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We present a new observation of photorefractive (PR) effects in bent-core nematic (BCN) liquid crystal (LC) materials, where two kinds of optical-induced gratings are demonstrated and compared in pure and surface-doped BCN systems. The experimental results showed that these two kinds of gratings exhibit distinctive different polarization-dependent and angular-dependent behaviors, respectively. Furthermore, we supplied the pure and surface-doped rodlike LC systems for comparison, which revealed that V shape molecular structure of BCN can produce charge carrier more efficiently than rodlike molecular structure does. Thus BCN materials can offer an exciting potential for optical information processing.
©2013 Optical Society of America
1. Introduction
Liquid crystal (LC) is one of the best-known classes of functional materials. Besides the applications in displays, they are also used to manipulate light such as beam shaping or steering, switchable holograms and adaptive optics. This field is becoming increasingly important for future information processing.
The manipulation of light using LC is usually associated with the interaction between light and molecules [1,2]. Excited by a light field, the reoriented LC molecules produce a large birefringence change and induce a nonlinear optical effect. However, the needed exciting light intensity is rather high. For practical application, a low optical field is highly desirable. Thus LC with dopants have drawn much attention because the optical field threshold for nonlinear optics can be drastically reduced [3–7].
Among the various doped systems, the orientational photorefractive (PR) effect is most commonly encountered that gives rise to the large optical nonlinearity [8–14]. PR effects in LC have been observed with applied dc voltage in the order of 1V and optical power in the microwatt to milliwatt range. So far, the bulk- and surface-PR effects in traditional rodlike LC are thoroughly investigated, and described well by the following models. For the bulk PR effect, optical-induced charge carrier diffusion models [15–18] were introduced by Rudenko, Sukhov, Tabiryan, Umeton, Pagliusi, Cipparrone and Khoo, where charge carriers are generated by dopants or impurities under the illumination of the writing beams, then drift and diffusion to form a space-charge field, producing a phase grating corresponding to the light intensity pattern. In addition, this Kerr-like optical nonlinearity associated with the director reorientation can be enhanced by an external dc electric field. For the surface PR effect, surface-charge distribution models were usually applicable [19–21]. Under the illumination of the writing beams, charge carriers are generated and distributed near the LC-polymer interface, and the resulting surface electric field induces a reorientation of the LC film. The main difference between bulk- and surface-PR effect is the different charge distribution: in the former case, a charge density is modulated in the bulk of the film, while in the latter case, modulated surface-charges are produced near the interface.
Previous investigations on PR effects were limited to rodlike LC system. In order to achieve strong PR effects, dopants in bulk or surface of LC are essential, which make the systems unstable and light absorptive. Therefore novel pure LC are highly desirable to make system stable and transparency. Recently, a new kind of bent-core molecules (BCN) LC was invented [22–24], and have generated considerable excitement because of their potential to provide new functional capabilities. The BCN has a V-shape molecular structure, which is made up of one bent central unit and two calamitic wings. Due to its peculiar molecular structures, BCN are expected to induce some new PR effects. However, up to date, observations of the PR effect in BCN materials are scarce. Here we observed strong PR effects in both pure BCN and surface-doped BCN system. Especially, we examined the temperature effect on the behaviors of PR effects in BCN systems.
2. Experiment arrangement
Two kinds of LCs were used in our study. One was BCN material 9P-CF2O-OPBP[2,5-bis (4-((4-nonylphenyl) difluoromethoxy) phenyl)-1, 3, 4-oxadiazole], whose molecular structure and phase transition sequence are shown in Fig. 1(a) ; the other was rodlike material 5CB, as shown in Fig. 1(b). Two kinds of cell configurations, planar cell and surface-doped planar cell (both are 9μm thickness) were exploited, and the corresponding schematic diagrams are shown in Figs. 1(c) and 1(d). The planar cell comprises of two ITO glass substrates with a 9μm spacer in between, and these two ITO glass substrates were coated with polyimide (PI) and rubbed unidirectionally to obtain planar alignment. Besides, in order to prevent the occurrence of charge carriers injection, a passivation layer of SiO2 is placed over ITO electrodes. The rubbing direction is defined to be n0 along the X-axis, and the Z-axis is chosen as the direction of the applied electric field Edc, perpendicular to the device substrates. The surface-doped planar cell was identical to the planar cell, except that 0.05wt % nanorod ZnO was doped in one of PI alignment layers. The nanorod ZnO is a semiconductor with anisotropic geometry, whose length and diameter were approximately 180 nm and 20 nm, respectively.
