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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26956–26961
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Polymerized micro-patterned optical birefringence film and its fabrication using multi beam mixing

Jeong-Ku Lim and Jang-Kun Song  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26956-26961 (2011)
http://dx.doi.org/10.1364/OE.19.026956


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Abstract

The photo-polymerized liquid crystal (LC) film aligned on a photo-alignment layer was investigated with varying polarizability of UV light exposing on the photo-alignment layer. Interestingly, the polarizability of UV light required to induce bulk LC alignment on the photo alignment layer was found to be very low down to 0.1, and UV light greater than 0.3 polarizability produced outstanding optical performance of the film. The films fabricated with low polarizability light exhibited comparable thermo-stability with one fabricated with high polarizability light. The results suggest that micro-patterned optical birefringence films (MP-OBFs) can be fabricated by using an incoherent multi beam mixing method, where the direction of polarization of UV light can be spatially modulated. A simple MP-OBF was fabricated by using a two beam mixing method, and it exhibited a quality 3D film performance. The method will be highly useful in various optical components such as the MP-OBF, optical retarders, polarization grating etc.

© 2011 OSA

1. Introduction

As the stereoscopic 3-dimensional (3D) display has recently become a commercial success, MP-OBF have attracted much research interest. This is because MP-OBF is a key component of passive type stereoscopic 3D displays, where the MP-OBF combined with polarizing glasses produces two offset stereoscopic images perceived separately by a human’s binocular vision [1

1. S. Pastoor and M. Wopking, “3D displays: a review of current technologies,” Displays 17(2), 100–110 (1997). [CrossRef]

]. Further, MP-OBFs may be applicable to optical retarders, polarization converters, interference filters as well as display components.

Micro-patterned optical birefringence films were first demonstrated about 17 years ago by Shannon et al [2

2. P. J. Shannon, W. M. Gibbons, and S. T. Sun, “Patterned optical properties in photopolymerized surface-aligned liquid-crystal films,” Nature 368(6471), 532–533 (1994). [CrossRef]

]. They introduced photo-alignment technology to the MP-OBF fabrication [3

3. W. M. Gibbons, P. J. Shannon, S. T. Sun, and B. J. Swetlin, “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351(6321), 49–50 (1991). [CrossRef]

6

6. H. Yu and T. Ikeda, “Photocontrollable liquid-crystalline actuators,” Adv. Mater. (Deerfield Beach Fla.) 23(19), 2149–2180 (2011). [CrossRef] [PubMed]

], wherein polarized UV light was illuminated through a patterned photo-mask on a photo-alignment layer containing photo-reactive polymers, followed by coating with liquid crystals (LCs). The LCs were aligned with the photo-induced anisotropic surface, resulting in optical anisotropy and were then solidified by an additional photo-polymerization process. The photo-alignment method combined with a micro-patterned photo-mask makes it possible to fabricate a micrometer-scale pattern of varying optical axis, which is not achievable via conventional polymer stretching methods.

The fabrication of an MP-OBF requires at least two polarized UV irradiation processes with precise photo-mask aligning steps in order to acquire at least two different optical birefringence areas. Lee and Clark suggested another cost-effective method for implementing micro-patterned LC alignment using a patterned self-assembled monolayer [7

7. B. W. Lee and N. A. Clark, “Alignment of liquid crystals with patterned isotropic surfaces,” Science 291(5513), 2576–2580 (2001). [CrossRef] [PubMed]

], which requires only a single photo-lithography process. However, the method exhibited several weaknesses coming from uncontrollable disclination lines [5

5. N. Kawatsuki, T. Kawakami, and T. Yamamoto, “A photoinduced birefringent film with a high orientational order obtained from a novel polymer liquid crystal,” Adv. Mater. (Deerfield Beach Fla.) 13(17), 1337–1339 (2001). [CrossRef]

]. Another method for MP-OBF fabrication using lithographically controlled wetting was suggested [8

8. M. Cavallini, A. Calo, P. Stoliar, J. C. Kengne, S. Martins, F. C. Matacotta, F. Quist, G. Gbabode, N. Dumont, Y. H. Geerts, and F. Biscarini, “Lithographic alignment of discotic liquid crystals: a new time-temperature integrating framework,” Adv. Mater. (Deerfield Beach Fla.) 21(46), 4688–4691 (2009). [CrossRef]

], but the technology is in initial stage and remains many weaknesses to overcome including temporal instability. Thus, the photo-alignment method using two consecutive polarized UV irradiation is the only way to practically fabricate MP-OBFs at the moment.

