Open Access
Add to BibSonomy
Abstract
Two different types of azo dye-doped liquid crystal mixtures were separately nano-confined and characterized and then dispersed in the same host matrix to enhance the wavelength tunability of the laser-induced transparency of the fabricated polymeric thin film nanocomposites. The obtained results indicated that the transmitted intensity can be controlled separately by applying dual-wavelength pump lasers irradiation based-on reorientation of liquid crystal mesogens followed by trans-cis photoisomerization of the different azo dye dopants inside the core of the polymeric nanocapsules. Since the fabricated thin films qualify the demand for improvement of laser-induced tunability, it is feasible to use the introduced nanocomposite as a smart light-responsive thin film layer that can be widely implemented in all-optical tunable nanophotonics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced reorientation of liquid crystal (LC) mesogens in the presence of trans-cis-trans photoisomerization reaction of the azobenzene organic dyes have been a subject of interest to investigate the optically controllable composite materials [1–8] and because of their efficient light-control capabilities, the detailed description of molecular photo-dynamics of the azo dyes and also principles of photoinduced reorientation of dye-doped liquid crystals (DDLCs) have been studied both theoretically and experimentally in numerous literature [9–18]. Furthermore, previous studies demonstrated that the micro-structured dye-doped polymer-dispersed LCs (DD-PDLCs) is capable of acting as tunable laser [19], smart textile [20], and flexible displays [21].
However, utilization of DDLC mixtures in a polymer matrix is limited by few technical challenges like solubility problem and dye diffusion which some dyes are dissolved not only in the liquid crystal but also in the polymer matrix [22], UV absorption of dye molecules which in turn leads to a poor polymerization followed by photo-decomposition of the azo dyes [23,24], lack of precise control of the size and morphology of LC droplets simultaneously with the consideration of different fabrication parameters such as UV-light intensity, curing time and temperature, LC and dye concentration, etc. [25–28] which decrease the fabrication reproducibility of the samples.
In addition, several studies have been published in recent years on LCs nanoscience [29–36] especially on the confined LCs in nanometer-sized droplets for the production of novel nanomaterials such as well-developed nano-structured polymer-dispersed LCs (nano-PDLCs) as an optically isotropic liquid crystal (OILC) nanocomposites which have drawn much attention for use in tunable nanophotonics by reason of their promising applications in various fields including polarization-independent optical shutters [37], random lasing [38], tunable microlens [39,40], flexible displays [41–43], diffractive devices [44,45], optical switches [46], optical phase modulators [47], etc. Nevertheless, in general, it is difficult to fabricate a nano-PDLC with a large LC/polymer weight ratio besides completely desired morphology and also the size distribution of LC nanodroplets [37,48].
Moreover, the light-responsive polymeric nano core/shell structures recently used as an interesting nanocarriers for encapsulation of either drugs in nano-pharmaceutical [49–52], fluorescence agents in nano-bioimaging and nanotheranostics [53,54], food ingredients [55], and especially optical materials [56–62]. Subsequently, incorporation of DDLC mixtures inside the polymeric nanocapsules is even more interesting because it became possible to adjust the fabrication parameters in order to obtain the specific optical behavior of these core/shell nanostructures before dispersing in a host matrix to form a polymeric nanocomposite thin film layer.
Consequently, the use of pre-characterized polymeric nanocapsules (NCs) containing different types of DDLC mixtures can provide a novel solution to overcome the impediments and improve the optical tunability of DDLC-based photonic materials. Therefore, in this work a binary mixture of nano-encapsulated DDLCs dispersed in a host matrix was demonstrated which to the best of our knowledge has not been reported before. Subsequently, the investigation of dual-wavelength tunability of the transmitted optical signal levels was carried out after proper wavelength selection for pump and probe lasers based on the absorbance and extinction measured spectra of the fabricated nanocomposite polymeric thin films.
The combination of different DDLC-NCs as a nanocomposite material with adjustable fabrication and film-forming parameters that provide multi-wavelength tunability of the transmitted intensity can be utilized not only as a light-responsive smart material in nanophotonic devices but also in form of a laser-tunable free-standing polymeric thin film in all-optical applications.
2. Experimental
2.1 Materials
The materials used in this work include Poly (vinyl alcohol) (PVA; Mw = 3.0 × 104 to 7.0 × 104 g.mol-1, 87%-90% hydrolyzed, Sigma-Aldrich Chemical Co.) with excellent film-forming properties and good chemical and thermal stability, was selected to stabilize nanoemulsions and furthermore, as an isotropic matrix for DDLC-NCs. Poly (methyl methacrylate) (PMMA; [Mw] = 15.0 × 103 g.mol-1, Sigma-Aldrich Chemical Co.) which was used as polymeric nanoshell. Disperse Yellow7 (DY7; Mw=316.37 g.mol-1, Sigma-Aldrich Chemical Co) and Disperse Red1 (DR1; Mw=314.35 g.mol-1, Sigma-Aldrich Chemical Co) were used as azobenzene derivative organic dyes (Fig. 1). The nematic liquid crystal E7 (TNI = 60 °C, Δn = 0.22, Merck) as a eutectic commercial mixture of cyanobiphenyl derivatives was used as the main component inside the core of the fabricated nanocapsules. Ethyl acetate (EA, Merck Company) and ultrapure deionized (DI) water were used for the preparation and for the dilution of the nanoemulsions. All chemicals were of analytical grade and they were used as received without further purification.
