October 17, 2008
Davis Auditorium, Schapiro Center
Speaker: Ursula Keller, ETH Zürich
The tunneling process is one of the most fundamental quantum phenomena. It is important for many different fields such as intense laser field interactions, nuclear physics, nanostructures, superconductors and chemical reaction dynamics. However, the question if tunneling through an energetically forbidden region takes a real time or is instantaneous has been an ongoing controversial discussion over the last 60 years. In our experiment a helium atom is ionized by an intense laser field and a tunneling delay time DtD is defined as the time delay between the lowering of the barrier and the time at which the escaping electron first experiences acceleration by the external field of the laser. We could demonstrate intensity-independent “instantaneous” ionization with an upper limit of 30 attoseconds over a Keldysh parameter variation of 1.45 to 1.17. For this measurement we have used attosecond angular streaking, a new technique to achieve attosecond time resolution using close to circularly polarized, intense pulses. We can confirm such “instantaneous” ionization with quantum mechanical simulations using time-dependent Schrödinger equations. This result is compatible with a zero delay time based on a Wigner-Eisenbud phase analysis but is far shorter than the Buttiker-Landauer traversal time DtT, which for the present conditions is predicted to range between 450 to 560 as. This talk will explain how attosecond angular streaking works and how it was used to measure such fast dynamics. In addition, we also discuss some of the different issues with regards to tunneling times.
Ursula Keller was born in Zug, Switzerland, in June 1959. She received the "Diplom" in physics from the Federal Institute of Technology (ETH) Zürich, Switzerland in 1984. From late 1984 to 1985 she received an ETH research fellowship to work on optical bistability at Heriot-Watt University, Edinburgh, Scotland. She then continued to earn her M.S. and Ph.D. degree in Applied Physics from Stanford University, Stanford, CA in 1987 and 1989, respectively. Her Ph.D. research was in optical probing of charge and voltage in GaAs integrated circuits and in low-noise ultrafast laser systems.
In 1989, she joined AT&T Bell Laboratories, Holmdel, NJ, as a Member of Technical Staff where she conducted research on photonic switching, ultrafast laser systems, and semiconductor spectroscopy.
Since March 1993, she has been an associate Professor, and since Oct. 1997 a full Professor in the Physics Department at the Swiss Federal Institute of Technology (ETH) in Zürich Switzerland. Her current research interests are in ultrafast lasers, high harmonic generation and attosecond science, ultrafast spectroscopy and novel devices for applications in optical information processing and communication.
She has published more than 240 peer-reviewed journal papers and 11 book chapters and she holds or applied for 17 patents.