An optical lens, less than one thousandth of a millimeter in diameter, produced with a laser whose pulses are shorter than a millionth of a second, and used in endoscopes which are thinner than a human hair. Sound like a sequel to the movie ‘Honey, I shrunk the kids’ from the year 1989? Now, more than three decades later, it has become reality. Harald Giessen, Director of Physics Institute 4, and his young research team move in a world of other dimensions when working in their high-tech laboratory at the University of Stuttgart’s campus in Vaihingen. And what they have worked out there is a ‘tiny’ scientific research sensation: micro-lenses from a 3D printer.
The naked eye can barely make them out, but they themselves see everything. The resulting applications are endless: autonomous robots with mini-sensors, cell phones with 360° cameras, spy spectacles, and 20-20 digital eyes for vehicles or infinitesimal flying objects. Mini-lenses constitute a breakthrough above all in medical technology: in minimal-invasive diagnostic procedures and surgical interventions where medical technology quickly reached its limits in the past, ‘impossibly’ thin endoscopes now open up new horizons. They can examine organs and areas of the body that were formerly inaccessible. Just a few examples: the tear duct of the eye, the interior of a tooth root, or middle ear diagnosis via an eardrum hole with a diameter of only 100 micrometers. The endoscope lenses are produced in a single process by printing out the lens together with its frame directly on a tiny glass fiber.
The 3D printer used to make such optical lenses has little in common with the conventional printer. The basis of the lens is a type of liquid photoresist coating which is poured onto a glass slide or a glass fiber. This is then hardened with the aid of a technique jestingly named the ‘dentist’s trick’ by Harald Giessen, namely with UV light in the red (not blue) spectrum.ht.
2 x red = blue
Since blue UV light hardens everything found within its light cone, an extremely precise method is required. The solution: the femto-second laser, which emits light impulses lasting only a thousandth of a millionth of a millionth of a second. ‘That’s about as long as an electron needs to orbit 100 times around an atom,’ says Harald Giessen. The technique requires using a microscope to focus the laser, with a pulse duration of less than 100 femto-seconds, on the liquid photoresist coating. Two photons of the red laser beam meet at the focal point, cast light upon it, and cause the coating to harden. Giessen explains it in layman’s terms this way: ‘When the polymer (the coating) sees the two red light particles, it thinks, ‘They’re just as good as one blue one!’ So two times red equals blue.’ A series of hardening laser ‘shots’ brings the lenses into the desired shape, one point after another, with more than 1,000 layers per hour. This technique makes it possible to create even incredibly complex devices quickly, easily, and with a precision that even makes it possible to create free-form surfaces.
From the bull to the lens
The cornerstone for all this was laid long ago: the method is based on the two-photon absorption process discovered back in 1931 by Nobel Prize winner Maria Goppert-Mayer. Then, around the turn of the millenium, the Japanese researcher Satoshi Kawata used a femto-second laser for the first time to harden photoresist coatings and showed that an infinite number of shapes could be produced in this way - from bulls to naked women. But when asked how he and his team were the first to get the idea of printing out lenses and positioning them on glass fibers, Giessen answers: ‘We simply asked ourselves why nobody had done it up to now. Sometimes the solution is right in front of our noses!’
Only a select, privileged few are allowed to view the ‘holy halls’ where the lenses are printed. After all, as Giessen points out: the machines there are worth millions. Just one example: the high-precision 3D printer with its integrated femto-second laser from Nanoscribe, the Karlsruhe startup company, costs half a million Euros. ‘You won’t see anything like this in any other laboratory in the world,’ says Giessen, visibly enthused about his room of other-worldly treasures. Up to now, several hundred lenses have been printed out here with the 3D technique.
Interdisciplinary team
It all began with Giessen’s former doctoral candidate Timo Gissibl, whose work is now being carried on by Master’s Degree student Simon Ristock. The structural designs for the lenses are created by doctoral candidate Simon Thiele, a member of Alois Herkommer’s task force at the Institute for Technical Optics. There was a moment when the team of researchers reached a point in Giessen’s view where no further progress was possible with the resources of his own discipline, and so he turned to Thiele, at the allied institute. ‘They can all be calculated,’ announced Thiele, and true to his word, the first design was worked out and printed in only a single day. That was enough to convince Harald Giessen. ‘Simon knew how to make good optical designs, and that put us in business.’ As a whole, the team now includes ten researchers working on the project interdisciplinarily at the Stuttgart Center for Photonic Engineering (SCoPE). Their abilities range from optical design and 3D printing expertise to the areas of materials science, measurement, and control, and the team includes mechatronics and electronics specialists.
Revolutionarily fast results
The results are small. Tiny, in fact. The researchers test the resolution of the lenses with the so-called ‘Air Force Test Target Scale’ and an image created by the U.S. Air Force in order to test the resolution capabilities of optical instruments. A look into the laboratory’s microscope shows what ‘Simon’s 35th lens’ is capable of: the image is so clear that even tiniest lines are recognizable. The quality is thus similar to that of a professional microscope lens but costs only a fraction of the latter’s price.
As Giessen points out, however, the really revolutionary thing about it is not the size or the low cost, but the speed. ‘We need only a day to go from idea and optical design to the CAD model. That means a lot to us as natural scientists and physicists. Never before have we arrived so quickly at applications’ When asked how often they print out these new lenses, the answer comes like a shot from three mouths at once: ‘As often as possible!’ A lens is used up in one to three hours, so that up to 10 may be needed in a single night. For a ‘bigger’ lens, in contrast, about 10 hours are required. And lenses yielding ultra-high-quality images due to multiple focal points have now become feasible for the first time, along with free-form optics. ‘It amounts to computer-aided manufacturing for optics,’ is how Giessen sums it up.
Small lenses in gigantic demand
The project is subsidized by the Baden-Württemberg Foundation as part of its ‘Top Research Initiative’. Once the data were ‘hard and fast’, different aspects of the project were discussed nearly simultaneously in diverse publications. ‘It was like a lightning bolt,’ says Giessen; ‘the telephone was ringing off the hook!’ So many inquiries came in from all over the world that Simon Ristock was forced to created a database for them. This has led, among other things, to close collaboration with numerous companies, including such high-tech enterprises as Trumpf, Carl Zeiss in Oberkochen (with advisory services to all areas of optic research), and Storz, the medical technology and world endoscopy leader in Tuttlingen. One set of technical specifications in a commission by this company, for example, required the team to produce an endoscopic lens that can illuminate all surrounding areas. The answer was ready a week later: uniformly illuminated, high-resolution images. A project for which the company itself would otherwise have needed at least half a year. ‘We were pleased - and Storz too, of course!’
Without the others, none of us alone could have reached this result. An interaction like this exists only here, in the region of Stuttgart.
Harald Giessen, Director of Physics Institute 4
Many perspectives
But the research team is by no means finished: both the method itself and the areas of application harbor a huge potential for development. For example, the team is currently working on a way to print the lens out directly onto a microchip. If it were to be positioned on such a CMOS chip, compact sensors could be produced for such devices as minidrones. Another challenge for the research team is to create long-lasting, reflection-free images in true-to-life colors. Here they need ‘a few tricks,’ as they currently admit. This project has become a trend-setter for interdisciplinary collaboration between engineers and physicists in research and industry, a fact which induced the German Federal Ministry of Education and Research (BMBF) to subsidize the researchers and their industrial partners with more than two million Euros in a new joint project coordinated by four physics institutes. ‘Without the others, none of us alone could have reached this result. An interaction like this exists only here, in the region of Stuttgart,’ sums up Giessen. It is realistic even now to speak of a new era in the production of miniature optical devices. Katja Welte