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We report a simple method to prepare an array of polarization converters using a twisted-azimuthal nematic liquid crystal (NLC) in cylindrical polymer cavities. When a NLC is filled in a cylindrical polymer cavity, LC in the cavity presents concentrically circular orientations. By treating LC on one side of the cavity with homogeneous alignment, a twisted-azimuthal texture is formed. Such a LC texture can convert a linear polarization light to either radial or azimuthal polarization light depending on the polarization direction of the incident light. The LC surface on the other side of the cavity is convex, so the light after passing through the cavity can be focused as well. The LC texture can be fixed firmly using polymer network. In comparison with previous polarization converters, our polarization converter has the merits of individually miniature size, array of pattern, and lens character. Our polarization converter array has potential applications in tight focusing, imaging, and material processing.
©2013 Optical Society of America
1. Introduction
A polarization converter, which can convert a linear polarization light into a radialand/orazimuthal polarization light, is a useful optical device. Unlike a linearly polarized light, radial and azimuthal polarization light possess a high degree of symmetry. Due to this unique characteristic, they have found widespread applications in biological tissue analysis [1], particle trapping [2,3], tight focusing [4–6], material processing [7],and polarization camera imaging [8]. Except some special lasers which can create radial or azimuthal polarization beams [9,10], most of light sources do not produce such polarization lights. To generate radial and/or azimuthal polarization lights, a polarization converter is required. Various techniques, such as optical processing [11], interferometric systems [12], metal stripe grating [13], and nematic liquid crystal (NLC) [14–22], have been demonstrated. Due to large optical activity, strong twist power, and easy fabrication with low cost, LC has become the desired material for fabricating various polarization converters.
In a conventional LC polarization converter cell, LC is treated with special orientations. If LC presents homogeneous alignment on one substrate surface and exhibits radial or azimuthal orientations on the opposite substrate surface, then a twisted-radial or twisted-azimuthal LC orientation can be obtained. Both LC textures can convert a linear polarization light to radial and/or azimuthal polarization light. To treat LC with radial or azimuthal orientations on one substrate surface, circular rubbing [14,15,20], shearing [16] electric-field inducing [17], and photo-alignment processing [18,19] have been proposed. Based on these approaches, only a single converter can be created in one LC cell. To process multiple beams at the same time, a polarization converter array integrated in one cell is required. Moreover, the aperture size of individual converter should be in miniature size. It would be difficult to prepare a converter array based on the above approaches.
In this work, we report a simple method of preparing a polarization converter array. When a LC is filled into a cylindrical polymer cavity, LC in the cavity presents concentrically circular (azimuthal) orientations. By treating LC on one side of the cavity with homogeneous alignment, a twisted-azimuthal texture is formed. LC with such an orientation texture functions as a polarization converter which can convert a linear polarization light to either radial or azimuthal polarization light. By filling the LC in a polymer cavity array, a converter array can be obtained easily. The size and aperture shape of each converter is dependent on the pattern of the photomask. By employing polymer network in the LC host, the formed LC texture in each cavity can be fixed firmly. Since one surface of LC in each cavity is convex, each converter has a lens character too. Our converter array has potential applications in beam focusing, imaging, and material processing.
2. Device fabrication
When a NLC is filled into a cylindrical polymer cavity, the side polymer surface will exert an anchoring force on the LC molecules. As a result, LC in the cavity can present a special alignment rather than disordered orientation. To investigate the alignment of LC in a polymer cavity, we first pattern a polymer cavity on a substrate surface. The cell fabrication procedure is shown in Fig. 1. First a UV curable monomer, such as NOA65 (Norland Optical Adhesive), is chosen as the polymer material. The monomer is coated on a glass substrate surface to form a thin film (Fig. 1(a)). Then a photomask is placed above the film (Fig. 1(b)). The photomask is made by coating a circular metal spot on a glass plate surface. After that, the film is exposed to UV light through the photomask (Fig. 1(c)). The monomer covered by the metal spot is not polymerized. Then the uncured monomer is removed using a solvent (such as ethanol) and a solid polymer film with a cavity is obtained (Fig. 1(d)). To easily fill a LC in the cavity, a small amount of solvent is doped in the LC. The solvent is used to decrease the LC surface tension and control the amount of LC in the cavity (Fig. 1(e)) without distorting the polymer film. Finally, a small amount of LC is remained in the cavity after thorough solvent evaporation (Fig. 1(f)).
