We report the observation of knots and links in the mixture of a chiral nematic liquid crystal and micrometer-sized silica microspheres. We have observed that in these mixtures, topological defect loops spontaneously form knots and links, spanned on the colloidal scaffold. We used the laser tweezers to analyze and rewire knots and links using the effect of a strongly focused laser light. Using the theoretical approach, based on phenomenology and topology, we have explained the formation and stability of colloidal knots and links observed. To our surprise, arbitrary knot and link can be created on a sufficiently large colloidal array.
COBISS.SI-ID: 2336868
We demonstrate a tunable and omnidirectional microlaser in the form of a microdroplet of a dye-doped, cholesteric liquid crystal in a carrier fluid. The cholesteric forms a Bragg-onion optical microcavity and the omnidirectional 3D lasing is due to the stimulated emission of light from the dye molecules in the liquid crystal. The lasing wavelength depends solely on the natural helical period of the cholesteric and can be tuned by varying the temperature. Millions of microlasers can be formed simply by mixing a liquid crystal, a laser dye and a carrier fluid, thus providing microlasers for soft-matter photonic devices.
COBISS.SI-ID: 24377895
We demonstrate a new class of soft matter optical fibers, which are self-assembled in a form of smectic-A liquid crystal microtubes grown in an aqueous surfactant dispersion of a smectic-A liquid crystal. The diameter of the fibers is highly uniform and the fibers are highly birefringent. They are characterized by a line topological defect in the core of the fiber with an optical axis pointing from the defect core towards the surface. We demonstrate guiding of light along the fiber and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The light guiding as well as the lasing threshold are significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low (≈ 75μJ/cm2) when the pump beam polarization is perpendicular to the direction of the laser dye alignment and is similar to the lasing threshold in nematic droplets. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits.
COBISS.SI-ID: 27532583
We demonstrate the resonant transfer of light from a planar waveguide to a nematic liquid-crystal microdroplet immersed in water. A wide spectrum of light from a supercontinuum laser source is coupled into a high-refractive-index polymer waveguide using a prism-film coupler. The waveguide is in contact with a water dispersion of droplets from the nematic liquid-crystal 5CB. The evanescent field of the light in the waveguide is resonantly coupled to the whispering-gallery mode resonances, sustained by 5 − 20μm-sized nematic liquid-crystal droplets, which are in close proximity to the waveguide. The resonant transfer of light is tuned by the temperature-induced shifting of the WGM resonances due to the temperature dependence of the refractive index of the nematic liquid crystal. The measurements are compared to the calculations of the coupled-mode
COBISS.SI-ID: 2577252
We demonstrate that control over spatial and temporal positioning of topological defects allows for the design and assembly of 3D nematic colloidal crystals, giving some unexpected material properties, such as giant electrostriction and collective electrorotation. Using laser tweezers, we have assembled 3D colloidal crystals made up of 4 micrometer spheres in a bulk nematic liquid crystal, implementing a step-by-step protocol, dictated by the orientation of point defect. The 3D colloidal crystals have tetragonal symmetry with antiparallel topological dipoles and exhibit giant electrostriction, shrinking by 25-30% at 0.37V/micrometer. An external electric field induces a reversible and controllable electrorotation of the crystal as a whole, with the angle of rotation being 30 degrees at 0.14V/micrometer when using liquid crystal with negative dielectric anisotropy. This demonstrates a new class of electrically highly responsive soft materials.
COBISS.SI-ID: 26543143