Empirical expression for the minimum residual reflectance of normal- and oblique-incidence antireflection coatings

A new empirical expression for estimating minimum achievable residual reflectance of antireflection (AR) coatings is presented. The expression gives an accurate approximation of the minimum residual reflectance for normal- and oblique-incidence AR coatings in the visible and infrared spectral ranges.     

可见光聚合物基增透膜的研究进展

可见光增透膜是制造很多光学元件的必要材料之一。简要介绍了增透膜增透的原理,在使用单层和多层增透膜增透效果的计算基础上,重点综述了聚合物基增透膜技术的研究进展。这些技术主要包括:(1)在聚合物中引入低折射指数元素;(2)使用纳米孔聚合物薄膜调节折射指数;(3)使用高低折射指数组成的复合膜。此外还分析了国内外增透技术的发展趋势、市场需求及其发展前景。     

梯度折射率宽带减反射光伏玻璃研究

采用二次腐蚀法制备了梯度折射率减反射光伏玻璃。SEM分析表明,多孔减反射层的微结构尺度在20～30nm左右,膜层厚度为100～300nm。通过计算模拟,确定了膜层折射率分布形式为Gaussian-like型。制备的梯度折射率减反射光伏玻璃透过率>96%的带宽超过了1200nm;在390～1022nm波段的透过率>99%;在350～1084nm的双面反射率<1%,其中624～922nm的双面反射率低于0.2%。     

Optimal single-band normal-incidence antireflection coatings

Mathematical and computational evidence that strongly suggests that optimal solutions exist to single-band, normal-incidence antireflection coating problems is presented. It is shown that efficient synthesis and refinement techniques can quickly and accurately find such solutions. Several visible and infrared antireflection coating examples are presented to support this claim. Graphs that show the expected optimal performance for different representative substrates, refractive-index ratios, wavelength ranges, and overall optical thickness combinations are given. Typical designs exhibit a pronounced semiperiodic clustering of layers, which has also been observed in the past. Explanations of this phenomenon are proposed.     

Further guidance for broadband antireflection coating design

Recent investigations have added to and refined the understanding of the behavior of broadband antireflection coating designs and provided further guidance for achieving more nearly optimal designs. The ability to optimize designs wherein the overall optical thickness of the design is constrained to a specific value has allowed this investigation. A broader bandwidth than previously reported has been studied and statistically fit more precisely by a polynomial equation, and also two linear equations for routine approximations have been derived. It has also been found that the optimal number of layers in the design can be predicted as a function of the bandwidth.     

Expanded viewpoint for broadband antireflection coating designs

Examining spectral regions outside of the band where an antireflection coating is specified can aid in finding optimal design solutions. The reflectance versus wavenumber plots at low frequencies indicate the overall thickness of the design. These plots also point to whether the design will provide the minimum possible average reflectance in the specified band. It has been discovered that these patterns are nearly replicated by the plot of a quarter-wave stack at peak frequency. It is also found that optimal solutions exist only at quantized intervals.     

The theory of double-layer antireflection coatings

The theory of double-layer antireflection coatings using non-quarter-wave films is reviewed and extended. Normal incidence only is considered.The spectral reflectance of a double-layer coating depends on the refractive indices of the layers and the adjacent media, and on Ø₁ and Ø₂, the phase thickness of the layers. It is known that the reflectance is theoretically zero if the refractive indices satisfy certain inequalities and if the phase thicknesses are those calculated from certain equations.To obtain a fuller appreciation of double-layer coatings, the reflectance throughout the whole (Ø₁, Ø₂) plane is studied analytically. The pattern formed by the lines of constant reflectance exhibits translational symmetry, and the unit pattern which generates the whole pattern has a twofold axis of symmetry. The shape of the unit pattern depends on the refractive indices of the four media. Numerical examples are given to illustrate the similarities and differences between the unit patterns of some double-layer systems.     

Porous broadband antireflection coating by glancing angle deposition

We deposited graded-index SiO2 films using glancing angle deposition to produce high-transmission antireflection coatings on glass. Because of the accurate control over the thin-film microstructure provided by this technique, we were able to create graded densities with a Gaussian profile resulting in transmission values greater than 99.9% for a single-layer interface with bandwidths up to 460 nm. The graded-index layer also provides low reflectance at nonnormal angles of incidence with transmission values degrading little for incidence angles up to 30 degrees.     

Triple-layer broad band antireflection coatings using homogeneously mixed dielectric coatings

Mixtures of ZnS and MgF₂ of different compositions were successfully used for the innermost and intermediate layers in triple-layer (quarter-half-quarter) antireflection coatings. The suitability of this mixture for optical interference coatings is discussed. The transmittance of the sample was found to be 99–99.25% over a broad range in the visible region (490–725 nm). This is in good agreement with theoretical predictions.     

