1. Introduction

Thanks to their versatility, liquid crystal materials have been used in many applications besides displays, data storage, [1, 2] image processing. [3] They are also useful in nano- or micro-scale applications such as electro-optical devices [4] or the fabrication of photonic crystals. [5] Polymer liquid crystals can be used for three-dimensional storage under two-photon illumination. [1] 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, 7] 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, 9]. 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. [10,11]. 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].

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]. 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.

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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.

Two different methods of photopolymerization based on the absorption either of one photon (UV, λ = 365 nm), or of two photons (λ = 780 nm) were used to obtain liquid crystal elastomers with spatially controlled micro-patterns. The focused beam of the microscope will polymerize and crosslink only monomers located in the illuminated volume of the cell. The same liquid crystalline mixture was used for one-photon and two-photon polymerizations since Irgacure 369, a typical UV photoinitiator, has already shown its efficiency for two-photon absorption in the photopolymerization of acrylates. [15]

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.

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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]

$V={\left(\frac{\pi }{2}\right)}^{\frac{3}{2}}{{\omega }^2}_{\mathrm{xy}}{\omega }_z$, where ${\omega }_{\mathrm{xy}}=\frac{0.52\lambda }{\sin \theta }$ and ${\omega }_z=\frac{0.76\lambda }{1-\cos \theta }$ 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.

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In order to stabilize the patterns, a post-photopolymerization of the whole sample can be performed by using UV light of weak energy. Under these conditions, the pattern written by photopolymerization using a high energy beam can be preserved, as demonstrated in previous work.¹⁷ In a fourth series of experiments, conducted for the present work, a 170 μm step grating was first written by using a UV beam of 80 μW.cm^-2 in an aligned monomer sample. The post-polymerization of the whole sample was then conducted by using UV light at 8 μW∙cm^-2. The resulting rubber-like film was removed from the cell. By heating this sample up from room temperature to 120°C, a clear uniaxial deformation of the film is observed in the direction of alignment, as shown in Fig. 4. The step of the grating was found to gradually change from 170 μm to 120 μm upon temperature change. This phenomenon is completely reversible as demonstrated in the movie (supplementary information). Therefore, variable step-gratings which are responsive to an external stimulus were obtained in a nematic elastomer. Here, the one-photon polymerization technique has been used in order to present well resolved pictures.

 

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.

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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.

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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.