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ESA FORUM Mission to deploy ‘thermometer turned high precision satellite’

19 Dec 2023

Fraunhofer IOF researchers develop diamond structures to enable climate change research.

Measuring how Earth is losing its cool – or how it is warming up – is the objective of the planned FORUM mission.

The European Space Agency’s satellite mission, scheduled for launch in 2027, aims to analyze Earth’s heat budget in order to better understand global warming and Earth’s climate system.

For the spectrometer on board the satellite, researchers from Jena, Germany, have developed a novel diamond structure that enables precise measurements on the far-infrared spectrum. The first flight hardware has now been delivered.

Earth is starting to sweat – literally, say the researchers. The years 2018 and 2022 were among the warmest since records began. An important piece of the puzzle in the study of global warming is the radiation of heat from Earth to space.

To better understand these complex mechanisms, the FORUM mission will record Earth’s radiation budget with precision to a local level. “Figuratively, that means FORUM is a thermometer turned satellite with extreme precision,” said Eilenberger.

A spectrometer is employed on the satellite to function as a thermometer. It records Earth’s heat radiation on the far-infrared spectrum – between about 10 to 100 µm. The key component is the spectrometer’s beam splitter.

Etched diamond structure

This is where the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) and the Institute of Applied Physics (IAP) at Friedrich-Schiller-University, Jena, are playing a crucial role.

For this beam splitter, researchers from both institutes have developed and manufactured an innovative diamond microstructure for the high-precision measurement of spectra in the far-infrared range. A special technology is used for this, in which microscopic pyramids are etched into a diamond.

“For the extreme range to be recorded by FORUM, we need a beam splitter that is transparent over the entire spectral range,” said Dr. Falk Eilenberger, who heads the Department of Micro- and Nanostructured Optics at IOF. He added, “There is no optical material that has this property except diamond.”

Accordingly, the researchers used a diamond measuring 43mm x 64 mm (about the size of a credit card). Its shiny surface acts as a beam splitter. And here the next challenge awaits, because: “Only one surface of the diamond is allowed to shine,” said Eilenberger. “So, our job was to make the second surface antireflective.”

Conventional antireflection processes, such as those used for eyeglass lenses, are unsuitable for this application because they consist of layers of different materials and are not transparent over the entire spectral range. Because of this, the researchers have developed a special etching process to place the necessary structures into the diamond.

Inspired by nature

To do this, they drew inspiration from nature – specifically, from the eye of the moth. “Moth eyes are antireflective on a broad spectrum,” said Eilenberger. “They achieve this antireflection through microscopically small pyramids on the surface. The IOF has already adapted this model from nature some years ago for optics that are visible.”

The requirements for application with the FORUM mission are extreme, as Eilenberger explained further: “Diamond is very difficult to structure. The extreme range also requires pyramids with extremely accurate shapes. This is why we had to develop an etching process that can produce the structures of the necessary shape in an exact and reproducible way.”

The researchers achieved this with the help of a reactive ion etching process, masked by an electron beam lithography mask. As a result, they achieved an antireflection coefficient of more than 96% thanks to a structural depth of more than 7 micrometers and a precisely defined slope steepness.

The colleagues from the Institute of Applied Physics at the Friedrich-Schiller-Universität Jena were significantly involved here: "Without this absolutely excellent collaboration and the outstanding etching technology developed at the IAP, the structuring of the diamond would not have been possible or so successful," says Eilenberger.

‘Diamonds are photonics’ best friend’

For Falk Eilenberger and his team, the development shows great potential in the use of diamonds as an optical material: “This project also shows that we can only really start to use the potential of diamond as an optical material through nanostructuring,” he said.

He added, “With this project we also want to show that nanostructured diamond is the material of the future for optics in the extreme range. Our processes enable us exceptionally well to shape this future: diamond meta-optics, waferscale diamond magnetic-field sensors, resonant diamond mirrors and much more. You could say: Diamonds are photonics’ best friend.”

The research team from Jena – consisting of IOF and IAP staff – has spent four years on the development of the novel diamond structure. The development is carried out in close consultation with the client OHB SE, as well as ESA as the mission’s responsible body. The airworthy beam splitter was handed over to OHB in December 2023.

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