Spatial self-phase modulation of a Gaussian beam transmitted through a ferrofluid.

We report on the experimental observation of the diffraction pattern formed in the far-field region when a high-power continuous-wave laser convergent or divergent Gaussian beam passes through a cuvette with ferrofluid. Two different types of diffraction rings with opposite light-intensity distribution are shown in the far field. The difference between the diffractive patterns is attributed to the interaction of the strong spatial self-phase modulation caused by the refractive index change of the medium with wavefront curvature of the input Gaussian beam. The observed behavior of the diffraction pattern dynamics is interpreted theoretically based on the Fresnel-Kirchhoff integral. The negative polarity of nonlinear refraction can be identified by the central interference profiles and the diffraction pattern. At the same time, the self-defocusing phenomena of the ferrofluid can be determined by the type of pattern. The nonlinear refraction coefficients of the ferrofluid were estimated to be ∼-2.89×10-5cm2/W (convergent Gaussian beam) and ∼-3.53×10-5cm2/W (divergent Gaussian beam). In addition, the corresponding third-order nonlinear optical susceptibility of the sample was also estimated as ∼1.43×10-5esu and ∼1.75×10-5esu, respectively. The experimental results imply a novel potential application of ferrofluid in nonlinear phase modulation devices.

Study of optical rotation generated by the twisted nematic liquid crystal film: based on circular birefringence effect.

The optical behavior of twisted nematic liquid crystals (TNLCs) is revealed through an angular scanning technique. Experimental results show that the optical rotation and degree of polarization of transmitted light are dependent on the polarization direction of incident light. The optical rotation is reciprocal, i.e., the polarization direction of incident and transmitted light can reciprocate when optical rotation is π/2. In some cases, the optical rotation is zero. The orientation of alignment layers in the TN cell can be determined from the behavior of optical rotation, which agrees with the measurement by an atomic force microscope. The experimental results are explained with the model of circularly polarized light based on the circular birefringence effect. Linearly polarized incident light is the superposition of right- and left-handed circularly polarized light. The propagation velocity of circularly polarized light in the LC is relevant to the polarization direction of incident light, so that the refractive indices of left- and right-handed circularly polarized light, n- and n+, or circular birefringence Δn(=n--n+) are not constants. As a result, when a linearly polarized light with the wavelength λ propagates through a TN cell with the cell gap l, the polarization direction of transmitted light is rotated to an angle Δθ. The optical rotation Δθ(=π(n--n+)l/λ) is dependent on the polarization direction of incident light, whereas the averaged refractive index ⟨n⟩(=(n-+n+)/2) can be independent of that. The incident light is partially linearly polarized light in our experiments, so that the degree of polarization of transmitted light varies with the polarization direction of incident light because the optical rotatory rates for the primary and secondary light beams are different.

Nanotechnology for purifying nematic liquid crystals based on magnetic separation accompanied by phase transition.

Free ions are generally unfavorable in liquid crystal (LC) displays, and LC purification technologies are critically important. The colloidal γ-Fe2O3 magnetic nanoparticles coated with oleic acid (γ-Fe2O3@OA MNPs) have a high ratio of surface to volume, which may adsorb more free ions and are uniform in the LC at room temperature. In this work, the precipitation and separation of the doped colloidal γ-Fe2O3@OA MNPs resulting from the magnetic field accompanied by an isotropic-nematic phase transition are more efficient than in the single case of the phase transition or the magnetic field. The residual ion concentrations have decreased distinctly using the low gradient magnetic field (∇ B âˆ¼ 2 T/m) with the phase transition. In addition, when the doped colloidal γ-Fe2O3@OA MNPs are 0.4 % and 0.2 % by weight, the former concentrations of the residual ions and γ-Fe2O3@OA MNPs are lower than the latter. As a result, the commercial nematic LC can be purified by this approach based on nanotechnology in our study.

Evolution of Nonlinear Optical Characteristics of Magnetic Nanoparticle Colloidal Suspensions after Laser-Induced Clusters.

