April 5, 2007, Garching, Germany--By drawing on newly developed tools of attosecond metrology, a German-Austrian-Dutch collaboration, led by Ferenc Krausz at the Max Planck Institute of Quanum Optics, has observed electrons in real time as they tunnel through the potential binding them to the atomic core under the influence of laser light (Nature, April 5, 2007).
Using ultrashort laser pulses the scientists were able to reveal distinct steps of ionization each lasting several hundred attoseconds. This result represents a milestone in gaining deeper insight into electronic motion inside atoms and molecules, which is relevant to advancing many areas of science, technology and medicine.
The key to this feat has been an intense pulse of red laser light comprising merely a few, well controlled oscillations of its electric field and an attosecond pulse of extreme ultraviolet light coming in perfect synchronism with the few cycle laser wave. The electric field of the red laser wave exerts a strong force on an electron at the periphery of an atom. The electric force of the light wave suppresses the atomic potential binding the electron to the nucleus, which gives the electron an opportunity to tunnel through the barrier and escape from the atom.
This opportunity for tunnelling exists near the wave crests only, within a brief time interval of only a fraction of a femtosecond. As a consequence, as the few-cycle pulse passes through the atom, the probability of the electron being set free is expected to increase stepwise: within a period of several hundred attoseconds the probability rises each time the laser wave crest hits the atom. This is what theory predicted but it has been awaiting experimental verification for more than four decades.
No instrument can capture this inconceivably fast process directly. So to measure the final product, atoms that have disintegrated into an electron and a positively charged ion after the laser pulse, the research team used a gas of neon atoms, in which the electrons reside in closed shells, are very tightly bound and resist the attempt of the laser field to free them. Only electrons hit by an ultraviolet light pulse are promoted to the periphery of the atom to be detached from the atom via tunnelling. Thus only neon atoms that have first been "prepared" by an attosecond ultraviolet pulse can later be ionized by the red laser pulse.
With an ultraviolet pulse lasting merely 250 attoseconds and precisely timed to the red laser wave, the German-Austrian-Dutch team was able to place an electron at the periphery of the atom at any instant during the laser wave with attosecond accuracy. Scanning this instant across the laser pulse and measuring the number of atoms ionized by the laser wave at each setting enabled them to reconstruct the temporal evolution of the "periphery" electron leaving the atom under the influence of the strong field of the red laser light. Just as predicted by theory, the measurements revealed three distinct steps of ionization coincident with the central wave crests of the laser pulse and lasting less then 400 attoseconds each.
The experiments provide the first direct insight into the dynamics of electron tunnelling and reveal how light-field-induced tunnelling can be exploited for real-time observation of intra-atomic or intramolecular motion of electrons. Gaining increasing insight into and control over the atomic-scale motion of electrons will be instrumental in developing compact sources of X-ray light, thereby pushing the frontiers of microelectronics into the multi-THz regime and advancing biological imaging and radiation therapies.
For more information, contact the Max Planck Institute of Quantum Optics.