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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 15 — Jul. 16, 2012
  • pp: 16684–16689
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Arbitrary photo-patterning in liquid crystal alignments using DMD based lithography system

Hao Wu, Wei Hu, Hua-chao Hu, Xiao-wen Lin, Ge Zhu, Jae-Won Choi, Vladimir Chigrinov, and Yan-qing Lu  »View Author Affiliations


Optics Express, Vol. 20, Issue 15, pp. 16684-16689 (2012)
http://dx.doi.org/10.1364/OE.20.016684


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Abstract

We propose and implement a technique for arbitrary pattern fabrication in liquid crystal (LC) alignments and local polarization control for light wavefront. A micro-lithography system with a digital micro-mirror device as dynamic mask forms arbitrary micro-images on photoalignment layers and further guides the LC molecule orientations. Besides normal phase gratings, more complex 2D patterns such as quasicrystal and checkerboard structures are demonstrated. To characterize the optical performances of the fabricated structures, the electro-optically tunable diffraction patterns and efficiencies are demonstrated in several 1D/2D phase gratings. Compared to other techniques, our method enables the arbitrary and instant manipulation of LC alignments and light polarization states, facilitating wide applications in display and photonic fields.

© 2012 OSA

1. Introduction

Liquid crystal (LC) devices have been widely used in flat panel display, optical communication, information processing and sensing [1

1. D. K. Yang and S. T. Wu, Fundamentals of Liquid Crystal Devices (Wiley, 2006).

5

5. Y. H. Wu, Y. H. Lin, Y. Q. Lu, H. W. Ren, Y. H. Fan, J. R. Wu, and S. T. Wu, “Submillisecond response variable optical attenuator based on sheared polymer network liquid crystal,” Opt. Express 12(25), 6382–6389 (2004). [CrossRef] [PubMed]

] for their advantages of low voltage, light weight, low cost, no moving parts, and low power consumption. LC alignment is the foundation of a wide variety of LC components. Recently, photoalignment technique has attracted intensive interests for it avoids the problems resulting from the conventional rubbing method, such as contamination, electrostatic charges and mechanical damage [6

6. V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, England, 2008).

]. Besides, photoalignment also brings the possibility of fabricating certain patterns with different LC alignments, which contributes to the wide viewing angles in liquid crystal displays [7

7. M. Schadt, H. Seiberle, and A. Schuster, “Optical patterning of multi-domain liquid-crystal displays with wide viewing-angles,” Nature 381(6579), 212–215 (1996). [CrossRef]

] and various photonic structures such as LC gratings [8

8. S. Y. Huang, S. T. Wu, and A. Y. G. Fuh, “Optically switchable twist nematic grating based on a dye-doped liquid crystal film,” Appl. Phys. Lett. 88(4), 041104 (2006). [CrossRef]

10

10. W. Y. Wu and A. Y. G. Fuh, “Rewritable liquid crystal gratings fabricated using photoalignment effect in dye-doped poly(vinyl alcohol) film,” Jpn. J. Appl. Phys. 46(10A), 6761–6766 (2007).

]. Different approaches have been proposed to obtain patterned alignments. Presnyakov et al realized a grating using azo-dye alignment layer exposed with two interfering laser beams of opposite circular polarizations [11

11. V. Presnyakov, K. Asatryan, T. Galstian, and V. Chigrinov, “Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer,” Opt. Express 14(22), 10558–10564 (2006). [CrossRef] [PubMed]

]. Exploiting UV photoalignment and amplitude mask, Kapoustine et al demonstrated switchable LC gratings through introducing a periodic variation of the alignment direction into one substrate of cells [9

9. V. Kapoustine, A. Kazakevitch, V. So, and R. Tam, “Simple method of formation of switchable liquid crystal gratings by introducing periodic photoalignment pattern into liquid crystal cell,” Opt. Commun. 266(1), 1–5 (2006). [CrossRef]

]. Zhao et al reported a micropolarizer array by using a linearly polarized light to define the orientation of LCs [12

12. X. Zhao, A. Bermak, F. Boussaid, T. Du, and V. G. Chigrinov, “High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum,” Opt. Lett. 34(23), 3619–3621 (2009). [CrossRef] [PubMed]

]. Hu et al presented both one dimensional (1D) and two dimensional (2D) liquid crystal gratings with alternate twisted nematic (TN)/homogeneously alignment (PA) and orthogonal PA regions respectively by two-step photo exposure [13

13. W. Hu, A. Srivastava, F. Xu, J. T. Sun, X. W. Lin, H. Q. Cui, V. Chigrinov, and Y. Q. Lu, “Liquid crystal gratings based on alternate TN and PA photoalignment,” Opt. Express 20(5), 5384–5391 (2012). [CrossRef] [PubMed]

