A unique feature of liquid crystals (LCs) is orientational order of molecules that can be controlled by electromagnetic
fields, surface modifications and pressure gradients. We describe a new effect in which the orientation of LC molecules
is altered by thermal expansion. Thermal expansion (or contraction) causes the LC to flow; the flow imposes an
orienting torque on the LC molecules and the optical axis. The optical and mechanical responses activated by a simple
temperature changes can be used in sensing, photonics, microfluidic, optofluidic and lab-on-a-chip applications as they
do not require externally imposed gradients of temperature, pressure nor electromagnetic fields.
We show how a tightly focused laser beam can serve as a tool to image complex patterns of the director using the technique of fluorescence confocal polarizing microscopy (FCPM). We expand the capabilities of FCPM into the domain of real-time scanning in order to study the dynamic processes at the time scale of about 1ms. In this approach
which we call Fast FCPM, confocal imaging is performed using a modified Nipkow-disc scanning confocal microscope. In the Fast FCPM set up, we use a twisted nematic cell as a fast achromatic polarization rotator to change the polarization of probing light by 90°. The achromatic polarization rotator switches between two orthogonal polarization states when a sufficiently strong electric field is applied to reorient the director structure from the twisted to the homeotropic state. Both FCPM and Fast FCPM employ the property of anisotropic media to align fluorescent dye molecules. When observation is performed in polarized light, the measured fluorescence signal is determined by orientation of the dye molecules. As the dye molecules are aligned by the liquid crystal, the detected fluorescence signal visualizes the spatial patterns of the director rather than concentration gradients of dyes. Finally, we present 3D patterns of director associated with both static and dynamic processes in liquid crystals, anisotropic emulsions, and colloidal suspensions.
We developed electrically-switchable two-dimensional diffractive gratings with periodic refractive index modulation arising from layers undulations in cholesteric liquid crystal. Two-dimensional layer undulations occur above the threshold voltage when a planar cholesteric cell of thickness much larger than the cholesteric pitch is subjected to an electric field. The periodic structure of the layers undulations and corresponding spatial modulation of an average refractive index in the plane of a cell allows us to produce diffraction patterns with a square-type arrangement of diffraction maxima. The cholesteric cell can be switched by pulses of ac voltage between two states: one with flat layers of a planar cholesteric texture and another with square lattice of periodic director modulation associated with layer undulations that produces two-dimensional diffraction patterns. The periodicity of the developed two-dimensional phase gratings and intensities of the diffraction maxima can be tuned by changing the applied field magnitude. The diffractive properties of gratings are practically independent of the polarization state of the incident beam and can be optimized for different wavelengths by appropriately choosing the cholesteric pitch, cell thickness, and surface treatment. The potential applications include beam steering devices, optical waveguides, devices for splitting monochromatic beams and beam multiplexing.
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