Fig. 1 The fabrication of a polymer cavity on one substrate surface and a method to fill LC in the cavity. (a) UV monomer film coated on a substrate, (b) photomask placing above the film, (c) UV exposure, (d) a polymer cavity, (e) a mixture filled in the cavity, and (f) LC remained in the cavity.
3. Experiment
According to the fabrication procedure of Fig. 1, a polymer film with a cavity was prepared. The diameter of the cavity is ~0.35mm and the thickness of the film is ~100μmmeasured using a polarizing optical microscope (POM).nematic LC SLC-9023 (no = 1.522, Δn = 0.251, Chengzhi Yonghua Display Material, China) was chosen as the LC material. The LC was mixed with dichloromethane (γ~28 mN/m at 20 °C)at ~6 wt%. A small amount of the mixture could be easily filled in the cavity due to the reduced surface tension. After solvent evaporation at 80 °C, only a small amount of LC was remained in the cavity. LC in the cavity was cooled down slowly to room temperature.
4. Results and discussion
To observe the orientation of LC in the polymer cavity, the cell was placed on the stage of the POM between crossed polarizers. A white light was used to illuminate the cell from the substrate side. The output light intensity was monitored through eyepiece and a two-dimensional light intensity pattern was recorded using a CCD camera. Fig. 2(a) shows the observed crosshair pattern. The black areas imply that the LC directors are either parallel or perpendicular to the optical axis of the analyzer. From the black region to the bright region, light intensity changes gradually. When the LC cell on the stage was rotated in azimuthal direction, the observed pattern remains unchanged. Such a result means the orientation of LC in the cavity is highly symmetrical although the substrate surface is not specially treated and the top LC surface exposes to air directly. From the pattern of Fig. 2(a), LC in the cavity may presents either radial orientation (Fig. 2(b)) or azimuthal orientation (Fig. 2(c)), because both textures can present the similar crosshair pattern when they are observed between crossed polarizers [23,24].Undoubtedly, the untreated substrate surface could not induce LC with such a symmetrical alignment. While at the LC/air interface, the surface tension forces caused the LC molecules to align perpendicularly with respect to the plane of the substrate [25]. Because homeotropic alignment is optically isotropic, a dark field is observed between crossed polarizers. Therefore, the wall of the cavity plays the key role for the radial or azimuthal alignment.
Fig. 2 LC in a polymeric cavity and possible alignment from the top view. (a) crosshair pattern observed between crossed polarizers, (b) radial texture, and (c) azimuthal texture.
To conclude which texture it should be in the polymer cavity, we prepared another polymer film with a cavity according to the fabrication procedure of Fig. 1. The polymer film was peeled-off and attached on a glass substrate with a gap. The gap was controlled using 15-μm Mylar film. The substrate surface was overcoated with a polyimide (PI) layer and buffed in one direction, so that LC can present a homogeneous alignment on the bottom substrate surface. We then filled a small amount of the LC mixture in the empty cavity as shown in Fig. 3(a). After solvent evaporation, only pure LC was remained in the cell cavity.
Fig. 3 LC in a cell chamber and light intensity patterns observed between the polarizer and analyzer. (a) the cross-sectional cell structure. The PI layer coated on the bottom substrate surface was buffed in one direction. (b)(Media 1), (c), and (d) show the observed intensity patterns by setting the axis of the analyzer to be 0°, 45°, and 90° with that of the polarizer, respectively.