2.2 Preparation and characterization of nanocapsules
Fabrication procedure and characterization of azo dye-doped nematic liquid crystal-loaded polymeric nanocapsules by using the emulsification solvent diffusion (ESD) method [63], have been completely described and demonstrated in the previous work [56]. Briefly, the organic phase consisted of 0.35 g E7 and 0.45 g PMMA was first dissolved in 25 ml ethyl acetate and then water-insoluble organic dye with 1 wt% of the E7 was added. Then, the solution was added into 100 ml of 0.6 wt% PVA aqueous solution as a continuous phase to obtain an oil-in-water (O/W) emulsion. Subsequently, the obtained emulsion was tip-sonicated for 20 min by using a sonicator (UP400-A, TOP sonics Co.) at 400W. Afterward, another 500 ml of DI water was added to the O/W nanoemulsion in order to initiate the PMMA deposition around the nanodroplets as a polymeric nanoshell leading to the formation of nanocapsules. Then, the prepared homogeneous nanosuspension was rotary evaporated (Laborota 4000; Heidolph Instruments) under reduced pressure to remove volatile solvent and a part of the water. Finally, the resulting aqueous nanosuspension was purified by repetitive microcentrifugation technique (4,500 rpm, 30 min) and then redispersed in DI water.
This nanoencapsulation process was employed to fabricate two kinds of DDLC-NCs with different types of azo disperse organic dyes. The mean diameter and particle size distribution of fabricated nanocapsules with different core materials according to the dynamic light scattering (DLS) test results (NANOPHOX, SympaTEC Co.) is summarized in Table 1.
Table 1. DLS measurement results for different types of DDLC-NCs
The morphology of the obtained DDLC-NCs was investigated by a field-emission scanning electron microscope (FE-SEM; TESCAN Mira3, Czech Republic). FE-SEM images of well-formed spherical shape and surface morphology of the nanocapsules are shown in Fig. 2.
Fig. 2. The morphological observation of purified DDLC-NCs: (a) S1, (b) S2. The insets are the digital photographs of the corresponding concentrated aqueous nanosuspensions prepared by the ESD method.
2.3 DDLC-NCs film forming
In order to obtain homogeneous nanocomposite mixtures, the specified amount of purified nanocapsules were added and stirred at room temperature for 20 min at 100 rpm with an aqueous PVA solution (5 wt%) according to the weight ratios in Table 2.
Table 2. Weight composition of the prepared samples
The resulting homogeneous and bubble-free mixtures were then coated on cleaned UVO-treated substrates by using the blade-coating as a fast film-forming method [64,65] in a dust-free environment and 40 μm-thick flat spacers were used to obtain a suitable film thickness. The schematic of the sample preparation process is shown in Fig. 3.
Fig. 3. Schematic illustration of the fabrication process of the polymeric thin-film and core/shell nanostructure of the DDLC-NCs.
After being coated, the wet films were dried in a vacuum chamber at 25 °C for about 20 min to obtain uniform and smooth thin films. Thereafter, the formed thin films were carefully peeled off at room temperature without any wrinkle or degradation during the detachment process. Afterward, flexible samples were further dried in an oven at 60 °C for 1 hour and finally, the completely dried polymeric thin films were kept in a desiccator for further investigations. Figure 4 depicts the clear appearance of text under the detached films which indicates the transparency of the obtained samples.
Fig. 4. Photographs showing the transparency of the free-standing polymeric thin films containing the DDLC-NCs. The top, middle, and bottom peeled off samples represent the F1, F2, and F3, respectively.
The thickness of the obtained flexible polymeric films was measured at approximately 10 µm based on spectral interferometry technique.
2.4 Spectral measurements
In order to perform spectral investigations of the materials inside the core of obtained nanocapsules, organic dyes (DY7 and DR1) mixtures with 1% wt concentrations in E7 liquid crystal were prepared separately and their absorbance spectra were obtained by using a UV/Vis spectrometer (AvaSpec-2048, Avantes, Netherlands) and using a deuterium-halogen light source (AvaLight-DHS, Avantes, Netherlands) across the 200–700 nm wavelength range. Moreover, UV/Vis spectroscopy technique utilized to determine the extinction spectra of the fabricated nanocomposite polymeric thin films.
2.5 Experimental setup and procedure
The dual-wavelength pump-probe experimental setup for the investigation of photoinduced transparency of the DDLC-NCs thin films is shown in Fig. 5. Two linearly polarized diode lasers (λ=405 nm and λ=532 nm) were employed as excitation laser sources and a He-Ne laser (λ=632.8 nm) was used as the probe beam. The pump and probe beams were combined by dichroic mirrors (M2 and M3 in Fig. 5) and were collimated by using long focal length spherical lenses. In addition, all beams were collinear and incident normally with respect to the surface of the thin films. The beam diameter of the probe laser on the surface of the samples was about 3 mm, and the spot was covered by the pump beams to obtain a complete spatial overlap.
Fig. 5. Schematic illustration of the dual pump-probe experimental setup to determine photoinduced transparency of the DDLC-NCs thin film. Key components include CW laser source (LS), gradient ND filter (GN), collimating lens (L), mirror (M), polymeric thin film samples (SA), temperature controller (TC), 5x microscope objective lens (OL), band-pass filter (BF), and detector (DE).
The intensity of the probe beam was 10 mW/cm2 and the samples were illuminated by the pump beams with an equal intensity of 100 mW/cm2. Intensity adjustment of pump lasers were also performed using gradient neutral-density (ND) filters that were placed before the collimating lenses.