The photo-alignment process has been carried out mostly using highly polarized UV light, which has a polarizability greater than 0.9 [2

2. P. J. Shannon, W. M. Gibbons, and S. T. Sun, “Patterned optical properties in photopolymerized surface-aligned liquid-crystal films,” Nature 368(6471), 532–533 (1994). [CrossRef]

5

5. N. Kawatsuki, T. Kawakami, and T. Yamamoto, “A photoinduced birefringent film with a high orientational order obtained from a novel polymer liquid crystal,” Adv. Mater. (Deerfield Beach Fla.) 13(17), 1337–1339 (2001). [CrossRef]

] and occasionally using slanted unpolarized UV light with photo-isomerization materials [9

9. Y. Wu, T. Ikeda, and Q. Zhang, “Three-dimensional manipulation of an azo polymer liquid crystal with unpolarized light,” Adv. Mater. (Deerfield Beach Fla.) 11(4), 300–302 (1999). [CrossRef]

]. It was also reported that the birefringence of polymer films containing cinnamoyl groups can be controlled by exposure to UV light of varying polarizability [10

10. N. Kawatsuki, H. Takatsuka, and T. Yamamoto, “Coplanar alignment of mesogenic moieties in a photocrosslinked liquid crystalline polymer film containing cinnamoyl groups,” Appl. Phys. Lett. 75(10), 1386–1388 (1999). [CrossRef]

], where the polymer film itself exhibits low range optical birefringence; however, this technique has not been used for aligning LC layers.

Here, we investigate the optical properties of retardation films fabricated under UV light illumination of varying polarizability. Surprisingly, our experimental results show that the UV light with low polarizability of 0.3 or less can effectively generate surface anisotropy in the photo-alignment layer to control the bulk LC alignment. Moreover, the method suggests that the fabrication of MP-OBT having arbitrary-shaped domains is achievable by using a single photo-irradiation process, which will provide a cheap manufacturing process.

2. Polarizability dependency of photo-aligned OBF and its thermal stability

In the first experiment, the liquid crystalline alignment on a UV exposed photo-alignment layer was investigated. The polarizability of UV light was controlled by the number of quartz plates inserted between the UV light source and substrate, where the quartz plate was slanted with a Brewster angle, as shown in Fig. 1
Fig. 1 UV illumination set-up; the sample can be replaced by a detector with a pivot-able polarizer in order to measure the polarizability of the UV light.
. The energy of UV light was measured using a UV power-meter equipped with a pivot-able nano-grid polarizer which enables measurement of the polarizability of UV light. The polarizability was calculated as
P=(IVIH)/(IV+IH),
(1)
where IV and IH are the intensities of vertically polarized UV light and horizontally polarized UV light, respectively. The number of quartz plates varied from 1 to 6, and the resulting polarizability was measured from 0.108 to 0.554, respectively.

The photoalignment layer (a commercial photosensitive cinnamate polymer, ROLIC) was spin-coated onto a glass substrate and was soft-baked in 100°C for 1 minute on a hot stage in order to remove solvents. The thickness of polymer layer was measured as 70~80 nm after baking. The surface was exposed to 365 nm UV light, the radiant energy of which was 1.125 J (15 mW × 75 sec). The UV exposure time was adjusted based on the UV energy so as to illuminate with the same UV energy for different polarizability conditions. The UV-treated substrates were coated with the liquid crystalline reactive mesogen (RM), for which a commercial RM solution (RMS10-021, Merck Korea) containing 25 wt% RM in propylene-glycol-methyl-ether-acetate (PGMEA) was used. The RM was spin-coated onto the UV-treated photo-alignment layer and was soft-baked in 80°C for several tens of seconds until the isotropic RM layer was converted into a nematic state. Finally, the film was exposed to 15 mW 365 nm UV light for 500 sec in order to solidify the RM layer, the thickness of which was about 0.8~1.2 μm. The optical birefringence (Δn) of RM layer is 0.16 at 550 nm.

