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Optical Design & Engineering

Narrowband coatings for the extreme UV

New interference filters based on the low absorption of some lanthanides enable band selection within most wavelengths in the complicated ∼50–105nm range.
24 May 2011, SPIE Newsroom. DOI: 10.1117/2.1201104.003693

Everyone is familiar with optical filters that select a single color from a visible light beam. These can simply be colored glass pieces that transmit the desired hue and absorb all others. But light can also be filtered with a coating composed of several layers of different materials, typically certain oxides. Such multilayer coatings, which can be transmissive or reflective, are called interference filters. To select a narrow portion of the visible spectrum—to identify a material through its emitted or absorbed light band, for instance—requires narrowband filters.

Interference filters for visible light can be purchased off-the-shelf. But for applications ranging from defense to biology, scientists and technologists need to select virtually any part of the electromagnetic spectrum. One can find high-performance narrowband filters when moving from the visible to the near-IR or the near-UV. The only requirement is materials that are transparent to a wavelength of interest, with appropriately tailored layer thicknesses. Radiation then reflects at each interface of the coating, and the reflected components interfere constructively to produce the desired color.

Unfortunately, materials in nature do not behave the same way over the whole spectrum. Most materials that are transparent in the near-UV are not so in the far-UV. In fact, no material is transparent at wavelengths just below 105nm, where electrons absorb energy and transition from the valence to the conduction band.

Meanwhile, at ∼50nm and below, every material has absorption onsets at specific energy levels associated with core electron transitions, right above which it is relatively transparent. That means transparent materials exist for most of the electromagnetic spectrum except in the ∼50–105nm range, where materials absorb radiation and which we will refer to here as the extreme UV (EUV). Narrowband coatings are therefore not available in this range. Such coatings would benefit applications where imaging at specific spectral lines or bands is required, such as plasma diagnostics, astrophysics, solar physics, and atmospheric physics.

A source of relatively transparent materials in the EUV has recently been found in the lanthanides, or rare-earth metals.1,2 This has enabled the development of the first narrowband optical coatings with peak reflectance at wavelengths as long as 69nm. They contain a lanthanide such as terbium combined with a material such as silicon.3 This has shortened the range with no narrowband filters to ∼70–105nm.

We have developed, for the first time, narrowband multilayer coatings with peak reflectance at wavelengths longer than 70nm, where the problem of material absorption is highest.4 Our optical characterization of ytterbium (Yb)5 and europium (Eu)6 disclosed that these two lanthanides have the least absorption in the EUV above 56nm. Hence, we developed coatings based on Yb or Eu. We combined these materials with aluminum (Al) due to its contrasting refractive index, and also included multiple silicon oxide (SiO) layers both to avoid degradation in the atmosphere and as barrier layers to passivate the interface of Al with the lanthanide. Figure 1 displays the reflectance of a multilayer based on Yb, Al, and SiO as a function of wavelength, with a peak at 78nm. It corresponds to an aged coating, which is stable enough for practical applications. With these novel multilayers we can provide narrowband filters for the spectral range ∼50–100nm.

Figure 1. Reflectance versus wavelength of a multilayer coating based on ytterbium, aluminum, and silicon oxide.

In order to fully fill the 50–105nm EUV gap, we are beginning new research to develop coatings that can select a band centered at wavelengths just longer than 100nm. In this range, there are important spectral lines, such as the far-UV lines of hydrogen atoms, whose observation should improve solar physicists' and astrophysicists' understanding of the sun, the solar system, and the galaxy. Our objective for the future is to develop the necessary narrowband filters.

This work was supported by the National Programme for Space Research, Subdirección General de Proyectos de Investigación, Ministerio de Ciencia y Tecnología, project AYA2010-22032.

Juan I. Larruquert, Manuela Vidal-Dasilva, Sergio García-Cortés, Luis Rodríguez-de Marcos, José A. Aznárez, José A. Méndez
Instituto de Óptica
Consejo Superior de Investigaciones Científicas (CSIC)
Madrid, Spain

Juan I. Larruquert received his PhD in physics from Universidad Autónoma de Madrid in 1993. He is now a scientific researcher at Instituto de Óptica-CSIC, where his main interests are multilayer coatings for the far/EUV, space optics, roughness, and scattering.

José A. Méndez has been with the staff of CSIC since 1979. He has developed research activities in physical optics, optical imaging, and interferometry. At present his main research activity is in thin-film optics.

Mónica Fernández-Perea
Instituto de Óptica-CSIC
Madrid, Spain
Mónica Fernández-Perea
Lawrence Livermore National Laboratory
Livermore, CA 

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