Equal Refractive Index Research Articles

Transparent Cellulose Nanofibrils Composites with Two-layer Delignified Rotary-cutting Poplar Veneers (0\xb0-layer and 90\xb0-layer) for Light Acquisition of Solar Cell

Our transparent cellulose nanofibrils composites (TCNC) directly from rotary-cutting poplar veneer (RPV) whose lignin can be easily stripped by our treatment. This TCNC is prepared by stripping lignin of original RPV and infiltrating epoxy resin (ER) into delignified RPV. This TCNC with two-layer delignified RPVs whose grains perpendicular (0/90°) to each other, which were solidified on solar cell while infiltrating ER. This TCNC with high transmittance (~90%), high haze (~90%), and equal refractive index fluctuation. Comparing with epoxy resin (ER), this TCNC can enhance open circuit voltage (VOC) from 1.16 to ~1.36 and short circuit density (JSC) from 30 to ~34 for the solar cell, and can enhance test fore from 0.155 kN to ~0.185 kN and displacement from 43.6 mm to ~52.5 mm.

A polarization- and direction-independent phase-matching for third harmonic generation

We proposed a nearly perfect phase-matching for third harmonic generation (THG) in photonic crystals (PhCs), which is independent of the polarization and propagation direction of the fundamental wave. Taking an AlN-like Kerr medium as an example, equal effective refractive indices of the fundamental and the triple frequencies were achieved for both the TM and TE modes simultaneously by fabricating this dispersive medium into a two-dimensional square-lattice PhC. Furthermore, the THG conversion efficiency were numerically calculated by using the nonlinear finite-difference time-domain method to simulate Gaussian pulses with TM and TE polarizations propagating along the high-symmetry directions of the PhC (ΓX and ΓM), respectively. The simulation results show that all THG conversion efficiencies increase with the propagation length like a parabola regardless of the polarization and the propagation direction of the incident fundamental wave, indicating a nearly perfect phase matching.

A novel method to create high density stratification with matching refractive index for optical flow investigations

Turbulent mixing in stratified environments represents a challenging task in experimental turbulence research, especially when large density gradients are desired. When optical measurement techniques like particle image velocimetry (PIV) are applied to stratified liquids, it is common practice to combine two aqueous solutions with different density but equal refractive index, to suppress particle image deflections. While refractive image matching (RIM) has been developed in the late 1970s, the achieved limit of 4% density ratio was not rivalled up to day. In the present work, we report a methodology, based on the behavior of excess properties and their change in a multicomponent system while mixing, that allows RIM for solutions with higher density differences. The methodology is then successfully demonstrated using a ternary combination of water, isopropanol and glycerol, for which RIM in presence of a density ratio of 8.6% has been achieved. Qualitative PIV results of a turbulent buoyant jet with 8.6% density ratio are shown.

Transparency of chrysotile nanofiber-reinforced nanocomposite films: Effect of Rayleigh scattering

Abstract Chrysotile nanofibers, which are derived from the natural mineral chrysotile, are a promising reinforcement material used to improve the mechanical properties of composite films, causing minimal reduction in visible transparency. The fabricated films were characterized through scanning electron microscopy, atomic force microscopy, and UV–Vis spectroscopy. Results showed that the composite films exhibited high luminous transmittance at fiber contents ranging from 10 wt.% to 40 wt.%, with an average transparency above 75%. Transparency showed low sensitivity to the nanofiber content of the composite films and was dependent on refractive index difference between the nanofiber and the resin. The refractive index distribution of the resins ranged from 1.496 to 1.619, the resin matched better with nanofiber the obtained composites got higher transparency. The composite film exhibited the highest transparency of approximately 85%, with transparency loss less than 5%, under equal refractive index between the chrysotile nanofiber and the resin. This study illustrates that the high transparency of the composite film mainly resulted from the reduction of Rayleigh scattering effect.

