Principal Refractive Indices Research Articles

A simulation study on investigation the optical and structural properties of polyethylene fiber

Multiple-beam Fizeau Interference technique, in transmission, is one of the precise techniques used in the examination of fibers. The modeling equations to simulate multiple-beam Fizeau interferograms, which express the fringe shift or phase modulation of the drawn polyethylene (PE) fibers in the case of using an immersion liquid with a refractive index greater than and less than of PE fiber, were presented. The simulated interferograms were analyzed by our previously published algorithm, which built on the use of the Fourier transform technique for fringe analysis. The simulated interferograms were used to study the effect of the drawing process on the structural properties of PE fibers. Some physical structural parameters, such as the refractive indices, the birefringence, the orientation function, and others, are significantly affected by the drawing process. The density, the degree of crystallinity, the mass fraction crystallinity, the mean square density fluctuation, the molar refractivity, and others are not affected or slightly affected by the drawing process. A new simple approach to determine the isotropic refractive index (μiso) and the principal refractive indices (ΔμoII&Δμo⊥) of fully oriented PE fiber is presented. The maximum draw ratio (Dmax) and some constants for PE fiber were obtained. The results obtained are in fair agreement with the published results of PE fiber, which indicates the validity of the simulation equations presented. To illustrate the idea of the work, convenient interferograms, figures, and tables were presented.

Some approximate relations in the photoelasticity of strongly anisotropic crystals

The knowledge of the principal refractive indices and their dependence on the applied stress is the starting point for any experimental analysis of transparent crystals. Because for strongly anisotropic crystals and a general state of stress an explicit analytical solution can be difficult to obtain, then we need some approximated solutions. Here, we propose a coherent approximation procedure that generalizes results already obtained and give a simple example of application.

Exact and linearized refractive index stress-dependence in anisotropic photoelastic crystals.

For the permittivity tensor of photoelastic anisotropic crystals, we obtain the exact nonlinear dependence on the Cauchy stress tensor. We obtain the same result for its square root, whose principal components, the crystal principal refractive index, are the starting point for any photoelastic analysis of transparent crystals. From these exact results we then obtain, in a totally general manner, the linearized expressions to within higher-order terms in the stress tensor for both the permittivity tensor and its square root. We finish by showing some relevant examples of both nonlinear and linearized relations for optically isotropic, uniaxial and biaxial crystals.

Morphology and Optical Properties of Precipitated Vivianite, Fe3(PO4)2·8H2O

AbstractCrystals of vivianite, Fe3(PO4)2∙8H2O, belonging to the monoclinic prismatic class 2/m, are grown by precipitation from aqueous solution at temperatures from 25 to 56 °C. At 25 °C aggregates are dominating, but at 35 °C and above single crystals are more common, being tabular along {010} and most often with parallelogram outline; in a few cases twins are observed. The outline is distinct and consistent with published lattice parameters, but at low temperature the {010} faces are very rough as judged from interference colors seen in the polarizing microscope. Lateral faces belonging to the {110} form are visible on crystals grown at 45 and 56 °C, and the widths of their projections on {010} as measured with a micrometer eyepiece served for estimating crystal thickness, making use of the face angle from lattice parameters. Optical parameters are determined by a combination of three methods: 1) measurement of birefringence with an Ehringhaus 6λ compensator, 2) Kamb's method for the optic axis angle determined by the angle‐of‐flight method in conoscopic illumination, and 3) visual interferometry using a polarizing microscope incorporating a Mach–Zehnder interference device. Reasonably precise values of principal refractive indices in the expected range are found.

A More Efficient Method for Synthetic Textile Fiber Analysis Using Polarized Light Microscopy.

The efficiency of conventional polarized light microscopy (PLM) methods for analyzing synthetic fiber evidence analyses is improved. Historically, using PLM for fiber identification relied on measuring refractive index. This prior PLM technology is reliable, but it is not efficient. Most fibers are optically anisotropic, having two principal refractive index values, N(High) and N(Low) . When the fiber is mounted in intermediate refractive index medium, efficiency is improved by observing the change in contrast while the polarized light's vector is rotated relative to the fiber's axis. Minimum contrast occurs when the refractive indices of the mounting medium and fiber are equal. This angle of equality is determined by orienting the fiber's highest refractive index parallel to the polarized light's electric field vector, rotating the fiber or polarizing element, observing minimum contrast and measuring the angle of equality. This method is rapid, reduces remounting fibers in different mounting media and provides a quantitative measure for fiber comparisons.