Fig. 1 (a) and (b) are molecular structures and phase transition sequences of BCN and rodlike materials, respectively; (c) and (d) are schematic diagrams of planar cell and surface-doped planar cell, respectively.
Four samples were studied in our experiments: Sample_A (pure BCN system), where BCN material was contained inside planar cell; Sample_B (surface-doped BCN system), where BCN material was contained inside surface-doped planar cell; Sample_C (pure rodlike system), where 5CB was contained inside planar cell; Sample_D (surface-doped rodlike system), where 5CB was contained inside surface-doped planar cell.
The photoelectric characteristic measurements of the samples were carried out at different temperatures (T) across the nematic phase. The temperature of the samples was controlled using a heating stage of ± 0.5þC accuracy. A holographic optic setup was constructed to demonstrate the PR effects, as shown in Fig. 2 . A cw linearly polarized laser beam at 532nm, with the polarization E, was split into two writing beams I1 and I2 with equal intensities, which were combined to overlap on the LC film with a spot diameter of 2 mm. With cross angle θ, the two writing beams produced a light intensity grating with periodicity Λ(θ) (Here we choose as an example to demonstrate the diffraction effect). The bisector of the two writing beams was tilted at an angle α with respect to the normal of cell surface. The unperturbed LC director n0 was parallel to the cell surfaces and in the plane of incidence. A weak probe beam I0 at 633 nm along bisector of writing beams was diffracted by the optical-induced phase grating (PR effect) of samples. The dynamic and static diffraction characteristics were recorded by a digitizing storage oscilloscope and light meter, respectively. And the diffraction efficiency η is defined as a ratio of intensity of 1st order diffraction to the intensity of incident probe beam.
3. Experiment results
The behaviors of PR effects in four samples were significantly different, and exhibit strong polarization-, angular- and dc-dependences:
3.1 PR behaviors of sample_A
With a combined application of dc voltage (Vdc) and writing beams (the intensities are as low as 6 mW/mm2), an intense diffraction can be easily and quickly established when Vdc exceeds a threshold (Vth), which reveals an optical-induced grating (called grating_1) has been produced. However, when the dc field was replaced by ac field, the diffraction disappeared under any ac voltage and light intensity. These contrast results proved that a PR effect was provoked under a dc field. The static PR behaviors of sample_A are demonstrated in Fig. 3 , which reveal the following characteristics of PR effects:
- 3.1.1 The PR effects can be produced in pure BCN system, without bulk or surface doping;
- 3.1.2 The dc voltage range (Vrange) for producing PR effect is rather wide, from several to more than one hundred Volts. On the contrary, as will be shown later, we observe that the corresponding Vrange in rodlike system was rather narrow, only several Volts;
- 3.1.3 Polarization–dependence of PR effect. Curve 1 and 2 in Fig. 3 indicate that when the polarization E of writing beams was perpendicular to the orientation n0 of pure BCN, the diffraction effect is strongest; however, if the E was placed to parallel to n0, the diffraction effect is weakest. Also, the PR dynamics in curve 1 and 2 in Fig. 4 demonstrated that the buildup and decay times were irrespective of the polarization of writing beams;
- 3.1.4 Angular-dependence of PR effect. Curve 1, 2, 3 in Fig. 6 indicate that with a increased incident angle α, the PR effects in pure BCN are enhanced, characterized by increased diffraction efficiency η and decreased Vth. Besides, the PR dynamics with different α are shown in curve 1, 2 in Fig. 7.
It is noted that the η exhibits a wavy behaviors under high voltages (curve 1 in Fig. 3). At the same time, a strong light scattering was observed, characterized by an intensity stray light in the background, which superposed on the PR diffraction pattern. The occurrence of light scattering impairs the diffraction performance and results in a wavy η.
In order to examine the occurrence of light scattering, we checked the dc dependence of BCN texures under polarizing microscope, as shown in Fig. 5 . It is shown that with a increased Vdc, the BCN went through the unperturbed, electroconvection [25, 26] and dynamic scattering stages, which are characterized by different textures and corresponding Vdc. As the light scattering induced by dynamic scattering becomes obvious in a high Vdc regime, different BCN materials are needed and compared to obtain a low Vth, making them available to applications.
