Press release | 24.4.2018
Munich, 24 April 2018 - For over three decades, Swiss physicist, inventor and university professor Ursula Keller has been at the forefront of the field of ultra-fast-laser technology - working to reduce the short bursts of energetic light these lasers can emit to infinitesimal snippets of time. Keller invented the "semiconductor saturable absorber mirror" (SESAM), the first practical method for creating ultra-fast pulses in solid-state lasers that is now employed in applications across fields from electronics and automotive manufacturing to medical surgery and diagnostics. Building on the SESAM concept, Keller subsequently extended its use into new compact and easy-to-manufacture laser designs for consumer electronics and other applications. She has also pioneered sensitive laser equipment that explores the wonders of the universe down to quantum scale.
For her achievements, Ursula Keller has been nominated for the European Inventor Award 2018 as one of three finalists in the category "Lifetime achievement". The winners of this year's edition of the European Patent Office's annual innovation prize will be announced at a ceremony in Paris, Saint-Germain-en-Laye, on 7 June 2018.
"The technology developed by Ursula Keller has paved the way for the application of fast-paced lasers at industrial scale in a broad range of sectors ranging from computers and smartphones to vehicles to medical technology. Her work has been highly influential in charting new directions in laser technology and is even likely to extend the technical boundaries for scientific research, as well as strengthen Europe's role as a leader in ultra-fast laser research and implementation."
Keller's wide-reaching impact on the field of laser technology began in the early 1990s. Soon after receiving her PhD in applied physics from Stanford University, Keller joined AT&T Bell Laboratories in New Jersey in 1989, and built up her own lab. Keller set her sights on an unsolved technical challenge that had long attracted laser scientists: to develop a practical method for shortening a continuous wave of laser light into ultra-fast pulses. One potential answer, called passive mode-locking, involved modifying the mirrors within a laser to exploit light waves' peaks and valleys. This technique would absorb all but the highest sections of a light wavelength's peaks until the absorber was 'saturated' and reflected back only the sharpest spikes for further amplification. When a threshold was reached, tiny energetic pulses lasting less than a trillionth of a second could be delivered in rapid succession.
The method had already been tried 25 years earlier, six years after the invention of the first laser, but had yielded little success. Keller turned to the wonders of semiconductor technology for an answer. By replacing the mirrors in a conventional laser cavity with what she would name the semiconductor saturable absorber mirror (SESAM), Keller turned a continuous wave laser into an ultra-fast laser with mind-bogglingly fast pulse durations ranging from picoseconds (10-12 sec) down to below 5 femtoseconds (10-15 sec). Keller not only solved a vexing problem in laser science; the high intensity and ultra-fast pulse rate that SESAM-equipped lasers were able to produce opened up a wealth of new applications ranging from detained fabrication in manufacturing and highly accurate cutting in medical surgeries to faster and more accurate data transmission for telecommunications. "I knew from the moment SESAM was developed that it would likely have a high impact," the inventor says.
The short bursts of light from an ultra-fast laser last only a few trillionths of a second (picosecond) or less and are delivered in fast repetition of up to several billion times a second (GHz cycle). This enables very small, thin slices of a material to be removed – not by heat, as with other types of lasers – but through a process called cold ablation. Thanks to cold ablation and pinpoint accuracy down to a thousandth of the width of a human hair, ultra-fast lasers can create the fine details on glass, polymer and silicon substrates that would otherwise crack or become brittle under higher temperatures. They are important, for example, in cutting the toughened glass used in smart phones touchscreens and to create the thin patterning in flat panel displays for monitors and television sets.
Employed in the automotive industry, they can optimise the spray pattern of fuel injectors and improve fuel economy by 10% or more without affecting performance. In medical applications, especially in eye surgery, SESAM-equipped lasers deliver the precise amount of energy required to make fine incisions without damaging the surrounding tissue. In the processing of advanced materials, ranging from high-tech ceramics to labs-on-chip to cutting-edge solar cells, they help extend the range of materials that can be used while improving manufacturing techniques and reducing waste. In Keller's own words: "There is hardly anything that is not processed with short-pulsed lasers and the applications continue to grow. It is one of the fastest growing markets in laser processing."
After leaving AT&T Bell Laboratories to accept a professorship at ETH Zurich's physics department in 1993, Keller continued to build on the SESAM idea and incorporate it into other laser types. In 2000, she demonstrated the first passively mode-locked vertical external cavity surface emitting laser (VECSEL), which was followed in 2007 by a mode-locked integrated external-cavity surface emitting laser (MIXSEL). Both approaches led to several important European patents, extended SESAM technology to include inexpensive laser light sources and further boosted performance, making them suitable for applications such as laser displays, telecommunications and as the light source for medical imaging technologies. Moreover, MIXSEL's wafer-type design is easier to mass produce and compact enough to broaden ultra-fast lasers' roles to include consumer electronics applications such as lasers in gaming consoles that are not only able to recognise body movement but also facial expressions and the laser guidance systems (LIDAR) employed in self-driving vehicles.
