Didier Martin (promotion date: 7 November 2007)

Development of superconducting tunnel junction arrays for astronomical observations

Promotion date: 7 November 2007

We are trying, and this has been an ongoing process over the last 15 years, to make superconductors into imaging detectors for radiation in general and optical radiation in particular. We optimized the fabrication method of the existing detectors. As a result we discovered new effects in our measurements, and tried to explain these theoretically.

These newly optimized detectors are not as yet deployed on a satellite. We first wanted to demonstrate that this type of detector is suitable for future satellites, carefully establishing the limiting factors. These new detectors do not only make pictures of an object, but also measure the colour of the light. They can also analyse the light in a millionth of a second, while measuring the wavelength of each photon that enters. The fact that you can both measure each individual photon at the very moment they enter and analysing the colour at the same time make these detectors unique. We optimized the instrument around these detectors as well. Also this is a new aspect and part of the project, because so far these detectors were either experimented with in a laboratory setting or the existing instruments were too small. Imagine that these detectors are extremely sensitive. They react to about everything, down to very low energies. Every environment radiates infrared light and this radiation is so strong that it would prevent the detector from doing real measurements. You need filters to stop the infrared but allow the visible light to enter. Optimizing this and optimizing and engineering the cooler, necessary for superconducting, was also done in my thesis.

What is your background?

I have been working at ESA (European Space Agency), in the ESTEC centre (the technical and engineering centre in Noordwijk), for 17 years now. So I am not the traditional PhD student, who applies for and gets a position for three or four years at a university.

Originally I am an electronics engineer. I studied in Belgium in a different field altogether: telecommunication. Years ago I applied at ESA and was hired at an R&D department to build scientific instruments for satellites.

Where is the connection with Twente University?

At one stage my superior wanted the research into superconducting detectors to continue. At that time there was a PhD student who studied with Professor Horst Rogalla. So the link with Twente already existed.

Did you have frequent contacts?

Not that much with Horst Rogalla himself, but with others in the group: yes. I worked a lot with Sacha Golubov, who is also involved in superconducting detectors.

Was it difficult to combine the job at ESA and doing a PhD?

It definitely was. In three years time doing a project like this and working at the same time is virtually impossible. Originally I did not want to do a promotion study at all. I wanted to work and not limit myself to one specific area of research. Then I went to the US as I had received a grant to do a second Master degree. I was advised against it, doing a PhD would be more in my line. But I had to return to Belgium after one year for my military service, so nothing much came of my PhD in Telecommunication. But somehow the idea of a PhD stuck.

When I started to work with superconductors I started to write up and publish the things I developed in a way that could be useful for a promotion. Finally in 2003 we formalized matters with Horst Rogalla. But as I said, you cannot be Head of a section and do a project like this in three years. It involved quite a lot of work in the evenings and in weekends and it was good that I had already unofficially started a couple of years before.

Doing a PhD in this way is something of a luxury, because you are financially safe in a well paid job, but it is very hard on the family.

So there is more reason than one to celebrate on your promotion day?

I am sure of it.