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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 2, Iss. 6 — Jun. 13, 2007
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External stimulus driven variable-step grating in a nematic elastomer

Emel Sungur, Min-Hui Li, Gregory Taupier, Alex Boeglin, Michelangelo Romeo, Stéphane Méry, Patrick Keller, and Kokou D. Dorkenoo  »View Author Affiliations


Optics Express, Vol. 15, Issue 11, pp. 6784-6789 (2007)
http://dx.doi.org/10.1364/OE.15.006784


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Abstract

We report on the creation of micro-patterns in an oriented nematic elastomer (an artificial muscle material) by photopolymerization of surface aligned nematic liquid crystal monomers. We demonstrate that microscopic techniques are able to create accurate patterns in rubber-like liquid crystal materials. Two approaches, based on one and two-photon excitations respectively, are implemented using a microscope-based setup. Due to its high spatial selectivity, the two-photon excitation mode yields finer patterns. Benefitting from the intrinsic, thermally-induced, contractile properties of the material, gratings with variable steps in response to temperature changes were fabricated.

© 2007 Optical Society of America

1. Introduction

Thanks to their versatility, liquid crystal materials have been used in many applications besides displays, data storage, [1

1. D. McPhail and M. Gu, “Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination,” Appl. Phys. Lett. 81, 1160–1162 (2002). [CrossRef]

, 2

2. A. Y.-G. Fuh, C.-Y. Tsai, and C.-L. Lu, “Fast optical recording in dye doped polymer dispersed liquid crystal films,” Opt. Lett. 26, 447–449 (2001). [CrossRef]

] image processing. [3

3. M. Y. Shih, A. Shishido, and I. C. Khoo, “All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-substraction,” Opt. Lett. 26, 1140–1142 (2001). [CrossRef]

] They are also useful in nano- or micro-scale applications such as electro-optical devices [4

4. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tamlinnd, and T. J. Bunning, “Holographic formation of electro-optical polymer liquid crystal photonic crystal,” Adv. Mater. 14, 187 (2002). [CrossRef]

] or the fabrication of photonic crystals. [5

5. I. Divliansky, T. S. Mayer, K. S. Holliday, and V. H. Crespi, “Fabrication of three-dimensional polymer photonic crystal structures using single diffraction element interference lithography,” Appl. Phys. Lett. 82, 1667–1669 (2003). [CrossRef]

] Polymer liquid crystals can be used for three-dimensional storage under two-photon illumination. [1

1. D. McPhail and M. Gu, “Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination,” Appl. Phys. Lett. 81, 1160–1162 (2002). [CrossRef]

] In this case, the liquid crystal molecules, which present an optical anisotropy, adopt a direction of alignment that depends on the polarization state of the excitation light. With the help of two-photon polymerization, such polarization states can be frozen in small focal regions allowing the stable recording of 3D bit arrays. This method, which can be used for the fabrication of diffractive optical elements, presents the advantage of being erasable but the size of the dots or the step of the gratings cannot be changed.

On the other hand, liquid crystal materials have been the object of much interest in the pursuit of so called “smart materials”. Indeed, since the first proposition by de Gennes to use nematic liquid crystal elastomers to elaborate “artificial muscles”, [6

6. P.-G. de Gennes, “Réflexions sur un type de polymères nématiques,” C. R. Acad. Sci. Paris, Ser. B 281, 101–103 (1975).

, 7

7. P.-G. de Gennes, “A semi-fast artificial muscle,” C. R. Acad. Sci. Paris, Ser. IIb 324, 343–348 (1997).

] many authors have attempted to synthesize liquid crystalline polymer-based materials capable of modifying their macroscopic shape or size when receiving an external stimulus. [8

8. M.-H. Li and P. Keller, “Artificial muscles based on liquid crystal elastomers,” Phil. Trans. R. Soc. A 364, 2763–2777 (2006). [CrossRef] [PubMed]

, 9

9. Y. Yu and T. Ikeda, “Soft actuators based on liquid-crystalline elastomers,” Angew. Chem. Int. Ed. 45, 5416–5418 (2006). [CrossRef]

]. We have recently developed an approach which uses a photopolymerization/photocrosslinking reaction to directly obtain, in a single step, a “monodomain” nematic side-on elastomer starting from a mixture of a nematic side-on monomer, a crosslinking agent, and a photoinitiator. [10

10. D. L. Thomsen III, P. Keller, J. Naciri, R. Pink, H. Jeon, D. Shenoy, and B. R. Ratna, “Liquid crystal elastomers with mechanical properties of a muscle,” Macromolecules 34, 5868–5875 (2001). [CrossRef]