The POM images in Fig. 2
Fig. 2 The polarized optical microscopic (POM) images of the film in a cross Nicole state. The optic axis of the film is parallel to the polarizer in images. The number in each image represents the polarizability of light exposed on the alignment layer.
were taken in a cross-polarized state, where the optical axis of the film is parallel to the polarizer. The number written in each image is the polarizability value of UV light exposed on the photo-alignment layer of the sample. The film exposed to a UV light of 0.108 polarizability exhibits many white spots. As the polarizability increases, the density of white spots decreases rapidly. The film exposed to a UV light of 0.407 or higher polarizability is largely even and black, with no white defects.

The black area represents the region where the RM molecules align well along the exposed UV polarization direction. One the other hand, white spots represent an uncontrolled RM domain. In our precise microscopic inspections, only small portions of white spots are due to dust or particles coming from imperfect experimental circumstances, and most spots do not contain any particles acting as a seed source to disrupt the alignment. Hence, one can conclude that the white spots arise from the weak azimuthal anchoring of the alignment layer.

In order to quantitatively analyze the quality of the film, the optical birefringence was measured using an Axoscan instrument (APM 42H, Axometrics), as shown in Fig. 3(a)
Fig. 3 (a) Retardation values (Ro and Rth) of films with the polarizability of exposed UV light on photo-alignment layer. The error bars are the standard deviation of data of 5 film samples. (b) The stability of retardation values under thermal shock. The numbers are the thermal cycling number, and ‘H’ and ‘L’ mean the high temperature (60°C) and the room temperature (25°C), respectively, in the labels of horizontal axis. High and low polarizability values are 0.554 and 0.208, respectively.
. The birefringence, Ro and Rth are respectively defined as R0 = d(nx - ny), and Rth = d{(nx + ny)/2 – nz}, where nx, ny and nz represent the refractive indices along two in-substrate axes parallel and perpendicular to UV polarization and normal to the substrate, and d is the thickness of film.

As shown in Fig. 3(a), the retardation values increase or decrease in the range of low polarizability and approach saturation near polarizabilities of 0.3~0.4. The data show that Ro value of the sample using 0.1 polarizability UV light is as large as 147 nm, which means that the RMs on the photo-alignment layer align quite well in the UV polarization direction. Interestingly, one can see that the photo-alignment layer exposed to weakly polarized UV light enables alignment of RMs. In the film exposed to a UV light of 0.108 polarizability, photo-induced polymerization in the direction of UV polarization occurs only about 20% faster than that in the perpendicular direction. Hence, the weak polymerization anisotropy is sufficient to induce bulk LC alignment even though many uncontrollable defects appear.

Ichimura et al. reported that a low density of azobenzene molecules on a surface can align 15000 times more LC molecules [11

11. K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, “Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer,” Langmuir 4(5), 1214–1216 (1988). [CrossRef]

]. This coincides with our experimental results, in that the surface anisotropy required for aligning the bulk LCs is not large. LC, itself has a tendency to align together without external enforcement. The functionality of surface anchoring provides guidance in the preferable direction of the self-alignment of LCs, but it does not compulsorily enforce the intermolecular ordering of LC molecules. Hence, the required energy for aligning LCs is not large. Note that the photo-alignment layer may contain the information regarding all of the intensity, polarizability, and polarization direction of exposed UV light, but, interestingly, only the polarization direction is recorded on RM layers while the intensity and polarizability information are saturated and not recorded as long as the intensity and polarizability are not too low.

In order to analyze and compare the stabilities of the films exposed to high and low polarization UV lights, we measured the retardation of the films under repeated thermal shock conditions, for which the temperature periodically fluctuated from room temperature to 60 °C. The optical birefringence values of the films during thermal shock conditions are plotted in Fig. 3(b), where the red square data set corresponds to the film exposed to high polarizability UV light (0.554) and the blue diamond data set is for the film exposed to low polarizability UV light (0.208). In each thermal shock cycle, the birefringence of films decreases with increasing temperature and recovers with decreasing temperature, as shown in Fig. 3(b). The initial birefringence of the films decreased slightly after the first thermal cycle, and these values were preserved after additional thermal cycles in both films. The slight decrease in birefringence observed during the first thermal cycle is interpreted as the stabilization process of polymerized films. However, little difference was observed between the two films exposed to high and low polarizability UV lights. It means that even the film with a low anisotropic surface alignment has good stability up to 60 °C, though further tests will be required for fully verify the thermal stability. In this way, we verified that the photo-alignment layer exposed to low polarizability UV light can be applicable for LC or RM alignment, which will significantly expand the applicability of the technology.

