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Nonlinear optics (NLO) has attracted much attention for its immense potential in applications. However, it typically requires very high light intensities. Recently, we reported that polymer stabilization efficiently enhances the self-focusing effect for dye-doped liquid crystals (LCs). Here we studied the effect of polymer concentration on self-focusing in the dye-doped polymer-stabilized LC (PSLC). Oligothiophene (TR5)-doped PSLCs with different polymer concentrations were prepared, and self-diffraction ring measurement was performed. As polymer concentration increased, the threshold light intensity for self-focusing effect first decreased then slightly increased. The lowest threshold light intensity of 3.4 W/cm2 was obtained at 10 mol%. Polymer stabilization of the LCs decreased the threshold intensity by a factor of 7. We also discussed the possible mechanism for the polymer concentration dependence of NLO response in the TR5-doped PSLC.
© 2015 Optical Society of America
1. Introduction
Liquid crystal (LC) technology has had a major impact on many areas of science and engineering, as well as device technology [1, 2]. Variety of LC based applications have been realized thanks to recent progress [3, 4], such as, the most common application LC display (LCD) [5],optical imaging [6], LC thermometer [7]. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems. In the past decades, optical sensitive LC materials have shown high potential as promising solutions for many problems and novel directions for innovative applications due to their unique properties, such as, fast response, high efficiency, remotely controllability, as well as large optical nonlinearity [8–11].
Nonlinear optics (NLO) of LCs describes the behavior of light in LCs, in which the dielectric polarization density of LCs responds nonlinearly to the electric field of the light [12, 13]. LCs has been well recognized as highly efficient NLO materials due to its giant optical nonlinearity [9, 14]. Molecular reorientation of LCs takes place only when the light intensity is above a threshold light intensity. Continuous efforts have been made on designing novel LC materials with larger optical nonlinearity in order to lower the threshold light intensity. In 1990, Jánossy et al. reported that a small amount of anthraquinone dichroic dye doped into nematic LCs could greatly enhance the nonlinearity by two orders [15]. The accepted mechanism of the dye-enhanced reorientation is selective photoexcitation of the dye molecules, followed by a consequent large change in guest-host interactions by photoexcited dye molecules [16, 17]. Since then, lots of attention was paid on dye-induced nonlinearity enhancement in LCs [18–21]. So far azobenzene dye molecules have been most extensively studied and shown the highest efficiency for LC nonlinearity enhancement [22–24]. On one hand, photoisomerization of azobenzene can take place easily in the system upon photoirradiation. On the other hand, novel azobenzene related material systems have been explored based on not only chemical structure but also initial molecular alignment. Hence, so far molecular reorientation in azobenzene-doped LCs can be driven with the lowest light intensity.
One unique effect of NLO responses in LCs is self-focusing effect. Molecular reorientation is brought about by a single Gaussian beam, and the LC material itself acts as a lens above a threshold light intensity. It allows us to control the transmitted beam profile by simply changing the light intensity above or below the threshold intensity. Combined with an iris, transmittance can also be tuned by the self-focusing effect. In addition, the NLO molecular reorientation of LC is an order-to-order process (molecular director turns from one direction to another), which means that the optical manipulation of LC can be precisely controlled [25]. However, despite those attractive advantages from the viewpoint of optical materials, the self-focusing effect seems still less noticed because of high threshold light intensity.
In 2000, a non-photoisomerizable oligomer-type thiophene derivative (TR5) was designed and subsequently doped into LCs to evaluate the self-focusing effect [26]. It effectively decreased the threshold light intensity of photoinduced molecular reorientation by 135 times for polar 5CB and 180 times for non-polar MBBA, respectively.
Recently, we prepared a TR5-doped polymer stabilized liquid crystal (PSLC) [27]. Polymer stabilization refers to addition of a small amount of polymerizable units into the anisotropic LC matrix, and subsequent photopolymerization. This design is based on the fact that PSLC bears several advantages compared to conventional low-molecular-weight LCs, for instance, stabilization of LC blue phase [28], faster response toward light [29] and enhancement of performance of cholesteric LC-based reflective devices [30]. The results illuminated that not only the threshold light intensity was reduced by a factor of 6 but also the refractive-index change was increased for 56%. Samples became thermally more stable through polymer stabilization. Three possible mechanisms were proposed for the enhancement of the optical nonlinearity in such TR5-doped PSLC system: (1) the optical torque was enhanced due to the enhancement of guest-host interaction between photoexcited dye molecules and the PSLCs; (2) the molecular alignment was disordered by the polymer, which increased absorbance and increased the optical torque needed for photoreorientation; (3) the director torque of the molecules was decreased in PSLC system because of the decreasing of surface anchoring. Therefore, polymer stabilization of LCs showed promising direction for the significant enhancement of the self-focusing effect.
