnyak, I. P. Pinkevych, G. Cook, D. R. Evans, and T. J. Sluckin, “Two-Beam Energy Exchange in a Hybrid Photorefractive Inorganic-Cholesteric Cell,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 560(1), 8–22 (2012). [CrossRef]
22. A. Anczykowska, S. Bartkiewicz, M. Nyk, and J. Mysliwiec, “Study of semiconductor quantum dots influence on photorefractivity of liquid crystals,” Appl. Phys. Lett. 101(10), 101107 (2012). [CrossRef]
23. Y. Xiang, Y. K. Liu, Z. Y. Zhang, H. J. You, T. Xia, E. Wang, and Z. D. Cheng, “Observation of the photorefractive effects in bent-core liquid crystals,” Opt. Express 21(3), 3434–3444 (2013). [CrossRef] [PubMed]
24. J. Zhang, V. Ostroverkhov, K. D. Singer, V. Reshetnyak, and Y. Reznikov, “Electrically controlled surface diffraction gratings in nematic liquid crystals,” Opt. Lett. 25(6), 414–416 (2000). [CrossRef] [PubMed]
25. P. P. Korneychuk, O. G. Tereshchenko, Y. A. Reznikov, V. Y. Reshetnyak, and K. D. Singer, “Hidden surface photorefractive gratings in a nematic-liquid crystal cell in the absence of a deposited alignment layer,” JOSA B 23(6), 1007–1011 (2006). [CrossRef]
26. Y. Xiang, Y. K. Liu, T. Li, S. L. Yang, and Z. J. Jiang, “Laser induced gratings enhanced by surface-charge mediated electric field in doped nematic liquid crystals,” J. Appl. Phys. 104(6), 063107 (2008). [CrossRef]
27. I. C. Khoo, K. Chen, and Y. Zhang Williams, “Orientational photorefractive effect in undoped and cdse nanorods-doped nematic liquid crystal—bulk and interface contributions,” IEEE J. Sel. Top. Quant. 12(3), 443–450 (2006). [CrossRef]
28. K. T. Cheng, C. K. Liu, C. L. Ting, and A. Y.-G. Fuh, “Optical addressing in dye-doped cholesteric liquid crystals,” Opt. Commun. 281(20), 5133–5139 (2008). [CrossRef]
29. L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag, Berlin, 1994).
30. Z. Zheng, D. Zhang, X. Lin, G. Zhu, W. Hu, D. Shen, and Y. Lu, “Bistable state in polymer stabilized blue phase liquid crystal,” Opt. Mater. Express 2(10), 1353–1358 (2012). [CrossRef]
