Some Observations on Absorbing Multilayers

Three aspects of absorbing optical thin films are discussed. First, the reflectance of a multilayer necessary to maximize the potential transmittance of a single metal film is obtained explicitly in terms of the thickness and refractive index of the film. Second, the form of the standing wave in a metal film at maximum potential transmittance is determined. Third, a technique for the design of an asymmetric mirror is presented. A numerical example is included.     

Equivalent refractive index of multilayer films of different materials

The variation of the equivalent refractive index with composition of multilayer films of ZnS-MgF₂-SiO, ZnS-Na₃AlF₆ and Ge-ZnS, prepared by depositing alternate layers of different materials, has been investigated. It has been established that, for small step thicknesses (much less than the wavelength of light to be used for measurements) of each layer, the composite multilayer films are optically equivalent to homogeneously mixed films of the same materials of corresponding relative compositions. Further, the results show that the indices of the multilayer films of ZnS-MgF₂-SiO and ZnS-Na₃AlF₆ are in good agreement with the values predicted on the basis of the Lorentz-Lorenz theory. The refractive index of Ge-ZnS multilayer films is in agreement with the Drude theory. Our studies also show that the refractive index of a multilayer film composed of three materials, two of which react chemically in the molten and vapour states, as, for example, ZnS and SiO in ZnS-MgF₂-SiO films, is equivalent to that expected theoretically for a homogeneously mixed film of the same materials. The controlled and predictable equivalent behaviour of multilayer films suggests their use to produce variable refractive index optical coatings by selecting any number of materials which may or may not react with each other chemically.     

Microstructure-related properties at 193 nm of MgF2 and GdF3 films deposited by a resistive-heating boat

MgF2 and GdF3 materials, used for a single-layer coating at 193 nm, are deposited by a resistive-heating boat at specific substrate temperatures. Optical characteristics (transmittance, refractive index, extinction coefficient, and optical loss) and microstructures (morphology and crystalline structure) are investigated and discussed. Furthermore, MgF2 is used as a low-index material, and GdF3 is used as a high-index material for multilayer coatings. Reflectance, stress, and the laser-induced damage threshold (LIDT) are studied. It is shown that MgF2 and GdF3 thin films, deposited on the substrate at a temperature of 300 degrees C, obtain good quality thin films with high transmittance and little optical loss at 193 nm. For multilayer coatings, the stress mainly comes from MgF2, and the absorption comes from GdF3. Among those coatings, the sixteen-layer design, sub/(1.4L 0.6H)8/air, shows the largest LIDT.     

Comparative study of physical properties of vapor chopped and nonchopped Al2O3 thin films

Aluminium oxide being environmentally stable and having high transmittance is an interesting material for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness, optical transmission, band gap, refractive index and intrinsic stress of Al₂O₃ thin films were studied. The cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor chopped Al₂O₃ thin film showed higher transmittance than the nonchopped film. The optical band gap of vapor chopped thin film was higher than the nonchopped Al₂O₃, whereas surface roughness and refractive index were lower due to vapor chopping.     

A study of silicon oxynitride film prepared by ion beam assisted deposition

A SOI-based optoelectronic device needs a high-quality antireflection coating on both faces of the device to minimize the optical reflectance from the face. In this work amorphous silicon oxynitride films were deposited on silicon substrates by ion beam assisted deposition (IBAD). The main purpose was to use silicon oxynitride film as single layer anti-reflection coating for SOI-based optoelectronic devices. This application is primarily based on the ability to tune the silicon oxynitride optical functions to the optimal values by changing deposition parameters. The chemical information was measured by X-ray photoelectron spectroscopy (XPS). Spectroscopic ellipsometry (SE) was applied to measure the refractive index and thickness. Single-side polished silicon substrate that was coated with silicon oxynitride film exhibited low reflectance. Double-side polished silicon substrate that was coated with silicon oxynitride film exhibited high transmittance. In addition, the Fresnel losses could be reduced to 0.08 dB by depositing silicon oxynitride films onto double-side polished silicon substrates. The results suggested silicon oxynitride film was a very attractive single layer anti-reflection coating for SOI-based optoelectronic device.     

