Thin-film optical filters, 3rd Edition

It is shown how the discussion about antireflection coatings for the visible and near infrared region has been changed
dramatically with recent experimental applications of nanostructures that realize media with effective refractive indices
less than the 'magic border' of 1.34. Using the so-called binary optics as an example, a glass-like nanostructure similar to
the moth-eye structure is theoretically designed as antireflection coating for the visible and near infrared region. With the
aim of this example and considering only known design principles of thin-film optics, a connection between
nanostructures and thin films regarding their alternative or combined application as antireflection coatings is presented.
As summary regarding the nanostructures vs. thin film discussion, a reference list is presented that cited different types
of antireflection coatings presented in the past 70 years with respect to their applications, designs, and deposition
technologies.

Nanostructures versus thin films in the design of antireflection coatings

The devices which are dealing with the thermal radiation (such as emitters [1–4], receivers [5–8], filters [9–12], transformers [13–15], etc.) are mainly working in a finite range of frequencies. Such devices are frequently need to be optimized for getting the best possible value for the specific thermal radiative or thermodynamic characteristics that can be reached in the given spectral range. For example, the devices can be optimized for obtaining the maximal possible amount of the emitted (or received) total radiation energy, or, alternatively, the maximal number of the emitted (or received) number of photons. Potentially, the devices can be also optimized for some other thermal radiative or thermodynamic parameters.

Optimization Problems for the Devices Using the Black-Body Thermal Radiation

Market drivers for the increasing demand for optical interference coatings on planar substrates and lenses include data communication systems and consumer electronics. Magnetron sputtering can deliver environmentally stable coatings with high throughput. Low defect densities combined with high stability of the process and coating system enable complex optical layer designs with very accurate reproducibility. Cylindrical targets are well known for coatings on large area such as window glass, but are rarely used for coating stacks with high layer counts. With the OPTA X, we are introducing a rotary target-based platform that takes advantage of the long-term stable deposition rate and high material utilization of these sputtering sources. Cylindrical targets have almost no re-deposition zone and no deep racetracks due to rotation. This strongly reduces the potential of particle generating arcs that are caused by surface charges in oxidized target areas. We also use substoichiometric target material sputtered in a pure argon atmosphere without the need for further reactive gas process control. Oxidation is performed by a plasma source in a pressure-separated plasma oxidation zone. The platform can be equipped with sputter sources in a sputter-up or sputter-down configuration or in a combination of both. Interesting applications for double-sided filter coatings include beam splitters for aerospace applications or filter deposition on thin glass substrates to compensate for layer stresses and associated geometric deformations. The elimination of substrate flipping for double-sided coatings also enables shorter production times and offers potential for cost savings. In this paper, we are presenting preliminary optical properties as well as deposition rates of SiO2 and Nb2O5 oxide films. Furthermore, we are showing results of optical single layers as well as examples of filter coatings prepared by broadband transmittance monitoring and process control.

Sputtering platform for double-sided precision optical coatings

We present hereafter a study of complex bandpass optical interference filters with central wavelengths ranging in blue region or in the near infrared. For these applications, the required functions are particularly complex as they must present a very narrow bandwidth as well as a high level of rejection over a broad spectral range. Moreover, these components must have a good flatness meaning that the stress induced by the different layers has to be taken in account in the filter design. We present a thorough study of these filters including their design, fabrication using Plasma Assisted Reactive Magnetron Sputtering (PARMS) and characterization. Excellent agreement between experimental and theoretical spectral performances associated with a final sag of 326 and 13 nm, and uniformity from −0.05 to 0.10 and −0.10 to 0.20% are demonstrated for the two manufactured filters.

Complex optical interference filters with stress compensation for space applications

For more than four decades band-pass filters are important components of microscopes used for the fluorescence
spectroscopy. During all the time this special field of application has been one of the main drivers for research and
development in thin-film optics, particularly for the thin-film design software and the coating technology. With a
shortwave pass filter, a multi-notch filter, and a classical band-pass filter as examples of such filters provided for the
latest generation of fluorescence microscopes we present the state-of-the-art in coating design and technology.
Manufacturing these filters is a great challenge because the required spectral characteristics need necessarily multilayers
with up to 300 layers and overall thicknesses up to 30 μm. In addition, the designs require also 3 to 5 nm as thinnest
layers and all the layers are completely of non-quarterwave type. The filters were manufactured in a rapid-prototyping
regime by a Leybold Helios plant using plasma-assisted reactive magnetron sputtering of thin films of different metal
oxides. Designed and real spectra are compared and differences are discussed. Measurement results of other optical and
non-optical characteristics as film stress, total integrated scattering, and micro roughness are presented.

Exclusive examples of high-performance thin-film optical filters for fluorescence spectroscopy made by plasma-assisted reactive magnetron sputtering

Besides the typical channels in the visible and near infrared spectrum, optical remote sensing of the earth from air and
space utilizes also several channels in the short-wave infrared spectrum from 1000 nm to 3000 nm. Thin-film optical
filters are applied to select these channels, but the application of classical multiple-cavity band-pass filters is impossible.
Because of their additional blocking elements they are disallowed due to geometrical or other non-optical reasons.
Within the sensitivity region of an MCT detector as typical detector device, the selection and blocking of radiation by the
filter has to be provided by a single multilayer system. The spectral region of the SWIR as well as blocking width and
depth require necessarily designs with overall thicknesses of more than 20 μm, with layer numbers up to 100. SiO 2 and
TiO 2 were used as thin-film materials deposited with reactive e-beam evaporation under ion assistance in a Leybold
SyrusPro box coater. A special challenge was the thickness measurement of the thin films by an optical broadband
monitoring device in the visible range. The results of manufacturing and characterizing of such filters are presented by
three examples for the center wavelengths of 1375 nm, 1610 nm, and 2190 nm.

Manufacturing and characterizing of all-dielectric band-pass filters for the short-wave infrared region

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Exclusive examples of high-performance thin-film optical filters for fluorescence spectroscopy made by plasma-assisted reactive magnetron sputtering

Manufacturing and characterizing of all-dielectric band-pass filters for the short-wave infrared region