Nanoplasmonic response of porous Au-TiO2 thin films prepared by oblique angle deposition

In this work, a versatile method is proposed to increase the sensitivity of optical sensors based on the localized surface plasmon resonance (LSPR) phenomenon. It combines a physical deposition method with the oblique angle deposition technique, allowing the preparation of plasmonic thin films with tailored porosity. Thin films of Au–TiO2 were deposited by reactive magnetron sputtering in a 3D nanostructure (zigzag growth), at different incidence angles (0° ≤ α ≤ 80°), followed by in-air thermal annealing at 400 °C to induce the growth of the Au nanoparticles. The roughness and surface porosity suffered a gradual increment by increasing the incidence angle. The resulting porous zigzag nanostructures that were obtained also decreased the principal refractive indexes (RIs) of the matrix and favoured the diffusion of Au through grain boundaries, originating broader nanoparticle size distributions. The transmittance minimum of the LSPR band appeared at around 600 nm, leading to a red-shift to about 626 nm for the highest incidence angle α = 80°, due to the presence of larger (scattering) nanoparticles. It is demonstrated that zigzag nanostructures can enhance adsorption sites for LSPR sensing by tailoring the porosity of the thin films. Atmosphere controlled transmittance-LSPR measurements showed that the RI sensitivity of the films is improved for higher incidence angles.

Optical, mechanical, and photoelastic anisotropy of biaxially stretched polyethylene terephthalate films studied using transmission ellipsometer equipped with strain camera and stress gauge

ABSTRACTThe anisotropic properties of polyethylene terephthalate film resulting from its manufacturing process are quantitatively investigated in terms of its optical, mechanical, and photoelastic aspects. Transmission ellipsometers and a Jones‐matrix‐based analysis software together with a 4 × 4 Berreman‐matrix‐based analysis software are adopted to determine the wavelength‐dependent in‐plane birefringence, the principal refractive indices, and the orientation of the optical axis. Mechanical anisotropy is characterized in terms of the elastic compliance tensor components using the measured azimuthal angle‐dependent Young's modulus and Poisson's ratio. From the measured variation of the wavelength‐dependent in‐plane birefringence as a function of tensile stress, the dispersive photoelastic coefficients are obtained for a few sample azimuthal angles, and the components of the photoelastic tensor are determined. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 152–160

Polymeric Layers for Optical Structures with Controllable Waveguide Parameters

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Laser Conoscopy Study of Optical Anomalies in Uniaxial Crystals

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Quadratic nonlinear optical properties of the organic N-benzyl-2-methyl-4-nitroaniline (BNA) biaxial crystal.

We performed the direct measurement of second harmonic generation and sum frequency generation phase-matching directions in the organic N-benzyl-2-methyl-4-nitroaniline crystal over its visible and near-infrared transparency range. The fit of these data allowed us to refine the Sellmeier equations of the three principal refractive indices in this range. With these equations, we improved the calculated tuning curves of terahertz emission from a phase-matched difference frequency process. We also determined the absolute magnitude of the d24 nonlinear coefficient.

Phase-matching properties and refined Sellmeier equations of La3Ga55Nb05O14

We directly measured the phase-matching angles of second-harmonic generation and difference-frequency generation up to 6.5 µm in the Langanate crystal La3Ga5.5Nb0.5O14 (LGN). We also determined the nonlinear coefficient and damage threshold. We refined the Sellmeier equations of the ordinary and extraordinary principal refractive indices, and calculated the conditions of supercontinuum generation.

Non-critically phase-matched second harmonic generation and third order nonlinearity in organic crystal glucuronic acid γ-lactone

The linear, second order, and third order nonlinear optical properties of glucuronic acid γ-lactone single crystals were investigated. The optic axes and principal dielectric axes were identified through optical conoscopy and the principal refractive indices were obtained using the Brewster's angle method. Conic sections were observed which is perceived to be due to spontaneous non-collinear phase matching. The direction of collinear phase matching was determined and the deff evaluated in this direction was 0.71 pm/V. Open and closed aperture Z-scan measurements with femtosecond pulses revealed high third order nonlinearity in the form of self-defocusing, two-photon absorption, as well as saturable absorption.

Frequency conversion in the acentric tetragonal nonlinear crystal NaI_3O_8

We performed the first study of phase-matched second harmonic generation and difference frequency generation in the acentric nonlinear crystal NaI3O8. By fitting all the recorded phase-matching tuning curves, we determined the Sellmeier equations describing the wavelength dispersion of the ordinary and extraordinary principal refractive indices. We also measured the absolute value of the quadratic nonlinear coefficients and we calculated the conditions of supercontinuum generation in NaI3O8.

Temperature-Dependent Sellmeier Equations of IR Nonlinear Optical Crystal BaGa4Se7

The thermal dependent principal refractive indices of a new promising IR nonlinear optical crystal BaGa4Se7 at wavelengths of 0.546, 0.5806, 0.644, 0.7065, 1.530, 1.970, and 2.325μm were measured by using the vertical incidence method within the temperature range from 25 to 150 °C. We derived equations of thermal refractive index coefficients as a function of wavelength that could be used to calculate the principal thermal refractive indices at different wavelengths. The temperature-dependent Sellmeier equations were also obtained and used to calculate the phase matching angles for the optical parametric process of BaGa4Se7 crystal at different temperatures.