Transmittance analysis of three-dimensional photonic crystals by the effective medium theory

A simple method for calculating the transmittance of three-dimensional photonic crystals is proposed. The crystals are divided into multilayer thin films, and each film is divided into rectangles with a minute width to calculate the effective permittivity of the film by the effective medium theory. Transmittance of the multilayer thin films is calculated with the matrix method. As the number of atomic layers increases, remarkable stop bands appear. When the refractive index of photonic atoms increases, the stop band shifts to a lower frequency, the band widens, and the number of bands increases. Polarization and incident angle dependences are also analyzed. The limit of application for this calculation method is also discussed.     

热处理对钛酸锶钡薄膜光学性能的影响

采用无水溶胶-凝胶技术制备了钛酸锶钡光学薄膜,系统研究了预处理和退火温度对钛酸锶钡薄膜的相结构、微观形貌和光性能的影响,优化了薄膜的光学性能,结果表明,膜的结构和形貌直接影响其光学性能。通过150℃预处理和850℃退火获得了晶化程度良好、表面形貌平整致密的钛酸锶钡薄膜。该膜在350-1000nm的最大光透过率为80．72％,在600-1000nm的折射率稳定在-1．93左右,消光系数最小,最小值为4x10-5。     

Multifunctional transparent ZnO nanorod films

Transparent ZnO nanorod (NR) films that exhibit extreme wetting states (either superhydrophilicity or superhydrophobicity through surface chemical modification), high transmittance, UV protection and antireflection have been prepared via the facile ammonia hydrothermal method. The periodic 1D ZnO NR arrays showed extreme wetting states as well as antireflection properties due to their unique surface structure and prevented the UVA region from penetrating the substrate due to the unique material property of ZnO. Because of the simple, time-efficient and low temperature preparation process, ZnO NR films with useful functionalities are promising for fabrication of highly light transmissive, antireflective, UV protective, antifogging and self-cleaning optical materials to be used for optical devices and photovoltaic energy devices.     

Studies on aluminum-doped ZnO films for transparent electrode and antireflection coating of β-FeSi2 optoelectronic devices

β-FeSi₂ can be used for various optoelectronic devices owing to its superior material features including high optical absorption coefficient and direct band gap of about 0.8 eV. Due to its high refractive index (>5.6), however, suitable antireflection coating (ARC) is necessary for practical device applications. In order to increase the effective areas of optoelectronic devices, transparent electrodes should be also developed. In this work, Al-doped ZnO (AZO) films were fabricated by sputtering on β-FeSi₂ thin films and were found suitable for both transparent electrodes and ARC films. Choosing optimum substrate temperature and sputtering rate, high quality AZO films were formed. The conductivity of AZO films was as high as 3×10³ S/cm and ohmic contact was easily achieved between AZO and β-FeSi₂ films, indicating AZO film as an ideal transparent electrode for β-FeSi₂. The transmittance of 400-nm-thick AZO films was >80% and >70% in the wavelength ranges 400-1400 and 1400-1600 nm, respectively. By changing the thickness of AZO film, the central wavelength of minimum reflectance was adjusted to 1550 nm where the total reflectance of AZO/β-FeSi₂/Si structure was reduced below 2%.     

Symmetrical periods in antireflective coatings for plastic optics

Plastic optical parts require antireflective as well as hard coatings. A novel design concept for coating plastics combines both functions. Symmetrical three-layer periods with a phase thickness of 3/2pi are arranged in a multilayer to achieve a step-down refractive-index profile. It is shown mathematically that the equivalent index of symmetrical periods can be lower than the lowest refractive index of a material used in the design, if the phase thickness of the symmetrical period is set equal to 3/2pi instead of the usual pi/2. The straightforward application of the concept to the design of antireflection coatings in general is demonstrated by example.     

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.     