In addition to the wall surface of the cavity, the rubbed PI surface can affect the alignment of LC as well. According to the elastic deformations of the continuum theory [26], LC in the chamber can generate a new alignment which can be analyzed by observing the light transmission between two polarizers. Firstly, the polarization axis of the polarizer was set to parallel to the rubbing direction of the bottom substrate. As the axis of the analyzer was parallel to that of the polarizer, a fan-shaped intensity pattern was observed using the POM, as shown in Fig. 3(b).The black axis of the intensity pattern through the fan-shape center is along the axis of the analyzer. A disclination line passing through its center along the rubbing direction was also observed. By rotating the analyzer gradually in counter-clockwise direction, the axis of the fan-shaped pattern would rotate synchronously except the observed disclination line. As the analyzer is rotated continuously, the shape of the observed intensity pattern remains unchanged. Figures 3(c) and 3(d) show the observed intensity patterns as the analyzer was rotated 45° and 90°, respectively. To observe the intensity pattern change versus the axis rotation of the analyzer, a dynamic video (Media 1) was given in Fig. 3(b). The symmetrical fan-shaped pattern implies that the linear polarization light has been converted to either radial or azimuthal polarization light.
According to previous reports [14,15,23], when a linear polarization beam passes through a LC converter, the output intensity presents a fan-shaped pattern, as shown in Fig. 4. For the LC cell with homogeneous alignment on the left substrate surface and concentrically circular orientation on the right substrate surface, if the polarization direction of a linear polarization incident light is along the LC directors of the left substrate, an azimuthal polarization light is converted. When such a polarization light passes through an analyzer, a fan-shaped pattern is obtained (Fig. 4(a)). If the right substrate surface of the cell is treated with radial alignment, then LC cell can convert the linear polarization light to radial polarization light (Fig. 4(b)). When such a polarization light passes through the analyzer, a fan-shaped pattern is observed as well (Fig. 4(b)). However, the two patterns have different positions. The black axis of the left pattern is parallel to the axis of the analyzer, while the black axis of the right is perpendicular to the axis of the analyzer. Comparing Figs. 4(a)-4(b) with Fig. 3(d), the beam after passing through the polymer cavity should be azimuthally polarized. Therefore, LC in the polymer cavity must present azimuthal orientation as depicted in Fig. 2(c).
Fig. 4 The output light intensity pattern of a polarized light beam after passing through a linear analyzer. (a) azimuthal polarization light and (b) radial polarization light.
To convert a linear polarization light to an azimuthal polarization light, LC in the cavity should present a twisted-azimuthal alignment. Because LC is generally homeotropic at the air/LC interface, alignment of LC in the region near the top surface should be hybrid. Such hybrid alignment can be equivalently treated as pure azimuthal orientation. By doing so, the thickness of the latter will be half as that of the former (hybrid alignment). Because the thickness of the cavity is ~100 μm, such an equivalent treatment will not induce additional issues. To describe such an alignment, we establish a xyz coordinate with its original point located at the apex (center) of the LC/air interface, as illustrated in Fig. 5(a). To clearly explain the LC orientation, we choose LC in xoz and yoz planes for analysis. Figure 5(b) shows the equivalent cross-sectional LC alignment in xoz plane. LC on the bottom substrate surface is along x-axis and LC in the cavity is alongy-axis. Therefore, LC presents a 90°twisted orientation starting from the substrate surface to the bottom of the cavity. If the top surface of LC in the polymer cavity is convex, then LC molecules may not be on the same plane, but they present the same orientation.
Fig. 5 LC in the cell aligned by the substrate and polymeric cavity surfaces. (a) xyz coordinate definition, (b) 90° twisted alignment in xoz plane, and (c) homogeneous alignment in yoz plane.