The Pockels effect in TmAl3(BO3)4

ABSTRACTThe Pockels effect was studied in TmAl3(BO3)4 crystal. The electro-optic coefficient of the compound was determined. The principal refractive indexes of the crystal were determined by measuring the Brewster angles. Both contributions to the electric field induced birefringence associated with the redistribution of connected charges and deformation were estimated.

Dielectric frame, Sellmeier equations, and phase-matching properties of the monoclinic acentric crystal GdCa4O(BO3)3.

We directly measured the phase-matching properties of the biaxial GdCa4O(BO3)3 (GdCOB) crystal using the sphere method. We studied second-harmonic generation and difference frequency generation in two principal planes of the crystal. All these data allowed us to refine the Sellmeier equations of the three principal refractive indices. These equations are valid over the entire transparency range of GdCOB and then could be used to calculate the tuning curves of infrared optical parametric generation.

Refined orientation of the optical axes as a function of wavelength in three monoclinic double tungstate crystals KRE(WO_4)_2 (RE = Gd, Y or Lu)

We measured a rotation of the dielectric frame as a function of wavelength of about 2° in the spectral range of 0.4-1.6 μm in the monoclinic double tungstate crystals, KRE(WO4)2 where RE = Gd, Y or Lu. We also refined the Sellmeier equations of their three principal refractive indices. By taking into account these data, we calculated the real orientation of the optical axes as a function of wavelength which is of key importance for the design of conical refraction lasers based on monoclinic double tungstate crystals.

Properties of LiGa0.5In0.5Se2: A Quaternary Chalcogenide Crystal for Nonlinear Optical Applications in the Mid-IR

LiGaSe2 (LGSe) and LiInSe2 (LISe) are wide band-gap nonlinear crystals transparent in the mid-IR spectral range. LiGa0.5In0.5Se2 (LGISe) is a new mixed crystal, a solid solution in the system LGSe–LISe, which exhibits the same orthorhombic structure (mm2) as the parent compounds in the same time being more technological with regard to the growth process. In comparison with LGSe and LISe its homogeneity range is broader in the phase diagram. About 10% of the Li ions in LGISe occupy octahedral positions (octapores) with coordination number of 3. The band-gap of LGISe is estimated to be 2.94 eV at room temperature and 3.04 eV at 80 K. The transparency at the 0-level extends from 0.47 to 13 µm. LGISe crystals exhibit luminescence in broad bands centered near 1.7 and 1.25 eV which is excited most effectively by band-to-band transition. From the measured principal refractive indices and the fitted Sellmeier equations second-harmonic generation from 1.75 to 11.8 μm (fundamental wavelength) is predicted. The nonlinear coefficients of LGISe have values between those of LGSe and LISe. 6LGISe crystals are considered promising also for detection of thermal neutrons.

Acentric langanite La3Ga5.5Nb0.5O14 crystal: a new nonlinear crystal for the generation of mid-infrared parametric light.

The mid-infrared spectral range extending from 2 to 6 μm is significant for scientific and technological applications. A promising nonlinear oxide crystal La3Ga5.5Nb0.5O14 (LGN) is proposed and fully characterized for the first time to our knowledge. The transparency range extends between 0.28 and 7.4 μm. The two principal refractive indices were measured and we found that the nonlinear coefficient d11 = 3.0 ± 0.1 pm/V at 0.532 μm. The simultaneous fit of data allowed us to refine the Sellmeier equations of LGN and to calculate the tuning curves for optical parametric generation (OPG) pumped at 1.064 μm. Calculations are consistent with recorded data and also show the generation of a supercontinuum between 1.5 and 3.5 μm when pumped at 0.98 μm by a Ti:Sapphire laser.

Investigation on crystalline perfection, optical transmittance, birefringence, temperature-dependent refractive index, laser damage threshold and pyroelectric characteristics of inversely soluble lithium sulfate monohydrate single crystals

Bulk prismatic lithium sulfate monohydrate (LSMH) single crystals were grown by seed rotation with slow heating method from aqueous solution. Small FWHM obtained from high-resolution X-ray diffraction spectrum shows that the crystals grown by this method have less defects and absence of low-angle grain boundaries. The high transmittance and low reflectance nature of the grown crystal was observed using UV–Vis–NIR spectrometer. The principal refractive indices of a LSMH crystal have been measured by a prism coupling method for the wavelengths of 0.407, 0.532, 0.828, 1.064 and 1.551 µm at room temperature, and Sellmeier equations are determined from the fitting of the data point. The refractive index data confirm that LSMH crystal is negative biaxial and the optic axis lies in YZ plane with an angle (2Vy) of 51.74° with respect to y axis at 532 nm wavelength. The thermo-optic coefficients were determined from the temperature-dependent refractive indices measured in the range of 30–125 °C for the wavelengths of 532 and 1064 nm. The surface laser damage threshold studies reveal the higher optical radiation stability against 532-nm laser. The pyroelectric coefficients and pyroelectric figure of merit were determined from the pyroelectric current measurement by the Byer and Roundy method.