The nanocomposite thin films were vertically mounted in an aluminum holder with the surface faced orthogonally to the probe beam and then, have been placed on a thermoelectric cooling temperature controller (TECDRV-1515, Fnm Co.) in order to maintain the temperature of the thin films at approximately 20 °C during the measurements.
After the incident probe laser passed through the samples, a 5× objective lens with a focal length of 25.4 mm (NA=0.1, Newport Co.) was employed to focus the optical signal into the detector and a bandpass filter (central wavelength λ=632.8 nm, band width=10 nm) was also used to block the transmitted pump lasers from entering the detector.
Finally, the variation of the transmitted probe beam signal before and after the sample excitation was measured as a function of time by a silicon photodetector (DET10A, Thorlabs Inc.) which was connected to a digital oscilloscope (TDS2014B, Tektronix). In addition, the scattering patterns on the output side of the samples were captured as a transverse intensity profile of probe beam by a laser beam profiler (Beamprox-1600, Polaritech Co.) equipped with an appropriate ND filter.
3. Results and discussion
3.1 Spectral characterization
The obtained absorbance spectra of constituents inside the core of fabricated nanocapsules between 200 - 650 nm wavelength range are shown in Fig. 6. The measured spectrums were normalized to the absorbance peak of E7 since liquid crystal concentration is identical in both mixtures. The absorbance peaks are located around 390 nm and 510 nm correspond to DY7 and DR1 respectively [66,67], and considering these results, the 405 nm and 532 nm diode lasers were utilized as excitation sources to match the maximum absorption peaks of the organic dyes and consequently to achieve the wavelength selectivity of dual-pump investigations. In addition, E7 exhibits a well-recognized absorbance peak at around 285 nm [68] whereas, the increase in absorbance below 250 nm is due to the UV absorption spectra overlap of organic dyes and the liquid crystal.
Fig. 6. Normalized absorbance spectra of the pure DDLC mixtures, in the same dye concentration (1%wt). Black curve: DY7 in E7, red curve: DR1 in E7. The inset shows the magnified absorption peaks of the organic dyes and corresponding selected excitation laser lines.
The measured extinction spectra of the purified DDLC-NCs dispersed in PVA thin film samples are displayed in Fig. 7. Considering the fact that the absorptions of both PMMA as the shell of the fabricated nanocapsules and PVA as the polymeric matrix can be neglected at the wavelength range greater than 250 nm [56,69,70], the significant difference between obtained absorbance spectra of the core constituents of the NCs (Fig. 6) and the extinction spectra of the fabricated thin films, mainly results from the broadband scattering contribution of DDLC nanocapsules in the polymeric matrix [56] that occurs since the mean diameters of the obtained nanocapsules were comparable to the wavelength of the broadband incident light source [39,42,71,72] in addition to the overlaid of nanocapsules inside the polymeric host.
Fig. 7. Normalized extinction spectra of the fabricated polymeric thin films containing DDLC-NCs of F1 (red curve), F2 (gold curve), and F3 (black curve) samples under the illumination of UV/Vis light source. The red dashed line represents the selected wavelength of the probe laser.
Based on the spectral measurement results and in order to avoid any undesirable absorption in laser-induced transparency investigations of the samples, the appropriate wavelength of the probe laser (λ=632.8 nm) was selected adequately far from the measured absorbance peaks and shoulders of the liquid crystal and organic dyes. Consequently, in spite of the scattering portion, the absorption of the samples is too weak at the wavelength range greater than 580 nm (especially under the wavelength of the probe laser) and it is ignored in the measurements.
It should be noted here that as depicted in Fig. 7, the extinction of the F2 sample is slightly more than the obtained result of the same measurement for the F1 sample. Indeed, the larger mean diameter of the S2 nanocapsules (Table 1) inside the F2 sample in comparison to the S1 nanocapsules dispersed in the F1 sample could explain such a result that reveals the reason for the scattering difference between the two thin-film samples.
3.2 All-optical reorientation process in DDLC-NCs
The schematic illustration of a two-step laser-induced cooperative molecular reorientation process of a binary mixture of DDLC-NCs dispersed in a polymeric host is shown in Fig. 8. Since in the absence of an external stimulus, the director vector of nano-confined DDLC mixture in different NCs is randomly oriented (Fig. 8(a)), and considering the mean diameter of obtained NCs (Table1) which is comparable to the wavelength of probe laser, partially light scattering occurs at the interface between the PVA matrix and nanocapsules due to the difference of the refractive indices between them. Consequently, the sample presents a partially light scattering state that is semi-transparent for the probe laser beam. Therefore, as shown in Fig. 8, the probe beam simultaneously experiences forward and backward scattering loss along with the attenuated laser beam pathing through the fabricated thin films.
Fig. 8. Schematic illustrations of laser-induced cooperative molecular reorientation inside of nanocapsules in a binary mixture of DDLC-NCs in F3 sample before and after irradiation by linearly polarized pump lasers: (a) random director vectors (initial state), (b) aligned director vectors of DY7 doped LC nanocapsules (first reorientation state), (c) further aligned director vectors of DR1 doped LC nanocapsules (second reorientation state)
When the sample was irradiated by the pump lasers (Fig. 8(b, c)), liquid crystal mesogens followed by dye molecules in DDLC mixture show a realignment which is perpendicular to the electric field direction of the incident excitation beam due to the trans-cis photoisomerization and cistrans thermal relaxation continuously iterated cycles of azo dyes [73–76]. Considering the polarization direction of pump lasers and due to the birefringence properties of the E7 as a nematic liquid crystal, this laser-induced realignment of the ingredients inside the core of NCs leads to a reduction in refractive index mismatch between the polymer matrix and the nanocapsules. Therefore, the scattering and subsequently the attenuating of the probe beam will decrease when passing through the sample, which results in an increase of the transmitted intensity of the optical signal.
