Super-flat silicon device enables new far-infrared spectrometer

SRON Netherlands Institute for Space Research has designed a new type of Fabry-Pérot interferometer. This device is able to filter just one wavelength of far-infrared light from a light beam and can be used in astronomical observations. The project was conducted by different astronomical and engineering institutes as well as Bachelor’s student Carolien Feenstra, who first built a prototype and then the real instrument, which was tested in Canada in late November.

Willem Jellema (Senior Instrument Scientist at SRON Groningen) and Carolien Feenstra (now in the first year of a Master’s programme in Applied Physics at the University of Groningen) just came back from a visit to the University of Lethbridge in Canada. They went there for the assembly and testing of an instrument that can filter photons at a particular wavelength in the far-infrared and can easily be adjusted to different wavelengths. ‘It was a great conclusion to Carolien’s time with us’, says Jellema.

Feenstra built a prototype etalon, an instrument used to control and measure the wavelengths of light, in a research project during her Bachelor’s programme in Astronomy two years ago. The initial device was designed for the SAFARI instrument on the SPICA infrared space telescope. SAFARI is a far-infrared spectrometer which will allow astronomers to search for faint signals of the first and oldest galaxies, as well as for ice and water vapour in protoplanetary discs. The instrument should be able to study light from different wavelengths with high precision and accuracy.

Bouncing

Jellema holds up a box with a round mirror inside: high-quality monocrystalline silicon, the stuff that the wafers for the computer chip industry are made of. ‘This is the prototype that we used to test our idea.’ This idea is based on what is called a Fabry-Pérot etalon . Silicon is almost fully transparent for infrared light. However, when a number of layers of silicon are stacked with air gaps in between, the change in refractive index between silicon and air will affect the photons. They will start bouncing between the layers and, depending on their wavelength, either bounce back or eventually pass through the material.

‘The size of the air gap between the layers and the layer thickness determines which frequency will pass through’, explains Jellema. The system only works when the silicon is totally flat and smooth. In the prototype, the wavelengths were longer which meansthe tolerance for error was. ‘This showed us that the principle worked, even when we cooled it down to 4 Kelvin.’ This is minus 269 ° C.

When Feenstra’s prototype had proven itself, a real prototype operating at the wavelengths for SAFARI was constructed. A number of ultrathin silicon layers, as thin as a human hair, were stacked onto both sides of a piece of silicon some 8 millimetres thick. The layers were extremely straight and smooth. ‘A specialist in lens grinding and polishing, Rik ter Horst, produced them’, says Jellema. Ter Horst, who works in the Optical and Infrared group of the Netherlands Research School for Astronomy (NOVA) in Dwingeloo, prepared the silicon parts in a workshop at his home in Zuidwolde.

Wavelengths

Further processing of the ultrathin silicon parts was done by Marcel Ridder in the clean room at the SRON site in Utrecht. Finally, micromachining equipment at the Delft Technical University Kavli Nanolab was used to etch the cavities into the silicon layers.

After the silicon layers were stacked, these cavities trapped photons. ‘We also needed to be able to switch between wavelengths’, explains Jellema. The easiest way to do this is to change the angle of the light: when the light is perpendicular to the strips, the distance the photons travel is shortest. When the light is at an angle, the distance increases, thereby selecting photons with a different wavelength.

The trip to Canada showed that the entire system worked very well, even at 4 degrees Kelvin. ‘However, it will not be used on the spectrometer on SAFARI’, says Jellema. ‘This system was a backup solution, but the baseline system already worked very well, and the potential ultra-high spectral resolution isn’t needed for that.’ However, it may still be used for the calibration source on the SAFARI instrument or during ground testing. ‘And French astronomers at the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse have expressed an interest in our system for use in a balloon mission that will fly over Canada and possibly Australia from 2019 to 2021, with an instrument that will measure ionized carbon in interstellar space.’ Furthermore, the robust and compact design makes the instrument a candidate for the next-generation infrared space telescope that is to succeed the James Webb Space Telescope – which itself is due to be launched in about three years’ time.

Change

For Feenstra, the project meant a change of direction: starting out in a Bachelor’s programme in Astronomy, she enjoyed the technical challenges of the build so much that she is now studying for a Master’s programme in Applied Physics. It meant taking a year after graduation to gain more technical knowledge. She combined this with working on the prototype. ‘I may end up in space research after graduation, but I’m keeping my options open’, she says.

The entire project has had a very good pay-off, says Jellema. ‘First and foremost, it has further strengthened the cooperation between SRON, the Kapteyn Institute for Astronomy at the University of Groningen and the Optical and Infrared group in Dwingeloo. Furthermore, we are working closely with the University’s engineering research institute ENTEG on other optical technologies and control. This technology opens up new opportunities, combining the knowledge present at different research groups like ENTEG, ZIAM and Kapteyn at the University of Groningen, SRON and the Optical and Infrared group in Dwingeloo. This design already combines the best of different disciplines of engineering.’

Also, the collaboration with the Kavli Nanolab in Delft has sparked plans to work together on metamaterials, smart optical components and systems. ‘And there is close collaboration in the teaching as well, for example in the new Master’s programme in Mechanical Engineering’, concludes Jellema. ‘It all shows how much quality and knowledge in engineering is available in this part of the country.’

This project was a joint collaboration between SRON, the University of Groningen, the University of Lethbridge and the University of Cardiff

See also the news report on the SRON webiste and a news report by the University of Lethbridge.