In this study, we investigated the effect of polymer concentration on the self-focusing effect in the homeotropic-aligned dye-doped PSLC (Fig. 1).TR5-doped polymer-stabilized 5CB was employed for the evaluation. Films having various polymer concentrations were prepared by varying the A4CB monomer concentration and subsequently photopolymerizing the monomers. Main results are expected not only to optimize the composition of the material but also shed some light on understanding the mechanisms of the nonlinearity enhancement in dye-doped PSLC.
2. Experiment
2.1 Materials
Chemical structures of the compounds used in this study are shown in Fig. 2. 4-[4-(4’-Cyanobiphenyl)oxy]butyl acrylate (A4CB) was synthesized according to a method similar to a previous report [31] and used as the monomer for photopolymerization. 5,5”-Bis-(5-butyl-2-thienylethynyl)-2,2’:5′,2”-terthiophene (TR5) was synthesized [19] and used as an effective dye molecule. 4-Cyano-4’-pentyl biphenyl (5CB) was purchased from Merck Ltd. and employed as the nematic host LC without further purification. Irgacure 651 was obtained from Nagase & Co., Ltd. and served as a photoinitiator for photopolymerization without further purification.
2.2 Fabrication of films
First, nematic 5CB and A4CB monomer were mixed at different molar ratios (A4CB percentages: 0, 2, 4, 6, 7, 8, 9, 10, 11, 13 mol%) as hosts. Then TR5 and Irgacure 651 were doped into the host at the concentration of 0.1 mol% and 0.5 mol%, respectively. After fully stirring, the mixtures were injected into a 100-μm thick glass cell. The inner surfaces of the substrates were treated with octadecyltrimethoxysilane as a silane coupler to obtain homeotropic LC molecular alignment. Photopolymerization was conducted under irradiation from a high-pressure mercury lamp with a wavelength of 366 nm (1 mW/cm2, 60 min). Finally, the sample was heated at an isotropic temperature, and then slowly cooled to room temperature.
2.3 Characterization
The molecular alignment was examined with a polarized optical microscope (POM, Olympus, BX50) equipped with a hot stage. We observed orthoscopic and conoscopic images of the glass cell after photopolymerization to confirm the homeotropic molecular alignment at room temperature. To examine phase transition behavior of LCs, the sample was taken out from the cell, put on an untreated glass and covered with a cover glass. We observed orthoscopic images of the sample in the heating and cooling processes to determine the phase transition behavior. Polarized absorption spectra of the films were measured using a UV-Vis spectrometer (JASCO, V-650st).
2.4 Evaluation of self-focusing effect
The threshold light intensity to show self-focusing effect was evaluated by self-diffraction ring measurements. The optical setup is illustrated in Fig. 3.A spatially filtered, vertically polarized Ar+ laser beam (488 nm, Spectra Physics, BeamLok, 2060ZGB) with a Gaussian intensity distribution was focused onto the sample cell at normal incidence. The diffraction ring pattern due to self-phase modulation was observed on a screen placed behind the sample, and the number of rings was used to indicate the refractive index modulation at different light intensities. The threshold intensity, at which the first diffraction ring formed, was determined with a beam profiler (Ophir, BeamStarFX-50). The time needed for the stable diffraction-ring formation strongly depended on the light intensity and became slower at lower intensities. Hence, we waited for several minutes at relatively low intensities before counting the number of the rings.
Fig. 3 Top view of the optical setup for self-diffraction ring measurement. M, mirror; ND, neutral density filter; L1, plane-convex lens; PH, pinhole; GP, Glan-Thompson prism; L2, biconvex lens.
3. Results and discussion
3.1 Characterization
Homeotropic-aligned cells of TR5-doped PSLC with polymer concentrations from 0 to 13 mol% were successfully obtained. For the convenience of description, the fabricated cells are denoted as P-X, where X represents the polymer concentration; e.g. P-0 represents the sample of TR5-doped 5CB with no polymer in the system. All the obtained LC cells at polymer concentrations from 0 to 13 mol% exhibited optical transparency and uniform homeotropic alignment over the area, as provided in Fig. 4(a) as an example.The cells having polymer concentrations at more than 13 mol% became opaque due to a phase separation. LC molecular alignments in the cells were confirmed to be homeotropic by POM and UV-Vis spectrometry. The results of P-10 are provided in Fig. 4(b) and Fig. 4(c) as a representative. UV-Vis spectra revealed that all the samples had similar profiles independent from the polymer concentration (Fig. 5).All TR5-doped PSLCs showed a blue shift of the maximal absorption wavelength by about 10 nm compared with TR5-doped 5CB with no polymer. Although the profile of adsorption spectra slightly changed, the absorbance around 488 nm remained unchanged, which means that this wavelength can be used for investigation of the molecular reorientation behavior.
