The subterranean Einstein Telescope will be Europe’s most advanced observatory for gravitational waves, with a sensitivity more than ten times greater than that of existing observatories. This groundbreaking facility will enable researchers to detect black hole collisions and gain deeper insights into the early universe. As an expert in high-energy optics technology, cosine is leading the consortium that will develop the silicon mirrors for the Einstein Telescope.
Several European countries are currently working on a proposal to build the Einstein Telescope. The Netherlands, Belgium, and Germany are leading the race, as their shared border region provides an ideal location due to its low seismic activity, stable soil, and strong ecosystem of research institutions and high-tech companies. The final decision on the telescope’s location is expected in 2026 or 2027.
A telescope of this scale is not only a boost for science but also a magnet for researchers from around the world. Studies suggest that every euro invested in the Einstein Telescope is expected to generate a return of three to four times its value. Additionally, the project will create approximately 2,000 direct and indirect jobs.
Recognizing its significance for science, society, and the economy, the Dutch government designated the telescope as a national priority end of 2024, reinforcing its commitment to the project’s realization.
Precise silicon mirrors for gravitational waves
The Einstein Telescope detects gravitational waves by continuously monitoring the lengths of its three measurement arms using highly sensitive lasers and vibration-free, suspended mirrors. A precise change in these lengths, following a specific pattern, signals the passage of a gravitational wave through the Earth.
The Einstein Telescope will push the limits of technological innovation. At its core lies a series of ultra-precise silicon mirrors, each 45 cm in diameter and weighing 200 to 300 kilograms. Their substantial mass minimizes vibrations caused by individual light particles in the laser beam.
These mirrors are polished to an extraordinary flatness of ±2 nm and a surface roughness of less than 0.1 nm—deviations on the atomic scale. Two of the mirrors will have a concave shape with an exceptionally long focal length of five kilometers, allowing laser light to be reflected back and forth to amplify the signal. Special coatings will ensure 99.999% reflectivity, maximizing the telescope’s sensitivity to gravitational waves.
Higher accuracy near absolute zero
The Einstein Telescope must be ten times more sensitive than its predecessors to detect gravitational waves that are currently too weak for existing detectors to observe. Achieving this level of sensitivity requires, among other advancements, an entirely new mirror design.
Current gravitational wave observatories, LIGO and Virgo, operate at room temperature. However, to enhance measurement accuracy and minimize heat-induced vibrations, the Einstein Telescope’s mirrors will be cooled to just 10 °C to 20 °C above absolute zero—a temperature range of -250 °C to -260 °C. Under these extreme conditions, crystalline silicon outperforms the fused silica (silicon dioxide) used in LIGO and Virgo’s mirrors. However, this material also necessitates the use of longer-wavelength laser light (1550 nm) for optimal performance.
Consortium SPACE-ECHO – materials and fabrication techniques
The R&D consortium SPACE-ECHO, consisting of leading companies and research institutions—including cosine innovations , SRON, VSL, Maastricht University, NOVA, and TNO—will develop the advanced knowledge and expertise required for the fabrication of the Einstein Telescope’s mirrors.
“Silicon can withstand the cryogenic temperatures required for the Einstein Telescope, but there is still limited global experience with silicon mirrors for such a cutting-edge application,” says Dr Boris Landgraf, who is managing the project at cosine innovations, being an expert on optics made from silicon. “Our approach is to compare different polishing methods for crystalline silicon and its coatings, then analyze their performance and later on fabricate the optimum mirror configuration.”
Within the consortium, multiple work packages focus on:
- Developing super-polishing techniques for crystalline silicon to achieve the required accuracy.
- Creating specialized measurement equipment to characterize the silicon mirrors.
cosine innovations has an in-house Ion beam polishing technique in which a beam of argon atoms removes surface irregularities of the material. This technique is also used for manufacturing the mirrors of the European Space Agency’s (ESA) NewAthena telescope.
Other partners contribute additional cutting-edge methods:
- TNO utilizes magneto-rheological finishing (MRF), a polishing process using a magnetic fluid to achieve sub-nanometer precision.
- SRON is researching wet-chemical etching.
- NOVA’s optics workshop is applying single-point diamond turning polishing.
- Maastricht University and the national metrology institute (VSL) investigate each polishing method to determine laser reflectivity and absorption levels in silicon.
- NOVA is developing an automated defect detection system for large mirror surfaces.
- cosine innovations is testing a fabrication technique to fuse smaller mirrors into a larger one.
- Maastricht University, also, is investigating how bonding of the silicon material affects mirror performance.
The consortium also supports ongoing coating research at Maastricht University through semi-annual workshops. Once the silicon mirrors achieve the necessary accuracy, they will receive reflective coatings for performance testing, ensuring they meet the stringent requirements of the Einstein Telescope.
Spin-off potential – telescopes, semiconductor industry, and special optics applications
According to Dr Landgraf, the knowledge and expertise gained by the SPACE-ECHO consortium will have significant potential beyond the Einstein Telescope. One possible application is the development of more precise mirrors for space telescopes, which must achieve high performance under challenging conditions. It could also greatly benefit the semiconductor industry and the development of special optics for materials research or medical applications.
Exploring potential spin-offs remains an ongoing focus for the consortium. They will meet every six months to discuss their progress and identify opportunities at an early stage.
About the Einstein Telescope
The R&D scheme is part of the Einstein Telescope valorization program for high-tech companies – a program of the National Growth Fund. Regional development company LIOF is leading the national effort, including on behalf of the Dutch Ministry of Economic Affairs (EZ), the Dutch Ministry of Education, Culture and Science (OCW) and the research institute Nikhef. With three service points at the regional development agencies in North Brabant (BOM), South Holland (InnovationQuarter) and East Netherlands (Oost NL), a national connection is being sought with high-tech ecosystems around the technical universities.
Six consortia with a total of 26 Dutch high-tech companies and knowledge institutions will develop crucial technology for the Einstein Telescope, Europe’s most advanced observatory for measuring gravitational waves from space. The consortia will focus on challenges in optics, vacuum technology, vibration-free cooling, vibration damping and thermal deformations. For this, they will receive over €12 million in funding. Together, they will provide groundbreaking technology for the future observatory, as well as economic opportunities by driving high-tech innovation. The technology domain Optics focuses on ultra precision mirrors that are stable at ten to twenty degrees above absolute zero.
Read more information here.
About cosine
cosine is a leading worldwide company in the development of space instrumentation, such as Silicon Pore Optics for astronomy and remote sensing solutions with onboard analytics for Earth Observation and planetary science. The company combines physics and technology to bring out-of-the-box solutions to its clients. cosine has been developing and delivering innovative measurement systems for space and industrial applications since 1998. cosine operates 1,000 m2 of cleanrooms and high-tech assembly facilities to build and test the systems we produce for customers at our headquarters in Sassenheim, The Netherlands.