,11

11. M.-H. Li, P. Keller, B. Li, X. Wang, and M. Brunet, “Light-driven side-on nematic elastomer actuator,” Adv. Mater. 15, 569–572 (2003). [CrossRef]

]. Those nematic elastomers, which are “macroscopic” (their size is in the millimeter/centimeter range), exhibit a large contraction (up to 40%) parallel to the nematic director when stimulated thermally or photochemically. Using this approach, microstructured “artificial muscles” made of nematic liquid crystalline elastomers have recently been fabricated. [13–14

13. A. L. Elias, K. D. Harris, C. W. M. Bastiaansen, D. J. Broer, and M. J. Brett, “Photopatterned liquid crystalline polymers for microactuators,” J. Mater. Chem. 16, 2903–2912 (2006). [CrossRef]

].

In this letter, we propose to realize optical diffractive elements in artificial muscle materials in order to take advantage of their contraction and elongation properties and thus obtain step changing gratings. We have fabricated such gratings by one- and two-photon patterning using a microscope-based setup. In particular, we show that a contraction of 20% can be obtained in the grating steps when the sample is thermally actuated.

2. Materials and sample preparation

The photopolymerizable liquid crystal was a mixture of three compounds: a liquid crystal monomer, a crosslinker, and a UV photoinitiator. The structure of the liquid crystal monomer is shown in Fig. 1. It was synthesized according to a previously published method. [10

10. D. L. Thomsen III, P. Keller, J. Naciri, R. Pink, H. Jeon, D. Shenoy, and B. R. Ratna, “Liquid crystal elastomers with mechanical properties of a muscle,” Macromolecules 34, 5868–5875 (2001). [CrossRef]

]. The crosslinker is 1,6-hexanediol diacrylate (from Aldrich) at a concentration of 10mol%. The UV photoinitiator, added at 1mol%, is 2-benzyl-2-(dimethylamine)-4’-morpholinobutyrophenone (Irgacure 369) (from Aldrich).

Fig. 1. Chemical structure of the nematic liquid crystal monomer.

The nematic-isotropic transition of this mixture occurred at 81.5°C. Both aligned and non-aligned liquid crystal monomer samples were used for photopolymerization. To prepare the aligned samples, the monomer mixture was introduced into a rubbed poly (vinyl alcohol)-coated glass cell of 10 or 20 μm gap, at a temperature of 90°C (liquid state). The filled cell was slowly cooled down (1°C/min) to the nematic phase (63°C) to achieve a uniaxial planar alignment where the long axis of the liquid crystal moieties is aligned parallel to the rubbing direction. To prepare the non-aligned samples, the monomers mixture was introduced into non-treated but otherwise identical glass cells. The photopolymerization under the confocal microscope was then made in the nematic phase at 63°C.

3. Experimental setup

The experimental setup is based on a confocal microscope (see Fig. 2). The liquid crystal cell is mounted on a motorized heating stage that is placed under the objective of the microscope and can execute computer-controlled 3D translation along the X, Y, and Z axes. The UV or IR beam focused by the objective can then “draw” the desirable pattern on the sample at a defined speed (200 μm/s in this study).

The two-photon photopolymerization is induced by focusing the beam of a Spectra-Physics Tsunami laser, set at λ = 780 nm, inside the sample. The laser output consists in a train of pulses of 100 fs duration with a 80 MHz repetition rate, corresponding to 0.9 W of average power. The beam power is measured before the beam is enlarged through a telescope. The signal uniformity at the telescope output is monitored in order to ensure a homogeneous power distribution. The beam is then sent through the microscope objective and focused inside the sample. For this experiment, an objective with a magnification of 50 and a numerical aperture of 0.45 was used. The shape of the voxel where the two-photon polymerization will actually occur is defined by the power density near its waist (focal point). For a standard Gaussian beam, the polymerized region is an ellipsoid with its major axis aligned along the propagation direction z. For the one-photon polymerization, the IR laser is replaced by an Argon laser at 365 nm, all the geometrical parameters being unchanged.

Fig. 2. Schematic presentation of the experimental setup based on a confocal microscope. Femtosecond laser (80 MHz at 780 nm) or UV argon laser (365 nm) were used as excitation beams. The sample is placed on a thermally controlled heating stage, mounted on (X, Y, Z) 3D computer controlled translation stages.

4. Results and discussion

Different sets of experiments have first been performed, depending on the sample alignment state (using treated or untreated glass cells) and the excitation source (UV or IR light).