In addition, considering randomly oriented director of DDLC mixture inside the core of nanocapsules at the initial state and similar to PDLC and nano-PDLC systems [37,39,77] the scattering-based optical response of the fabricated samples is independent of the incident probe beam polarization. However, it is noteworthy to mention that because the laser-induced reorientation of azo dye molecules due to the photoisomerization used in this work is a polarization dependent process, the polarization direction of the pumps and probe beams were selected the same and set parallel to each other, in order to have the maximum transmitted optical signal.
3.3 Laser-induced transparency measurements
The wavelength dependence of laser-induced transparency behavior of the DDLC-NCs dispersed polymeric thin films were investigated by measuring the intensity variations of the transmitted probe laser before and after irradiation of the samples with various combinations of two exciting wavelengths were measured as a function of irradiating time. Samples were irradiated with green (532 nm) and violet (405 nm) pumps in turn including 10 s of sample exposure and 10 s obstruction and then dual-wavelength excitation was performed with 10 s irradiation time overlapping of two pumps. As shown in Fig. 9, when the excitation wavelengths were applied, the transmittance of the nanocomposite thin-film samples starts to increase due to the laser-induced reorientation of DDLCs discussed in section 3.2.
Fig. 9. Laser-induced transparency measurement results corresponding to the on/off irradiation of dual pump lasers recorded in a 90 s time interval. The inset shows the transmitted intensities for each sample separately to elucidate the difference in optical signal levels.
The measured transmitted intensity levels are also corresponded to what has previously been discussed in section 3.1 for the absorbance and also extinction spectrums (Figs. 6 and 7). Considering the first 40 seconds of irradiating, it is clear that the degree of induced transparency and consequently the transmitted intensity of the probe laser in the F2 sample due to the violet pump irradiation is considerably more than in the F1 sample due to the irradiation of the green pump, in the same NCs concentration and also the same intensity of pump lasers. This could be explained by two main reasons: (i) as presented in Fig. 6 the measured absorbance of DY7 at 405 nm is slightly more than the absorbance of DR1 at 532 nm, (ii) DY7 organic dye has two azo-functional groups compared to the DR1 with only one azo-functional group [66,78] which leads to a more effective reorientation of LC inside the core of the S2 nanocapsules.
In the case of the F3 sample, which contains a binary mixture of S1 and S2 nanocapsules, the increment of the optical signal levels was achieved due to both 405 nm and 532 nm pump lasers, and consequently, it is seen that this sample has all the laser-induced transparency characteristics of both F1 and F2 samples as well. Based on the obtained results, the percentage of increase in the transmitted optical signal levels due to the laser-induced transparency effect are summarized in Table 3.
Table 3. The percentage of increase in the transmitted optical signal levels of the samples for different wavelengths of the pump lasers
It should be noted that the initial intensity level of the transmitted probe passing through the samples without applying any excitation wavelengths is not zero because as discussed in section 3.2., the fabricated nanocomposite samples are semi-transparent for the probe laser.
Additionally, in further measurements by using a He-Ne laser line bandpass filter to block the transmitted pump lasers, the images of the transmitted optical signal on the output surface of the F3 sample were captured as a transverse intensity profile of the probe beam before and after the sample illumination by the pump lasers.
As depicted in Fig. 10 the optical signal images show typical scattering patterns formed by DDLC nanocapsules dispersed in the PVA matrix. In fact, the obtained results indicated that after pump irradiation, the transmitted intensity of the probe beam passing through the sample is increased especially close to the center of the images due to the laser-induced transparency process and consequently diffuse scattering reduction of the thin film. These results are completely consistent with the transmitted intensity variation measurements presented in Fig. 9.
Fig. 10. Captured images of the transmitted probe laser beam at the output surface of the F3 sample obtained from the same irradiated spot before (top) and after (middle) violet/green laser excitation separately. The bottom image shows the result of laser-induced transparency in the sample due to the dual-wavelength exposure.
4. Conclusion
By combining DDLC photoalignment behavior and the partially light-scattering effect together and utilizing the benefits of the nanoencapsulation technique a novel free-standing polymeric nanocomposite thin film was fabricated based on a binary mixture of polymeric nanocapsules containing the same LC but different organic dyes. Subsequently, dual-wavelength laser-induced transparency of the fabricated samples was investigated after spectral characterization. The obtained results indicate that the transmitted intensity can be optically controlled by applying pump lasers with separate or combination of two wavelengths which is due to the photoinduced reorientation of completely independent nanoconfined different DDLC mixtures. Indeed, the absorbance and extinction spectra analysis indicated the appropriate wavelength for the pumps and probe lasers, which ultimately affects the all-optical response tunability of the fabricated samples.
Moreover, by using the proposed DDLC-NCs based nanocomposite, the optical signal levels, which is crucial in the optical diffusers or optical valves, can be easily adjusted by using a custom ratio composition of a binary mixture of already prepared DDLC nanocapsules, varying the NCs concentration dispersed in a polymer matrix, the different mean diameter of NCs, varying the wavelength or intensity of the pump beams considering the absorbance spectra of the organic dyes, or even use more than two types of DDLC nanocapsules in a polymer host.