Compositional analysis and optical properties of Ag-doped ITO films

ITO and Ag-ITO composite films were deposited on glass substrates by DC magnetron sputtering. Scanning electron microscope images indicated that Ag existed in the form of nanoparticles. X-ray photoelectron spectroscopic analysis illustrated that both Ag doping and heat treatment had obvious influence on the composition and element chemical state of the films. Ag doping dramatically decreased the transmittance of the film. But after being annealed, Ag-ITO films exhibited the high average transmittance of 80-90% in visible wavelength range. By using the spectroscopic ellipsometry optimization, the optical constants were extracted from the transmittance spectra. The results suggested the potential use of Ag-ITO films for antireflection coating system.     

Fabrication of silica-based multilayer films with self-cleaning and antireflective properties

The silica-based multilayer films exhibiting both self-cleaning property and antireflection in the visible and near infrared regions have been deposited onto glass substrate by layer-by-layer deposition of PAH/PAA polyelectrolyte bilayers, followed by sequential deposition of PAH/SiO2 nanocomposite layers to create the nanoporous structure in the film, and finally treating with fluorosilane. To obtain the appropriate porosity and surface roughness so as to satisfy both antireflection and self-cleaning properties, the deposition of PAH/SiO2 nanocomposite layers was varied from 2 to 8 layers. Scanning electron microscopy and atomic force microscopy analysis revealed that a highly nanoporous structure was obtained from the more deposition of loosely agglomerated SiO2 nanoparticles which increased with the increased cycle of PAH/SiO2 deposition. The film consisting of 6 and 8 PAH/SiO2 layers exhibited a very low reflection of only 1-2% in the visible region and approximately 2-4% in the near infrared region, a maximum transmittance of approximately 93% in the visible region and approximately 94% in the near infrared region, and a high advancing contact angle of approximaetly 150 degrees. It is anticipated that the film which exhibited both self-cleaning and antireflection properties enables the application of the antireflection coatings under humid environments.     

真空度对TiO_2薄膜光学和结构特性的影响

为制备高性能TiO2光学薄膜,采用离子束辅助,电子束蒸发沉积的方法,研究不同真空度对薄膜性能的影响。真空度随真空室内通氧量增大从1.3×10-3Pa变化到2.5×10-3Pa,得到不同光学和结构特性的TiO2薄膜。采用分光光度计测试其光谱,SEM测试其表面形貌。测试结果发现,TiO2薄膜的透过率峰值随真空度降低而增大,折射率和消光系数随真空度降低而减小,薄膜表面形貌随真空度降低从致密变粗糙。在真空度2.0×10-3Pa的工艺条件下,成膜质量最佳,此时最大透过率92%,折射率在2.45～2.20之间,消光系数在10-4以下。根据Cauchy公式拟合其色散规律,拟合曲线和采用包络法计算得到的曲线较好重合,折射率随波长的变化公式为n(λ)=2.12+5.69×104/λ2+8.07×107/λ4。     

High quality transparent conductive ZnO/Ag/ZnO multilayer films deposited at room temperature

We have fabricated, by simultaneous DC and RF magnetron sputtering, multilayer transparent electrodes having much lower electrical resistance than the widely used transparent conductive oxide electrodes. The multilayer structure consists of three layers (ZnO/Ag/ZnO). Ag films with different film thickness were used as metallic layers. Optimum thicknesses of Ag and ZnO films were determined for high optical transmittance and good electrical conductivity. Several analytical tools such as spectrophotometer, atomic force microscopy, scanning electron microscopy and four-point probe were used to explore the possible changes in electrical and optical properties. A high quality transparent electrode, having resistance as low as 3 Ω/sq and high optical transmittance of 90% was obtained at room temperature and could be reproduced by controlling the preparation process parameters. The electrical and optical properties of ZnO/Ag/ZnO multilayers were determined mainly by the Ag film properties. The performance of the multilayers as transparent conducting materials was also compared using a figure of merit.     