Radar albedos and circular-polarization ratios for realistic inhomogeneous media using the discrete-dipole approximation

Electromagnetic scattering is simulated using the discrete-dipole approximation for inhomogeneous spheres and closely-packed aggregates of spherical particles with an emphasis on backscattering and planetary radar applications. The simulations are utilized to study how different parameters, including the composition and distribution of a homogeneous medium, affect the circular-polarization ratio and the radar albedo. Two different pairs of refractive indices are chosen, using values relevant for asteroids and meteorites of silicaceous and basaltic materials at microwave wavelengths. The results show substantial variance due to the geometry at backscattering, which is why analytic formulae cannot be deduced. However, some tendencies are visible: for example, the circular-polarization ratio increases when the structure becomes less isotropic as well as when the effective refractive index increases due to an increase in the phase shift. As for the radar albedo, the values computed for the homogeneous spheres with an equal effective refractive index correlate to some extent, but also differ from the radar albedos of the inhomogeneous spheres systematically. For the aggregates of spheres, the effect of the general shape and differences between the algorithms that dominate the effects caused by the inhomogenization. We find that the general scattering features remain regardless of the inhomogeneous internal structure.

Guided-mode resonant Brewster filter consisting of two homogenous layers and a single grating with an equal refractive index

In this paper, a new type of resonant Brewster filter (RBF) consisting of two homogenous layers and a single grating with an equal refractive index is presented. The properties are studied by using the plane waveguide method (PWM) and rigorous coupled-wave analysis (RCWA). It is found that the variation of the grating thickness does not effectively change the position of the resonant wavelength, however it has a remarkable effect on the line width, and the resonant peak can be adjusted back to its original position by slightly tuning the grating period. Moreover, by simultaneously tuning the thicknesses of the homogeneous layers above and beneath the grating structure, multiple channels can also be obtained when the RBF is illuminated at the Brewster angle calculated with the effective medium theory (EMT) of subwavelength grating. The adjacent optical thickness for acquiring the multiple channels is about three-quarters of the resonant wavelength. Furthermore, it is demonstrated that the line width at the operating resonant wavelength can be appreciably narrowed by tuning the thickness of the homogenous layer to its corresponding thickness without fine tuning the grating period or the thickness. Therefore, it is very useful for designing filters with different line widths at the desired wavelength. In addition, it is shown from our calculations that the symmetrical line feather can be obtained if the total optical thickness for the homogeneous layer meets the special condition.

Triple-layer guided-mode resonance Brewster filter consisting of a homogenous layer and coupled gratings with equal refractive index

A triple-layer guided-mode resonance Brewster filter consisting of a homogeneous layer and two identical gratings with their refractive indices equal to that of the homogeneous layer is presented. The spectral properties of this filter are analyzed based on the coupling modulation of two identical binary gratings at Brewster angle for a TM-polarized wave. The grating layer between substrate and homogeneous layers can significantly change the linewidth and resonant mode position, which are due to the asymmetric field distribution inside the grating layers. The tunability of the resonance can be altered on different resonant channels and a practical filter can be obtained in TM2 waveguide mode. Variation of filling factor can alter the field localization in the grating structure and significantly adjust the linewidth of the filter.

The theory and implications of the biaxial model of corneal birefringence

A theoretical model of biaxial optical anisotropy is derived and its structural, biomechanical and developmental implications are discussed with reference to known corneal anatomy. METHODS A concise review of the theory of optical crystallography is followed by the derivation of a theoretical model of the optical anisotropic properties of a dome of biaxial birefringent crystalline material. The model is then applied to parameters relevant to the biaxial model of human corneal birefringence. Theoretical distributions of refractive indices and vibration directions for transmitted monochromatic light are derived for the central and paracentral zones of a model human cornea. Contours of equal refractive index (equirefringence curves) are found to have orthogonal confocal spheroconic geometry. A novel model of corneal structure is proposed in which discrete uniaxial positive birefringent fibre-like elements conform to the derived spheroconic geometry. Biomechanical implications and the relationship of the birefringent elements to known corneal anatomy are discussed. The crystallographic conventions are proposed as a standard for further investigations of corneal birefringence.

