Thomas Elsaesser is a director at the Max-Born-Institute, Berlin, Germany, and a professor for experimental physics at Humboldt University, Berlin. He received a Dr. rer. nat. degree from the Technical University of Munich in 1986 and worked there as a research associate until 1993. He spent a postdoc period at AT&T Bell Laboratories, Holmdel, in 1990 and joined the newly established Max-Born-Institute in 1993. Thomas is a fellow of the American Physical Society and the Optical Society of America and has received numerous scientific awards. Klaus Reimann has worked as a scientist at the Max-Born-Institute, Berlin, Germany in the field of ultrafast mid-infrared and THz spectroscopy since 1999. He received a Dr. rer. nat. degree from the University Dortmund in 1987. Afterwards he joined the Max-Planck-Institut fur Festkorperforschung in Stuttgart. After receiving a five-year Heisenberg- Stipendium of the Deutsche Forschungsgemeinschaft, he spent time at the Universitat Dortmund and at the University of California at Berkeley doing research on nonlinear optics of semiconductors under high pressure before joining the MBI. Woerner is a department head at the Max-Born-Institute, Berlin, Germany, and holds a lecturer qualification (Habilitation) in physics at Humboldt University, Berlin. He received a Dr. rer. nat. degree from the Technical University of Munich in 1991 and worked there as a postdoc until 1993. He then joined the Max-Born-Institute in 1993 and spent a postdoc period at Bell Laboratories (Lucent Technologies), Holmdel, in 1997.