433 research outputs found
Realignment-enhanced coherent anti-Stokes Raman scattering (CARS) and three-dimensional imaging in anisotropic fluids
We apply coherent anti-Stokes Raman Scattering (CARS) microscopy to
characterize director structures in liquid crystals.Comment: 14 pages, 11 figure
Diversity of knot solitons in liquid crystals manifested by linking of preimages in torons and hopfions
Topological solitons are knots in continuous physical fields classified by
non-zero Hopf index values. Despite arising in theories that span many branches
of physics, from elementary particles to condensed matter and cosmology, they
remain experimentally elusive and poorly understood. We introduce a method of
experimental and numerical analysis of such localized structures in liquid
crystals that, similar to the mathematical Hopf maps, relates all points of the
medium's order parameter space to their closed-loop preimages within the
three-dimensional solitons. We uncover a surprisingly large diversity of
naturally occurring and laser-generated topologically nontrivial solitons with
differently knotted nematic fields, which previously have not been realized in
theories and experiments alike. We discuss the implications of the liquid
crystal's non-polar nature on the knot soliton topology and how the medium's
chirality, confinement and elastic anisotropy help to overcome the constrains
of the Hobart-Derrick theorem, yielding static three-dimensional solitons
without or with additional defects. Our findings will establish chiral nematics
as a model system for experimental exploration of topological solitons and may
impinge on understanding of such nonsingular field configurations in other
branches of physics, as well as may lead to technological application
Three dimensional optical manipulation and structural imaging of soft materials by use of laser tweezers and multimodal nonlinear microscopy
We develop an integrated system of holographic optical trapping and
multimodal nonlinear microscopy and perform simultaneous three-dimensional
optical manipulation and non-invasive structural imaging of composite
soft-matter systems. We combine different nonlinear microscopy techniques such
as coherent anti-Stokes Raman scattering, multi-photon excitation fluorescence
and multi-harmonic generation, and use them for visualization of long-range
molecular order in soft materials by means of their polarized excitation and
detection. The combined system enables us to accomplish both, manipulation in
composite soft materials such as colloidal inclusions in liquid crystals as
well as imaging of each separate constituents of the composite material in
different nonlinear optical modalities. We also demonstrate optical generation
and control of topological defects and simultaneous reconstruction of their
three-dimensional long-range molecular orientational patterns from the
nonlinear optical images
Anisotropic electrostatic screening of charged colloids in nematic solvents
The physical behaviour of anisotropic charged colloids is determined by their
material dielectric anisotropy, affecting colloidal self-assembly, biological
function and even out-of-equilibrium behaviour. However, little is known about
anisotropic electrostatic screening, which underlies all electrostatic
effective interactions in such soft or biological materials. In this work, we
demonstrate anisotropic electrostatic screening for charged colloidal particles
in a nematic electrolyte. We show that material anisotropy behaves markedly
different from particle anisotropy: The electrostatic potential and pair
interactions decay with an anisotropic Debye screening length, contrasting the
constant screening length for isotropic electrolytes. Charged dumpling-shaped
near-spherical colloidal particles in a nematic medium are used as an
experimental model system to explore the effects of anisotropic screening,
demonstrating competing anisotropic elastic and electrostatic effective pair
interactions for colloidal surface charges tunable from neutral to high,
yielding particle-separated metastable states. Generally, our work contributes
to the understanding of electrostatic screening in nematic anisotropic media.Comment: 15 pages, 5 figures, SM under ancillary file
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