Development of interference filters based on multilayer porous silicon structures

Porous silicon (PS) has a great potential in optical applications due to its tuneable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indices can be used as interference filters for applications in the field of optoelectronics and sensors. In the present work, the optical properties of PS single layers and multilayer structures were studied. Since the refractive index of PS varies depending on the air content of the porous matrix, the PS structures were modelled as an homogeneous mixture of silicon and air, according to the effective medium theories (EMTs). By adjusting the refractive index and thickness of each individual layer, we can obtain a stack of PS layers with the desired optical properties, resulting in interference filters of predetermined bandwidth.     

Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

Metal/dielectric transmission interference filters with low reflectance. 1. Design

For many applications the high reflectance of conventional multilayer interference filters is undesirable. It is possible to design and construct interference filters in which the transmittance and reflectance are independently controlled and in which the reflectance is very low. However, to do this it is necessary to employ absorbing coating materials and, as a result, a reduction in the peak transmittance must be accepted. We describe three different methods for the design of such coatings. Examples are given of a number of bandpass filters, neutral density filters, cutoff filters, and of an xˉ(λ) tristimulus filter, all having a low reflectance. This reflectance is frequently 15 to 50 times lower than that of conventional filters.     

Induced metal-dielectric selective reflecting filter for miniprojectors

Metal-dielectric multiple-band high-reflection coatings are designed as induced filters and fabricated by reactive deposition. Ta(2)O(5) and SiO(2) are used as high- and low-refractive-index layers, and Cr and Al are used as bonding and reflective layers, respectively, for constructing the filters. The metal-dielectric coatings are deposited on a light-shaping flexible plastic substrate for use as a screen with high-contrast enhancement performance. This screen was suitable for miniprojectors with red, green, and blue LEDs as light sources. Mechanical properties such as stress, hardness, and adhesive strength of these multilayer films are investigated also.     

Narrow-bandpass filters with broad rejection band for single-mode waveguides

Fabry-Perot bandpass filters made of mirrors with both high- and low-Dn (refractive-index modulation) have simultaneously a broad rejection band and a narrow passband. The higher Dn's are obtained with multilayer mirrors and the lower with Bragg gratings (BG's). Implementation of a sampling calculation technique based on the characteristic matrix formalism used for interference coatings allows for simulation of hybrid filters constructed from multilayer mirrors and BG's. The possible defects of hybrid filters are extensively analyzed. Bandpass filters made purely of both high- and low-Dn BG's are also simulated. All these filters are useful for wavelength division multiplexing applications.     

Laser-Induced Damage of a 1064-nm ZnS/MgF(2) Narrow-Band Interference Filter

The laser-induced damage thresholds, absorptances, and damage morphologies of ZnS/MgF(2) interference filters that were designed to allow radiation at wavelengths near 1064 nm to pass through them have been examined. The damage morphologies as well as their laser behaviors suggest that the initial damage is located not at the surface layers but near the interface of the spacer layer where ZnS is sublimed to form many little bubbles. The electric field distribution and the temperature rise in the multilayer was calculated to model this interesting phenomenon. Various explanations for the thermodynamic coupling are presented.     

空气过滤用微纳米聚丙烯腈/皮芯型聚乙烯-聚丙烯双组分纤维多层复合材料的制备与性能

采用静电纺丝技术将聚丙烯腈(PAN)纳米纤维收集在皮芯型聚乙烯-聚丙烯(PE-PP)双组分微米纤维网上,制备PAN/PE-PP单层复合纤维网,再将多个单层复合纤维网层层堆叠,经热黏合加固,制备PAN/PE-PP多层复合空气过滤材料,研究了PAN/PE-PP复合纤维网的层数和纺丝时间对其孔径及过滤性能的影响。结果表明:多层复合的方式可得到与单层复合材料相似的孔径参数,但两种材料的孔道结构不同。在总面密度和总纺丝时间一定时,当PAN/PE-PP复合纤维网的层数大于10层时,PAN/PE-PP多层复合过滤材料的过滤效率和品质因子Q_F均明显大于PAN/PE-PP单层复合过滤材料,阻力略微增大;其中,相较PAN/PE-PP单层复合过滤材料,20层PAN/PE-PP复合过滤材料对≥0.3 μm颗粒的过滤效率提高了33%,阻力增加了5 Pa,Q_F值提高了30%。当总面密度和层数一定时,延长静电纺丝时间≥210 min,20层PAN/PE-PP复合过滤材料对颗粒的过滤效率可提高至90%以上,但阻力也急剧增大,因此静电纺丝时间为210 min的PAN/PE-PP多层复合材料的过滤性能最佳。因此,与相同面密度的PAN/PE-PP单层复合过滤材料相比,PAN/PE-PP多层复合过滤材料的过滤性能明显提高;微纳米纤维多层复合法是制备高效低阻复合空气过滤材料的有效方法。      