, 14

14. W. Hu, A. Kumar Srivastava, X.-W. Lin, X. Liang, Z.-J. Wu, J.-T. Sun, G. Zhu, V. Chigrinov, and Y.-Q. Lu, “Polarization independent liquid crystal gratings based on orthogonal photoalignments,” Appl. Phys. Lett. 100(11), 111116 (2012). [CrossRef]

]. With multi interfering beams method, only periodic patterns such as 1D and 2D gratings have been demonstrated, for more complex structures it will be difficult to realize. While it would be arduous and cost inefficient to accomplish complex structures in multi LC orientations with conventional amplitude mask because of the employment of multiple masks as well as the inevitable registration issue. Besides, the beam expansion limits the resolution. Above shortcomings considerably constrain their applications.

To address these limitations, the Digital Micro-mirror Device (DMD) based micro-lithography could be considered [15

15. C. Sun, N. Fang, D. M. Wu, and X. Zhang, “Projection micro-stereolithography using digital micro-mirror dynamic mask,” Sensor. Actuators, A. 121(1), 113–120 (2005).

, 16

16. C. Culbreath, N. Glazar, and H. Yokoyama, “Note: Automated maskless micro-multidomain photoalignment,” Rev. Sci. Instrum. 82(12), 126107 (2011). [CrossRef] [PubMed]

]. Herein, we use the DMD as a dynamic mask, generating arbitrary patterns by individually tilting angle control of each mirror, thus supply a one-mask-for-all method that complex patterns will be generated with great ease. Furthermore, mask registration problem in multi-exposure could be eliminated because no mechanical movements occur when changing masks, which also greatly simplifies the fabrication process. High resolution could be reached with a projection lens for it avoids beam expansion problem.

In this work, we utilize a DMD based micro-lithography system to control the LC alignments. Micro patterns generated by DMD are projected onto sulfonic azo-dye (SD1) films through a polarizer. By this means, arbitrary patterns such as 1D Fibonacci grating, checkerboard pattern and 12-fold symmetric quasi-period pattern are demonstrated. High resolution up to 5 μm is achieved, and further improvement is possible. Thanks to the image generating ability of DMD, combined with the good alignment rewritability of SD1, arbitrary azimuthal angle control of LC is also realized by rotating the polarizer. TN regions with twist angles varying from 10° to 90° with an interval of 10° are accomplished in a single LC cell. The electro-optical properties of 1D phase grating are measured. The diffraction efficiency of the first order reaches over 22% and the intensity of the same order could be electrically suppressed by over 20 dB. New applications relying on free manipulation of LC alignments and light polarizations are achievable by this approach.

2. Design and fabrication

As shown in Fig. 1
Fig. 1 Schematic illustration of DMD based micro-lithography system.
, the DMD based micro-lithography system consists of several sub-systems, including a light emission part, a dynamic pattern generation part, an image focusing part and a monitor part, all of which function in cooperation to provide correct exposure on the substrate. A mercury lamp (S1000, EXFO, Canada) filtered at 320-500 nm along with a collimating lens provide a uniform and collimated light beam. The beam illuminate onto the DMD surface through an iris. DMD (1024 × 768, Discovery 3000, Texas Instruments), as a dynamic mask, consists of more than 786,000 micro-mirrors. Independently tilted by an electrostatic force, each mirror (13.68 μm × 13.68 μm in size) can be switched between “on” and “off” states by toggling the applied voltage. The bundle of light reflected by the “on” state mirrors will form a desired light pattern [17

17. J. W. Choi, Y. M. Ha, S. H. Lee, and K. H. Choi, “Design of microstereolithography system based on dynamic image projection for fabrication of three-dimensional microstructures,” J. Mech. Sci. Technol. 20(12), 2094–2104 (2006). [CrossRef]

]. Then the pattern will be focused by an apo-chromatically corrected projection lens (10 × , NA = 0.3, WD = 34 mm, Cinv Optics Co., China) and recorded on the substrate placed at the image plane. The light reflected by the substrate is then collected into a CCD, which is used to monitor the focusing of the image. In this system, several parameters are adjustable: the focal length of the collimating lens, the distance between the collimating lens and DMD and that between DMD and the objective lens. We optimized these parameters by ZEMAXTM thus achieved a uniform illumination on DMD and a clear image at the substrate.