3. MP-OBF fabrication using novel two beam mixing method

It was reported that two beam interference method can produce spatially modulated polarization grating of light, which can be used for producing polarization grating film [12

12. S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, and B. R. Kimball, “Polarization insensitive imaging through polarization gratings,” Opt. Express 17(3), 1817–1830 (2009). [CrossRef] [PubMed]

,13

13. G. Crawford, J. Eakin, M. D. Radcliffe, A. Callan-Jones, and R. Pelcovits, “Liquid-crystal diffraction gratings using polarization holography alignment techniques,” J. Appl. Phys. 98(12), 123102 (2005). [CrossRef]

]. In the method, the interference pattern made by two oppositely circular polarized laser beams has spatial polarization grating with perfect polarizability. However, such an interference method has less flexibility in the formation of pattern shape, compared to the usual photo-lithographical method, and it is almost impossible to produce an arbitrary-shaped polarization pattern. Meanwhile, our experimental results in the section 2 exhibit that the required polarizability of light for inducing well controlled LC alignment is not high, and only the direction of polarizability is recorded on the LC films after making a birefringence film. Therefore, we can use a multi-beam mixing method with incoherent lights for fabricating MP-OBF or polarization grating film. Note that it is not difficult to produce a pattern with modulated direction of polarization by mixing two beams or three beams, because the required polarizability is low.

Here we demonstrate a simple example of two beam mixing method for fabricating MP-OBF. In this method, only a single photo-mask aligning process with UV exposure is necessary. The concept of the technology is illustrated in Fig. 4(a)
Fig. 4 (a) Incoherent two beam mixing method, where ‘A’ and ‘B’ lights are simultaneously exposed on the film. (b) Polarizability profile near the slit edge, where the polarization direction is sharply changed near the edge. (c) 3D film observed with polarized microscopy without (c-1) and with a quarter wave plate (c-2).
. Two polarized UV lights are simultaneously illuminated on the photo-alignment layer coated on a quartz substrate. The two lights are not coherent, and so they do not interfere with each other. The polarization directions of these two lights are perpendicular to each other, as illustrated in Fig. 4(a). Hence, the polarization direction and the polarizability of a mixed UV beam are determined by the intensity of UV light A and B from top and bottom, respectively, as
EM//EA,whenIA>IB,EM//EB,whenIA<IB,
(2)
and the polarizability is defined as Eq. (1), where EM, EA and EB are the main polarization directions of the mixed light, light ‘A’ and light ‘B’, respectively. In order to maintain the polarizability of the mixed light above 0.3 in whole area except boundaries, ‘B’ light intensity should be less than about 50% of ‘A’ light. In actual experiment, ‘B’ light was approximately 20% of ‘A’ light for producing large polarizability as shown in Fig. 4(b).

The polarization of the mixed light near the edge of the mask slit is plotted in the bottom image of Fig. 4(b), where the left side corresponds to the open area of the photo-mask, and the right side is for the closed area of the photo-mask. In the open area, ‘A’ light contributes to the total polarization more than ‘B’ light, and so the polarization direction of the light is 45°. In the closed area of the mask, only the ‘B’ light is illuminated on the surface, and the polarization direction is 135°. The polarizability of the light is greater than 0.5 in the whole area except at the slit edge. The low polarizability near the edge is due to the light diffracted from the slit edge, and the width of low polarizability less than 0.3 depends on the distance between the mask and sample.

