“No, it’s not just science joke after science joke flying across the table at home,” Arthur Hendriks laughs. Cursor has sat down with him before, when he had earned his master’s degree and talked about his technically-inclined family. In the 1970s, his grandfather obtained his PhD with the then Solid-state Physics research group, and his father pursued a PhD track in Semiconductor Physics. His mother also studied Applied Physics, but took her PhD in Leiden. “There’s a link to Eindhoven though, she conducted her experiments at the NatLab,” says Hendriks with a wink. While his parents always encouraged him to follow his own path, science really is in the family’s blood. “My brother graduated from Applied Physics a few years ago, one of my sisters studied Mechanical Engineering at TU/e, and my other sister studied Aerospace Engineering in Delft.”

It was definitely not a predetermined path for Hendriks to add another dissertation to the family collection. “I wanted to do something different for a long time, but during my graduate internship at Photonics and Semiconductor Nanophysics, I found myself captivated by the research. I wanted more of that. And so, another Hendriks thesis was added to the collection.”

Attaching without glue

Hendriks spent a lot of time in the cleanroom over the past four years, working with photonic sensors. “These sensors convert a change in the environment into an optical signal,” explains Hendriks. This has some major advantages, which he goes on to list. “You can take measurements at a distance, at far distances even. That’s very useful for measuring water or air quality, for example, but also for all sorts of medical applications. Even inside the human body. We can create minuscule sensors – on the micrometer scale – which are also extremely sensitive. All in all, optical sensors offer a great deal of flexibility.”

The fiber optic sensors that are currently used on an industrial scale are integrated into the fiber optic itself. However, by placing a sensor at the very end of the fiber optic, environmental factors can be measured even more effectively. That is what Hendriks and his colleagues are working on. “The dimensions are a real challenge. The fiber optics we use have roughly the same diameter as a human hair. And on top of that, we wanted to attach a sensor in a reproducible way. And it had to be secured properly.”

Optimal hole structure

A method has now been developed within the research group to do so, says an enthusiastic Hendriks. “We use a sensor based on a semiconductor chip as our foundation. In a very thin membrane, we make photonic crystals using lithography and etching machines. These reflect only some of the wavelengths of light. The holes we make are a few hundred nanometers in size. Around them, we place what we call a “transfer structure”. Then, during a second lithography step, we create a larger hole at the bottom where we can insert the fiber optic. The final membrane has four refractive points that align precisely with the cross-section of the fiber optic. It’s very thin, about 250 nanometers, and extremely flat; it stays firmly in place even in liquid. Without glue.” What exactly keeps the sensor so firmly attached is currently being investigated, according to Hendriks. “We think Van der Waals forces – relatively weak attractive forces between molecules, Ed. – are responsible for that. But the design works, which I’ve demonstrated in several applications.”

He also examined the sensor structures to further refine the sensitivity of the measurements. We took inspiration from nature. The photonic crystals we use as the basis for our tip are also found in peacock feathers, butterfly wings and beetle shells. The hole pattern we then created required great precision. A hexagonal pattern turned out to work best.”

Kitchen table

Together with the Molecular Biosensing research group, Hendriks tested the sensors he developed and they were able to identify antibodies in a liquid in a very precise way in several experiments. “This is useful for quickly measuring something on a large scale, for example the presence of virus particles in a patient sample.” He also used his extremely sensitive photonic sensor to detect nanoparticles, for example in ultrafine dust. “What we demonstrate here is that we can truly measure the ultrasmall – we can even individually measure particles as small as 50 nanometers.” The technique to create these ingenious sensors is currently only being used in the TU/e research group, and Hendriks has high expectations. “It’s reproducible, flexible and relatively simple and therefore highly suitable for industrial applications.”

Hendriks puts his dissertation on top of the stack he brought from home; the theses of his mother, father and grandfather. He briefly flips through his grandfather’s dissertation. “Grandpa dictated, and grandma typed it out by hand. Unfortunately, my grandfather passed away last year, but he found it fascinating that I was taking my PhD in “his” research group. Although my work is very specific, “the ways of science” are still mostly the same, which is also something I notice when we all sit around the kitchen table together. It’s truly special that we can share this in our family.”