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Passive mode-locking with an intracavity saturable absorber was first demonstrated in 1966 [1], six years after the first laser was discovered. However, passive modelocking in solid-state lasers suffered from a fundamental problem: the so-called Q-switched modelocking behavior, where an overlying large Q-switched pulse modulated the train of clean mode-locked pulses. This meant that the laser would turn itself off at regular intervals. The underlying mode-locked pulses could only be used in special circumstances and with very limited applications. The semiconductor saturable absorber mirror (SESAM) solved this problem more than 25 years later in 1992 for the first time for diode-pumped solid-state lasers [2].
I invented and led the development of SESAMs, which are a novel family of optical devices that allow for very simple, self-starting, passive modelocking of ultrafast solid-state and semiconductor lasers. I invented the first member of the SESAM family in 1992 when I was still at Bell Labs in New Jersey, USA [2]. This breakthrough device allowed the first demonstration of a passively mode-locked neodymium:YLF laser - without Q-switching. After moving to ETH Zurich in Switzerland, we developed the theoretical underpinnings of the performance of SESAMs in solid-state lasers, worked out design guidelines for application to practical laser systems, and took this know-how to demonstrate unprecedented laser performance improvements in several key directions: shortest pulse widths (in the 5-fs regime with only about two optical cycles), highest average and peak power from a passively mode-locked laser (nanojoules extended to more than 10 microjoules), and highest pulse repetition rate to (~1 GHz extend to >160 GHz) [3]. Today, SESAM modelocked solid-state lasers fulfill the requirements for industrial applications and are being used for many different applications (see for example overview Table 2 in Ref [4]).
With the help of Dr. Thomas Südmeyer (senior postdoc in my group) and Sergio Marchese (graduate student), we were able to increase the pulse energy of passively modelocked solid-state lasers by more than four orders of magnitude: We demonstrated more than 10 µJ directly out of a SESAM modelocked diode-pumped solid-state laser oscillator [5], and we believe that we can scale this concept even further, well above the 100 µJ regime. This pulse energy will enable material processing and high field laser physics at very high pulse repetition rates (1 – 100 MHz). Previously, such pulse energies were only obtained with modelocked oscillators, followed by one or several amplifier stages at much lower pulse repetition rates of ~ 1 kHz. A higher pulse repetition rate increases the possible scan rate in material processing (e.g. marking, waveguide writing etc.) and increases the signal-to-noise ratio in high field physics experiments. This feature will enable many new measurements that were not previously possible because space charge effects smeared out the important dynamics.
This JSTQE publication was the first longer review paper describing SESAMs and also introduced the acronym which contains all relevant information: first it is based on a semiconductor saturable absorber material (which is ideally suited for this application) and second the semiconductor saturable absorber is embedded into a mirror structure. In the simplest case this is a Bragg mirror (which was first demonstrated with a single quantum well absorber in 1995 [6]). However, the SESAM concept is much broader and can be extended to any other mirror structure. A more recent review of SESAM design concepts is given in Ref. 7.

[1] A. J. D. Maria, D. A. Stetser, and H. Heynau, “Self mode-locking of lasers with saturable absorbers,” Appl. Phys. Lett., vol. 8, pp. 174-176, 1966.
[2] U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson, and M. T. Asom, “Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable absorber,” Opt. Lett., vol. 17, pp. 505-507, 1992.
[3] U. Keller, “Recent developments in compact ultrafast lasers,” Nature, vol. 424, pp. 831-838, 14.08.2003 2003.
[4] U. Keller, “Ultrafast solid-state lasers,” Progress in Optics, vol. 46, pp. 1-115, April 2004.
[5] S. V. Marchese, S. Hashimoto, C. R. E. Bär, M. S. Ruosch, R. Grange, M. Golling, T. Südmeyer, U. Keller, G. Lépine, G. Gingras, B. Witzel, “Passively modelocked thin disk laser reach 10 µJ pulse energy at megahertz repetition rate and drive high field physics experiments”, CLEO Europe 2007, Munich, Germany, June 17-22, 2007
[6] L. R. Brovelli, I. D. Jung, D. Kopf, M. Kamp, M. Moser, F. X. Kärtner, and U. Keller, “Self-starting soliton modelocked Ti:sapphire laser using a thin semiconductor saturable absorber,” Electron. Lett., vol. 31, pp. 287-289, 1995.
[7] G. J. Spühler, R. Grange, L. Krainer, M. Haiml, V. Liverini, M. Golling, S. Schön, K. J. Weingarten, and U. Keller, “Semiconductor saturable absorber mirror structures with low saturation fluence,” Appl. Phys. B, vol. 81, pp. 27-32, July 2005.

Biography: Ursula Keller
Ursula Keller joined ETH as an associate professor in 1993 and has been a full professor of physics since 1997. She received the Ph.D. in Applied Physics from Stanford University in 1989 and the Physics “Diplom” from ETH in 1984. She was a Member of Technical Staff (MTS) at AT&T Bell Laboratories in New Jersey from 1989 to 1993. Her research interests are exploring and pushing the frontiers in ultrafast science and technology: ultrafast solid-state and semiconductor lasers, ultrashort pulse generation in the one to two optical cycle regime, frequency comb generation and stabilization, reliable and functional instrumentation for extreme ultraviolet (EUV) to X-ray generation, attosecond experiments using high harmonic generation, and attosecond science. She has published more than 260 peer-reviewed journal papers and 11 book chapters and she holds or has applied for 18 patents. She was a “Visiting Miller Professor” at UC Berkeley in 2006 and a visiting professor at the Lund Institute of Technologies in 2001. She received the Philip Morris Research Award in 2005, the first-placed award of the Berthold Leibinger Innovation Prize in 2004, and the Carl Zeiss Research Award in 1998. She was the “2006 Ångström lecturer” supported by the Royal Swedish Academy of Sciences and the LEOS Distinguished Lecturer for modelocked solid-state lasers in 2000. The Thomson Citation Index highlighted her as the third-place top-cited researcher during a decade (1991-1999) in the field of optoelectronics in 2000. She is an OSA Fellow and an elected foreign member of the Royal Swedish Academy of Sciences.

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