SRON and Nikhef receive an ENW-M grant from NWO for the development of detectors for the gravitational wave telescope LISA. The grant will enable them to further develop the existing prototype into a space-worthy version that meets the stringent accuracy requirements.
The two institutes are working with Dutch companies BRIGHT Photonics and SMART Photonics to create a photodiode large enough and noise-free enough to pick up LISA’s weak laser signals of roughly a nanoWatt.
Astronomers and physicists use gravitational waves to observe events in the universe that are usually invisible to our telescopes, such as collisions between stellar black holes or neutron stars. But observatories on Earth are unable to observe gravitational waves with a length of more than 10,000 kilometers, so supermassive black holes, for example, still remain invisible.
ESA is therefore launching the Laser Interferometer Space Antenna (LISA) in the mid-2030s, which can detect much longer wavelengths with its 2.5 million kilometer measuring arms. Using those enormous measurement arms-formed by laser beams between three separate satellites-LISA detects gravitational waves via picometer changes in the distances between them.
SRON and Nikhef are joint candidates to provide the detectors for LISA’s laser beams. In recent years, they have developed a prototype and are now receiving an ENW-M grant of 500 thousand euros from NWO to develop this into a fully-fledged detector for LISA. This must be able to withstand the extreme conditions in space, demonstrate high reliability and meet the high requirements set by LISA to detect the minute vibrations that race across our solar system.
After LISA’s laser beams have traveled their 2.5 million kilometers, roughly only a nanoWatt remains, so the detectors need a low noise level so as not to overpower the signal. In addition, they must be two millimeters in diameter to keep LISA’s optics design as simple as possible. Such a diameter is ten to one hundred times larger than usual in photonics applications.
Because of the specific requirements, SRON and Nikhef must build the detectors from scratch, together with the Dutch companies BRIGHT Photonics and SMART Photonics. They make a photodiode by growing micrometer-thin layers of the semiconductor material indium-gallium arsenide on a wafer. To keep noise low, the capacitance of the diode must be low, and so must the doping of the semiconductor. The prototype already has the required low noise for diameters up to 1.5 mm, but improvement is still needed for a diameter of 2 mm.
Once the photodiode has been fully developed using the ENW-M grant, the detector must also be fitted into a single housing with the readout electronics. This housing is also being developed by SRON and Nikhef. This is a challenge in itself because the housing must not expand and contract too much due to temperature fluctuations and it must keep the photodiode stable to within a micrometer. Ultimately, LISA needs 72 units for its three spacecraft, plus several dozen as backup.
Dutch (astro)physicists involved in gravitational waves have united in the LISA-NL consortium, which will hold its first meeting on Friday, October 1. In addition to developed expertise, involvement in the construction of LISA also means better access to the data that LISA produces once it floats through space.