Watching electrons tunnel

Every-day experience teaches us that overcoming a hill implies its climbing (figure 1). Quantum physics knows another way. Objects may get to the other side of the hill without climbing it up: by penetrating the hill horizontally. The phenomenon, dubbed tunneling, has been understood in terms of the wave-like nature of matter. For macroscopic objects its probability is very low, that's why we have never observed it. By sharp contrast, in the microcosm particles may – with significant probability – "tunnel" through regions of space where they could not be according to the laws of classical physics (e.g. in the bowels of the hill in figure 1). 

Just as a valley stabilizes the position of a body by gravity, the nuclear force binding protons and neutrons together to form the nucleus of an atom, or the electric force binding (negatively charged) electrons to the (positively charged) nucleus to form an atom stabilizes the position of these particles within a tiny volume of space. This stabilizing effect can also be represented by a valley, which physicists call a potential. Tunneling through a binding potential is ubiquitous in the microscopic world: it is believed to be responsible for nuclear as well as electronic phenomena^[1], but – because of its awesome rapidity – it has never been observed in real time. Until recently. By drawing on the newly-developed tools of attosecond metrology, a German-Dutch collaboration led by Ferenc Krausz managed to capture electrons as they tunnel through the potential binding them to the atomic core under the influence of laser light [1]. 

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 [2] and an attosecond pulse of extreme ultraviolet light [3] coming in perfect synchronism with the few-cycle laser wave.

Impinging on the atoms, the electric field of this red laser wave exerts a strong force on an electron at the periphery of the atom^[2] (illustrated by the green cloud around the atomic core in figure 2). The way this force varies in time is sketched in figure 2. This variation is extremely fast: it takes the force about one femtosecond (1 femtosecond is equal to one trillionth of a second, 1fs = 10^-15 s) to switch direction^[3] - from pointing to the right with maximum strength (at time t₁) to the left (at time t₂) and then back to the right again (at time t₃). At these moments the electric force of the light wave suppresses the atomic potential binding the electron to the nucleus (black line in the central panel of figure 2) on the right (at t₁ and t₃) or the left (at t₂). 

Whilst the slope is suppressed (i.e. around the most intense crests of the laser wave) the electron has the chance to tunnel through the barrier and escape from the atom. This chance for tunneling exists near the wave crests only, i.e. just within a brief time interval of 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 (1 attosecond = one thousandth of a femtosecond, 1 as = 10^-18 s) the probability rises each time the laser wave crest hits the atom, as sketched by the green line in the lower panel of figure 3. This is what theory predicted [4] and has been awaiting experimental verification for about four decades.

No instrument can resolve this inconceivably fast process. Measurable is only the final product, the atoms that are disintegrated into an electron and a positively charged ion after the laser pulse is over. The research team had to resort to a trick. They used a gas of neon atoms, in which the electrons reside in closed shells, are very tightly bound and resist to 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^[4] and can be detached from the atom via tunneling. Thus, only neon atoms "prepared" by an attosecond ultraviolet pulse first can be ionized by the red laser pulse later. With an ultraviolet pulse lasting merely 250 attoseconds and precisely timed to the red laser wave, the German-Dutch team were able to put the "periphery" electron in place at any instant during the laser wave with attosecond accuracy. By measuring the number of atoms ionized by the laser wave step by step, with the attosecond ultraviolet pulse being scanned across the laser wave, they were able 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 and sketched in figure 3 (green line), 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 (see inset in the lower panel of figure 3). 

The experiments provide the first direct insight into the dynamics of electron tunneling and reveal how light-field induced tunneling can be exploited for real-time observation of intra-atomic, or intra-molecular 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, pushing the frontiers of microelectronics into the multi-THz regime and advancing biological imaging and radiation therapies.

References
[1] M. Uiberacker et al., Nature, in press.
[2] A. Baltuska et al., Nature 421, 611 (2003).
[3] R. Kienberger et al., Nature 427, 817 (2004).
[4] L. V. Keldysh, Sov. Phys. JETP 20, 1307 (1965).

For more information, please contact:
Prof.dr. Marc Vrakking, FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam, telefoon +31 (0)20 608 13 49/06 48 80 85 58 or
Prof.dr. Ferenc Krausz, Managing Director, Max Planck Institute of Quantum Optics, Garching, Germany, telephone: +49 893 290 56 12.

You can also find information on www.attoworld.de, www.munich-photonics.de.