ATTOSECOND PHYSICS: Ultrafast-laser methods reveal electrons tunneling in real time

June 1, 2007

After high-harmonic generation (HHG) of the EUV pump signal from a near-IR source (left), collinear, linearly polarized EUV and near-IR (NIR) light beams in the attosecond beamline are reflected in a concentric double mirror arrangement (right) that introduces a variable delay between the EUV and the NIR fields, isolates either a single or twin EUV pulse of less than 250 as duration and focuses both beams into the sample. The absolute delay between the EUV and the NIR signals is determined with an accuracy of better than 0.5 fs and ions created in the common focus of the two beams are detected by a time-of-flight ion spectrometer that combines high mass resolution with the capability of analyzing particles within a micron-scale detection volume [2].

Just over four decades ago, Leonid Keldysh predicted that ionization of atoms in strong electromagnetic fields occurs via a quantum process in which electrons overcome the attractive force of the nucleus by tunneling through atomic potential barriers. A tool that would ultimately enable observation of this process, the laser, had been invented just a few years earlier, but the typical time scale of electron motion in atoms and molecules, a few hundred attoseconds, is not only shorter than a pulse of visible light; it is also shorter than a single electric-field period at optical wavelength.

Subsequent advances in ultrashort light-pulse generation and control, however, have made it possible to control electron motion with attosecond precision, which enabled direct measurement of the electric field of a light pulse, imaging of an electronic orbital, and production of extreme-ultraviolet (EUV) pulses of a few hundred attoseconds duration and timed relative to the laser field with a precision of a few tens of attoseconds.

In April, a German-Austrian-Dutch collaboration led by Ferenc Krausz, director of the Max Planck Institute of Quantum Optics (Garching, Germany), reported successful use of these emerging tools of attosecond or time-resolved atomic physics within a gas of neon atoms to achieve the first real-time observation of the electron tunneling process predicted by Keldysh in 1965.^{1, 2}

The researchers used subfemtosecond EUV pump pulses to excite valence or core electrons in the gas sample. They then probed the excited populations with strong, few-cycle near-IR pulses arriving at the sample at a precisely controlled delay time after the subfemtosecond EUV excitation. Conventional pump-probe techniques could not be used in this spectral range because of the relatively low flux of available subfemtosecond EUV pulses and relatively low two-photon transition probabilities at EUV and x-ray wavelengths.

High-harmonic generation

The femtosecond-laser source for the experiment, consisted of a Ti:sapphire laser system, delivering 25 fs, 1 mJ carrier-envelope-phase-stabilized pulses. The pulses were focused into a hollow fiber filled with neon for spectral broadening to enable compression of the pulses into the few-cycle regime. The pulses were then compressed with chirped mirrors to precompensate for dispersion that would be introduced by passing through air and glass in the experiment. High-harmonic generation (HHG) of the resulting 300 mJ, 750 nm, waveform-controlled, 5.5 fs duration and 3-kHz-repetition-rate laser pulses produced the EUV pump signal (see figure).

The success of this experimental approach enables observation and testing of all forms of optical-field ionization—both tunneling (quantum phenomena in which electrons pass through rather than over atomic potential barriers) and barrier suppression ionization (in which field strengths grow strong enough to allow classical detachment of electrons). Ultimately such observation and testing is expected to equip researchers with a full understanding of field ionization, further enabling them to use attosecond tunneling techniques to gain direct time-domain insight into a wide range of multi-electron dynamics and electron-electron interactions at resolution scales ultimately approaching tens of attoseconds. Potential applications include developing compact sources of x-ray light, pushing the frontiers of microelectronics into the multi-terahertz regime and advancing biological imaging and radiation therapies.