Broadband transmission filters from the 2013 Optical Interference Coatings manufacturing problem contest [invited].

A broadband transmission filter from 400 to 1100 nm was selected for the manufacturing problem contest. The purpose of the contest is to test the state of the art of current optical thin film manufacturing capabilities. A total of 37 people from 15 teams participated in the contest and submitted 17 samples. Diverse approaches were taken by participants to tackle the problem. A range of different solutions was obtained where the number of layers varied from 22 to 608, and the total layer thickness ranged from 1.859 to 23.099 µm. Two independent laboratories performed sample evaluation measurements. Three teams shared the best result with the lowest average measured merit function.

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.

Implementation of long-wavelength cut-off filters based on critical angle.

Our first attempts at the fabrication of long-wavelength infrared cut-off filters with extended transmission and rejection regions that are based on the use of the critical angle, the dispersion of refractive indices, and on thin-film interference were not very successful. The design of the filter consisted of layers placed at the interface between two high-index prisms. Using the available deposition equipment, the layers produced were porous and very rough. The pores adsorbed water vapor, which resulted in absorption. The roughness made the process of optical contacting very difficult. In this paper we describe the adjustments in the design and deposition processes that allowed us to obtain filters with a better and more stable performance.

2010 topical meeting on optical interference coatings: manufacturing problem.

For the 2010 Manufacturing Problem, the participants were required to produce a filter that had normal incidence transmittances of 0.001, 0.01, 0.1, and 0.96, respectively, in four separate 60 nm wide bands in the 400 to 700 nm wavelength region. The problem is not unlike those that need to be routinely solved in the telecommunication industry. Nine groups submitted a total of 11 different filters for the contest. The number of layers in the filters received ranged from 28 to 678, and the total metric thicknesses varied between 4,038 and 22,513 nm. The transmittances of the filters were measured at two independent laboratories. Some of the performances were quite close to the specifications.

Optical coatings with an integral FTIR air layer.

The use of solid frustrated total internal reflection (FTIR) layers to control the polarization effects in optical coatings usually results in large substrate sizes and complicated designs. To overcome this problem, it is proposed to incorporate an air FTIR layer into the multilayer thin film coatings. The low refractive index of air not only helps to reduce the substrate sizes, but also simplifies coating designs or improves the performance. The principle and layer structures of the proposed multilayers are described. Examples of polarizing- and non-polarizing beam-splitters for the infrared spectral region are given. Some practical manufacturing issues are also discussed.

Gradient-index optical filter synthesis with controllable and predictable refractive index profiles.

The Fourier transform thin film synthesis method often results in solutions that call for indices that lie outside the range of values of the available materials. To make the resulting refractive index profiles always realizable in our meta-mode sputtering machine, a modified Fourier transform synthesis method is proposed with which the reflectance spectra can be accurately synthesized with controllable and predictable refractive index profiles. In our procedure, an optimal phase function is explored to yield acceptable refractive index profiles. Then the overall thickness is estimated using the Parseval theorem. Finally, several errors inherent to the Fourier transform method, including the imprecision of the spectral function, the truncation of the film and the apodization of the refractive index profiles, are compensated by successive corrections to the magnitude of the spectral function. An explicit iterative formula based on the derivative of the magnitude function is proposed for the compensation of the spectral mismatches. We show with a number of examples that, with the proposed method, it is possible to synthesize gradient-index optical filters with almost any desired spectral performance using experimentally realizable refractive indices.

Estimation of the average residual reflectance of broadband antireflection coatings.

We deal with optimal two-material antireflection (AR) coatings for the visible and adjacent spectral regions. It has been shown before that, for a given set of input parameters (refractive indices of the substrate, ambient medium and high- and low-index coating materials, and for a given spectral width of the AR coating), such designs consist of one or more clusters of layers of approximately constant optical thickness and number of layers. We show that, through the analysis of many different optimal coatings, it is possible to derive two parameters for a simple empirical expression that relates the residual average reflectance in the AR region to the number of clusters. These parameters are given for all possible combinations of relative spectral bandwidth equal to 2, 3, and 4; low-index to ambient-medium index ratio equal to 1.38 and 1.45; and high-to-low index ratio equal to 1.4, 1.5, and 1.7. The agreement between the numerically and the empirically calculated values of residual average reflectance is excellent. From the information presented the optical thin-film designer can quickly calculate the required number of layers and the overall optical thickness of an AR coating having the desired achievable residual average reflectance.

2007 topical meeting on optical interference coatings: manufacturing problem.

Measurements are described on the experimental filters submitted to the Third Optical Thin Film Manufacturing Problem in which the object was to produce multilayers with a measured colorimetric performance that is as close as possible to that specified. The perceived colors of the coating, when illuminated with randomly polarized light incident at 7 degrees by a source representing average daylight with a correlated color temperature of approximately 6500 K, were to be yellow and blue, respectively, in light reflected from its two surfaces, and the color was to appear white when viewed in transmission mode. Eleven teams from 7 different countries submitted a total of 18 samples.

Charles Keith Carniglia (1944-2006): in memoriam.

Chuck Carniglia was an industrial scientist, an educator, and a friend to all he met. He had an important impact on the optical thin-film community. This article highlights his career and accomplishments.