In yoz plane, LC has the same orientation along y-axis, i.e., LC exhibits homogeneous alignment, as depicted in Fig. 5(c). By rotating the xoz plane around the z-axis to yoz plane in counter clockwise direction, the twisted angle in the first quadrant changes continuously from 90 to 0°. By rotating the xoz plane 360°,LC in the second, the third, and the forth quadrants will present the similar alignment as that in the first quadrant due to the geometrical symmetry. However, along the rubbing direction, twist directions of clockwise and counter clockwise exist in the cell. As a result, a disclination line always appears in such a LC configuration. According to Mauguin condition, the plane of a linear polarization light can be rotated only if the below limit is satisfied [27]
Where ϕ is the twisted angle, d is the thickness of the LC layer, Δn is the birefringence of the LC material, and λ is the wavelength of light. For a 90° twisted LC, Eq. (1) reduces toFor our LC cell with d~15 μm and Δn = 0.251, the calculated dΔn (~3.765) is much larger than λ/4 (λ~0.55 μm). Because our LC cell satisfies this condition, the twisted LC can rotate the plane of a linear polarization light.When a linear polarization light is normally incident on the cell (along –z direction) with its polarization direction parallel to the rubbing direction of the substrate, the light polarization direction will follow the LC director. In xoz plane, the output polarization is twisted 90° by the LC. In yoz plane, the LC is homogeneous, so the output polarization direction does not change. From xoz to yoz plane, the output polarization direction is twisted continuously. Around z-axis with 360° rotation, the polarization of output light becomes azimuthal, as depicted in Fig. 6(a).If the polarization direction of the incident light is perpendicular to the rubbing direction of the cell, then the output light becomes radial, as shown in Fig. 6(b) [15].
Fig. 6 Linear polarization light converting to (a) azimuthal polarization light and (b) radial polarization light after passing through an azimuthal-twisted LC in the polymer cavity.
Like a conventional polarization converter prepared by circular rubbing [14, 15, 20] or non-contact photo-alignment method [18, 23], LC with twisted-azimuthal alignment in a polymer cavity functions as a polarization converter too. In addition to a single polarization converter, a converter array can also be prepared using our method. Unlike a single polarization converter, a converter array can process multiple beams at the same time.
Based on our method, it is easy to prepare a polarization converter array if the LC is filled in multiple polymer cavities. To prepare polymer cavities, a photomask with an opaque metal spot array is used. The procedure of preparing multiple polymer cavities and the cell preparing conditions are the same as that shown in Fig. 1 and Fig. 3(a). To stabilize the LC texture formed in the polymer cavities, we doped ~6wt% diacrylate monomer (RM257,containing ~1wt % photoinitiator) in LC SLC-9023. The monomer has reactive double bonds at both sides and can highly align with the LC. After filling a small amount of LC mixture in the cavities, the solvent was thoroughly evaporated at 80 °C for 5 minutes. Only LC, the diacrylate monomer, and the photoinitiator were remained in the cavities. Then the monomer in the cavities was exposed under UV light (~20 mW/cm2) for 10 minutes. After UV exposure, the prepared cell was observed using POM.
Similar to the evaluating method as shown in Fig. 3, the rubbing direction of the LC cell was set to parallel to the polarization axis of the polarizer. Here a 2 × 2 polymer cavity array was chosen to be observed, as given in Fig. 7. As the axis of the analyzer is parallel to that of the polarizer, LC in each cavity presents a fan-shaped intensity pattern, as shown in Fig. 7(a).Such a result implies that LC in each cavity functions as a polarization converter. As the analyzer is rotated, each fan-shaped pattern rotates with the analyzer in phase and their intensity patterns remain unchanged. Figs. 7(b)-7(c) show the recorded intensity patterns when the analyzer was rotated 45° and 90° in clockwise direction, respectively. The four converters have the same character as shown in Fig. 3. To observe the intensity pattern change as the analyzer is rotated, a dynamic video (Media 2) was recorded as shown in Fig. 7(a).Although ~6 wt% monomer is doped in the LC host, LC in the polymer cavity still presents twisted-azimuthal alignment.
Fig. 7 The light intensity patterns observed by setting the polarization direction of the analyzer with (a) 0°(Media 2),(b)45°, and (c)90° with that of the polarizer.