Therefore, in contrast to the conventional DD-PDLCs or nano-PDLCs, the utilization of a binary mixture of DDLC nanocapsules presented in this work, has considerable advantages such as multi-wavelength tunability, straightforward and easy film-forming, using different azo dye dopants in the same polymer matrix regardless of solubility or aggregation considerations, and also the ability to adjust the initial transmitted intensity level by optimizing the fabrication parameters. In other words, this two-color combination of DDLC-NCs enhances the all-optical response tunability and simplifies the fabrication process of thin films. In addition, because the nanocapsules are pre-characterized, the polymeric thin films fabricated in this way will have high sample reproducibility.
The introduced and characterized nanocomposite in this work, expect to pave way for developing the use of DDLC mixtures in the area of light-responsive smart materials and designing new polymeric thin film layers in the area of all-optical nanophotonics.
Acknowledgments
The authors would like to thank Hassan Aghajani for valuable comments and lab work assistance and also Kazem Ayoubi for the useful discussions.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
No data were generated or analyzed in the presented research.
References
1. J.-a. Lv, Y. Liu, J. Wei, E. Chen, L. Qin, and Y. Yu, “Photocontrol of fluid slugs in liquid crystal polymer microactuators,” Nature 537(7619), 179–184 (2016). [CrossRef]
2. A. Y.-G. Fuh, T.-H. Lin, Y.-Y. Chen, C.-C. Li, and H.-C. Jau, “Optical control of the rotation of cholesteric liquid crystal gratings,” Opt. Express 27(8), 10806 (2019). [CrossRef]
3. O. Francescangeli, L. Lucchetti, F. F. Simoni, S. Slussarenko, E. Ouskova, Y. A. Reznikov, S. Shiyanovskii, and J. L. West, “Light-induced anchoring and reorientation effects in dye-doped liquid crystals,” Proc. SPIE 4799, 83 (2002). [CrossRef]
4. L. Deng, K. He, W. Su, H. Sun, R. Wang, H. Zhang, and H.-K. Liu, “Optical limiting performances of the methyl-red-dye-doped nematic liquid crystal films,” Materials, Devices, and Systems for Display and Lighting, 4918 (2002).
5. C.-T. Wang, Y.-C. Wu, and T.-H. Lin, “Photo-controllable tristable optical switch based on dye-doped liquid crystal,” Dyes Pigm. 103, 21–24 (2014). [CrossRef]
6. K. Goda, M. Omori, and K. Takatoh, “Optical switching in guest–host liquid crystal devices driven by photo- and thermal isomerisation of azobenzene,” Liq. Cryst. 45(4), 485–490 (2018). [CrossRef]
7. J. R. Talukder, Y.-H. Lee, and S.-T. Wu, “Photo-responsive dye-doped liquid crystals for smart windows,” Opt. Express 27(4), 4480 (2019). [CrossRef]
8. Y. J. Liu, G. Y. Si, E. S. Leong, N. Xiang, A. J. Danner, and J. H. Teng, “Light-driven plasmonic color filters by overlaying photoresponsive liquid crystals on gold annular aperture arrays,” Adv. Mater. 24(23), OP131 (2012). [CrossRef]
9. T. Ikeda, “Photomodulation of liquid crystal orientations for photonic applications,” J. Mater. Chem. 13(9), 2037–2057 (2003). [CrossRef]
10. S. K. Fegan, P. Kirsch, and F. Müller-Plathe, “The alignment of dichroic dyes in a nematic liquid crystal: a molecular dynamics investigation,” Liq. Cryst. 45(9), 1377–1384 (2018). [CrossRef]
11. M. T. Sims, L. C. Abbott, S. J. Cowling, J. W. Goodby, and J. N. Moore, “Dyes in liquid crystals: experimental and computational studies of a guest-host system based on a combined DFT and MD approach,” Chem. - Eur. J. 21(28), 10123–10130 (2015). [CrossRef]
12. S. L. Oscurato, M. Salvatore, P. Maddalena, and A. Ambrosio, “From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials,” Nanophotonics 7(8), 1387–1422 (2018). [CrossRef]
13. M. Kreuzer, E. Benkler, D. Paparo, G. Casillo, and L. Marrucci, “Molecular reorientation by photoinduced modulation of rotational mobility,” Phys. Rev. E 68(1), 011701 (2003). [CrossRef]
14. D. Pirone, N. A. Bandeira, B. Tylkowski, E. Boswell, R. Labeque, R. Garcia Valls, and M. Giamberini, “Contrasting photo-switching rates in azobenzene derivatives: how the nature of the substituent plays a role,” Polymers 12(5), 1019 (2020). [CrossRef]
15. D. Statman and J. C. Lombardi, “Modeling the dynamics of photo-induced reorientation of nematic liquid crystals doped with azo-dye,” Mol. Cryst. Liq. Cryst. 494(1), 1–10 (2008). [CrossRef]
16. C.-Y. Huang, Y.-R. Lin, K.-Y. Lo, and C.-R. Lee, “Dynamics of photoalignment in azo-dye-doped liquid crystals,” Appl. Phys. Lett. 93(18), 181114 (2008). [CrossRef]
17. F. Ahmad, M. Jamil, and Y. J. Jeon, “Advancement trends in dye-doped polymer dispersed liquid crystals—a survey review,” Mol. Cryst. Liq. Cryst. 648(1), 88–113 (2017). [CrossRef]
18. C. Manzo, D. Paparo, L. Marrucci, and I. Jánossy, “Light-induced rotation of dye-doped liquid crystal droplets,” Phys. Rev. E 73(5), 051707 (2006). [CrossRef]
19. G. Petriashvili, M. D. Bruno, M. P. De Santo, and R. Barberi, “Temperature-tunable lasing from dye-doped chiral microdroplets encapsulated in a thin polymeric film,” Beilstein J. Nanotechnol. 9, 379–383 (2018). [CrossRef] .