3. Results and discussions

Beside flexible pattern manipulation, local alignment direction control is also realized by rotating the polarizer. We demonstrated a polarization rotator array with different angles in a single LC cell. The two substrates are photoaligned in parallel and then 9 regions on one of the substrates are realigned sequentially through changing the DMD pattern (disk position) as well as the polarization of activating light. Thus, 9 TN disks with twist angles varying from 10° to 90° at an interval of 10° are formed, observed as different gray scales in Figs. 3(a)
Fig. 3 Micrographs of LC polarization rotators with different twist angles under a polarizing microscope with a) crossed and b) parallel polarizers.
and 3(b). During observation, the light is incident to the unpatterned substrate first, with its polarization parallel to the alignment direction, and then when passing through the cell, only components along the analyzer directions could be observed. The results suggest the possibility of arbitrary polarization control of any part of the wavefront. Furthermore, in the whole exposure process, no mechanical movements of either DMD or substrate are needed, avoiding the registration problem of conventional multi-step lithography.

Experimental results reveal the following advantages of our system: 1) Arbitrary pattern fabrication and local polarization control are practical. 2) High resolution of 5.5 μm has been reached. The theoretical optical resolution of current system is 1.4 μm, according to the DMD pixel size and the minification of the projection lens. By further optimization such as substituting the projection lens with one of higher magnification, resolution up to diffraction limit could be achieved. 3) With the projection system, the substrate is exposed at the focus plane; so beam expansion problem can be overcome. Therefore, even after the substrates being assembled to a cell, high quality replication can still be obtained, allowing instant control of the alignment [6

6. V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, England, 2008).

]. 4) No mechanical movements are needed in multi-step exposure, avoiding the registration problem of conventional lithography technique.

Above advantages of this technique permit wide range of applications, from LC display to photonic fields. Quasicrystals are attracting considerable interests because of their high level of symmetry, which makes them excellent candidates of photonic bandgap materials [18

18. Y. Y. Liu and R. Riklund, “Electronic properties of perfect and nonperfect one-dimensional quasicrystals,” Phys. Rev. B Condens. Matter 35(12), 6034–6042 (1987). [CrossRef] [PubMed]

]. This should be the first demonstration of quasi-periodic structures in LC, which opens a new door to realize tunable photonic crystal devices. The technique also enables locally control of polarization states, which is significant for the use in generating vector beams [20

20. G. Montemezzani and M. Zgonik, “Light diffraction at mixed phase and absorption gratings in anisotropic media for arbitrary geometries,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55(1), 1035–1047 (1997). [CrossRef]

], realizing integrated [21

21. Y. Liao, M. Huang, Y. F. Ju, F. F. Luo, Y. Cheng, Z. Xu, K. Sugioka, and K. Midorikawa, “Alignment of liquid crystal molecules in a micro-cell fabricated by femtosecond laser,” Chem. Phys. Lett. 498(1–3), 188–191 (2010). [CrossRef]

] and multi-stable LC devices [19

19. G. J. Parker, M. E. Zoorob, M. D. B. Charlton, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature 404(6779), 740–743 (2000). [CrossRef] [PubMed]

].

4. Conclusion

Acknowledgments

References and links

1.

D. K. Yang and S. T. Wu, Fundamentals of Liquid Crystal Devices (Wiley, 2006).

2.

E. J. Jang, H. R. Kim, Y. J. Na, and S. D. Lee, “Multistage optical memory of a liquid crystal diffraction grating in a single beam rewriting scheme,” Appl. Phys. Lett. 91(7), 071109 (2007). [CrossRef]

3.

X. W. Lin, J. B. Wu, W. Hu, Z. G. Zheng, Z. J. Wu, G. Zhu, F. Xu, B. B. Jin, and Y. Q. Lu, “Self-polarizing terahertz liquid crystal phase shifter,” AIP Advances 1(3), 032133 (2011). [CrossRef]

4.

J. Feng, Y. Zhao, S.- S. Li, X. W. Lin, F. Xu, and Y. Q. Lu, “Fiber optic pressure sensor based on tunable liquid crystal technology,” IEEE Photon. J. 2(3), 292–298 (2010). [CrossRef]

5.

Y. H. Wu, Y. H. Lin, Y. Q. Lu, H. W. Ren, Y. H. Fan, J. R. Wu, and S. T. Wu, “Submillisecond response variable optical attenuator based on sheared polymer network liquid crystal,” Opt. Express 12(25), 6382–6389 (2004). [CrossRef] [PubMed]

6.

V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, England, 2008).

7.

M. Schadt, H. Seiberle, and A. Schuster, “Optical patterning of multi-domain liquid-crystal displays with wide viewing-angles,” Nature 381(6579), 212–215 (1996). [CrossRef]

8.