Fig. 4 (a) Photograph of P-10. (b) Conoscopic (left) and orthoscopic (right) polarized optical micrographs of P-10. (c) Polarized absorption spectra of P-10. The two spectra of orthogonal polarizations are well overlapped, which indicates that the molecules are homeotropic-aligned in the cell.
Phase transition behavior of the samples was observed with a POM equipped with a hot stage (Fig. 6).Samples were taken out from the glass cell, put on an untreated glass and covered with a cover glass for the evaluation. The samples were heated from 25 °C to 70 °C, then cooled to 25 °C at a rate of 2 °C/min. The clearing temperature of P-0 was 34 °C, which was identical to that of pure 5CB. However, in the TR5-doped PSLC, the phase transition temperature range became broader. As shown in Fig. 6, by heating the PSLC samples, some region changed from anisotropic to isotropic around 34 °C, the other region was stabilized by polymer and remained anisotropic. The clearing temperature, at which LC-isotropic phase transition completes over the whole area, was raised up to 60 °C. It indicates that TR5-doped LCs were thermally stabilized by polymer stabilization in the host, and the degree of the stabilization was enhanced as the polymer concentration increased.
In order to estimate the order parameter of TR5-doped PSLC molecules, homogenous cells with a thickness of 20 µm were prepared, and their polarized UV-Vis spectra were recorded (Fig. 7).Order parameter of the TR5 molecules were calculated according to the following equation [32]:
where A// and A⊥ are the polarized absorbances parallel and perpendicular to the rubbing direction, respectively. The maximum absorbance was used for evaluation of the order parameter at 437 nm for P-0 and at 430-432 nm for P-2 to P-13 due to blue shift. The results clearly indicated that polymer stabilization in the host had a significant influence on the order parameter of the guest molecules. As polymer concentrations increased from 0 to 13 mol%, the order parameter first decreased then slightly increased. The smallest order parameter in TR5-doped PSLC was obtained at the polymer concentration around 9 mol%. According to this result, the initial molecular alignment was disordered through polymer stabilization of the host, which is in favor of increase in the dye excitation probability and the subsequent increase of optical torque. As a result, the light intensity needed to start photoinduced reorientation can be reduced in samples with lower order parameter. It has been proved that the surface anchoring energy can be significantly decreased by the photopolymerization shielding the surface potential [33], which leads to a lower order parameter. Slight increase in the order parameter at higher polymer concentration might be due to immobilization of the guest and host molecules by photopolymerization, which suppresses the disorder. Further investigation is required to clarify this point.All the samples had homeotropic alignments and similar absorbances at 488 nm. Dye-doped PSLCs with higher polymer concentration showed higher clearing temperatures and disordered molecular alignment below 10 mol%.
3.2 Self-focusing effect
Typical diffraction rings observed on the screen are presented in Fig. 8 and systematic measurements of number of diffraction rings as a function of light intensity are given in Fig. 9(a).Obviously, polymer concentration in the host had a great influence on the threshold intensity of exhibiting the self-focusing effect of the TR5-doped PSLC. The threshold intensity and the maximum number of diffraction rings as a function of polymer concentration are summarized in Fig. 9(b). The threshold intensity was first decreased as the polymer concentration increased, then reached a minimal value. Further increase of the polymer concentration rather increased the threshold intensity. The lowest threshold intensity (3.4 W/cm2) was obtained at 10 mol%, at which the threshold was reduced by a factor of 7 compared to that of TR5-doped 5CB with no polymer (23.5 W/cm2).
Fig. 9 (a) Number of diffraction rings as a function of light intensity in P-X with different polymer concentrations in dye-doped PSLCs. (b) Threshold intensity (black) and maximum number of rings (red) as a function of polymer concentration.
Furthermore, the maximum number of diffraction rings first increased then slightly decreased as polymer concentration increased. The largest maximum number of diffraction rings (29) was observed in P-10, which was increased by 38% compared to that of TR5-doped 5CB with no polymer (21). The number of rings N relates to photoinduced refractive-index change Δn through ∣Δn∣ = NλL−1 (λ: wavelength; L: cell thickness) [34]. As λ and L are constant here, larger number of rings means larger Δn. This indicates that the TR5-doped PSLC with 10 mol% of polymer showed the largest refractive-index change. Estimated change in refractive index was found to be 0.14 in P-10.