20. M. Sheng, L. Zhang, D. Wang, M. Li, L. Li, J. L. West, and S. Fu, “Fabrication of dye-doped liquid crystal microcapsules for electro-stimulated responsive smart textiles,” Dyes Pigm. 158, 1–11 (2018). [CrossRef]
21. K.-J. Yang, S.-C. Lee, and B.-D. Choi, “Dye-doped polymer dispersed liquid crystal films for flexible displays,” Jpn. J. Appl. Phys. 49(5), 05EA05 (2010). [CrossRef]
22. J. Eun Jung, G. H. Lee, J. Eun Jang, K. Y. Hwang, F. Ahmad, M. Jamil, L. Jin Woo, and Y. Jae Jeon, “Optical property enhancement of dye-PDLC using active reflector structure,” J. Appl. Polym. Sci. 124(1), 873–877 (2012). [CrossRef]
23. Z. Shi, L. Shao, F. Wang, F. Deng, Y. Liu, and Y. Wang, “Fabrication of dye-doped polymer-dispersed liquid crystals with low driving voltage based on nucleophile-initiated thiol-ene click reaction,” Liq. Cryst. 45(4), 579–585 (2018). [CrossRef]
24. F. Ahmad, M. Jamil, Y. Jeon, L. Woo, J. Jung, and J. Jang, “Investigation of nonionic diazo dye-doped polymer dispersed liquid crystal film,” Bull. Mater. Sci. 35(2), 221–231 (2012). [CrossRef]
25. P. Kumar, V. Sharma, C. Jaggi, and K. K. Raina, “Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal,” Liq. Cryst. 44(4), 757–767 (2017). [CrossRef]
26. S. Ohta, S. Inasawa, and Y. Yamaguchi, “Size control of phase-separated liquid crystal droplets in a polymer matrix based on the phase diagram,” J. Polym. Sci., Part B: Polym. Phys. 50(12), 863–869 (2012). [CrossRef]
27. S.-I. Park, N.-H. Park, and K.-D. Suh, “The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules,” Liq. Cryst. 29(6), 783–787 (2002). [CrossRef]
28. K.-J. Yang, K.-P. Kim, D.-H. Kim, and B.-D. Choi, “The effects of conditions for polymerization induced phase separation processes on the electro-optic characteristics of polymer dispersed liquid crystals,” Mol. Cryst. Liq. Cryst. 498(1), 83–88 (2009). [CrossRef]
29. Y. Li, J. Jun-Yan Suen, E. Prince, E. M. Larin, A. Klinkova, H. Thérien-Aubin, S. Zhu, B. Yang, A. S. Helmy, O. D. Lavrentovich, and E. Kumacheva, “Colloidal cholesteric liquid crystal in spherical confinement,” Nat. Commun. 7(1), 12520 (2016). [CrossRef]
30. P. Kumar, V. Sharma, C. Jaggi, P. Malik, and K. K. Raina, “Orientational control of liquid crystal molecules via carbon nanotubes and dichroic dye in polymer dispersed liquid crystal,” Liq. Cryst. 44(5), 843–853 (2017). [CrossRef]
31. G. Pawlik, A. C. Mitus, P. Karpinski, and A. Miniewicz, “Laser light-induced molecular reorientation in 90° twisted nematic liquid crystal: Classic approach, Monte Carlo modeling and experiment,” Opt. Mater. 34(10), 1697–1703 (2012). [CrossRef] .
32. P. Khushboo, P. Sharma, K. K. Malik, and Raina, “Textural, thermal, optical and electrical properties of Iron nanoparticles dispersed 4′-(Hexyloxy)-4-biphenylcarbonitrile liquid crystal mixture,” Liq. Cryst. 44(11), 1717–1726 (2017). [CrossRef]
33. L. Zhou, M. H. Saeed, and L. Zhang, “Optical diffusers based on uniform nano-sized polymer balls/nematic liquid crystals composite films,” Liq. Cryst. 47(5), 785–798 (2020). [CrossRef]
34. B. Liu, Y. Ma, D. Zhao, L. Xu, F. Liu, W. Zhou, and L. Guo, “Effects of morphology and concentration of CuS nanoparticles on alignment and electro-optic properties of nematic liquid crystal,” Nano Res. 10(2), 618–625 (2017). [CrossRef]
35. W.-Y. Teng, S.-C. Jeng, C.-W. Kuo, Y.-R. Lin, C.-C. Liao, and W.-K. Chin, “Nanoparticles-doped guest-host liquid crystal displays,” Opt. Lett. 33(15), 1663 (2008). [CrossRef]
36. O. Tongcher, R. Sigel, and K. Landfester, “liquid crystal nanoparticles prepared as miniemulsions,” Langmuir 22(10), 4504–4511 (2006). [CrossRef]
37. W.-K. Choi, S.-L. Hou, J.-Y. Chen, G.-D. J. Su, and Y.-M. Li, “Fast-response polarization-independent optical shutter using nano-PDLC inside a Fabry-Perot cavity,” Mol. Cryst. Liq. Cryst. 612(1), 232–237 (2015). [CrossRef]
38. Y. J. Liu, X. W. Sun, H. I. Elim, and W. Ji, “Gain narrowing and random lasing from dye-doped polymer-dispersed liquid crystals with nanoscale liquid crystal droplets,” Appl. Phys. Lett. 89(1), 011111 (2006). [CrossRef]
