In laser inertial fusion, high-precision and high-energy laser beams are used to compress and heat fuel capsules. The temperature and pressure in the capsules increase to such an extent that atomic nuclei fuse, releasing a large amount of energy. "If the amount of energy gained is greater than that expended, laser inertial confinement fusion can be a valuable source of clean energy for the future. For this to succeed, however, the laser technologies used must be further developed to meet the extreme challenges," says Dr. Christian Rieck from Glatt Ingenieurtechnik GmbH in Weimar. He is coordinating the joint project, which is scheduled to run until 2027 and is being funded by the German Federal Ministry of Education and Research (BMBF) with six million euros as part of the "Basic technologies for fusion - on the way to a fusion power plant" program.

Innovative anti-reflective coatings and structures to optimize laser beam transport

The laser beams must be aligned extremely precise in order to hit the fuel capsule evenly and ensure symmetrical compression. Their path is controlled by various materials and atmospheres that act like lenses. This leads to optical losses, which are higher the more lenses are used. Energy is also lost when the laser beam hits the fuel capsule because its material reflects some of the energy. Last but not least, the high energy of the laser causes thermal expansion, which varies depending on the material properties and can lead to cracks or other damage and thus have a negative impact on the precision and service life of the systems.

The project partners are therefore focusing on the anti-reflective coatings that are used to reduce reflection losses. For example: when a laser hits fused silica glass, the reflection losses are around 4 percent per interface. If the beam is directed 50 times by a glass lens, only 1.7 percent of the output power passes through the last interface. For this reason, lenses are already equipped with extremely thin, multi-layer anti-reflective coatings that effectively reduce losses.

"However, these solutions hardly seem suitable for use in future petawatt laser fusion reactors. This is because the significantly higher laser power increases the thermal load: if the substrate and the anti-reflective coatings expand to different degrees, there is a risk of defects," says Dr. Nadja Felde from the "Functional Surfaces and Coatings" department at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena. In addition, difficult to detect subsurface damage (SSD), which may occur during the manufacturing process and is less significant at lower laser powers, can become critical in laser fusion applications - especially in continuous operation at repetition rates of around 10 Hz.

Combination of anti-reflective coatings and nanostructured materials

In order to pave the way for viable solutions here, the project partners are focusing on nanostructured or nanoporous anti-reflective coatings based on materials with a high band gap, which should ensure the required laser damage threshold. On the other hand, they are testing a subtractive approach: instead of a combination of a substrate material and multi-layer anti-reflective coatings, they are relying on lenses made from a single material, which is to be given the desired anti-reflective properties by suitably nanostructuring its surface. Using the example of two materials with a large band gap (fused silica glass and calcium fluoride), large-area demonstrators are to be developed for different wavelengths and pulse lengths.

"We want to prove that the approach with anti-reflection structures can be specifically optimized for high-power laser applications such as laser inertial confinement fusion and achieve the best possible anti-reflection effects below 0.5 percent residual reflection. The technology also offers further exploitation opportunities in the field of high-power optics," says Prof. Dr. Thomas Höche, head of the "Optical Materials and Technologies" business unit at Fraunhofer Institute for Microstructure of Materials and Systems IMWS in Halle (Saale).

Partners from industry and research in the "nanoAR" project  

In the "nanoAR" project, the project partners are pooling their expertise from substrate manufacturing and processing methods for the effective reduction of SSD, to technology-open process development for nanostructure generation, including the use of simulations and modeling, to high-resolution material characterization and the development of new methods for quality assurance.

The following companies and research institutes are involved in the project "Antireflective metasurfaces on wide bandgap materials (nanoAR)": Glatt Ingenieurtechnik GmbH (Weimar), POG Präzisionsoptik Gera GmbH (Löbichau), FLP Microfinishing GmbH (Zörbig), Trionplas Technologies GmbH (Leipzig), Fraunhofer IOF (Jena), Fraunhofer IMWS (Halle/Saale), the Fraunhofer Institute for Mechanics of Materials IWM (Freiburg), the Leibniz Institute for Surface Modification IOM (Leipzig) and the Ernst Abbe University of Applied Sciences (Jena).