Magnetically tunable optical diffraction gratings based on a ferromagnetic liquid crystal.

Transmission optical diffraction gratings composed of periodic slices of a ferromagnetic liquid crystal and a conventional photoresist polymer are demonstrated. Dependence of diffraction efficiencies of various diffraction orders on an in-plane external magnetic field is investigated. It is shown that diffraction properties can be effectively tuned by magnetic fields as low as a few mT. The tuning mechanism is explained in the framework of a simple empirical model and also by numerical simulations based on the rigorous coupled wave analysis (RCWA). The obtained results provide a proof of principle of operation of magnetically tunable liquid crystalline diffractive optical elements applicable in contactless schemes for control of optical signals.

Photoisomerizable Guanosine Derivative as a Probe for DNA Base-Pairing in Langmuir Monolayers.

Mixtures of azo-functionalized amphiphilic derivatives of guanosine and of amphiphilic derivatives of other DNA nucleobases were deposited at an air-water interface and repeatedly irradiated with light of 340 and 440 nm wavelengths. The consequent switching between cis and trans configurations of the azobenzene moiety caused changes in the surface pressure of the film, which were analyzed using a model based on the two-dimensional Van der Waals equation of state. For mixed films of guanosine and cytidine derivatives, the analysis revealed a significant modification of the strength of intermolecular interaction caused by the optical irradiation, while no such modifications were identified in mixed films involving other nucleobases. The difference is attributed to light-induced breaking of the hydrogen bonding that is established only between specific nucleobases. The results demonstrate that photosensitive nucleoside derivatives can be used as an efficient probe for base-pairing in Langmuir monolayers.

Guanosine quadruplexes in solution: a small-angle x-ray scattering analysis of temperature effects on self-assembling of deoxyguanosine monophosphate.

We investigated quadruplex formation in aqueous solutions of 2'-deoxyriboguanosine 5'-monophosphate, d(pG), which takes place in the absence of the covalent axial backbone. A series of in-solution small angle X-ray scattering experiments on d(pG) have been performed as a function of temperature in the absence of excess salt, at a concentration just above the critical one at which self-assembling occurs. A global fit approach has been used to derive composition and size distribution of the scattering particles as a function of temperature. The obtained results give thermodynamical justification for the observed phase-behavior, indicating that octamer formation is essential for quadruplex elongation. Our investigation shows that d(pG) quadruplexes are very suitable to assess the potential of G-quadruplex formation and to study the self-assembling thermodynamics.

Temperature dependence of optical anisotropy of holographic polymer-dispersed liquid crystal transmission gratings.

We measured the angular dependence of the 0th, +/-1 st, and +/-2 nd optical diffraction orders from a 50 microm thick transmission grating recorded in a UV-curable holographic polymer-dispersed liquid crystal (HPDLC) made from commercially available constituents. The analysis was performed for two orthogonal polarizations of the probe beams. The emphasis was laid on the temperature dependence of the grating anisotropy. Above the nematic-isotropic phase transition, the grating is optically isotropic. At lower temperatures the grating strength for the optical polarization perpendicular to the grating vector decreases with decreasing temperature, while for orthogonal polarization it increases with decreasing temperature. As a consequence, a regime of diffraction with strongly overmodulated gratings is observed. Our investigations indicate that the anisotropy of the refractive-index modulation scales with the optical anisotropy of the liquid crystal medium forming the phase-separated domains. We further demonstrate that light scattering effects, which are profound only in the nematic phase, must not be neglected and can be taken into account via a Lorentzian line-shape broadening of the probing wave vector directions in the framework of the diffraction theory for anisotropic optical phase gratings.

Structural transitions in holographic polymer-dispersed liquid crystals.

Dynamic light scattering was used to analyze the structural and dynamic properties of nematic director field within liquid crystal domains formed in holographic polymer-dispersed liquid crystal transmission gratings. Samples prepared from two different types of prepolymer mixture: one curable with visible (VIS) and another curable with UV light were investigated. In both formulations a critical slowing down of thermal director fluctuations, signifying the second-order structural transition of the nematic director field was observed in the vicinity of some critical external electric field as well as close to some critical temperature. For VIS samples also the size and the shape of phase separated droplets and viscoelastic and surface anchoring parameters of the liquid crystalline (LC) material forming the droplets were deduced. The viscoelastic constants were found to significantly deviate from the viscoelastic parameters of the pure LC mixture.

Dynamic light scattering and 31P NMR study of the self-assembly of deoxyguanosine 5'-monophosphate: the effect of added salt.

Self-assembling and dynamical properties of deoxyguanosine 5'-monophosphate in isotropic aqueous solutions were studied by (31)P NMR spectroscopy and dynamic light scattering (DLS). All solutions had the same c = 4 wt% guanosine concentration, while the added KCl molarity ranged from 0 to 1.5 M. (31)P NMR measurements show that potassium ions strongly enhance the stacking process of guanosine tetramers until a saturation is reached at 0.1 M KCl with more than 70% of the molecules aggregated. Polarized light scattering reveals the presence of a fast relaxation mode that arises from the translational dynamics of the self-assembled stacks. The diffusion coefficient of this mode shows a strong dependence on molarity of added salt, which can be described in terms of the coupled mode and counterion condensation theories for polyelectrolyte solutions. Depolarized light scattering reveals the rotational dynamics of the self-assembled stacks which exhibits a pronounced slowing down with increasing the added salt content.

Structure and polarity of 8CB films evaporated onto solid substrates.

Brewster-angle reflection ellipsometry and surface optical second harmonic generation were used to study the growth of 4'-n-octyl-4-cyanobiphenyl (8CB) films evaporated in air onto polymeric and quartz glass substrates. The layer-by-layer growth of the films terminates after formation of two distinctive interfacial layers. Both of these two layers are polar and tilted. In the first layer the molecules lie nearly flat on the surface, while in the second layer they point on average about 50 degrees toward the surface normal. The dipole moment of the second layer has a lower magnitude and an opposite direction with respect to the dipole moment of the first layer.

Alignment of liquid crystals on a photosensitive substrate studied by surface optical second-harmonic generation

The adsorption kinetics and orientational ordering of 4(')-n-octyl-4-cyanobiphenyl (8CB) liquid crystal films evaporated onto photosensitive poly(vinyl-cinnamate) (PVCN) substrates were investigated by surface optical second-harmonic generation. The adsorption rate of the first monolayer decreases with increasing degree of photochemical modification of the polymer. The in-plane orientational anisotropy of the 8CB films grown on unidirectionally photopolymerized PVCN substrates is considerably lower than the intrinsic surface orientational anisotropy of these substrates, which can explain the generally found weak surface anchoring of liquid crystals on PVCN alignment layers.