Narrow-band interference filters with unconventional spacer layers

Classical narrow-band interference dielectric filters with all-dielectric reflectors have quarter-wave stacks separated by a half-wave thickness (or a multiple-) spacer layer. These filters are essentially Fabry-Perot filters; hence the theory developed for those filters applies in full. The theory of narrow-band interference dielectric filters with unconventional spacer layers is presented. This spacer layer consists of two different materials. The optical features of these filters are compared with the features of Fabry-Perot filters. The influence of the errors of the layers on spectral characteristics is analyzed. The theory presented can be applied to any spectral range as well as to any thin-film material, including absorbing and nonlinear materials.     

R.F. sputtering system for the preparation of multilayer optical filters

Thin film multilayer optical filters are usually prepared by vacuum evaporation of successive layers. The deposition of thin films by sputtering has many advantages over the vacuum evaporation technique and hence it is employed for the fabrication of better quality optical filters. However, the filters may be more time consuming to prepare if a single-target sputtering system is used because two different materials are required for alternate layers. For the preparation of multilayer optical filters, a special r.f. sputtering jig arrangement with two target holders is fabricated. Using this the successive layers can be deposited in one pumpdown. A simple coupling arrangement used to match the r.f. generator output impedance to the load is explained. With all the sputtering parameters such as the pressure, the temperature and the power maintained constant except the time, control of the thickness of the multilayers is achieved. A calibration chart of sputtering time versus thickness of the materials to be deposited is prepared for TiO₂ and SiO₂ for the above purpose. Using this calibration chart, the deposition time of different layers of any optical filters based on these dielectric materials can be fixed to obtain optical filters of acceptable quality and reproducibility.     

Simple computations involving two-component symmetric trilayers

The equivalent properties of a symmetric three-layer structure are derived by a nontraditional method that provides useful insights and a simplified application to some thin-film design problems. The equations derived from this method may be used to design a three-layer replacement for a single layer in an interference coating. The equations are especially helpful for cases that involve complex numbers, such as metal layers or above-critical angle propagation in dielectric layers. Several multilayer design problems are solved to demonstrate the application of this approach.     

Correlation of atomic force–distance microscopy and spectrophotometric techniques in the analysis of optical multilayer spectral aging process

Aging-related multilayer spectral instabilities can pose severe performance limiting constraints to optical multilayer devices. In this study such instabilities of some periodic Gd₂O₃/SiO₂ optical multilayer systems have been explored using scanning probe force–distance microscopy and spectrophotometric techniques. In the present case, a strong correlation between the spectral instabilities and the viscoelastic properties of the associated thin film layers has been distinctly noticed. From the experimental analysis it was quite evident that the spectral instability, which starts during the nucleation and growth stage in thin films, continues to persist at a much longer time scale following aging processes. In this study it is shown that the elastic properties of the constituent thin films, the layer design and the bilayer thickness have established a strong interrelation which ultimately contributes to the multilayer instabilities. These spectral instabilities also have strong interconnections with the morphological and viscoelastic changes in such multilayers. Other multilayer parameters like the total number of layers, the layer structure, the microroughness evolutions, related stiffness factors and the adhesion properties of the periodic layer systems contribute substantially to this instability process.     

Multilayered materials: a palette for the materials artist

Developments in the understanding of how materials behave enable us to design material structures to display specified properties. We introduce multilayered materials as systems in which new properties, not found in their constituents in bulk form, can emerge. The importance of transmission electron microscopy to determine structure-property relationships in nanoscale multilayers through characterization of their atomic and electronic structure is emphasized. Two examples of technologically useful multilayer systems are considered in more detail: hard coatings made from nitride multilayer films and the new structures and magnetic properties that are found in some metal multilayer systems. Finally, we discuss the future developments that are required to fully exploit the novel properties found in multilayered materials.     

Studying multilayer langmuir film structure by confocal laser scanning and atomic force microscopy techniques

Features of extracting information on the surface structure of a multilayer organic film from data on its optical properties and surface relief are considered. The object of investigation was a multilayer Langmuir-Blodgett film based on a prepolymer (polyamic acid salt). It is suggested that the formation of the film volume is influenced by inhomogeneities in the structure of layers.     