Figure 4(c) shows the film created using the polarization control with the two beam mixing method. The image in Fig. 4(c-1) was taken under crossed polarizers, the axes of which are illustrated in each image. Narrow white lines are observed between the two regions, and the lines correspond to the uncontrolled area when the polarizability of UV light is less than 0.3. The width of the white lines between the two regions is measured to be approximately 10 μm. An additional quarter wave (λ/4) plate was inserted to produce the image in Fig. 4(c-2). The optic axis of λ/4 plate is parallel to one of domains and is 45° with the polarizers as shown in Fig. 4(c). The retardation of RM layer is approximately λ/4, and so, the retardation of the dark domain is cancelled out and that of the other domain becomes about λ/2. The difference is clearly distinguished by POM as shown in Fig. 4(c-2).

4. Conclusion

We investigated the quality of LC films aligned on a photo-alignment layer exposed to UV light of varying polarizability. Interestingly, weakly polarized UV light to 0.1 polarizability induced surface anisotropy large enough to trigger bulk LC alignment, and UV light greater than 0.3 polarizability resulted in outstanding optical performance of the film. It means that only the polarization direction of light is recorded on RM layers, while the polarizability and intensity are saturated. Using this mechanism, we devised a polarization control method using an incoherent two beam mixing method, which exhibits a quality 3D film structure. The incoherent two beam mixing method does not generate interference patterns which usually appear in coherent beam mixing and is difficult to control; rather, our method produces a well controlled micro-polarization pattern with a clean domain boundary. The successful fabrication of MP-OBF suggests that a multi-beam mixing method using lights having different linear polarizations may be able to produce an arbitrary-shaped micro-birefringence film even with continuously varying birefringence. Thus, the method will be highly useful in various optical devices as well as in the production of MP-OBF.

Acknowledgments

This paper was supported by Samsung Research Fund, Sungkyunkwan University, 2010.

References and links

1.

S. Pastoor and M. Wopking, “3D displays: a review of current technologies,” Displays 17(2), 100–110 (1997). [CrossRef]

2.

P. J. Shannon, W. M. Gibbons, and S. T. Sun, “Patterned optical properties in photopolymerized surface-aligned liquid-crystal films,” Nature 368(6471), 532–533 (1994). [CrossRef]

3.

W. M. Gibbons, P. J. Shannon, S. T. Sun, and B. J. Swetlin, “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351(6321), 49–50 (1991). [CrossRef]

4.

M. Schadt, K. Schmitt, V. Kozinkov, and V. Chigrinov, “Surface-induced parallel alignment of liquid crystals by linearly polymerized photopolymers,” Jpn. J. Appl. Phys. 31(Part 1, No. 7), 2155–2164 (1992). [CrossRef]

5.

N. Kawatsuki, T. Kawakami, and T. Yamamoto, “A photoinduced birefringent film with a high orientational order obtained from a novel polymer liquid crystal,” Adv. Mater. (Deerfield Beach Fla.) 13(17), 1337–1339 (2001). [CrossRef]

6.

H. Yu and T. Ikeda, “Photocontrollable liquid-crystalline actuators,” Adv. Mater. (Deerfield Beach Fla.) 23(19), 2149–2180 (2011). [CrossRef] [PubMed]

7.

B. W. Lee and N. A. Clark, “Alignment of liquid crystals with patterned isotropic surfaces,” Science 291(5513), 2576–2580 (2001). [CrossRef] [PubMed]

8.

M. Cavallini, A. Calo, P. Stoliar, J. C. Kengne, S. Martins, F. C. Matacotta, F. Quist, G. Gbabode, N. Dumont, Y. H. Geerts, and F. Biscarini, “Lithographic alignment of discotic liquid crystals: a new time-temperature integrating framework,” Adv. Mater. (Deerfield Beach Fla.) 21(46), 4688–4691 (2009). [CrossRef]

9.

Y. Wu, T. Ikeda, and Q. Zhang, “Three-dimensional manipulation of an azo polymer liquid crystal with unpolarized light,” Adv. Mater. (Deerfield Beach Fla.) 11(4), 300–302 (1999). [CrossRef]

10.

N. Kawatsuki, H. Takatsuka, and T. Yamamoto, “Coplanar alignment of mesogenic moieties in a photocrosslinked liquid crystalline polymer film containing cinnamoyl groups,” Appl. Phys. Lett. 75(10), 1386–1388 (1999). [CrossRef]

11.

K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, “Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer,” Langmuir 4(5), 1214–1216 (1988). [CrossRef]

12.

