We investigated the effects of laser irradiation on sessile droplets of three well-known liquid crystalline materials (5CB, 8CB, E7) deposited on the surface of an iron-doped lithium niobate (LN:Fe) crystal. The static electric field, which is generated via the bulk photovoltaic effect in the LN:Fe substrate, produces the merging of smaller droplets into filaments oriented in the radial direction with respect to the laser spot. It also induces filament jetting from the rim of larger droplets toward the center of the illumination area. When the laser beam is focused directly onto the larger droplets, they abruptly disintegrate via the formation of several jet streams. The described effects are present in the nematic and also in the isotropic phase. We attribute them to a large gradient of the surface electric field that produces driving forces via the induced dipole moments of the droplets, analogous to electric field-based droplets transport mechanisms known for standard dielectric liquids.
Drops or shells of a planar-aligned short-pitch cholesteric liquid crystal exhibit unique optical properties due to the combination of Bragg reflection in the cholesteric helix and a radial orientation of the helix axis. If such a droplet is illuminated from above, light is reflected into a continuous set of cones, the opening angles of which depend on where on the droplet the light hits its surface. For the wavelength that fulfills the Bragg condition the reflection is dramatically enhanced, yielding the light cones colored. A photonic cross communication scheme arises for certain angles, reflecting light back to the observer from a different droplet than the one originally illuminated. This gives rise to an intricate pattern of colored and circularly polarized spots. A number of interesting applications may be developed based on this pattern, e.g. in identification and authentication devices. We have carried out a detailed spectrophotometric analysis of the patterns, localized to individual spot maxima. A quantitative comparison between the measured spectra and the reflection wavelength expected from a model for the pattern generation allows us to conclude that the droplets are in fact not spherical but slightly ellipsoidal.
We investigated one-dimensional and two-dimensional optical diffraction structures fabricated in thin films of a sidechain light-sensitive liquid crystal elastomer (SC-LS-LCEs) by optical holographic lithography methods. The emphasis was on analysis of modifications of the periodicity of the recorded patterns induced by application of an external strain and by temperature modifications. The results show that due to rubber elasticity of the LCE films, relative modifications of the periodicity by 10% can easily be reached. In most cases tuning is reversible and linear with respect to the strain. Temperature induced tuning is most efficient in the region of phase transition from the nematic to the paranematic phase and provides relative periodicity modifications up to 30%.
A computational model, which describes EM field formation in a pulsed laser from a randomly generated initial
spontaneous field inside the laser cavity has been developed. The model is based on a two-dimensional fast
Fourier transform and describes a real laser system taking into account a lensing and a diaphragm effect of the
laser rod. The laser cavity is described by five effective planes, which represent different laser cavity elements-the back and the front mirror, the Q-switch element and the laser rod. At each plane the EM field is calculated in
real space and propagation between the planes is achieved in Fourier space by multiplication with an appropriate
phase factor. The computational time needed for simulation of a realistic pulse formation is in order of minutes.
The model can predict the shape and the integral energy of the pulse, its transverse profile at different distances
from the front mirror (including near and far field) and beam divergence. The results of the model were found
to be in good agreement with measured parameters for a Q-switched ruby laser system running in stable as well
as unstable cavity configurations. The temporal shape of a laser pulse was measured and calculated not only for
the ruby laser, but also for a Nd:YAG laser. It was found that FWHM of a pulse produced by ruby laser is three
times longer than FWHM of a pulse produced by Nd:YAG laser.
The origin of optical diffraction in holographic polymer-dispersed liquid crystal (H-PDLC) transmission gratings was
investigated by optical two beam-coupling analysis based on the linear phase-shift technique. Gratings with the pitch of 1
micrometer and the thickness of 50 micrometers were fabricated from a UV curable mixture of commercially available
constituents. Photopolymerization in the interference field of two laser beams produces not only a periodic variation of
the refractive index, but also a periodic modulation of optical extinction due to light scattering. Both of them contribute
to the diffraction efficiency of the gratings. The magnitudes and relative phases of the two contributions were measured
as a function of a recording time of the grating and as a function of an applied external electric field. During the initial
stage of the grating formation phase modulation is predominant, while at longer exposures both contributions have the
same order of magnitude. They are phase shifted with respect to each other for around π/2. Application of an external
electric field causes a strong decrease of the amplitude modulation, while phase modulation is much less perturbed.
Photopolymerization-induced phase separation of the constituent components in holographic polymer-dispersed liquid
crystals (H-PDLCs) causes a huge variation of the refractive index for light as well as for neutrons. We demonstrated
that H-PDLCs with the thickness of only 30 micrometers act as extremely efficient gratings for neutrons. The lightinduced
refractive-index modulation for neutrons of about 10-6 was observed, which is nearly two orders of magnitude
larger than found in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs very promising
candidate for fabricating neutron-optical devices.
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