Design and manufacture of metal/dielectric long-wavelength cutoff filters

Thin films of high reflecting metal, such as Ag, have a high reflectance in the long-wavelength region. When they are combined with dielectric layers, it is possible, through thin film interference effects, to induce transmission in certain shorter wavelength regions. Thus, they are useful components for the design of long-wavelength cutoff filters with a broad rejection region. In this paper, metal/dielectric multilayer designs based on this principle are numerically investigated. Three designs with different cutoff wavelengths and with very broad transmission regions in the visible or near-IR spectral ranges are presented. An excellent rejection on the long-wavelength side extends beyond 20 μm. Experimental results for one of the designs produced in our magnetron sputtering system are given.     

Stepwise graded refractive-index profiles for design of a narrow-bandpass filter

A combination of stepwise graded refractive-index profiles and a cavity structure is used for designing narrow-bandpass filters of TiO(2)/SiO(2) multilayer films upon BK7 glass substrates. Symmetrical profiles of stepwise graded refractive indices result in high transmittance of passbands for the designed filters. The bandwidth of the narrow-bandpass filter is controlled by adjustment of parameters such as the thickness and the number of layers in the multilayer stack. This design is proposed as a new and simple method for coating synthesis of optical filters.     

Design and fabrication of a novel reflection filter

Reflection filters are useful in optical communication, display, and other systems. A novel reflection filter is designed and fabricated. Analytical design formulas have been put forward and show good agreement with the measured maximum reflectance as well as with the bandwidth at the central wavelength. The effective admittance and distribution of the electrical field intensity are also calculated to analyze the properties of the filter. This novel filter with high peak reflectance, narrow bandwidth, and deep cutoff, is simple to design, easy to fabricate and convenient to integrate, compared with the conventional dielectric multilayer reflection filters.     

Correlation of sidelobe suppression between rugate filters and multilayer mirrors

We present the study of the correlation between refractive index profiles and the optical response of rugate filters and multilayer mirrors. The conventionally used method in multilayer mirrors for ripple suppression in the passband will be compared with a similar simple method to remove the rugate filter sidelobes without apodization. The resulting layers are compared in performance with a typical quintic matching layer. An example based on silicon oxynitride alloys with refractive indices ranging between 1.47 and 1.83 was designed and deposited.     

Thin-film optical sensors with silicon-compatible materials

Antireflection filters based on multilayer stacks of dielectric and polysilicon films on monocrystalline silicon combined with charge collection in different (poly)Si layers can be used to realize sensors with a programmable spectral response controlled by weighted summing of the photocurrents detected in the polysilicon and the substrate. Thus, employing both interference and selective absorption of light yields increased photoelectric efficiency and improved flexibility of spectral control and enables on-chip integration of the detector(s) with the signal conditioning and processing circuits. The potential of thin-film color sensors has been evaluated for this purpose. However, for practical implementation of such structures the problems associated with the realization of reliable photodetectors in polysilicon must also be considered. Phosphorus passivation of the grain-boundary states has been employed to yield polysilicon photodiodes with improved electrical characteristics and reliable light and color detection. We present the design methods of thin-film color sensors employing silicon-compatible materials only. The measurement results of a fabricated structure fully demonstrate that such sensors can be realized with good spectral selectivity.     

Application of in situ ellipsometry in the fabrication of thin-film optical coatings on semiconductors

Thin-film interference filters, suitable for use on GaAs- and InP-based lasers, have been fabricated by use of the electron-cyclotron resonance plasma-enhanced chemical vapor deposition technique. Multilayer film structures composed of silicon oxynitride material have been deposited at low temperatures with an in situ rotating compensator ellipsometer for monitoring the index of refraction and thickness of the deposited layers. Individual layers with an index of refraction from 3.3 to 1.46 at 633 nm have been produced with a run-to-run reproducibility of 0.005 and a thickness control of 10 A. Several filter designs have been implemented, including high-reflection filters, one- and two-layer anitreflection filters, and narrow-band high-reflection filters. It is shown that an accurate measurement of the filter optical properties during deposition is possible and that controlled reflectance spectra can be obtained.     

Bandwidth reduction technique for multilayer wavelength division multiplexing (WDM) bandpass filters

The push to develop 100 GHz and smaller bandwidth WDM filters is at demanding levels. Currently, 200 GHz is the standard bandwidth for multilayer interference coatings with high efficiencies, and enormous processing effort is going into the development of standard 100 GHz filters. This paper outlines a simple design that will reduce bandwidth up to 40% when applied to 200 GHz bandpass filters. This design method can also be used in existing 100 GHz designs to achieve even smaller bandwidths.