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Laser-directed hierarchical assembly of liquid crystal defects and control of optical phase singularities.

Ackerman PJ, Qi Z, Lin Y, Twombly CW, Laviada MJ, Lansac Y, Smalyukh II - Sci Rep (2012)

Bottom Line: However, they are typically hard to control in a reliable manner.Here we describe facile erasable "optical drawing" of self-assembled defect clusters in liquid crystals.Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, USA.

ABSTRACT
Topological defect lines are ubiquitous and important in a wide variety of fascinating phenomena and theories in many fields ranging from materials science to early-universe cosmology, and to engineering of laser beams. However, they are typically hard to control in a reliable manner. Here we describe facile erasable "optical drawing" of self-assembled defect clusters in liquid crystals. These quadrupolar defect clusters, stabilized by the medium's chirality and the tendency to form twisted configurations, are shaped into arbitrary two-dimensional patterns, including reconfigurable phase gratings capable of generating and controlling optical phase singularities in laser beams. Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators.

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Director structure and POM images of periodic cholesteric finger arrays.(a) Director field in the vertical cross-section of a periodic finger array. (b) (c) POM textures obtained (b) by means of computer simulations and (c) experimentally for crossed polarizer and analyzer parallel and perpendicular to fingers, respectively. (d) (e) POM textures obtained (d) by means of computer simulations and (e) experimentally for crossed polarizer and analyzer at π/4 to fingers. Both experimental and computer-simulated POM textures were obtained for d = p = 10 μm.
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f2: Director structure and POM images of periodic cholesteric finger arrays.(a) Director field in the vertical cross-section of a periodic finger array. (b) (c) POM textures obtained (b) by means of computer simulations and (c) experimentally for crossed polarizer and analyzer parallel and perpendicular to fingers, respectively. (d) (e) POM textures obtained (d) by means of computer simulations and (e) experimentally for crossed polarizer and analyzer at π/4 to fingers. Both experimental and computer-simulated POM textures were obtained for d = p = 10 μm.

Mentions: In an LC cell with p/d≤1 and vertical boundary conditions, the sample regions outside of the drawn structures have uniform vertical director and appear black when viewed between crossed polarizers in polarizing optical microscopy (POM). The laser-realigned regions appear bright (Fig. 1a–c and supplementary Fig. S2). A close inspection of generated structures by means of rotating polarizers reveals a combination of effects of birefringence and polarization rotation, indicating the presence of director twist across the cell thickness in these sample regions (Fig. 2). To gain more direct insights into the director structure in the sample's vertical cross-section, we perform label-free 3D imaging by means of three-photon excitation fluorescence polarizing microscopy (3PEF-PM)24 and measure polarized fluorescence patterns emitted by the LC molecules themselves. An example of an image of the director field in the sample's vertical cross-section is shown in Fig. 1f, in which fluorescence intensity scales as ∝cos6φ, where φ is the angle between the spatially varying n(r) and the linear polarization direction of the excitation laser. Using this and other images obtained for various polarization states and cross-sectional planes (supplementary Figs. S3 and S4), we reconstruct the director structure schematically shown in Fig. 1e (see supplementary information for details). Experimental POM (Fig. 2c,e) and 3PEF-PM textures (Fig. 1f) closely resemble the corresponding computer-simulated textures (Fig. 2b,d and Fig. 1g) obtained for the minimum-energy n(r) configurations of either a single finger shown in Fig. 1h or a periodic finger array shown in Fig. 2a.


Laser-directed hierarchical assembly of liquid crystal defects and control of optical phase singularities.

Ackerman PJ, Qi Z, Lin Y, Twombly CW, Laviada MJ, Lansac Y, Smalyukh II - Sci Rep (2012)

Director structure and POM images of periodic cholesteric finger arrays.(a) Director field in the vertical cross-section of a periodic finger array. (b) (c) POM textures obtained (b) by means of computer simulations and (c) experimentally for crossed polarizer and analyzer parallel and perpendicular to fingers, respectively. (d) (e) POM textures obtained (d) by means of computer simulations and (e) experimentally for crossed polarizer and analyzer at π/4 to fingers. Both experimental and computer-simulated POM textures were obtained for d = p = 10 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3369196&req=5

f2: Director structure and POM images of periodic cholesteric finger arrays.(a) Director field in the vertical cross-section of a periodic finger array. (b) (c) POM textures obtained (b) by means of computer simulations and (c) experimentally for crossed polarizer and analyzer parallel and perpendicular to fingers, respectively. (d) (e) POM textures obtained (d) by means of computer simulations and (e) experimentally for crossed polarizer and analyzer at π/4 to fingers. Both experimental and computer-simulated POM textures were obtained for d = p = 10 μm.
Mentions: In an LC cell with p/d≤1 and vertical boundary conditions, the sample regions outside of the drawn structures have uniform vertical director and appear black when viewed between crossed polarizers in polarizing optical microscopy (POM). The laser-realigned regions appear bright (Fig. 1a–c and supplementary Fig. S2). A close inspection of generated structures by means of rotating polarizers reveals a combination of effects of birefringence and polarization rotation, indicating the presence of director twist across the cell thickness in these sample regions (Fig. 2). To gain more direct insights into the director structure in the sample's vertical cross-section, we perform label-free 3D imaging by means of three-photon excitation fluorescence polarizing microscopy (3PEF-PM)24 and measure polarized fluorescence patterns emitted by the LC molecules themselves. An example of an image of the director field in the sample's vertical cross-section is shown in Fig. 1f, in which fluorescence intensity scales as ∝cos6φ, where φ is the angle between the spatially varying n(r) and the linear polarization direction of the excitation laser. Using this and other images obtained for various polarization states and cross-sectional planes (supplementary Figs. S3 and S4), we reconstruct the director structure schematically shown in Fig. 1e (see supplementary information for details). Experimental POM (Fig. 2c,e) and 3PEF-PM textures (Fig. 1f) closely resemble the corresponding computer-simulated textures (Fig. 2b,d and Fig. 1g) obtained for the minimum-energy n(r) configurations of either a single finger shown in Fig. 1h or a periodic finger array shown in Fig. 2a.

Bottom Line: However, they are typically hard to control in a reliable manner.Here we describe facile erasable "optical drawing" of self-assembled defect clusters in liquid crystals.Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, USA.

ABSTRACT
Topological defect lines are ubiquitous and important in a wide variety of fascinating phenomena and theories in many fields ranging from materials science to early-universe cosmology, and to engineering of laser beams. However, they are typically hard to control in a reliable manner. Here we describe facile erasable "optical drawing" of self-assembled defect clusters in liquid crystals. These quadrupolar defect clusters, stabilized by the medium's chirality and the tendency to form twisted configurations, are shaped into arbitrary two-dimensional patterns, including reconfigurable phase gratings capable of generating and controlling optical phase singularities in laser beams. Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators.

Show MeSH