S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, and B. R. Kimball, “Polarization insensitive imaging through polarization gratings,” Opt. Express 17(3), 1817–1830 (2009). [CrossRef] [PubMed]

13.

G. Crawford, J. Eakin, M. D. Radcliffe, A. Callan-Jones, and R. Pelcovits, “Liquid-crystal diffraction gratings using polarization holography alignment techniques,” J. Appl. Phys. 98(12), 123102 (2005). [CrossRef]

OCIS Codes
(310.0310) Thin films : Thin films
(310.5448) Thin films : Polarization, other optical properties

ToC Category:
Thin Films

History
Original Manuscript: November 11, 2011
Revised Manuscript: December 1, 2011
Manuscript Accepted: December 1, 2011
Published: December 16, 2011

Citation
Jeong-Ku Lim and Jang-Kun Song, "Polymerized micro-patterned optical birefringence film and its fabrication using multi beam mixing," Opt. Express 19, 26956-26961 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26956


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References

  1. S. Pastoor and M. Wopking, “3D displays: a review of current technologies,” Displays 17(2), 100–110 (1997). [CrossRef]
  2. P. J. Shannon, W. M. Gibbons, and S. T. Sun, “Patterned optical properties in photopolymerized surface-aligned liquid-crystal films,” Nature 368(6471), 532–533 (1994). [CrossRef]
  3. W. M. Gibbons, P. J. Shannon, S. T. Sun, and B. J. Swetlin, “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351(6321), 49–50 (1991). [CrossRef]
  4. M. Schadt, K. Schmitt, V. Kozinkov, and V. Chigrinov, “Surface-induced parallel alignment of liquid crystals by linearly polymerized photopolymers,” Jpn. J. Appl. Phys. 31(Part 1, No. 7), 2155–2164 (1992). [CrossRef]
  5. N. Kawatsuki, T. Kawakami, and T. Yamamoto, “A photoinduced birefringent film with a high orientational order obtained from a novel polymer liquid crystal,” Adv. Mater. (Deerfield Beach Fla.) 13(17), 1337–1339 (2001). [CrossRef]
  6. H. Yu and T. Ikeda, “Photocontrollable liquid-crystalline actuators,” Adv. Mater. (Deerfield Beach Fla.) 23(19), 2149–2180 (2011). [CrossRef] [PubMed]
  7. B. W. Lee and N. A. Clark, “Alignment of liquid crystals with patterned isotropic surfaces,” Science 291(5513), 2576–2580 (2001). [CrossRef] [PubMed]
  8. M. Cavallini, A. Calo, P. Stoliar, J. C. Kengne, S. Martins, F. C. Matacotta, F. Quist, G. Gbabode, N. Dumont, Y. H. Geerts, and F. Biscarini, “Lithographic alignment of discotic liquid crystals: a new time-temperature integrating framework,” Adv. Mater. (Deerfield Beach Fla.) 21(46), 4688–4691 (2009). [CrossRef]
  9. Y. Wu, T. Ikeda, and Q. Zhang, “Three-dimensional manipulation of an azo polymer liquid crystal with unpolarized light,” Adv. Mater. (Deerfield Beach Fla.) 11(4), 300–302 (1999). [CrossRef]
  10. N. Kawatsuki, H. Takatsuka, and T. Yamamoto, “Coplanar alignment of mesogenic moieties in a photocrosslinked liquid crystalline polymer film containing cinnamoyl groups,” Appl. Phys. Lett. 75(10), 1386–1388 (1999). [CrossRef]
  11. K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, “Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer,” Langmuir 4(5), 1214–1216 (1988). [CrossRef]
  12. S. R. Nersisyan, N. V. Tabiryan, L. Hoke, D. M. Steeves, and B. R. Kimball, “Polarization insensitive imaging through polarization gratings,” Opt. Express 17(3), 1817–1830 (2009). [CrossRef] [PubMed]
  13. G. Crawford, J. Eakin, M. D. Radcliffe, A. Callan-Jones, and R. Pelcovits, “Liquid-crystal diffraction gratings using polarization holography alignment techniques,” J. Appl. Phys. 98(12), 123102 (2005). [CrossRef]

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