At an international scientific symposium at TU Ilmenau on April 15, 2024 to mark World Interferometry Day, scientists from all over the world will come together to share their enthusiasm for interferometry and the latest findings in precision measurement technology. More than any other technology, the invention of the laser interferometer by Albert Abraham Michelson over 140 years ago embodies the art of measuring exceptionally small quantities with high precision using light. We spoke to Prof. Eberhard Manske, Head of the Institute of Process Measurement and Sensor Technology, about the idea behind World Interferometry Day, his fascination with interferometry and current research at TU Ilmenau.
Research
The fascination of light: how interferometry can be used to precisely measure the smallest quantities
To commemorate the groundbreaking invention of the interferometer, you and the Managing Director of SIOS Meßtechnik GmbH, Dr. Denis Dontsov, launched World Interferometry Day in 2021. What exactly is interferometry?
Interference simply means superposition; in the case of physics, it means the superposition of waves. This means that the waves amplify or cancel each other out in certain areas. This effect occurs with all types of waves, including sound waves or gravitational waves, for example.
We know this from water waves: The ring-shaped waves of two stones thrown into the water overlap, so that where there should actually be a wave crest on the water, the wave is suddenly extinguished. This creates interesting "interference patterns". Finally, interferometry is a measuring method that makes technical use of the phenomenon of wave interference.
How can I imagine this phenomenon with light waves?
Just as with water waves, it also works with light. If I superimpose the light from two light waves, it is suddenly dark in certain places. That is interference. This allows you to achieve many beautiful effects, examine tiny details and measure the smallest distances, lengths or path changes that we cannot see with the naked eye. Whereas with water waves we are talking about wavelengths of several centimetres or meters, with light waves we are dealing with wavelengths of a few hundred nanometres - and that automatically puts us in the precision measurement range, i.e. in the nanometre range of a millionth of a millimetre.
What exactly can you investigate and measure with the help of such light or laser interference?
Innovative laser interferometers are used, for example, to measure lengths, angles, vibrations, but also temperatures, speeds and forces - in other words, a wide variety of physical quantities.
What fascinates you personally about this technology?
I was already interested in physics at school and was later fascinated by it during my studies at TU Ilmenau. I was already working on interferometry in my diploma thesis, and it has stayed with me ever since - from my doctorate and habilitation to many research projects and our nanomeasuring machines, which we are constantly developing further. There are no limits to physics. But even the mere sight of interference is simply fascinating: once you've looked at it, you can't let it go.
Nature also makes use of interference: the dazzling colors in the animal and plant kingdom, for example of the hummingbird or some insects and butterflies, are due to interference. In music, sound is only created through the interference of different sound waves. For example, when two tones with approximately the same frequency sound together, the volume periodically increases and decreases. However, only one tone can be heard, which pulsates. An effect that goes back to Giuseppe Tartini and has inspired many composers to create interesting works. But the superimposition of lines or dots is also used in art as an extremely decorative means of expression. Many a bridge has collapsed due to interference.
What was your idea behind the World Interferometry Day initiative?
With World Interferometry Day, we want to remind people of the fascinating physics behind interferometry, Michelson's outstanding achievement and his great invention of the interferometer and bring it closer to a wide audience, i.e. scientists, students, schoolchildren, but also the interested public - whether in a lecture at a university in Iowa, in physics lessons in Taiwan, in research institutions in Europe or companies in Germany. The topic can even be explained to children using soap bubbles, whose iridescent colors are created by interference on the soap skin.
The response to the first World Interferometry Day was great.
Yes, even in the first year we were very well received with this idea. The first response came from Andreas Kaufer, the director of the Paranal Observatory in Chile, where the Very Large Telescope, i.e. the large telescope of Europe's astronomy organization ESO, is located: "Great idea! When will it start?" The second came from one of Zygo's chief developers. We had encouragement from Japan, the USA and many other countries. That's when we knew: We are on the right track. This year, for example, there will be a special session on interferometry at the SPIE Photonics Europe conference from April 7 to 11 in Strasbourg, which I will be helping to organize.
Even today, more than 140 years after Albert Abraham Michelson, numerous developments are still based on laser interferometry. In which applications do we encounter interferometry in everyday life?
Interferometry is an extremely precise measuring method that is used wherever precision is required in manufacturing, for example in the semiconductor industry for the production of computer chips, which are found in every computer, every smartphone and an increasing number of technical and household appliances.
Such high-precision manufacturing processes can only be realized with laser interferometers. Automation systems for precision manufacturing must be regularly calibrated and checked with the help of laser interferometers. The aerospace industry benefits from the accuracy of laser interferometry just as much as coordinate measuring machines or production systems for large LCD screens.
Pushing interferometry to its limits and challenging it to ever new heights - that is also your credo at the Institute of Process Measurement and Sensor Technology at TU Ilmenau. For around 60 years, research has been carried out there into ever more precise measuring methods, including nanopositioning and nanomeasuring machines that can measure to an accuracy of a trillionth of a meter. What are the particular challenges here?