Optical constants of Yb films in the 23-1700 eV range

The optical constants of Yb films have been determined in the 23-1700 eV spectral range from transmittance measurements performed in situ on Yb films deposited by evaporation in ultrahigh vacuum conditions. Yb films were deposited over grids coated with a thin carbon film. Transmittance measurements were used to obtain the extinction coefficient of Yb films at each individual photon energy investigated. The energy range investigated encompasses Yb edges from M(4,5) to O(2,3). The current results, along with data in the literature, show that Yb has an interesting low-absorption band in the approximately 12-24 eV range, which may be useful for the development of transmittance filters and multilayer coatings. The current data along with literature data and extrapolations were used to obtain n, the real part of the complex refractive index, using a Kramers-Kr?nig analysis. The application of the sum rules showed a good consistency of the results.     

Transparent high refractive index nanocomposite thin films

This work reports the preparation of acetic acid-modified TiO₂ nanoparticles by sol–gel synthesis method. The nanoparticles can be incorporated directly into the polymer matrix to form transparent high refractive index nanocomposite thin films. The result shows that increasing the titania content in the hybrid nanocomposite thin films can significantly increase the refractive index. Hybrid nanocomposite thin film with refractive index value of 2.38 had been prepared. All prepared films also exhibit excellent optical transparency in the visible region.     

Transmittance and optical constants of erbium films in the 3.25-1580 eV spectral range

The optical constants of erbium (Er) films were obtained in the 3.25-1580 eV range from transmittance measurements performed at room temperature. Thin films of Er were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Substrates consisted of a thin C film supported on a grid. Transmittance measurements were used to obtain the extinction coefficient k of the Er films. The refractive index n of Er was calculated using the Kramers-Kr?nig analysis. k data were extrapolated both on the high- and low-energy parts of the spectrum by using experimental data and calculated k values available in the literature. Er, similar to other lanthanides, has a low-absorption band below the O(2,3) edge onset; the smallest absorption was measured at ~22.5 eV. Therefore, Er is a promising material for filters and multilayer coatings in the energy range below the O(2,3) edge, in which materials typically have an absorption stronger than at other energies. Good consistency of the data resulted from the application of f and inertial sum rules.     

Direct current reactive magnetron sputtered zinc oxide thin films —the effect of the sputtering pressure

ZnO thin films were deposited onto glass substrates by d.c. reactive magnetron sputtering from a metallic zinc target. A systematic study has been made on the influence of sputtering pressure in the range from 0.2 Pa to 3 Pa on the film structural and optical properties. At low sputtering pressure (0.2–0.4 Pa), the film was inhomogeneous, non-stoichiometric and had low refractive index and an almost amorphous structure. At high sputtering pressure (0.6–0.8 Pa), the film was homogeneous, stoichiometric and had high refractive index and the crystallinity was improved. As the sputtering pressure was further increased (1–3 Pa), the homogeneity and the refractive index of the film had no clear variation, but the crystallinity of the film went down. As the sputtering pressure was increased from 0.2 Pa to 3 Pa, the transmittance of the film increased and the deposition rate of the film decreased.     

Infrared Refractive Index and Extinction Coefficient of Polyimide Films

We have measured the transmittance of several polyimide (C₂₂H₁₀N₂O₄) films at wave numbers from 6000 to 500 cm^–1 (wavelengths from 1.67 to 20 m) using a Fourier-transform infrared (FT-IR) spectrometer. The free-standing polyimide films are made by spin coating and thermal curing processes. The thickness of the films ranges from 0.1 to 4 m. In the nonabsorbing region from 6000 to 4000 cm^–1, the minimum transmittance caused by interference is used to obtain the refractive index for film thicknesses greater than 1 m. The film thicknesses are determined by fitting the spectral transmittance using the refractive index. Molecular absorption strongly reduces the transmittance at wave numbers from 2000 to 500 cm^–1. The optical constants, i.e., the refractive index and the extinction coefficient, are determined from the measured transmittance for several films of different thickness using a least-squares method. A Lorentzian oscillator model is also developed, which in general agrees well with the measured transmittance at wave numbers from 6000 to 500 cm^–1. This study will facilitate the application of polyimide films in the fabrication of infrared filters and other optoelectronic applications. The methods presented in this paper can be used to determine the optical constants of other types of thin-film materials.