Stefan Witte
Having started in September 2024, Stefan Witte (1979) is a professor in Optics for nanoscale metrology in the department of Imaging Physics of the faculty of Applied Sciences. Partially thanks to the support from the Excellence Fund, he was able to move his research group and laboratory equipment from Amsterdam to Delft. Witte got his PhD in 2007 cum laude, from the Free University Amsterdam, for research into ‘intense ultrafast laser development and precision spectroscopy with frequency combs’. From this followed a postdoc at the Free University and a postdoc at the JILA (University of Colorado, 2010-2011). From 2014 to 2024, he worked at the Advanced Research Center for Nanolithography (ARCNL), where he co-led the ‘EUV Generation and Imaging Group’ and headed the department Metrology. Since 2019, he was also professor at the Free University of Amsterdam.
Over the course of the years, Witte has received almost €8 million in research grants, among which the prestigious ERC Starting Grant (2014), the ERC Consolidator Grant (2019), and the NWO VICI grant (2022). He also has more than 60 scientific publications to his name (among which almost half as first author). Witte is also head researcher in the NWO-TTW Perspective consortium LINX (Lensless Imaging with soft-X-rays) and a member of the NWO Advisory Committee Physics for Technology and Instrumentation. Last but not least, he has been able to register 10 patents.
The everyday use of optics (everyone uses light) combined with the potential it has to analyse matter on micro and nano-scale – that is what makes the science so fascinating to Witte. But laser experiments are also fun, because you can do many types of measurements in a relatively easy way. ‘And then you can go further than possible before, by viewing a problem differently or approaching an experiment differently,’ is how Witte describes the challenge.
The research so far
Imaging techniques are being used more and more for scientific and industrial diagnostics, but the necessary optical components (lenses) are often big and expensive and require high precision. Witte’s research focuses on ‘lensless imaging’, high resolution microscopy using numerical methods instead of physical optic components like lenses. Until his coming to Delft, Witte was involved with a series of experiments and studies in this field. For example, with the development of new imaging methods for biological microscopy. And with the development of non-linear microscopy methods aimed at ‘live’ imaging of the brains. From 2014 onwards, he worked at the then recently founded Advanced Research Center for Nanolithography (ARCNL), a public-private financed research centre of ASML together with FOM, NOW and both Amsterdam universities (Uva and VU). Along with prof. dr. Kjeld Eikema, Witte was group leader of the ‘EUV Generation and Imaging Group’. This group developed and develops imaging techniques for a range of different applications, such as very small microscopes, imaging through scattered materials, and very high resolution imaging with extreme ultraviolet (EUV) light.
Imaging nanostructures
The latter is something Witte continues in Delft. He will research methods to image 3D nanostructures. Such as those from computer chips, in which the smallest elements are only a few nanometres big and often consist of multiple layers. Microscopy using light is difficult, because these structures are smaller than the wavelength of visible light. That was why Witte uses extreme ultraviolet (EUV) light: this has a short enough wavelength to image such nanostructures. For this, they will first need a method to produce EUV light, for example, through intense, ultrafast laser pulses. Then they have to do this in a lensless way, because there are no good lenses for this sort of light. ‘But we understand how light scatters from small objects,’ states Witte. ‘By measuring that scatter with smart algorithms, we can reconstruct an image of the object.’ Additionally, Witte develops photo-acoustics techniques, to image non-transparent 3D structures. ‘In those cases, you can’t use light to view within the structures,’ says Witte. ‘That is why I generate sound waves at the surface of an object by using short light pulses. Those sound waves move through the object, and the ‘echo’s’ of hidden structures can then be detected with light on the surface.’
The setup for extreme ultraviolet imaging: one on the day of the lab move (September 2019), with the optical table just installed, and one from late 2025, with almost all components reinstalled. Witte: ‘We are almost ready to start these experiments.’
Impact for Delft and society
During his inaugural lecture in November 2025 (‘Measuring with light on a nanoscale’), Witte argued that microscopy is, in fact, a facilitating technology that enables other disciplines to develop further. These include materials science, (cell) biology, and the semiconductor industry. ‘In that sense, every step is important, and there is no end point,’ said Witte. With his arrival in Delft and the expansion of his research group, TU Delft hopes to take important, groundbreaking steps in 3D imaging on nanoscale. ‘Partially thanks to the Excellence Fund, I can hire researchers and build experiments. That is good to ensure getting the first research results quickly, and continue building the group from there. That way, I can quickly offer students challenging experimental dissertation projects. Educating new generations of scientists in optics will become very important in the next few years, and I look forward to contributing to that.’
Witte's Personal Passion Pride
Without a specific interest in science fiction, Stefan Witte always got enthusiastic about the image of lasers flashing through space. ‘I have always been interested in light, like using lasers to influence something kilometres away,’ he says. When he experimented with holograms in secondary school, the penny dropped. ‘I actually find that the most fun still, the experiments, the fiddling with set-ups. As group leader at TU Delft, I may be in the office most, but the lab is the most fun place,’ he laughs.
‘Such a wide variety’
‘TU Delft offers me a new, challenging opportunity,’ says Witte. ‘Science is only as useful as its application. Within TU Delft, there is a wide variety of science and applied research in which this imaging technology plays a role. This makes it very interesting to link my research to it and thus broaden its scope.’ The aim is certainly to put TU Delft on the international map. ‘As with the presentation by Matthijs Velsink – the postdoc I was able to hire thanks to the Excellence Fund – at the recent international conference on ultrasonic technology. The most important research groups in light-induced ultrasound were present there. Matthijs presented our work on using light to produce and detect ultrasound, and how this can be used to study nanostructures in metals and integrated circuits. The audience response was very positive. Leading researchers expressed their surprise and admiration at what we have already been able to achieve.’
'Nobel Prijs Verklaard'
The field of Stefan Witte is part of the internationally dynamic field of laser physics: how can you make laser pulses of increasingly short times at extremely short wavelengths and make these laser pulses interact with matter to generate images at nanometre-scale. In 2023, the Nobel Prize for Physics was awarded to the trio Pierre Agostini, Ferenc Krausz, and Anne L’Huillier, among other things, for making laser pulses of an attosecond. On 22nd April 2024, Stefan Witte gave a Nobel Prize Verklaard lecture at Diligentia, Royal Society for Physics since 1973, about this invention (‘Attosecond light pulses and following electrons at nanoscale). The lecture can be watched in this video.