Finally, we would like to discuss the possible mechanism related to the polymer concentration dependence of self-focusing threshold intensity in TR5-doped PSLC. First, it is important to understand the mechanism for the strong dye-induced nonlinearity enhancement in dye-doped LCs. So far, the well accepted physical mechanism is as follows [16, 17]: generally, the response of LCs to the optical field is generated by the effect of a molecular director n and optical torque τo (here, τo equals to electric torque τem) induced by optical electric field. Photoinduced molecular reorientation occurs when τo is strong enough to change the molecular director n toward the electric field direction. In dye-doped LCs, additional dye torque τdye contributes to τo (τo = τem + τdye). Dye molecules parallel to the electric field of the polarized light can be photoexcited, while others remain in the ground state. Because of the stronger interaction between photoexcited dye molecules and LC host, molecular reorientation towards the polarization direction of light happens much easier when τo is strong enough to change the LC molecular director n. Thanks to the “giant” τdye the photoinduced reorientation can be triggered with a much lower light intensity. Hence the nonlinearity of dye-doped LCs is greatly enhanced. The molecular director rotation is opposed to the surface anchoring strength and the bulk elasticity. On the other hand, the dye torque τdye is affected by the population of dyes in the photoexcited state and the interaction force between photoexcited dye molecules and the LC host.
According to the characterization discussed above, slight disorder of TR5 molecules was observed in dye-doped PSLCs. The order parameter first decreased then slightly increased as polymer concentration increased and the lowest order parameter was obtained in P-10. This tendency agrees very well with threshold intensity of showing the self-focusing effect. Therefore, what caused the change of order parameter must be related to the polymer concentration dependence of self-focusing effect. As we discussed earlier, increasing of polymer concentration could result in lower surface anchoring strength [33], which leads to a low molecular alignment strength (sa_anchoring). On the other hand, addition of polymer also stabilizes the molecular alignment of LCs [35], which leads to a high molecular alignment strength (sa_stabilization). In addition, the disorder of TR5 molecules can give rise to larger population of photoexcited TR5 upon light irradiation, which increases the total dye torque τdye (τdye_population). Moreover, the interaction force between photoexcited dye molecules and PSLC might affect the dye torque τdye (τdye_interaction).
As a result, there are four factors that affect the optical reorientation behaviour in the material system: sa_anchoring, sa_stabilization, τdye_population, and τdye_interaction. Among them, sa_anchoring and sa_stabilization are determined by the property of the film itself while τdye_population and τdye_interaction can be affected not only by the film property but also the incident light intensity.
Here, we propose a possible mechanism for the optical reorientation behavior. As polymer concentration increases: (a) sa_anchoring decreases, which is a positive contribution to the threshold intensity reduction; (b) sa_stabilization increases which leads to a negative contribution; (c) τe_population first increases then slightly decreases causing a positive contribution followed by a small negative contribution; (d) the change of τdye_interaction might occur. A comprehensive combination of those four factors resulted in the polymer concentration dependence of self-focusing effect in TR5-doped PSLC as shown in Fig. 9. The lowest threshold intensity obtained at 10 mol % was obtained by the balance of those four factors.
4. Conclusion
In summary, homeotropic-aligned TR5-doped PSLC with various polymer concentrations were fabricated. The polymer concentration dependence of the self-focusing effect was evaluated through self-diffraction ring measurements. Upon irradiation of the films with a 488 nm Ar+ laser beam, diffraction rings were clearly observed on the screen. According to the results of the number of diffraction rings as a function of light intensity, the threshold intensity first decreased then slightly increased as polymer concentration increased. In addition, the maximum number of diffraction rings first increased then slightly decreased as polymer concentration increased. The PSLC cell at polymer concentration of 10 mol% gave rise to the lowest threshold intensity and the largest refractive-index change for self-focusing effect. A detailed mechanism for the resulting phenomenon was discussed by a comprehensive effect of molecular alignment strength (sa_anchoring and sa_stabilization) and dye-contributed torque (τdye_population and τdye_interaction). The highest efficiency for the self-focusing effect in TR5-doped PSLC can be obtained by the best balance of those four factors.
Acknowledgments
This work was supported by Precursory Research for Embryonic Science and Technology (PRESTO), “Molecular Technology and Creation of New Functions”, Japan Science and Technology Agency (JST).
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