39. J. H. Yu, H.-S. Chen, P.-J. Chen, K. H. Song, S. C. Noh, J. M. Lee, H. Ren, Y.-H. Lin, and S. H. Lee, “Electrically tunable microlens arrays based on polarization-independent optical phase of nano liquid crystal droplets dispersed in polymer matrix,” Opt. Express 23(13), 17337 (2015). [CrossRef]
40. D.-M. Lee, Y.-J. Lee, H. B. Park, C.-J. Yu, and J.-H. Kim, “Optically isotropic microlens arrays using nanoencapsulated liquid crystals,” Mol. Cryst. Liq. Cryst. 647(1), 44–50 (2017). [CrossRef]
41. R. Manda, S. Pagidi, Y. J. Lim, R. He, S. M. Song, J. H. Lee, G.-D. Lee, and S. H. Lee, “Self-supported liquid crystal film for flexible display and photonic applications,” J. Mol. Liq. 291, 111314 (2019). [CrossRef]
42. N. H. Park, S. C. Noh, P. Nayek, M.-H. Lee, M. S. Kim, L.-C. Chien, J. H. Lee, B. K. Kim, and S. H. Lee, “Optically isotropic liquid crystal mixtures and their application to high-performance liquid crystal devices,” Liq. Cryst. 42(4), 530–536 (2015). [CrossRef]
43. H. Park, M. Park, Y.-J. Lee, C.-J. Yu, and J.-H. Kim, “P-102: flexible display using nano-encapsulated liquid crystal with low driving voltage characteristics,” Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 47(1), 1513–1515 (2016). [CrossRef]
44. R. Manda, J. H. Yoon, S. Pagidi, S. S. Bhattacharyya, D. T. Tran, Y. J. Lim, J.-M. Myoung, and S. H. Lee, “Paper-like flexible optically isotropic liquid crystal film for tunable diffractive devices,” Opt. Express 27(24), 34876 (2019). [CrossRef]
45. R. Manda, S. Pagidi, S. S. Bhattacharyya, C. H. Park, Y. J. Lim, J. S. Gwag, and S. H. Lee, “Fast response and transparent optically isotropic liquid crystal diffraction grating,” Opt. Express 25(20), 24033 (2017). [CrossRef]
46. B. Kim, H. G. Kim, G.-Y. Shim, J.-S. Park, K.-I. Joo, D.-J. Lee, J.-H. Lee, J.-H. Baek, B. K. Kim, Y. Choi, and H.-R. Kim, “Fast-switching optically isotropic liquid crystal nano-droplets with improved depolarization and Kerr effect by doping high k nanoparticles,” Appl. Opt. 57(2), 119 (2018). [CrossRef]
47. D. E. Lucchetta, A. Manni, R. Karapinar, L. Gobbi, and F. Simoni, “Nano-size polymer dispersed liquid crystals for phase-only optical modulation,” Mol. Cryst. Liq. Cryst. 375(1), 397–409 (2002). [CrossRef]
48. P. J. Hands, A. K. Kirby, and G. D. Love, “Phase modulation with polymer-dispersed liquid crystals,” Proc. SPIE 5894, 58940L (2005). [CrossRef] .
49. E. Wajs, T. T. Nielsen, K. L. Larsen, and A. Fragoso, “Preparation of stimuli-responsive nano-sized capsules based on cyclodextrin polymers with redox or light switching properties,” Nano Res. 9(7), 2070–2078 (2016). [CrossRef] .
50. Y. Tsuru, M. Kohri, T. Taniguchi, K. Kishikawa, T. Karatsu, and M. Hayashi, “Preparation of photochromic liquid core nanocapsules based on theoretical design,” J. Colloid Interface Sci. 547, 318–329 (2019). [CrossRef]
51. B. Iyisan and K. Landfester, “Modular approach for the design of smart polymeric nanocapsules,” Macromol. Rapid Commun. 40(1), 1800577 (2019). [CrossRef]
52. G. Sukhorukov, A. Fery, and H. Möhwald, “Intelligent micro- and nanocapsules,” Prog. Polym. Sci. 30(8-9), 885–897 (2005). [CrossRef]
53. R. Vecchione, G. Luciani, V. Calcagno, A. Jakhmola, B. Silvestri, D. Guarnieri, V. Belli, A. Costantini, and P. A. Netti, “Multilayered silica-biopolymer nanocapsules with a hydrophobic core and a hydrophilic tunable shell thickness,” Nanoscale 8(16), 8798–8809 (2016). [CrossRef]
54. X. Zhao, K.-C. Zhao, L.-J. Chen, Y.-S. Liu, J.-L. Liu, and X.-P. Yan, “pH reversibly switchable nanocapsule for bacteria-targeting near-infrared fluorescence imaging-guided precision photodynamic sterilization,” ACS Appl. Mater. Interfaces 12(41), 45850–45858 (2020). [CrossRef]
55. V. Marturano, V. Bizzarro, V. Ambrogi, A. Cutignano, G. Tommonaro, G. R. Abbamondi, M. Giamberini, B. Tylkowski, C. Carfagna, and P. Cerruti, “Light-responsive nanocapsule-coated polymer films for antimicrobial active packaging,” Polymers 11(1), 68 (2019). [CrossRef]
56. M. R. Sharifimehr, K. Ayoubi, and E. Mohajerani, “Fabrication, morphological investigation and spectral characterization of nano-encapsulated azo dye-doped nematic liquid crystals,” J. Mol. Liq. 313, 113576 (2020). [CrossRef]