The first set of experiments was made using UV excitation (Argon laser, λ= 365 nm, 8 μW∙cm-2) and the non-aligned nematic monomer sample. The polymerized and crosslinked part draws the capital letter “E.” The same microscope setup was used to visualize the quality of the patterning. Figures 3(a.1) and 3(a.2) show the images of the sample heated at 82°C and observed between uncrossed and crossed polarizers, respectively. At this temperature, the unpolymerized part is in the isotropic phase, while the polymerized part is in the nematic mesophase (The nematic to isotropic phase transition temperature is 120°C for the liquid crystal elastomer, i.e. the polymerized part). The dark domain in Fig. 3(a.2) corresponds to the isotropic phase of the unpolymerized part observed under crossed polarizers.

The bright letter “E” is an effect of the birefringence of the liquid crystal phase and thus demonstrates the success of the polymerization by the UV beam.

The second set of experiments was performed using the same conditions as described above (using UV light), excepted that an aligned nematic monomer sample was used [see Fig. 3(b)]. The polymerization was also successful (see Fig. 3(b.1) and Fig. 3(b.2). However, the polymerized letter “E” presents thinner (30 μm) and smoother lines than those obtained for the non-aligned sample (50–60 μm). The pattern lines are undistorted in the aligned sample. Therefore, using aligned nematic samples results in considerably improved micro-pattern formation.

In the third set of experiments performed again on aligned samples, a 780 nm-IR femtosecond laser delivering a power of 500 mW∙cm-2 was used instead of the Argon laser. The results presented in Fig. 3(c) show that such two-photon excitation processes produce a smooth pattern with even thinner lines measuring 5 to 10 μm across. The high resolution achieved in this case is indeed better than with UV illumination because two-photon excitation is defined by a voxel which is localized in the vicinity of the focal point. The size of the voxel is described by the following formula: [16

16. J. Mertz, “Molecular photodynamics involved in multiphoton excitation fluorescence microscopy,” Eur. Phy. J. D 3, 53–66 (1998). [CrossRef]

]

V=(π2)32ω2xyωz, where ωxy=0.52λsinθ and ωz=0.76λ1cosθ are respectively the transversal and longitudinal profiles of the waist, and θ the half-angle of the objective aperture. This formula gives a transversal size value of 0.9 μm, which is lower than the experimental value of ~7 μm. This higher experimental value may be attributed to the diffusion of the photo-created radicals which grows with the exposure time define by the speed of the writing beam (here 200 μm.s-1).

Fig. 3. Letter “E” patterns of liquid crystal elastomer produced by UV and two-photon photolithography under microscope (sample cell thickness: 10 μm; microscope objective: numerical aperture 0.45, magnification 50). (a) by UV polymerization at 8 μW.cm-2 in a non-aligned sample. (b) by UV polymerization at 500 mW∙cm-2 in an aligned sample. (c) by two-photon polymerization at 500 mW∙cm-2 in an aligned sample. For observation, polarizers were uncrossed in (a.1), (b.1) and (c.1), and were crossed in (a.2), (b.2) and (c.2). The width of the E-letter is 800 μm. The resolution of two-photon beam is 7 μm, while that of UV beam is around 20 μm. The nematic director orientation is indicated by the arrow. The observation temperature is 82°C.

Fig 4. (Avi movie MB) Liquid crystal elastomer with grating pattern created by UV polymerization. The grating pattern was first created by UV light of high energy (80 μW.cm-2) in the aligned monomer sample and a post-polymerization by UV light of low energy (8 μW.cm-2) was then carried out in the whole sample to produce the elastomer film. The sample thickness is 20 μm. The grating step changes reversibly as a function of temperature (from 170 μm at T = 63°C to 120 μm at T = 120°C) due to the thermal contraction of the liquid crystal elastomer along the direction of alignment (indicated by the arrow). The film floats on a thin layer of silicon oil and there is nearly no constraint during the contraction.

Using the two-photon polymerization process, gratings with steps below the diffraction limit can be fabricated but they can only be tested by monitoring the diffracted light. However, this technique can also produce large-step gratings and this has been done in Fig. 5. The diffraction efficiency in the first-order was about 2.5 % at room temperature, and increased to 4% when heated at above 100°C.

Fig. 5. Two-photon 50 μm grating (left) and its diffraction pattern (right) recorded at room temperature.

5. Conclusion

We have demonstrated that one photon “UV” photopolymerization as well as two-photon “IR” photopolymerization can be used to microstructure artificial muscle materials made of nematic liquid crystalline elastomers without losing the contraction/extension properties. The grating design generated in the sample can be used as a step changing grating when subject to an external stimulus such as a temperature increase. This kind of moving-step grating can be used as a template for distributed feedback lasers or for diffractive optics.