Keller's atypical career trajectory from applied research to a professorship gave her further opportunities to push the limits of laser technology. The inventor says: "Normally you do things the other way around: start with fundamental research and then move to applied. But it was easier for me as a female physicist to establish myself with a technical breakthrough first and then move to a university setting where I could work to push the performance even further."
Fuelled by intense curiosity, Keller turned to laser technology to also help explore some of the most puzzling questions of quantum physics. She created one of the world's most sensitive time-measuring devices, the "Attoclock", which records time intervals down to a few attoseconds or quintillionths of a second (1x10-18). For comparison, an attosecond is to one second, what one second is to about 31.7 billion years – seven times the age of the Earth. An attosecond is also about the length of time it takes for light to move between neighbouring atoms.
The Attoclock is helping to unlock the mysteries of quantum tunnelling, a phenomenon that allows an electron to pass through a thin solid barrier - counterintuitive to classical physics. Used as a virtual sub-atomic strobe light, the Attoclock "freezes" the motion of fast-moving objects such as electrons and allows processes including quantum tunnelling to be timed with great precision. "With these clocks we're also hoping that, at some point, we'll be able to measure whether our natural constants really are constant after all," says Keller. This scientific understanding could one day help us better comprehend photosynthesis and other photochemical reactions, or potentially lead to the creation of extremely fast supercomputers.
The elegance and simplicity of Keller's SESAM mode-locking technique, particularly combined with advances in diode-pumped lasers developed during the 1990s, resulted in new, practical, commercially available ultra-fast laser systems. Most commercial ultra-fast laser companies have since adopted SESAM techniques, either through internal development that was often supported by one or more of Keller's former students or through the acquisition of start-up companies, several of which her past doctoral graduates have launched. In order to commercialise her own developments on the SESAM concept, Keller founded the spin-off company Time-Bandwidth Products in 1994 - acquired by JDSU (now Lumentum) in 2014.
The rapidly growing global market for ultra-fast lasers was valued at EUR 2.2 billion in 2017, or about one fifth of the total laser market worldwide. It is expected to reach a value of EUR 8.3 billion by 2023 driven by sectors such as the automotive industry with its micro-machining applications that include spray pattern optimisation of fuel injectors.
Europe plays a major role in both the manufacture and research of ultra-fast lasers. Its high concentration of laser know-how has helped concentrate about 73% of the world's highly intense (petawatt-class), ultra-fast lasers in Europe. Europe is also home to the integrated LaserLab Europe consortium, which connects key research centres - such as ETH Zurich, the University of Jena, ParisTech's Institut d'Optique and the Max-Planck-Institute for Quantum Optics - with leading laser manufacturers in the field, for example, France's Amplitude Systemes and Germany's Trumpf.
One of the leading international scientists in the field of ultra-fast photonics, Keller has published more than 440 peer-reviewed journal papers and 11 book chapters,, and holds seven granted European patents. Among her numerous awards, she received the IEEE Photonics Award (2018), the Weizmann Women & Science Award (2017), the OSA Charles H. Townes Award (2015) and Geoffrey Frew Fellow of the Australian Academy of Science (2015). Since 2010, Keller has served as Director of Swiss National Centre of Competence for Research in Molecular Ultra-fast Science and Technology. In 2014, she became a member of the research council of the Swiss National Science Foundation. Keller is also supporting the next generation of laser innovators: she has already supervised over 72 PhD students and many junior postdoctoral researchers, 11 of whom have become professors themselves, and several others who have gone on to positions at laser manufacturing companies or have launched start-ups of their own.
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Over the years, lasers have played key roles in the innovations of several finalists and winners at the European Inventor Award. The list includes the inventors of optical coherence tomography (OCT) James G. Fujimoto, Eric A. Swanson and Robert Huber ( 2017; Non-EPO countries - winners); the electronic engineer behind the coding method for CD, DVD and Blu-ray, Kornelis Schouhamer Immink (finalist 2015, Lifetime achievement); the pioneer of laser eye surgery Josef Bille (2012; Lifetime achievement - winner); the inventors of Quantum Cascade lasers Federico Capasso, Jérôme Faist and team (2012; Non-EPO countries - finalists); and the inventors of the scanning laser ophthalmoscope Douglas Anderson and team (2008; SMEs/Research - finalists).
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