2004 Optical Society Of America's Topical Meeting on Optical Interference Coatings: Manufacturing Problem.

Results are presented for the second Optical Society of America's Optical Interference Coatings Manufacturing Problem. The participants were asked to produce multilayer coatings which, in the 450-650 nm spectral region and for light incident at 60 degrees, would have transmittances of 0.7 and 0.3 for p- and s-polarized light, respectively. Three different teams each submitted four solutions. Three different deposition processes were used to produce these coatings. The smallest average departure from the target transmission values was 0.79%. A number of interesting conclusions can be drawn from this exercise.

Alfred Josef Thelen--on the occasion of his seventy-fifth birthday.

Alfred Thelen has made many very important contributions to the field of optical thin films. To honor him on the occasion of his 75th birthday, I prepared a bibliography of his many contributions for the ninth Topical Meeting on Optical Interference Coatings. Here an updated list of Alfred Thelen's books, chapters, papers, conference reports, and patents is presented.

Philip Werner Baumeister (1929-2003): in memoriam.

Philip Baumeister pioneered the design of optical thin films by computer methods. He has published many important papers on various aspects of optical coatings, and he initiated the Optical Society of America's Topical Meetings on Optical Interference Coatings. Most important, during his time at the University of Rochester and thereafter, he trained and mentored generations of researchers and engineers in the theory and practice of thin-film interference coatings. Originally, a talk was prepared to celebrate his 75th birthday at the ninth Topical Meeting on Optical Interference Coatings. Because of his untimely demise on 22 October 2003, it was presented at the conference to honor his memory. This is a written version of the tribute that contains a comprehensive list of Philip Baumeister's various publications.

Toward perfect antireflection coatings. 3. Experimental results obtained with the use of Reststrahlen materials.

The equipment and methods used to produce wide-angle antireflection coatings based on Reststrahlen materials are described. The optical constants of the coating materials used in the construction of the multilayers were determined by spectrophotometric ellipsometry and are compared with the literature values. The measured performance of an experimentally produced antireflection coating is compared with the expected calculated performance. The reflectance is low over a wide range of angles, but only in the narrow-wavelength region at which the refractive index of the Reststrahlen material is close to unity.

Toward perfect antireflection coatings. 2. Theory.

Recently we performed a numerical investigation of antireflection coatings that reduce significantly the reflection over a wide range of wavelengths and angles of incidence, and we proposed some experiments to demonstrate their feasibility. We provide a theoretical description of omnidirectional antireflection coatings that are effective over a wide range of wavelengths.

Ion-beam etching for the precise manufacture of optical coatings.

We propose using ion-beam etching as an additional tool for the accurate control of the thickness of thin films during the manufacture of sensitive optical multilayer coatings. We use a dual ion-beam sputtering system in the deposition and etch modes. In the deposition mode both the assist and sputtering ion beams are used to produce dense films at deposition rates in the range of 0.1-0.3 nm/s. In the etch mode, only the assist ion beam is used to remove material at a rate of less than 0.1 nm/s. A very high precision in the layer thicknesses can be obtained by alternating between deposition and etch modes. We observed that etching did not significantly affect the surface quality and the uniformity of the coatings. We introduced etching into our current manufacturing process and demonstrated its potential for the fabrication of several optical multilayer systems with performances that are very sensitive to the thickness of their layers.

Topical meeting on optical interference coatings (OIC'2001): manufacturing problem.

Measurements are presented of the experimental filters submitted to the first optical thin-film manufacturing problem posed in conjunction with the Topical Meeting on Optical Interference Coatings, in which the object was to produce multilayers with spectral transmittance and reflectance curves that were as close as possible to the target values that were specified in the 400- to 600-nm spectral region. No limit was set on the overall thickness of the solutions or the number of layers used in their construction. The participants were free to use the coating materials of their choice. Six different groups submitted a total of 11 different filters for evaluation. Three different physical vapor deposition processes were used for the manufacture of the coatings: magnetron sputtering, ion-beam sputtering, and plasma-ion-assisted, electron-beam gun evaporation. The solutions ranged in metric thickness from 758 to 4226 nm and consisted of between 8 and 27 layers. For all but two of the samples submitted, the average rms departure of the measured transmittances and reflectances from the target values in the spectral region of interest was between 0.98% and 1.55%.

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.

Long-wavelength polarizing cutoff filters for the 275-550-nm spectral region.

The design and manufacture of a multiple-reflection-type multilayer element is described that efficiently removes all wavelengths higher than 550 nm from the incident radiation and that at the same time acts as a polarizer in the 275-550-nm spectral transmission region.

Black layer coatings for the photolithographic manufacture of diffraction gratings.

A black layer coating for an aluminum-photoresist interface with a reflectance less than 0.1% for 413-nm, s-polarized light incident at 25 degrees is described. It is made of space-compatible materials, and its rms roughness is less than 15 A.

High-rate automated deposition system for the manufacture of complex multilayer coatings.

A previously described automated thin-film deposition system based on rf-magnetron sputtering could deposit quite complex optical multilayer systems with good precision and with no one in attendance [Sullivan and Dobrowolski, Appl. Opt. 32, 2351-2360 (1993)]. However, the deposition rate was slow, and the uniform area on the substrate was limited. We describe an ac-magnetron sputtering process in which the same deposition accuracy has been combined with significantly better film uniformity and a fivefold or sevenfold increase in the deposition rate. This makes the equipment of commercial interest. Experimental results are presented for several difficult coating problems.