1,720,966 research outputs found
Structured, photosensitive PVK and PVCN polymer layers for control of liquid crystal alignment
No full textWe present characteristics of liquid crystal reorientation in cells with alignment layers made of different poly(vinyl)-type polymers. Mechanically-rubbed poly(N-vinyl carbazole) (PVK) produces planar alignment of liquid crystals with easy axis orthogonal to the rubbing direction and zero pretilt angle. Doping PVK with C60 makes this liquid crystal–polymer system extremely photosensitive for visible wavelengths. Illumination with a Gaussian beam reveals a complex structure of patterns of reoriented liquid crystal molecules. Using poly(vinyl-cinnamate) (PVCN), exposed to UV light, a periodic alignment of liquid crystals can be achieved via this all-optical method
Surface charge layers and beam coupling in photosensitive polymer-liquid crystal structures
No full textWe present results on DC field and light-induced photorefractive-like gratings in liquid crystal cells made with photosensitive polymer layers. Surface charge layers that develop on a liquid crystal-polymer interface are responsible for screening the liquid crystal bulk from an external DC field. These layers can be selectively discharged via polymer photoconductivity and lead to liquid crystal reorientation grating. Efficient two-beam coupling gain can arise from the reorientation grating for relatively high DC field (above 30 V). For lower voltages, strong diffraction can be observed that reduces the gain. Two-beam coupling gain and diffraction can also be controlled by experimental geometry and liquid crystal alignment. The simulated profile of electric field shows that its penetration depth depends on grating spacing
Surface screening layers and dynamics of energy transfer in photosensitive polymer-liquid crystal structures
No full textThe dynamics of energy transfer in photoconductive polymer liquid crystal structures can contain important information on interface effects and surface electric fields contributing to the strength of liquid crystal reorientation gratings. The characteristic, transient effects observed during switching on and off of incident light or electric field can be explained by the presence of surface screening layers. Screening layers play an important role in the reorientation of liquid crystal director in cells with different alignment layers. Strong screening of external DC field is present not only in cells with a photoconductive polymer (56 V), but in standard cells with thicker (0.3 ?m) polyimide, aligning layers
Photorefractive gratings in liquid crystals with polymer doped C<sub>60</sub> layers
No full textPhotorefractive-like liquid crystals [1], with their large diffraction efficiencies and nonlinear effects combined with thin film format, are ideal materials for integrated optics applications. Liquid crystals doped with dyes or with added photoconducting polymer layers' show high two-beam coupling gain.The reorientation process that liquid crystal molecules undergo is induced and controlled by the application of light and electric field. There is typically more than one mechanism involved in the reorientation of liquid crystal molecules and, with careful design, the photorefractive space-charge field can play an important role. Moreover, the reorientation can also be strongly influenced by surface-mediated effects and surface-charge modulation, as well as surface anchoring.Photorefractive liquid crystals structures with photoconductive layers on their substrates have shown to be very efficient, in particular using polyvinyl carbazole (PVK) polymer doped with photosensitiser trinitrofluorene (TNF). However, the practical application of this structure is minimised by the toxic nature of the TNF dopant
Enhanced two-beam coupling in colloids of ferroelectric nanoparticles in liquid crystals
No full textWe report on the first, to the best of our knowledge, studies of photorefraction in nematic liquid crystal (LC) doped with nanoferroelectric particles. We found the strong enhancement of two-beam coupling in the colloid of ferroelectric nanoparticles in LC. The effect originated from an increased birefringence of the colloid and a stronger LC reorientation torque. Our measurements allowed us to suggest that increased birefringence is caused by the contribution of polarizability anisotropy of the ferroelectric particles. Stronger reorientation torque is caused by the permanent dipole moment of the particles contributing to the dielectric anisotropy ofthe colloid a col. The enhancement of two-beam coupling in LCs by doping with ferroelectric nanoparticles at extremely small concentration shows the strong potential of ferroelectric nanoparticles for improving the optical response of LCs, especially for those materials where a method of chemical synthesis has reached its limit
Nonlinear shift of spatial solitons at a graded dielectric interface
No full textWe investigate total internal reflection of optical spatial solitons at the interface between two regions of nematic liquid crystals with different optical densities. Due to nonlinear molecular reorientation, the solitons experience a penetration depth, hence, a lateral shift that depends on the excitation, with lateral shifts from 0.7 to 1.2 mm as input powers increased from 1.6 to 9.3 mW
Escaping solitons from a trapping potential
No full textSolitons propagating within a confining potential undergo momentum-dependent scattering and eventually escape for large excitations. We experimentally highlight this phenomenon in the presence of a nonperturbative nonlinear response using self-confined light beams in a reorientational medium
Two-color vector solitons in nonlocal media
No full textWe investigate the interaction between two beams differing in wavelength and the properties of dual-frequency spatial solitons in nonlocal birefringent reorientational media. We report the first experimental observations of anisotropic nonlocal vector solitons in unbiased nematic liquid crystals. Model and simulations, based on the paraxiality along the Poynting vectors, include joint walk-off and breathing
Letter. Tuneable refraction and reflection of self-confined light beams
No full textLight filaments or optical spatial solitons are self-confined (non-spreading) beams that originate from the balance between diffraction and self-focusing in nonlinear optical media (those with a response dependent on the level of excitation)1, 2, 3. Owing to their ability to self-trap as well as to guide weaker signals (even if differing in colour or modulation format) within the waveguides or 'light-pipes' they induce, optical spatial solitons could form the basis of future all-optical processing networks4, 5. One of the most interesting challenges in soliton propagation and engineering concerns light filaments incident on linear/nonlinear or nonlinear/nonlinear interfaces. Here we report the robust propagation, refraction and reflection of optical spatial solitons at the interface between two regions of a nematic liquid crystal. The ability to independently tune the optical properties of each region enables us to steer the beams by refraction and total internal reflection by as much as -18 and +22 degrees, respectively. Moreover, the extended (nonlocal) and anisotropic response of our system supports polarization healing of the solitons across the interface as well as non-specular filament reflection. Finally, exploiting the inherent and all-optically tunable birefringence, we demonstrate unprecedented nonlinear Goos–Hänchen lateral shifts in excess of 0.5 mm
Nonspecular total internal reflection of spatial solitons at the interface between highly birefringent media
No full textWe investigate total internal reflection of nonlocal spatial optical solitons at the interface between two differently oriented regions of a highly birefringent nematic liquid crystal. The solitons survive the interaction with an induced index mismatch and undergo nonspecular reflection, with an emerging angle differing appreciably from the incidence angle
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