Acknowledgment

K. D. Dorkenoo wishes to thank J-Y Bigot, D. Guillon for helpful advice and scientific discussions. This research was supported in part by CNRS, ACI “Nanoscience” 2004 NR147.

References and links

1.

D. McPhail and M. Gu, “Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination,” Appl. Phys. Lett. 81, 1160–1162 (2002). [CrossRef]

2.

A. Y.-G. Fuh, C.-Y. Tsai, and C.-L. Lu, “Fast optical recording in dye doped polymer dispersed liquid crystal films,” Opt. Lett. 26, 447–449 (2001). [CrossRef]

3.

M. Y. Shih, A. Shishido, and I. C. Khoo, “All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-substraction,” Opt. Lett. 26, 1140–1142 (2001). [CrossRef]

4.

V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tamlinnd, and T. J. Bunning, “Holographic formation of electro-optical polymer liquid crystal photonic crystal,” Adv. Mater. 14, 187 (2002). [CrossRef]

5.

I. Divliansky, T. S. Mayer, K. S. Holliday, and V. H. Crespi, “Fabrication of three-dimensional polymer photonic crystal structures using single diffraction element interference lithography,” Appl. Phys. Lett. 82, 1667–1669 (2003). [CrossRef]

6.

P.-G. de Gennes, “Réflexions sur un type de polymères nématiques,” C. R. Acad. Sci. Paris, Ser. B 281, 101–103 (1975).

7.

P.-G. de Gennes, “A semi-fast artificial muscle,” C. R. Acad. Sci. Paris, Ser. IIb 324, 343–348 (1997).

8.

M.-H. Li and P. Keller, “Artificial muscles based on liquid crystal elastomers,” Phil. Trans. R. Soc. A 364, 2763–2777 (2006). [CrossRef] [PubMed]

9.

Y. Yu and T. Ikeda, “Soft actuators based on liquid-crystalline elastomers,” Angew. Chem. Int. Ed. 45, 5416–5418 (2006). [CrossRef]

10.

D. L. Thomsen III, P. Keller, J. Naciri, R. Pink, H. Jeon, D. Shenoy, and B. R. Ratna, “Liquid crystal elastomers with mechanical properties of a muscle,” Macromolecules 34, 5868–5875 (2001). [CrossRef]

11.

M.-H. Li, P. Keller, B. Li, X. Wang, and M. Brunet, “Light-driven side-on nematic elastomer actuator,” Adv. Mater. 15, 569–572 (2003). [CrossRef]

12.

A. Buguin, M.-H. Li, P. Silberzan, B. Ladoux, and P. Keller, “Micro-actuators: when artificial muscles made of nematic crystal elastomer meet soft lithography,” J. Am. Chem. Soc. 128, 1088–1089 (2006). [CrossRef] [PubMed]

13.

A. L. Elias, K. D. Harris, C. W. M. Bastiaansen, D. J. Broer, and M. J. Brett, “Photopatterned liquid crystalline polymers for microactuators,” J. Mater. Chem. 16, 2903–2912 (2006). [CrossRef]

14.

M. E. Sousa, D. J. Broer, C. W. M. Bastiaansen, L. B. Freund, and G. P. Crawford, “Isotropic “islands” in a cholesteric “sea”: patterned thermal expansion for responsive surface topologies,” Adv. Mater. 18, 1842–1845 (2006). [CrossRef]

15.

K. J. Schafer, J. M. Hales, M. Balu, K. D. Belfield, E. W. Van Stryland, and D. J. Hagan, “Two-photon absorption cross-sections of common photoinitiators,” J. Photochem. Photobiol. A 162, 497–502 (2004). [CrossRef]

16.

J. Mertz, “Molecular photodynamics involved in multiphoton excitation fluorescence microscopy,” Eur. Phy. J. D 3, 53–66 (1998). [CrossRef]

17.