Guided-mode resonance filters with shallow grating

A shallow grating triple-layer guided-mode resonance (GMR) filter and its properties are presented. For this type of GMR filter, the grating layer has the equal refractive index to the waveguide layer. It is shown that the GMR filter with shallow grating has good characteristics of narrow bandwidth, symmetrical line shape, and low-reflection sideband. The spectral FWHM of the GMR filter can be controlled by varying the grating thickness. The desired FWHM at the same resonance wavelength with symmetrical line shape and low-reflection sideband can be obtained by adjusting the grating thickness and the grating period with the total thickness of the grating and waveguide layers kept unchanged.

Fabrication of silicon nitride waveguides for visible-light using PECVD: a study of the effect of plasma frequency on optical properties

This paper presents work aimed at optimizing the fabrication of silicon nitride Si(x)N(y) thin-film visible-light planar waveguides using plasma-enhanced chemical vapour deposition (PECVD). The effects of plasma frequency, precursor gas ratio, and thermal annealing in relation to waveguide optical properties (refractive index, propagation losses) are studied. Experimental results over a wide range of precursor gas ratios show convincingly that waveguides fabricated using low-frequency PECVD have lower propagation losses in the visible range compared to waveguides of equal refractive index fabricated with high-frequency PECVD.

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.

AER lecture: Some reflections on corneal thickness

AbstractThe corneal thickness as an object for studies was recognized in the renaissance. A value of 1 mm, representing the maximally swollen human cornea, was reported. Optical in vivo measurements were done by Blix in 1880 reporting a thickness of about 0.5 mm, the value that we today know is correct.Blix lived in “the golden age of physiologic optics”. His interest was the contribution of the cornea to the optical refraction of the eye, and was thus the distance between the anterior and the posterior surface rather than the thickness of the cornea as such.A biomechanical interest in corneal thickness was initiated by the studies of tonometry, in particular Hans Goldmann’s development of applanation tonometry. He predicted correctly that corneal thickness would influence the estimated pressure reading.Another physiological aspect of the cornea is its transparency. Earlier explanations by equal refractive index was revolutionized by the interference theory by David Maurice. Optical transparency required a regular fiber pattern, and thus a stabilized thickness and stromal hydration. This led to extensive interest in the permeability of the limiting layers, in particular the transport of fluid across the endothelium. The physiological concepts required a regulated or stabilized thickness. The thickness as such became interesting.The human cornea is thinner in the center than more peripherally and the central, presumably regulated central thickness (CCT) became a biometric and clinical study object. The exact individual value became of interest. Several optical and later ultrasonic principles were presented.Questions addressed were: Is CCT a life‐long, age independent characteristics. Is CCT diagnostic for certain disease conditions (e.g. Macular dystrophy of Groenouw). Is CCT a useful clinical parameter to follow disease processes (e.g. progression in keratoconus or acute changes in graft rejections).Today refractive surgery has revived the interest in biomechanical and optical properties of the cornea. Modern computer technology allows for a description of the “thickness profile” of the entire cornea. This gives us access to an overwhelming amount of data, and reopen many issues of the past. We must realize, however, that what we see is the pendulum swinging back to the problems of the last century. The machinery is smarter but many of the basic questions remain to be solved.

Linewidth properties of double-layer surface-relief resonant Brewster filters with equal refractive index

In this paper, double-layer surface-relief resonant Brewster filters consisting of a homogenous layer and a grating with equal refractive index are obtained by adjusting the grating filling factor, and linewidth properties of these types of filters are investigated. It is shown etch depth error does not change the filter linewidth, but the grating filling factor and the substrate refractive index can significantly change the filter linewidth. Moreover, the coupling strength can be appreciately affected by the homogeneous layer thickness, and one can obtain different linewidths at the same operating wavelength by selecting different homogeneous layer thickness with other physical parameters maintained.