When the diacrylate monomers are exposed to UV light, the monomers are polymerized and form polymer network in the LC host. At the border, the polymer network adhered to the substrate/polymer wall surfaces as well. Due to high monomer concentration (~6 wt%) and high UV intensity (~20 mW/cm2), the formed the polymer network domains are very small. Therefore, the polymer network can firmly fix the LC texture without the concerns of shaking, vibrating, or shocking.
Since the top surface of LC in each polymer cavity contacts air directly, the converter may have a lens character. To evaluate the lens character, we choose the top-left LC converter (Fig. 7) for evaluation. The experimental setup is shown in Fig. 8(a). A collimated linearly-polarized He–Ne laser beam (λ~633 nm) is used to illuminate the cell from the substrate side. The polarization direction is parallel to the rubbing direction of the cell. The beam intensity is controlled by a neutral density filter (ND). After passing through the LC cell, the transmitted light is collected by an imaging lens (L) and detected by a CCD camera (BGS-USB-SP503, Newport).
Fig. 8 Experimental set up and the recorded output beam intensity. (a) simple setup, (b) 2D intensity spot, and (c) 3D intensity profile.
By precisely adjusting the position of the imaging lens and the CCD detector, a two dimensional (2D) focused spot was recorded, as shown in Fig. 8(b). Due to interference effect, some circular rings were also formed around the center spot. Figure 8(c) shows the three-dimensional (3D) intensity profile of the focused beam. From the 3D profile, the laser beam is focused after passing through the LC converter. Such a result implies that our LC polarization converter also has a lens character and the shape of the LC surface is convex. The focal length of the converter was measured to be ~35mm. In comparison to the diameter of the converter aperture (~0.35 mm), the top surface of LC in the cavity is fairly flat. As we have mentioned above, LC molecules near the top surface are hybrid-aligned. The effective refractive index of such aligned LC is smaller than the extraordinary refractive index of the LC. Due to this reason, the focus power of the lens is reduced. Although the LC cell was placed in vertical position, the performances of the converter/lens were not degraded by the gravity effect. This is owing to the strong stabilization of polymer network in the LC host.
Not only a single polarization converter, but also a polarization converter array can be easily prepared based on our approach, which would be difficult to achieve using previous methods. Moreover, the shape and size of a single converter can be easily controlled depending on the pattern of the photomask. Considering the anchoring effect of cavity wall on the LC orientation, the aperture of our converter is suitable to be prepared in miniature size, typically from micrometers to millimeters. Due to the convex shape of the top LC surface in the cavity, our polarization converter also has a lens character. Unlike a single converter, a polarization converter array has the advantage of processing multiple beams at the same time. Using polymer network, the formed LC texture in the cavity array can be firmly fixed. To get a polarization converter array with high fill factor and large area,a cavity array formed in a solid substrate (such as glass plate) is highly desired because polymer film is flexible.
5. Conclusion
We have demonstrated a polarization converter using a polymer cavity and a one-directional-rubbed PI glass substrate. LC in its cavity presents twisted-azimuthal alignment, which can convert a linearly polarized light to an azimuthal or radial polarized light depending on the polarization direction of the incident light. Since the shape of one LC surface is convex, it also has a lens character. By employing polymer network in the LC host, the formed LC texture can be fixed firmly without the concerns of shaking, vibrating or gravity effect. Based on our elementary approach, various polarization converters can be easily fabricated. The shape and size of the converter aperture are only dependent on the pattern of the employed photomask. In addition to a single aperture, an array of polarization converter can also be created. Our polarization converter array with lens character as potential applications in phase modulation, polarization-compensating system, tight focusing, imaging, and material processing.
Acknowledgments
The authors are grateful to the financial support of the National Research Foundation (NRF) of Korea, the Korea-China Joint Research Program under grant 2012-0004814, and partially support by the National Research Foundation of Korea under grant (2010–0021680).
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