S. Y. Huang, S. T. Wu, and A. Y. G. Fuh, “Optically switchable twist nematic grating based on a dye-doped liquid crystal film,” Appl. Phys. Lett. 88(4), 041104 (2006). [CrossRef]

9.

V. Kapoustine, A. Kazakevitch, V. So, and R. Tam, “Simple method of formation of switchable liquid crystal gratings by introducing periodic photoalignment pattern into liquid crystal cell,” Opt. Commun. 266(1), 1–5 (2006). [CrossRef]

10.

W. Y. Wu and A. Y. G. Fuh, “Rewritable liquid crystal gratings fabricated using photoalignment effect in dye-doped poly(vinyl alcohol) film,” Jpn. J. Appl. Phys. 46(10A), 6761–6766 (2007).

11.

V. Presnyakov, K. Asatryan, T. Galstian, and V. Chigrinov, “Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer,” Opt. Express 14(22), 10558–10564 (2006). [CrossRef] [PubMed]

12.

X. Zhao, A. Bermak, F. Boussaid, T. Du, and V. G. Chigrinov, “High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum,” Opt. Lett. 34(23), 3619–3621 (2009). [CrossRef] [PubMed]

13.

W. Hu, A. Srivastava, F. Xu, J. T. Sun, X. W. Lin, H. Q. Cui, V. Chigrinov, and Y. Q. Lu, “Liquid crystal gratings based on alternate TN and PA photoalignment,” Opt. Express 20(5), 5384–5391 (2012). [CrossRef] [PubMed]

14.

W. Hu, A. Kumar Srivastava, X.-W. Lin, X. Liang, Z.-J. Wu, J.-T. Sun, G. Zhu, V. Chigrinov, and Y.-Q. Lu, “Polarization independent liquid crystal gratings based on orthogonal photoalignments,” Appl. Phys. Lett. 100(11), 111116 (2012). [CrossRef]

15.

C. Sun, N. Fang, D. M. Wu, and X. Zhang, “Projection micro-stereolithography using digital micro-mirror dynamic mask,” Sensor. Actuators, A. 121(1), 113–120 (2005).

16.

C. Culbreath, N. Glazar, and H. Yokoyama, “Note: Automated maskless micro-multidomain photoalignment,” Rev. Sci. Instrum. 82(12), 126107 (2011). [CrossRef] [PubMed]

17.

J. W. Choi, Y. M. Ha, S. H. Lee, and K. H. Choi, “Design of microstereolithography system based on dynamic image projection for fabrication of three-dimensional microstructures,” J. Mech. Sci. Technol. 20(12), 2094–2104 (2006). [CrossRef]

18.

Y. Y. Liu and R. Riklund, “Electronic properties of perfect and nonperfect one-dimensional quasicrystals,” Phys. Rev. B Condens. Matter 35(12), 6034–6042 (1987). [CrossRef] [PubMed]

19.

G. J. Parker, M. E. Zoorob, M. D. B. Charlton, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature 404(6779), 740–743 (2000). [CrossRef] [PubMed]

20.

G. Montemezzani and M. Zgonik, “Light diffraction at mixed phase and absorption gratings in anisotropic media for arbitrary geometries,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55(1), 1035–1047 (1997). [CrossRef]

21.

Y. Liao, M. Huang, Y. F. Ju, F. F. Luo, Y. Cheng, Z. Xu, K. Sugioka, and K. Midorikawa, “Alignment of liquid crystal molecules in a micro-cell fabricated by femtosecond laser,” Chem. Phys. Lett. 498(1–3), 188–191 (2010). [CrossRef]

OCIS Codes
(110.3960) Imaging systems : Microlithography
(160.3710) Materials : Liquid crystals

ToC Category:
Optical Devices

History
Original Manuscript: May 11, 2012
Revised Manuscript: June 28, 2012
Manuscript Accepted: June 29, 2012
Published: July 9, 2012

Citation
Hao Wu, Wei Hu, Hua-chao Hu, Xiao-wen Lin, Ge Zhu, Jae-Won Choi, Vladimir Chigrinov, and Yan-qing Lu, "Arbitrary photo-patterning in liquid crystal alignments using DMD based lithography system," Opt. Express 20, 16684-16689 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-16684