57. S.-G. Kang and J.-H. Kim, “Optically-isotropic nanoencapsulated liquid crystal displays based on Kerr effect,” Opt. Express 21(13), 15719 (2013). [CrossRef]
58. V. Marturano, P. Cerruti, M. Giamberini, B. Tylkowski, and V. Ambrogi, “Light-responsive polymer micro- and nano-capsules,” Polymers 9(12), 8 (2016). [CrossRef]
59. M. R. Sharifimehr, K. Ghanbari, K. Ayoubi, and E. Mohajerani, “Preparation and spectral characterization of polymeric nanocapsules containing DR1 organic dye,” Opt. Mater. 45, 87–90 (2015). [CrossRef]
60. Y. Wu, X. Qu, L. Huang, D. Qiu, C. Zhang, Z. Liu, Z. Yang, and L. Feng, “Optically switchable organic hollow nanocapsules,” J. Colloid Interface Sci. 343(1), 155–161 (2010). [CrossRef]
61. M. Han, E. Lee, and E. Kim, “Preparation and optical properties of polystyrene nanocapsules containing photochromophores,” Opt. Mater. 21(1-3), 579–583 (2003). [CrossRef]
62. M. R. Sharifimehr, K. Ayoubi, and E. Mohajerani, “Third order optical nonlinearities characteristics of Disperse Red1 organic dye molecules inside of polymeric nanocapsules,” Opt. Mater. 49, 147–151 (2015). [CrossRef]
63. C. E. Mora-Huertas, H. Fessi, and A. Elaissari, “Polymer-based nanocapsules for drug delivery,” Int. J. Pharm. 385(1-2), 113–142 (2010). [CrossRef]
64. R. Ahmad, O. S. Wolfbeis, Y.-B. Hahn, H. N. Alshareef, L. Torsi, and K. N. Salama, “Deposition of nanomaterials: A crucial step in biosensor fabrication,” Mater. Today Commun. 17, 289–321 (2018). [CrossRef]
65. H. Yang and P. Jiang, “Large-scale colloidal self-assembly by doctor blade coating,” Langmuir 26(16), 13173–13182 (2010). [CrossRef]
66. H. Xie, L. Zhang, W. Zhang, B. He, X. Yuan, P. Song, Z. Chen, Z. Yang, and H. Yang, “Electro-switchable characteristics of broadband absorptive films based on multi-dichroic dye-doped nematic liquid crystal,” Liq. Cryst. 42(3), 309–315 (2015). [CrossRef]
67. Y.-J. Wang and G. O. Carlisle, “Optical properties of disperse-red-1-doped nematic liquid crystal,” J. Mater. Sci.: Mater. Electron. 13(3), 173–178 (2002). [CrossRef]
68. P.-T. Lin, S.-T. Wu, C.-Y. Chang, and C.-S. Hsu, “UV stability of high birefirngence liquid crystals,” Mol. Cryst. Liq. Cryst. 411(1), 243–253 (2004). [CrossRef]
69. R. M. Ahmed, “Optical Study on Poly(methyl methacrylate)/Poly(vinyl acetate) Blends,” Int. J. Photoenergy 2009, 1–7 (2009). [CrossRef]
70. T. Srimongkon, S. Mandai, and T. Enomae, “Application of biomaterials and inkjet printing to develop bacterial culture system,” Adv. Mater. Sci. Eng. 2015, 1–9 (2015). [CrossRef]
71. C. H. Park, E. J. Shin, R. Manda, S. C. Noh, M.-H. Lee, and S. H. Lee, “P-100: fast response and scattering free optically isotropic liquid crystal device for flexible display applications,” SID Symp,” Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 47(1), 1506–1508 (2016). [CrossRef]
72. Y. J. Lim, J. H. Yoon, H. Yoo, S. M. Song, R. Manda, S. Pagidi, M.-H. Lee, J.-M. Myoung, and S. H. Lee, “Fast switchable field-induced optical birefringence in highly transparent polymer-liquid crystal composite,” Opt. Mater. Express 8(12), 3698 (2018). [CrossRef]
73. T. Fukuda, J. Y. Kim, D. Barada, and K. Yase, “Photoinduced cooperative molecular reorientation on azobenzene side-chain-type copolymers,” J. Photochem. Photobiol., A 183(3), 273–279 (2006). [CrossRef]
74. S. Bagatur and T. Fuhrmann-Lieker, “Photoinduced supramolecular chirality and spontaneous surface patterning in high-performance azo materials,” J. Eur. Opt. Soc.-Rapid Publ. 15(1), 12 (2019). [CrossRef]
75. D.-Y. Kim and K.-U. Jeong, “Light responsive liquid crystal soft matters: structures, properties, and applications,” Liq. Cryst. Today 28(2), 34–45 (2019). [CrossRef]
76. N. Kawatsuki, “Photoalignment and photoinduced molecular reorientation of photosensitive materials,” Chem. Lett. 40(6), 548–554 (2011). [CrossRef]
77. H. Ren, Y.-H. Lin, Y.-H. Fan, and S.-T. Wu, “Polarization-independent phase modulation using a polymer-dispersed liquid crystal,” Appl. Phys. Lett. 86(14), 141110 (2005). [CrossRef]
78. B. Bahadur, R. K. Sarna, and V. G. Bhlde, “Guest-host interaction of pleochroic dyes in liquid crystal mixtures E8 and PCH-1132,” Mol. Cryst. Liq. Cryst. 75(1), 121–132 (1981). [CrossRef]