Double-layer resonant Brewster filters consisting of a homogeneous layer and a grating with equal refractive index

In this letter, the double-layer resonant Brewster filters and their properties were presented, which consist of a homogenous layer with a refractive index equal to that of the grating. By tuning the thickness of the homogeneous layer, a multichannel resonant Brewster filter with thin grating thickness can be obtained. For this type of filters, the variety of the grating thickness almost did not change the resonance location, but had a distinct effect on the filter linewidth. By changing grating period or tuning incident angle, different linewidths can be obtained at the same resonance wavelength without losing the low-reflection sideband feature.

Optical Microscope Absorbance Imaging of Carbon Black Nanoparticle Films at Solid and Liquid Surfaces

We have used an optical transmission microscope equipped with a digital camera and fitted with a narrow-band-pass filter to obtain absorbance images consisting of an array of pixel absorbance values. Absorbance images of films of carbon nanoparticles were used to derive spatially resolved images of the carbon film thicknesses with a resolution in the thickness dimension of a few nanometers. The technique was applied to the characterization of carbon nanoparticle films at cellulose-coated glass surfaces and at the oil-water interfaces of emulsion drops. For the emulsions, it was necessary to use oil and water phases of equal refractive index to avoid artifacts due to the drops acting as lenses.

Refractive index matching and clear emulsions

Refractive index (RI) matching is a unique way of making clear emulsions to meet market trends. However, RI matching has not been sufficiently investigated in terms of physical principles and methodologies. Snell's law (n2 sin r2= n1 sin r1) is applicable to cosmetic emulsions. When oil phase and water phase have equal RI (n2 = n1) values, light will not bend as it strikes obliquely at the emulsion interface. Instead, light is transmitted through the emulsion without refraction, which produces clarity. Theoretical RI values in solution can be calculated with summation of the product of the weight percentage and refractive index of each ingredient (RI(mix) = [W1 x n1 + W2 x n2 + W3 x n3 + + Wn x nn]Wtau). Oil-phase RI values are normally at 1.4 or higher. Glycols are used to adjust the water phase RI, since they typically have larger RI values than water. Noticeable deviations from calculated RI values are seen in experimentally prepared solutions. Three basic deviation types are observed: negative, positive, and slightly negative or positive, which can occur in glycol aqueous solutions at different concentrations. The deviations are attributed to changes in molecular interaction between molecules in solution, which can lead to changes in specific gravity. Negative RI deviation corresponds to a decrease in specific gravity, and positive RI deviation corresponds to an increase in specific gravity. RI values will deviate from calculated values since an increase or decrease in specific gravity leads to a change in optical density.

Ray invariants and wave equations for transverse modes in three-dimensional graded-index waveguides

A theory of the second ray invariant is proposed using the theory of plane Frenet curves. Its existence requires that the coordinate dependence of the refractive index in the waveguide cross section should satisfy the regularity condition: curves of equal refractive index differ only by an amount which can be obtained using an isotropic scaling transformation. The theoretical conclusions are illustrated using the example of waveguides having the generalized refractive index distribution n ( r ) = n [ (x/ a) + (y/ b)q].

Simple formula for the magneto-optic Kerr effect in large period magnetic multilayers

Abstract An elementary formula is derived from which the complex Kerr rotations can be calculated for a semi-infinite magneto-optic multilayer consisting of two types of magnetic media. The expression takes into consideration the absolute value of the sub-layer thicknesses and also of the order in which the layers appear in the system. The expression is exact for media of equal refractive index and also provides a very good approximation to cases where the indices of the media differ by as much as 20%. Consequently, the formulation presented provides a simple, quick and reliable means of calculating the magneto-optical properties of multilayered media.

Simple relations for a triple-layer antireflection coating

A specific triple-layer antireflection coating with equal refractive indices and thicknesses of the first and third layers is considered. Simple formulas for coating layer thicknesses are derived for nonabsorbing materials at normal incidence.

Elementary formula for the magneto-optic Kerr effect from model superlattices

An elementary formula is derived for the magneto-optic Kerr effect from model superlattices. An exact result is obtained for the case of equal refractive indices for the two constituents of the superlattice. Co/Pt is used as an example of such superlattices for wavelengths in the vicinity of 2 eV. First-order corrections in the difference of the refractive indices are derived which are suitable for treating a variety of materials.