Fig. 5 Photomicrograph of BCN texture under various dc voltage, where temperature is 107°C, A and P refer to the analyzer and polarizer, respectively. (a)Vdc = 0, unperturbed stage; (b) Vdc = 25, onset of electroconvection stage; (c) Vdc = 30, strong electroconvection stage; (d) Vdc = 40, dynamic scattering stage.
3.2 PR behaviors of sample_B
Likewise, a diffraction can be also observed in sample_B. However, the grating (called grating_2) of sample_B differs from grating_1 of sample_A in polarization- and angular-dependence:
- 3.2.1 Curve 3 and 4 in Fig. 3 indicate that polarization-dependence of grating_2 was inverse to that of grating_1. Grating_2 produces the weakest (or strongest) diffraction effect when E was perpendicular (or parallel) to n0;
- 3.2.2 Curve 4, 5 and 6 in Fig. 6 confirm that grating_2 also possesses angular-dependence, i.e., with the increased α, the corresponding diffraction effect was weaken, which exhibited a reverse trend to that of grating_1. For the dynamic process, curve 3 and 4 in Fig. 7 indicate that a smaller α will induce a quicker buildup process, whereas the decay process was independent of α.
3.3 PR behaviors of sample_C
In order to examine the underlying mechanism responsible for the PR effect in sample_A, we compared it with sample_C. These two systems are both pure LC systems, however differ in molecular structures. The experimental results demonstrated that no matter how strong the dc field and writing beams were, there is no diffraction observed from sample_C. This comparison demonstrated the decisive role of molecular structure in the production of PR effects.
3.4 PR behaviors of sample_D
In this case, a diffraction appears with the polarization- and angular-dependences similar to that in sample_B, see curve 5, 6 in Fig. 3 and curve 7, 8, 9 in Fig. 6. Since these two samples were both surface-doped system, together with the fact that no diffraction presents in sample_C, it is straightforward to infer that the origin of the PR effect in surface-doped systems were ascribed to the inclusion of nano dopant.
Despite of the above similarities, the dependences of PR effect on dc field in Fig. 3 and 6 demonstrate that these two surface-doped systems differ in terms of Vth and Vrange. In Fig. 3, the curve 4 and 6 indicate that the sample_D possesses a low Vth, and a narrow Vrange in the order of several Volt, whereas the sample_B possesses a high Vth, and a much wider Vrange, more than 100 Volt; then we compare the curve 4 for the sample_B with curve 1 for the sample_A, we can observe that these two BCN systems exhibit a similar Vth and Vrange. Therefore, it is reasonable to conclude that the high Vth and wide Vrange should correlate with the molecular structure of BCN. This unusual dc dependence is a distinctive characteristics of PR effects in BCN system, which was not observed before.
3.5 Temperature dependence of PR effect in pure BCN system
Traditionally, PR effects of rodlike LC system were measured at room temperature, rarely extended to the high temperature. However, due to the fact that nematic phase of BCN system falls inside high temperature range, it is highly desirable to perform the investigation at high temperature. The diffraction buildup and decay processes at different temperature are shown in Fig. 8 . With the increasing temperature, the buildup times decrease quickly; however, the decay times were almost unchanged. In general, PR effects in pure BCN system monotonically become weak as the temperature increases towards the clearing point Tc.
3.6 Photocurrent measurement in sample_A
In order to clarified the origination of PR effect in pure BCN material, we carried out a photocurrent measurement in sample_A. The experimental results are shown in the Fig. 9 , where the photocurrents closely correlate with the polarization E of writing beam and strength of Vdc: the productivity of photoelectron of pure BCN material is much stronger in the condition of E⊥n0 than that in the condition of E//n0; in addition, the productivity grows significantly with the enhanced Vdc. These two distinctive aspects are consistent with the experiment phenomena (curve 1 and 2 in Fig. 3), and provide the direct evidence for charge diffusion driven photorefractive effect.
Fig. 9 Polarization–dependence of photocurrents vary with different dc voltage in sample_A, where the currents are measured under uniform illumination of one writing beam.
4. Discussions
According to the characteristics of the above observed PR effects of the four kinds samples, two kinds of gratings are induced, which are characterized by the completely opposite polarization- and angular-dependences. Grating_1 only presents in pure BCN system, whereas grating_2 presents in both surface-doped BCN system and surface-doped rodlike system. We speculate that the formations of grating_1 and 2 are due to the bulk or surface PR effect, respectively. Since the space-charge fields is the key to understand the characteristics of gratings, we should clarify the origination of space-charge.