Toward perfect antireflection coatings: numerical investigation

A perfect antireflection (AR) coating would remove completely the reflection from an interface between two media for all wavelengths, polarizations, and angles of incidence. The degree to which this can be achieved is investigated numerically. It is shown that wideband solutions can be found provided that layers can be deposited with refractive indices that are close to that of the low-index medium. Thus realistic solutions exist for interfaces between two solid media. Narrow-band high-angle AR solutions are also possible for polarized light and for unpolarized light in the vicinity of certain reststrahlen bands.     

Multilayer antireflection coatings for the visible and near-infrared regions

With a high-refractive-index mixed-oxide dielectric material of ZrTiO(4) and ZrO(2) [Substance H2 (Sub2) from E. Merck, Darmstadt, Germany], in combination with magnesium flouride (MgF(2)), design optimization and experimental production of low-loss antireflection (AR) coatings are carried out. Design-optimization studies that make use of these materials as constituents of a seven-layer coating system demonstrate that when the useful bandwidth of an AR coating is extended to cover a wider spectral range, the designs are in general found to have increased integrated reflection loss, higher ripple, and increased spectral instability. The experimental studies on Sub2 material show that the films have excellent optical performance over a wider process window, the advantage of which is demonstrated in the production of different AR coatings on a variety of glasses with refractive indices that range from 1.45 to 1.784 and different mechanical, thermal, and chemical properties. The manufacturing process of AR coatings shows a consistency better than 99% with respect to optical properties and durability.     

Fluoride antireflection coatings deposited at 193 nm

Antireflection coatings for 193 nm composed of low-index (MgF(2) and AlF(3)) and high-index (LaF(3) and GdF(3)) materials are deposited by the resistive heating boat method at a substrate temperature of 300 degrees C. The optical characteristics (reflectance and optical loss), microstructure (morphology and surface roughness), stress, and laser-induced damage threshold (LIDT) of the coatings are investigated and discussed. The related reflection at 193 nm of the four kinds of antireflection coatings is smaller than 0.2% and the optical loss is less than 0.15%. Of the fluoride antireflection coatings, AlF(3)/GdF(3) had the lowest stress value. Antireflection coatings with AlF(3) as the low-index material had higher LIDT values than when MgF(2) was used.     

A note on multilayer acoustic antireflection coatings

As is well known, multilayer structures can be used as wide-band acoustical antireflection (AR) coatings. An investigation has been made of new approaches to the acoustical AR properties of multilayers based on optical techniques. We first present some results from standard acoustical theory as background. Then the use of symmetric multilayers is considered, the analogue of optical symmetric multilayers. Subsequently, characteristic matrices, a commonly used approach in optics, is applied to quarter- and half-wave layers to calculate acoustical AR properties. It is concluded that optical techniques provide a very useful approach to the design of acoustical AR coatings, which can be used to improve ultrasonic transducer performance.     

Two-layer wideband antireflection coatings with an absorbing layer

A method for designing antireflection coatings by the use of an absorbing layer and a dielectric layer is proposed. Theoretical and experimental results of three two-layer wideband antireflection coatings with Au, Cu, and TiO(x)N(y) are given. The antireflection coatings have low reflectance, good conductivity, and a simple structure. They are especially suitable for display applications.     

Universal antireflection coatings for substrates for the visible spectral region

It is possible to design normal-incidence antireflection coatings that reduce the reflectance of any substrate with a refractive index that lies in the range of 1.48 to 1.75. The performance of such coatings depends on the width of the spectral region over which the reflectance is to be suppressed, on the coating materials used for their construction, and on the overall optical thickness of the layer system. For example, the calculated average spectral reflectance of a set of six different substrates with refractive indices 1.48, 1.55, 1.60, 1.65, 1.70, and 1.75, when coated with a 0.56-μm-thick, eight-layer antireflection coating designed for the 0.40-0.80-μm spectral region, was 0.34%. This is higher than the average reflectance that is attainable with a conventional antireflection coating of similar optical thicknesses designed for a particular refractive index. However, it is an acceptable value for most applications. With the universal type of antireflection coating described, it is thus possible to coat a number of different refractive-index substrates in one deposition run, and this can result in considerable cost and time savings.     

Method for estimation of the average local working temperatures and the residual resource of metal coatings of gas-turbine blades

A new method is proposed for estimation of the average local operating temperatures and the residual service life (resource) of protective MCrAlY metal coatings of gas-turbine blades after a certain time of operation on the basis of experimental data on the residual content of aluminum in the coating of a blade and/or the width of its first depletion zone. A physical and a mathematical model of the processes of oxidation and diffusion of aluminum in the coating-blade alloy system, accounting for the kinetics of formation of the diffusion zone, are presented. Results of calculation of the local working temperatures and the residual resource of the coating of a blade at local points along the outline of the blade cross section by the method proposed with the use of data on the width of the first depletion zone are given. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 3, pp. 16–23, May–June, 2007.