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References

  1. D. K. Yang and S. T. Wu, Fundamentals of Liquid Crystal Devices (Wiley, 2006).
  2. E. J. Jang, H. R. Kim, Y. J. Na, and S. D. Lee, “Multistage optical memory of a liquid crystal diffraction grating in a single beam rewriting scheme,” Appl. Phys. Lett.91(7), 071109 (2007). [CrossRef]
  3. X. W. Lin, J. B. Wu, W. Hu, Z. G. Zheng, Z. J. Wu, G. Zhu, F. Xu, B. B. Jin, and Y. Q. Lu, “Self-polarizing terahertz liquid crystal phase shifter,” AIP Advances1(3), 032133 (2011). [CrossRef]
  4. J. Feng, Y. Zhao, S.- S. Li, X. W. Lin, F. Xu, and Y. Q. Lu, “Fiber optic pressure sensor based on tunable liquid crystal technology,” IEEE Photon. J.2(3), 292–298 (2010). [CrossRef]
  5. Y. H. Wu, Y. H. Lin, Y. Q. Lu, H. W. Ren, Y. H. Fan, J. R. Wu, and S. T. Wu, “Submillisecond response variable optical attenuator based on sheared polymer network liquid crystal,” Opt. Express12(25), 6382–6389 (2004). [CrossRef] [PubMed]
  6. V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, England, 2008).
  7. M. Schadt, H. Seiberle, and A. Schuster, “Optical patterning of multi-domain liquid-crystal displays with wide viewing-angles,” Nature381(6579), 212–215 (1996). [CrossRef]
  8. S. Y. Huang, S. T. Wu, and A. Y. G. Fuh, “Optically switchable twist nematic grating based on a dye-doped liquid crystal film,” Appl. Phys. Lett.88(4), 041104 (2006). [CrossRef]
  9. V. Kapoustine, A. Kazakevitch, V. So, and R. Tam, “Simple method of formation of switchable liquid crystal gratings by introducing periodic photoalignment pattern into liquid crystal cell,” Opt. Commun.266(1), 1–5 (2006). [CrossRef]
  10. W. Y. Wu and A. Y. G. Fuh, “Rewritable liquid crystal gratings fabricated using photoalignment effect in dye-doped poly(vinyl alcohol) film,” Jpn. J. Appl. Phys.46(10A), 6761–6766 (2007).
  11. V. Presnyakov, K. Asatryan, T. Galstian, and V. Chigrinov, “Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer,” Opt. Express14(22), 10558–10564 (2006). [CrossRef] [PubMed]
  12. X. Zhao, A. Bermak, F. Boussaid, T. Du, and V. G. Chigrinov, “High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum,” Opt. Lett.34(23), 3619–3621 (2009). [CrossRef] [PubMed]
  13. W. Hu, A. Srivastava, F. Xu, J. T. Sun, X. W. Lin, H. Q. Cui, V. Chigrinov, and Y. Q. Lu, “Liquid crystal gratings based on alternate TN and PA photoalignment,” Opt. Express20(5), 5384–5391 (2012). [CrossRef] [PubMed]
  14. W. Hu, A. Kumar Srivastava, X.-W. Lin, X. Liang, Z.-J. Wu, J.-T. Sun, G. Zhu, V. Chigrinov, and Y.-Q. Lu, “Polarization independent liquid crystal gratings based on orthogonal photoalignments,” Appl. Phys. Lett.100(11), 111116 (2012). [CrossRef]
  15. C. Sun, N. Fang, D. M. Wu, and X. Zhang, “Projection micro-stereolithography using digital micro-mirror dynamic mask,” Sensor. Actuators, A.121(1), 113–120 (2005).
  16. C. Culbreath, N. Glazar, and H. Yokoyama, “Note: Automated maskless micro-multidomain photoalignment,” Rev. Sci. Instrum.82(12), 126107 (2011). [CrossRef] [PubMed]
  17. J. W. Choi, Y. M. Ha, S. H. Lee, and K. H. Choi, “Design of microstereolithography system based on dynamic image projection for fabrication of three-dimensional microstructures,” J. Mech. Sci. Technol.20(12), 2094–2104 (2006). [CrossRef]
  18. Y. Y. Liu and R. Riklund, “Electronic properties of perfect and nonperfect one-dimensional quasicrystals,” Phys. Rev. B Condens. Matter35(12), 6034–6042 (1987). [CrossRef] [PubMed]
  19. G. J. Parker, M. E. Zoorob, M. D. B. Charlton, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature404(6779), 740–743 (2000). [CrossRef] [PubMed]
  20. G. Montemezzani and M. Zgonik, “Light diffraction at mixed phase and absorption gratings in anisotropic media for arbitrary geometries,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics55(1), 1035–1047 (1997). [CrossRef]
  21. Y. Liao, M. Huang, Y. F. Ju, F. F. Luo, Y. Cheng, Z. Xu, K. Sugioka, and K. Midorikawa, “Alignment of liquid crystal molecules in a micro-cell fabricated by femtosecond laser,” Chem. Phys. Lett.498(1–3), 188–191 (2010). [CrossRef]

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