4.1 Bulk effect
The facts that grating_1 presents only in the pure BCN system whereas absents in the pure rodlike system proved that the occurrence of photoelectrons and the resulting space-charge field in pure system is determined by the molecular structure. Namely, the contrast results demonstrated that V shape molecular structure has much superior productivity of photoelectron than that of rodlike structure. The polarization dependent PR effects and photocurrents further confirmed that in the pure BCN system, this productivity of photoelectron is anisotropic and correlate with V shape structure. The role of BCN molecular structure in producing photoelectron can be clarified in the following two aspects:
- 4.1.1 In BCN system with a V shape molecular structure, due to the oxadiazole core, a strong lateral dipole presents and locates at the centre of the molecular structure [27]. The presence of lateral dipole is demonstrated by the polarization-dependent PR effects, where the strongest diffraction effect appears when E⊥n0;
- 4.1.2 In addition, both the electron donator and acceptor groups present in the arms of V shape molecular structure. They are linked by conjugated electronic bridges and aromatic rings to facilitate the charge transference.
These two aspects in BCN molecular structure give rise to produce photoelectrons efficiently, without the helps of dopants. By establishing links between micro molecular structure and macro bulk properties, we can help the chemists to gain insights into how to create molecules with optimized properties.
Besides, the reorientations of LC director play a decisive role in the grating formation, and exhibit rather difference according to low or high voltages: at low voltages, due to the negative dielectric anisotropy of BCN material and condition of in planar cell, BCN molecules will be forced by Edc to retain in the unperturbed n0 without any deformations; whereas at higher voltages, the space-charge field Esc grows accordingly. Both Edc and Esc exert dielectric torques on the BCN molecules, resulting in a periodic bend deformation in the incident plane. The reorientation plane of director can be confirmed by means of changing the polarization of probe beam, where if the polarization is in (or perpendicular to) the incident plane, an evident (or weak) diffraction of probe beam can be observed.
4.2 Surface effect
However, in surface-doped systems, photoelectrons from the nano dopants were released and distributed in the surface according the light intensity pattern, and the resulting surface space-charge field changes the anchoring condition of neighboring BCN molecules, thereby produces grating_2 [28–30]. This surface space-charge induced PR effect is supported by the polarization-dependent characteristics. Since the long-axis direction of nano-rod dopants follows the orientation n0 of BCN molecules, and only the light polarization E parallel to the long-axis direction of nano-rod dopants can give rise to maximum photoelectrons (due to the anisotropic nano-structure of dopant) [31–36], it is reasonable that strongest diffraction effect appears when E//n0 (curve 4, Fig. 3).
Here we should reiterate that the diffraction efficiency η in pure or surface doped cell strongly depends on the conditions of polarization E and incident angle α. In the case of pure cell, η of bulk effect exhibits a maximum when E⊥n0 and α = 45°, and a minimum (almost zero) when E//n0 and α = 0°; however, in the case of surface doped cell, when E//n0 and α = 0°, η exhibits a maximum. Considering that at this moment, the contribution of bulk effect is negligible, the PR effect induced by surface effect plays a leading role.
Consequently, based on the two different geometries (pure and surface doped cells), we could understand the different PR mechanisms by comparing their own behaviors: in the case of pure cell, since only has the BCN material, the pronounced PR effect is mainly ascribed to the BCN material (a bulk effect); whereas in the case of surface doped cell, under the condition of E//n0 and α = 0°, the dopant dominates the PR effect of the system and supplies the photoelectrons in the surface of the cell (a surface effect), and the bulk BCN film only acts as a modulated and birefringent medium to produce refractive grating.
5. Conclusion
In conclusion, we have found that the characteristics of PR effects from a new LC system (BCN) are very different from that of traditional rodlike LC system. Various dependences, such as temperature, incident angle, polarization of writing beams and dc strength, on the PR effect have been preliminarily examined in this work. The measurements show that the PR effect can be obtained even in pure BCN system, in contrast to the rodlike system, where dopant must be present.
Based on the investigation of PR effect in BCN system, we showed the feasibility of use the BCN materials as a real-time optical information device. Due to the broadband birefringence and transparency throughout the entire visible spectrum, BCN materials are promising candidates for replacing the existing doped rodlike system. We hope our ground breaking work will start a new branch of PR effect field in liquid crystal materials.
Acknowledgments
Authors are indebted to Professor J. Y. Zhou for numerous useful discussions. This work is supported by the National Natural Science Foundation of China (11074054,10674033).
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