The topic of antireflection coatings is vast, and it is impossible to review it comprehensively in an article of this size. Here an attempt is made to show how antireflection coatings evolved from inhomogeneous layers, through homogeneous layers with optical thicknesses that were multiples of a quarter wave of some central wavelength, to the modern non-quarter wave designs that, at times, may consist of many tens of layers. Antireflection coatings for the solution of a number of special problems are presented.
Spectroscopic ellipsometry has become one of the most powerful tools of the investigation of the optical properties of thin films. In this paper we show that it enables one to successfully detect even fine properties of high-quality thin films, such as small bulk and surface inhomogeneities. In our experiments, niobium pentoxide films were deposited by ac magnetron sputtering on to quartz substrates with a thickness approximately equal to 500 nm. A J.A. Woolam variable angel spectroscopic ellipsometer was used to measure the ellipsometric parameters of the films over a spectral range from 400 nm to 850 nm. The measured ellipsometric data obtained at the different incidence angles were then analyzed using the OptiChar characterization software. All measurements indicated the presence of a surface micro-roughness in the film, estimated to be around 1 nm. This surface micro-roughness was modeled by a surface overlayer with a 50 percent packing density. The films also possess a small positive bulk inhomogeneity on the order of about 1.5 percent. In the specific case of Nb<SUB>2</SUB>O<SUB>5</SUB> films on a quartz substrate, the ellipsometric angle (Psi) at a 65 degree angle of incidence is the most sensitive to surface and bulk inhomogeneities. The presence of inhomogeneities of both types is clearly seen at certain wavelength points.
A novel high performance thin film polarizing beam-splitter (PBS) is described. This PBS is non-absorbing and has a broad band, a wide angular field and high extinction ratios for both reflected and transmitted beam. Several high efficiency projection displays based on this novel PBS will be presented.
A novel optical approach to predicting chemical and physical properties based on principal component analysis (PCA) is proposed and evaluated using a data set from earlier work. In our approach, a regression vector produced by PCA is designed into the structure of a set of paired optical filters. Light passing through the paired filters produces an analog detector signal directly proportional to the chemical/physical property for which the regression vector was designed. This simple optical computational method for predictive spectroscopy is evaluated in several ways, using the example data for numeric simulation. First, we evaluate the sensitivity of the method to various types of spectroscopy errors commonly encountered, and find the method to have the same susceptibilities toward error as standard methods. Second, we use propagation of errors to determine the effects of detector noise on the predictive power of the method, finding the optical computation approach to have a large multiplex advantage over conventional methods. Third, we use two different design approaches to the construction of the paired filter set for the example measurement to evaluate manufacturability, finding that adequate methods exist to design appropriate optical devices. Fourth, we numerically simulate the predictive errors introduced by design errors in the paired filters, finding that predictive errors are not increased over conventional methods. Fifth, we consider how the performance of the method is affected by light intensities that are not linearly related to chemical composition, and find that the method is only marginally affected. In summary, we conclude that many types of predictive measurements based upon use of regression vectors and linear mathematics can be performed more rapidly, more effectively, and at considerably lower cost by the proposed optical computation method than by traditional dispersive or interferometric instrumentation. Although our simulations have used Raman experimental data, the method is equally applicable to NIR, UV-Vis, IR, fluorescence and other spectroscopies.
A new optical computation method for the monitoring of chemical reactions requires filters with spectral transmittance curves that vary in a complicated way with wavelength. In this paper we consider the design of two different sets of filters, one of which could be used to predict the degree of curing of a polymer from an analysis of its Raman spectra. The problem is not easy because the required filters have sharp spectral features in a narrow spectral region. Two different design methods are used. The performance of one set designed by conventional means is very close to the specifications. However, current thin film deposition methods are probably incapable of producing filters of such thickness. The second solution is based on the use of several filters placed in series. It should be possible to implement this particular solution, but its performance is not nearly as good. Nevertheless, calculations indicate that this filter pair should also result in a satisfactory control of the curing process.
Infrared lenses, simultaneously color corrected at multiple wavelengths from 1.0 to 5.0 micrometer, can be designed using several different technologies such as color corrected lens triplets and three mirror anastigmats (TMA) telescopes. In this paper, several different broadband reflective and refractive lens design solutions are presented and compared. In addition, the problem of designing and producing broadband AR coatings for such a wide wavelength region will be considered. An assessment of the advantages and disadvantages for each of the lens design forms will be provided together with a discussion of producibility limitations.
The calculated performance is presented for a number of beamsplitters for the 10-200 micrometer spectral region. Designs based on pellicles as well as solid substrates are presented. Average efficiencies that are greater than 0.9 appear possible for this range of wavelengths. The implementation of the above designs will present considerable challenges. Difficulties that will have to be overcome in the manufacturing process are enumerated.
A novel device, the fast-scanning acousto-optic spectrophotometer, was used to measure the in situ reflectance of a thin film during deposition. The reflectance data, which was measured at different film thicknesses and over a wavelength region from 400 to 1100 nm, was used to study the refractive index profile of inhomogeneous ZrO<SUB>2</SUB> films. This data was subsequently analyzed using a homogeneous thin film model to derive an 'effective' refractive index profile.
The successful manufacture of metal/dielectric multilayer systems requires not only very close control of the thicknesses of the individual layers, but also a good knowledge of the optical constants of the materials they are made of. In the case of metal films, it is also essential to know whether any transition layers are formed at the interfaces and, if so, how their thicknesses and optical constants depend on the deposition conditions. Numerical modeling of the metal layers and their interfaces is a powerful tool for the determination of these parameters. To illustrate the method, a bandpass filer and a long-wavelength cut-off filter, both having a low reflectance for light incident on one side, were produced. Excellent agreement has been obtained between the calculated and the measured spectral transmittance and reflectance curves.
A series of antireflection coating designs for the 0.4 to 0.8 micrometers spectral region for a substrate of refractive index 1.52 are found by the Quadratic Problem Approximation method. This procedure yields optimal or near-optimal solutions. Parameters varied are the overall optical thickness of the systems and the acceptable ranges of the refractive indices of the coating materials. The calculated merit functions of inhomogeneous layer solutions and of homogeneous multilayer solutions before and after refinement are compared with the quadratic programming merit function.
Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 232 - 236 angstrom spectral region have been deposited by rf magnetron sputtering for use in a XUV Ge-laser. The mirrors had a peak reflectance of 26% in this wavelength region. Characterization by TEM and XRD indicates good thickness control in the deposition process and low interface roughness, although interdiffusion is present at the interfaces. Preliminary experiments indicate that the XUV laser output intensity was increased when a multilayer mirror was added to allow a double pass through the gain medium.
The NRCC Fourier transform synthesis method has been applied for the first
time to an optical thin film design problem in which the external media are different.
Graded index AR coatings for germanium substrates have been synthesized and
compared to multilayer solutions. No limits have been imposed on the refractive
indices. Although such systems cannot be put into practice, they provide basic
insight into the general nature of broadband AR coatings. Admittance diagrams have
been calculated for some of the systems to gain further understanding of their