When I joined the university in 1977, we started with relatively simple tools. Even with a helium neon laser, a few optical components and a photoreceiver, you can make interference visible and measure it. But the more you get into the technology, the more complex it becomes. Back then, we had a resolution, i.e. the smallest measurable steps, of 80 nanometers. Today we are at 20 picometers, which is 20 billionths of a millimeter. So we have improved the resolution by a factor of 4000. Of course, the demands are increasing in all directions, be it vibrations that we have to prevent or acoustic interference that has to be isolated. But the air pressure and theCO2 content of the air must also be taken into account. The deeper you delve into the matter, the more precisely you have to take all these error components into account, compensate for them, correct them and develop new processes.
What research are you currently involved in?
It's a mixture of continuous development, i.e. slowly making things better and better, and disruptive approaches. For example, we used frequency comb technology to improve the frequency, i.e. the precision of the lasers, by a factor of 1000 a few years ago, and four years later by a factor of 1000 again. And in recent years, we have also improved the refractive index measurement and correction by a factor of 1000. We also initially developed our nanomeasuring machine for a relatively small measuring range, i.e. 25 millimeters. Over time, we have extended this to 200 millimetres and are currently working on a machine with a measuring range of one meter while maintaining the same measuring accuracy. We have to rethink everything again. For example, the 1 meter by 1 meter mirror that we need for this nano-measuring machine weighs around 500 kilograms. This means that the main question here is how we can make this mirror thinner and much lighter with the help of additional measurement technology in order to still be able to measure with nanometer accuracy.
Can students at TU Ilmenau also take part in research on these topics?
The interferometric solutions are becoming increasingly complex and we have to investigate an incredible number of sub-components. That's why student work is always included in all research from the outset, in which students can solve small subtasks, for example as student assistants - be it a software module that they write or a construction that they make. When this works in the end, it is always a sense of achievement for the students.
Although even Michelson's first attempt didn't work. Well, it did work, but the interferometer didn't show anything. Through this "null experiment", which remains the most famous to this day, Michelson concluded that the previous theory of the "light ether" must be wrong. This was a phenomenal, unique result, which is why I always tell my students: If you plug the appliance into the socket and nothing happens, first think: where is the fault? Then you might come to the same conclusion as Michelson: my technique is right, but the theory is wrong.
What topics relating to interferometry are the focus of the international symposium on April 15, and who is the event aimed at?
Scientists, students and company representatives are invited, as well as interested members of the public. There will be many exciting presentations. For example, Peter de Groot, an internationally renowned expert from Zygo-Ametek in the USA, will start off with a firework display of stories about the turbulent history of interferometry and the precision of interferometers. But there will also be a presentation by Andrew Yacoot from the National Physical Laboratory in the UK, the world's leading expert in the field of X-ray interferometry. Jochen Hetzler from Carl Zeiss SMT GmbH, which is also a world leader with its expertise in key processes for the manufacture of microchips in the semiconductor industry, will talk about so-called picometer interferometry. Representatives from leading research institutes in Finland, Austria and Germany will also be giving exciting presentations. The range of topics will again be broad this year.
A few days before the symposium, on April 10, you will also be opening your laboratories to visitors from the region. What can they look forward to?
At TU Ilmenau, we will be celebrating World Interferometry Day on April 10, 2024 with a variety of activities in front of and inside the Ernst Abbe Center on the university campus. In addition to experiments on the topic of interferometry, there will also be a small exhibition on "Interferometry in Art" from 6 p.m., which will then also be on display in an expanded form in the university library from May. Visitors will also be able to visit the nanomeasuring machine. The event will conclude with a renewed and expanded Laserbaem show. Participation is free of charge, as is attendance at the symposium in the Faraday lecture hall on April 15.
We look forward to further strengthening scientific collaboration in the field of interferometry and precision metrology with this year's activities and call on everyone to participate in a variety of ways so that together we can bring the topic closer to a wide audience.
About the Michelson Interferometer
The measuring device named after the physicist Albert A. Michelson became known in 1881 through the Michelson-Morley experiment carried out in Berlin and Potsdam. The aim of the experiment was to prove the existence of ether. According to the state of research at the time, the ether was the carrier medium responsible for the propagation of light. After a failed attempt in Berlin, which was considerably impaired by the heavy traffic, the measuring apparatus was moved to the observatory in Potsdam and the series of measurements continued from April 5 to 15, 1881.
The results of the experiment were published in the American Journal of Science, and the measuring apparatus Michelson used in his experiments has been named after him ever since. Michelson's experiments did not produce the expected results, as the assumption of an ether flowing freely through solid matter did not apply and the expected changes in the interference pattern did not occur when the measuring device was rotated.
The correct interpretations were provided in 1905 by Albert Einstein with his theory of relativity, the important foundations of which are the observations of the Michelson experiment described above. Albert A. Michelson received a Nobel Prize for his work in 1907 and thousands of people all over the world use measuring devices to measure lengths, angles, straightness and vibrations, the measuring arrangement of which is known as a "Michelson".
Further information on the events on World Interferometry Day: www.world-interferometry-day.com.
Prof. Dr. Eberhard Manske
Head of the Institute of Process Measurement and Sensor Technology