In this paper, the nonlinear optical properties of magnetic nanoparticle colloidal solutions were studied by the Z-scan technique using 25 ps laser pulses at a wavelength of 1064 nm. Our results reveal that the formed magnetic nanoparticle clusters under high incident laser intensity will greatly affect the nonlinear optical characteristics of the solution. As the intensity of the pulsed laser decreases, the reverse saturable absorption coefficient and nonlinear refractive index of the sample tend to increase. The evolution of this nonlinear characteristic only occurs in liquid suspension. This is confirmed by fixing particles on a substrate upon which the responses observed in the liquid medium are no longer present. Besides, the possibility of generating optical trapping in the focus of the laser pulses is proposed to explain our experimental results.

Enhancement of Image Quality in LCD by Doping γ-Fe2O3 Nanoparticles and Reducing Friction Torque Difference.

Improving image sticking in liquid crystal display (LCD) has attracted tremendous interest because of its potential to enhance the quality of the display image. Here, we proposed a method to evaluate the residual direct current (DC) voltage by varying liquid crystal (LC) cell capacitance under the combined action of alternating current (AC) and DC signals. This method was then used to study the improvement of image sticking by doping γ-Fe2O3 nanoparticles into LC materials and adjusting the friction torque difference of the upper and lower substrates. Detailed analysis and comparison of residual characteristics for LC materials with different doping concentrations revealed that the LC material, added with 0.02 wt% γ-Fe2O3 nanoparticles, can absorb the majority of free ions stably, thereby reducing the residual DC voltage and extending the time to reach the saturated state. The physical properties of the LC materials were enhanced by the addition of a small amount of nanoparticles and the response time of doping 0.02 wt% γ-Fe2O3 nanoparticles was about 10% faster than that of pure LC. Furthermore, the lower absolute value of the friction torque difference between the upper and lower substrates contributed to the reduction of the residual DC voltage induced by ion adsorption in the LC cell under the same conditions. To promote the image quality of different display frames in the switching process, we added small amounts of the nanoparticles to the LC materials and controlled friction technology accurately to ensure the same torque. Both approaches were proven to be highly feasible.

Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Role of Treating Solution's Temperature.

Using FeOOH/Mg(OH)2 as precursor and FeCl2 as the treating solution, we prepared γ-Fe2O3 based nanoparticles. The FeCl2 treating solution catalyzes the chemical reactions, dismutation and oxygenation, leading to the formation of products FeCl3 and Fe2O3, respectively. The treating solution (FeCl2) accelerates dehydration of the FeOOH compound in the precursor and transforms it into the initial seed crystallite γ-Fe2O3. Fe2O3 grows epitaxially on the initial seed crystallite γ-Fe2O3. The epitaxial layer has a magnetically silent surface, which does not have any magnetization contribution toward the breaking of crystal symmetry. FeCl3 would be absorbed to form the FeCl3·6H2O surface layer outside the particles to form γ-Fe2O3/FeCl3·6H2O nanoparticles. When the treating solution's temperature is below 70 °C, the dehydration reaction of FeOOH is incomplete and the as-prepared samples are a mixture of both FeOOH and γ-Fe2O3/FeCl3·6H2O nanoparticles. As the treating solution's temperature increases from 70 to 90 °C, the contents of both FeCl3·6H2O and the epitaxial Fe2O3 increased in totality.

Improvement of Image Sticking in Liquid Crystal Display Doped with γ-Fe2O3 Nanoparticles.

Image sticking in thin film transistor-liquid crystal displays (TFT-LCD) is related to the dielectric property of liquid crystal (LC) material. Low threshold value TFT LC materials have a weak stability and the free ions in them will be increased because of their own decomposition. In this study, the property of TFT LC material MAT-09-1284 doped with γ-Fe2O3 nanoparticles was investigated. The capacitances of parallel-aligned nematic LC cells and vertically aligned nematic LC cells with different doping concentrations were measured at different temperatures and frequencies. The dielectric constants perpendicular and parallel to long axis of the LC molecules ε⊥ and ε//, as well as the dielectric anisotropy Δε, were obtained. The dynamic responses and the direct current threshold voltages in parallel-aligned nematic LC cells for different doping concentrations were also measured. Although the dielectric anisotropy Δε decreased gradually with increasing temperature and frequency at the certain frequency and temperature in LC state for each concentration, the doping concentration of γ-Fe2O3 nanoparticles less than or equal to 0.145 wt % should be selected for maintaining dynamic response and decreasing free ions. This study has some guiding significance for improving the image sticking in TFT-LCD.