K. D. Dorkenoo, F. Gillot, O. Crégut, Y. Sonnefraud, A. Fort, and H. Leblond “Control of the refractive index in photopolymerizable materials for (2+1)D solitary wave guide formation,” Phys. Rev. Lett. 93, 143905 (2004). [CrossRef] [PubMed]

OCIS Codes
(180.1790) Microscopy : Confocal microscopy
(190.4180) Nonlinear optics : Multiphoton processes
(230.3720) Optical devices : Liquid-crystal devices

ToC Category:
Microscopy

History
Original Manuscript: February 9, 2007
Revised Manuscript: March 23, 2007
Manuscript Accepted: March 24, 2007
Published: May 18, 2007

Virtual Issues
Vol. 2, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Emel Sungur, Min-Hui Li, Gregory Taupier, Alex Boeglin, Michelangelo Romeo, Stéphane Mery, Patrick Keller, and Kokou D. Dorkenoo, "External stimulus driven variable-step grating in a nematic elastomer," Opt. Express 15, 6784-6789 (2007)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-15-11-6784


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References

  1. D. McPhail and M. Gu, "Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination," Appl. Phys. Lett. 81, 1160-1162 (2002). [CrossRef]
  2. A. Y.-G. Fuh, C.-Y. Tsai and C.-L. Lu, "Fast optical recording in dye doped polymer dispersed liquid crystal films," Opt. Lett. 26, 447-449 (2001). [CrossRef]
  3. M. Y. Shih, A. Shishido and I. C. Khoo, "All-optical image processing by means of a photosensitive nonlinear liquid-crystal film: edge enhancement and image addition-substraction," Opt. Lett. 26, 1140-1142 (2001). [CrossRef]
  4. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland. D. Tamlinnd and T. J. Bunning, "Holographic formation of electro-optical polymer liquid crystal photonic crystal," Adv. Mater. 14, 187 (2002). [CrossRef]
  5. I. Divliansky, T. S. Mayer, K. S. Holliday, and V. H. Crespi, "Fabrication of three-dimensional polymer photonic crystal structures using single diffraction element interference lithography," Appl. Phys. Lett. 82, 1667-1669 (2003). [CrossRef]
  6. P.-G. de Gennes, "Réflexions sur un type de polymères nématiques," C. R. Acad. Sci. Paris, Ser. B 281, 101-103 (1975).
  7. P.-G. de Gennes, "A semi-fast artificial muscle," C. R. Acad. Sci. Paris, Ser.IIb 324, 343-348 (1997).
  8. M.-H. Li and P. Keller, "Artificial muscles based on liquid crystal elastomers," Phil. Trans. R. Soc. A 364, 2763-2777 (2006). [CrossRef] [PubMed]
  9. Y. Yu and T. Ikeda, "Soft actuators based on liquid-crystalline elastomers," Angew. Chem. Int. Ed. 45, 5416-5418 (2006). [CrossRef]
  10. D. L. Thomsen III, P. Keller, J. Naciri, R. Pink, H. Jeon, D. Shenoy and B. R. Ratna, "Liquid crystal elastomers with mechanical properties of a muscle," Macromolecules 34, 5868-5875 (2001). [CrossRef]
  11. M.-H. Li, P. Keller, B. Li, X. Wang and M. Brunet, "Light-driven side-on nematic elastomer actuator," Adv. Mater. 15, 569-572 (2003). [CrossRef]
  12. A. Buguin, M.-H. Li, P. Silberzan, B. Ladoux, and P. Keller, "Micro-actuators: when artificial muscles made of nematic crystal elastomer meet soft lithography," J. Am. Chem. Soc. 128, 1088-1089 (2006). [CrossRef] [PubMed]
  13. A. L. Elias, K. D. Harris, C. W. M. Bastiaansen, D. J. Broer, and M. J. Brett, "Photopatterned liquid crystalline polymers for microactuators," J. Mater. Chem. 16, 2903-2912 (2006). [CrossRef]
  14. M. E. Sousa, D. J. Broer, C. W. M. Bastiaansen, L. B. Freund, and G. P. Crawford, "Isotropic "islands" in a cholesteric "sea": patterned thermal expansion for responsive surface topologies," Adv. Mater. 18, 1842-1845 (2006). [CrossRef]
  15. K. J. Schafer, J. M. Hales, M. Balu, K. D. Belfield, E. W. Van Stryland and D. J. Hagan, "Two-photon absorption cross-sections of common photoinitiators," J. Photochem. Photobiol. A 162, 497-502 (2004). [CrossRef]
  16. J. Mertz, "Molecular photodynamics involved in multiphoton excitation fluorescence microscopy," Eur. Phy. J. D 3, 53-66 (1998). [CrossRef]
  17. K. D. Dorkenoo, F. Gillot, O. Crégut, Y. Sonnefraud, A. Fort and H. Leblond "Control of the refractive index in photopolymerizable materials for (2+1)D solitary wave guide formation," Phys. Rev. Lett. 93, 143905 (2004). [CrossRef] [PubMed]

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