Scalar limitations of diffractive optical elements

In this paper, scalar limitations of diffractive optic components are investigated using coupled wave analyses. Results are presented for linear phase gratings and fanout devices. In addition, a parametric curve is given which correlates feature size with scalar performance

Surface modification of natural fibers using bacteria: Depositing bacterial cellulose onto natural fibers to create hierarchical fiber reinforced nanocomposites

Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria (Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(L-lactic acid) was quantified using the single fiber pullout test

13. - Évacuation des eaux pluviales en rive droite du Rhône. Le cas des cantons de Roquemaure et Villeneuve-lez-Avignon

Les cantons de Roquemaure et Villeneuve lez Avignon sont le siège d’inondations fréquentes liées à la fois au climat méditerranéen mais également à une situation topographique défavorable et à des contraintes hydrauliques fortes résultant des différents aménagements mis en place au cours des siècles. Ce secteur a récemment fait l’objet d'une étude de schéma proposant des mesures techniques, institutionnelles et réglementaires, indispensables à une gestion cohérente de son développement. L'approche retenue a consisté à privilégier la rétention ; l’étude a privilégié la concertation avec l’ensemble des intervenants sur la zone dans le but à la fois de prendre en compte leurs contraintes nombreuses et variées et de renforcer l’esprit de solidarité amont-aval dans les bassins versants.Benedetti M., Pommet B., Prigent C., Reumaux Y. 13. - Évacuation des eaux pluviales en rive droite du Rhône. Le cas des cantons de Roquemaure et Villeneuve-lez-Avignon. In: Crues et inondations. 23èmes journées de l'hydraulique. Congrès de la Société Hydrotechnique de France. Nimes (France), 14-15-16 septembre 1994. Tome 2, 1994

Stable Implementation Of The Rigorous Coupled-Wave Analysis For Surface-Relief Gratings:Enhanced Transmittance Matrix Approach

An enhanced, numerically stable transmittance matrix approach is developed and is applied to the implementation of the rigorous coupled-wave analysis for surface-relief and multilevel gratings. The enhanced approach is shown to produce numerically stable results for excessively deep multilevel surface-relief dielectric gratings. The nature of the numerical instability for the classic transmission matrix approach in thepresence of evanescent fields is determined. The finite precision of the numerical representation on digitalcomputers results in insufficient accuracy in numerically representing the elements produced by invertingan ill-conditioned transmission matrix. These inaccuracies will result in numerical instability in the calculations for successive field matching between the layers. The new technique that we present anticipatesand preempts these potential numerical problems. In addition to the full-solution approach whereby all thereflected and the transmitted amplitudes are calculated, a simpler, more efficient formulation is proposed forcases in which only the reflected amplitudes (or the transmitted amplitudes) are required. Incorporatingthis enhanced approach into the implementation of the rigorous coupled-wave analysis, we obtain numericallystable and convergent results for excessively deep (50 wavelengths), 16-level, asymmetric binary gratings.Calculated results are presented for both TE and TM polarization and for conical diffraction. © 1995 Optical Society of America

Optimal Design For Antireflective Tapered Two-Dimensional Subwavelength Grating Structures

Techniques for the design of continuously tapered two-dimensional (2D) subwavelength surface-relief grating structures for broadband antireflection surfaces are investigated. It has been determined that the Klopfenstein taper [Proc. IRE 44, 31 (1956)] produces the optimum graded-index profile with the smallest depth for any specified minimum reflectance. A technique is developed to design the equivalent tapered subwavelength surface-relief grating structure by use of 2D effective-medium theory. An optimal Klopfenstein tapered 2D subwavelength grating is designed to reduce the Fresnel reflections by 20 dB over a broad band from an air-substrate (ns = 3.0) interface. The performance is verified by use of both a 2D effective- medium-theory simulation algorithm and rigorous coupled-wave analysis. These structures are also shown to achieve this low reflectance over a wide field of view (#fov \u3e 110°). The pyramidal spatial profile, which has generally been assumed to produce the optimal broadband antireflection grating structure, is shown to require a significantly larger depth to achieve the same performance as a Klopfenstein-designed tapered antireflection grating structure. © 1995 Optical Society of America

Formulation For Stable And Efficient Implementation Of The Rigorous Coupled-Wave Analysis Of Binary Gratings

The rigorous coupled-wave analysis technique for describing the diffraction of electromagnetic waves by periodic grating structures is reviewed. Formulations for a stable and efficient numerical implementation of the analysis technique are presented for one-dimensional binary gratings for both TE and TM polarization and forthe general case of conical diffraction. It is shown that by exploitation of the symmetry of the diffraction problem a very efficient formulation, with up to an order-of-magnitude improvement in the numerical efficiency, is produced. The rigorous coupled-wave analysis is shown to be inherently stable. The sources of potentialnumerical problems associated with underflow and overflow, inherent in digital calculations, are presented. A formulation that anticipates and preempts these instability problems is presented. The calculated diffraction efficiencies for dielectric gratings are shown to converge to the correct value with an increasing number of space harmonics over a wide range of parameters, including very deep gratings. The effect of the number ofharmonics on the convergence of the diffraction efficiencies is investigated. More field harmonics are shownto be required for the convergence of gratings with larger grating periods, deeper gratings, TM polarization, and conical diffraction. © 1995 Optical Society of America

Artificial Uniaxial And Biaxial Dielectrics With Use Of Two-Dimensional Subwavelength Binary Gratings

Two-dimensional symmetric and asymmetric subwavelength, binary gratings are investigated. A method for determining the three effective indices of a two-dimensional (2-D) subwavelength grating is presented, as well as a theoretical formalization for the effective index parallel with the normal to the surface. It is shown that a 2-D asymmetric binary grating on the surface of a dielectric substrate is analogous to a biaxial thin film. If the grating is symmetric, then the two effective indices perpendicular to the normal are equal, and the grating is analogous to a uniaxial thin film. Using these effective indices and the quarter-wave Tschebyscheff synthesis technique, we designed two- and three-level binary gratings to suppress reflections over a broad band. It is shown that for a substrate index of ns = 3.0 a three-level 2-D binary grating reduced reflections below 0.1% from 8μm to 12μm. © 1994 Optical Society of America

Limits Of Scalar Diffraction Theory For Diffractive Phase Elements

The range of validity and the accuracy of scalar diffraction theory for periodic diffractive phase elements (DPE’s) is evaluated by a comparison of diffraction efficiencies predicted from scalar theory to exact results calculated with a rigorous electromagnetic theory. The effects of DPE parameters (depth, feature size, period, index of refraction, angle of incidence, fill factor, and number of binary levels) on the accuracy of scalar diffraction theory is determined. It is found that, in general, the error of scalar theory is significant (e \u3e ±5%) when the feature size is less than 14 wavelengths (s \u3c 14l). The error is minimized when the fill factor approaches 50%, even for small feature sizes (A = 2l); for elements with an overall fill factor of 50% the larger period of the DPE replaces the smaller feature size as the condition of validity for scalar diffraction theory. For an 8-level DPE of refractive index 1.5 analyzed at normal incidence the error of the scalar analysis is greater than ±5% when the period is less than 20 wavelengths (A \u3c 20l). The accuracy of the scalar treatment degrades as either the index of refraction, the depth, the number of binary levels, or the angle of incidence is increased. The conclusions are, in general, applicable to nonperiodic as well as other periodic (trapezoidal, two-dimensional) structures. © 1994 Optical Society of America