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  • Lighting the way to ultrafast microprocessors

Photonics Spectra
Nov 2009
Marie Freebody,

MUNICH, Germany – The first milestone toward lightwave electronics has been achieved, thanks to a collaboration among physicists at Max Planck Institute of Quantum Optics in Garching, FOM Institute AMOLF (Institute for Atomic and Molecular Physics) in Amsterdam, the Netherlands, and chemists at Ludwig Maximilians University (LMU) in Munich.

Now, for the first time, light has been used to control single electrons within a molecular compound. The ability to pick out and guide individual electrons is the first step on the road to light-waveform electronics in which microprocessor speeds could reach attosecond timescales.

Attosecond research is at the frontier of laser physics, and the generation of short, phase-stable light pulses has been a key goal. Because electrons naturally travel at speeds on the order of attoseconds, marrying attosecond physics with electron motion was an obvious avenue of research.

The waveform of the laser pulses is controlled via two glass wedges. The laser pulses interact with the molecules in a velocity-map imaging spectrometer, in which the resulting distribution of ions (here, C ions) after the breakup of the molecules is imaged onto a detector (right). The image on the detector displays an up-down asymmetry along the vertical polarization axis of the laser. The symmetry of the ionized orbitals of CO becomes visible by the 45° contributions in the angular distribution, which can be predicted by theoretical calculations (black line). Courtesy of Christian Hackenberger/Ludwig Maximilians University.

“So far, attosecond control of electron dynamics has only been demonstrated for simple molecules containing two atoms and a single electron,” said professor Regina de Vivie-Riedle at LMU. “We extended this control to multielectron systems, where it becomes increasingly difficult to ‘pick out’ a single electron and control its motion.”

The ability to control electrons in multielectron systems not only opens the door to high-speed microprocessors but also marks an important step toward the control of chemical reactions using light as well as the control of electron motion in nanostructures.

In the German-Dutch collaboration, published in the journal Physical Review Letters on Sept. 4, 2009, individual electrons were singled out from a system containing 14 electrons. The key to the approach was to alter the waveform of the electromagnetic field of laser pulses to modify the forces that act on the electrons.

“The pulses that were used in our study are the shortest such pulses used for molecular control studies thus far,” said Dr. Matthias Kling of Max Planck. “Using laser pulses of just four femtoseconds, it also becomes feasible to image the structure of the outermost occupied molecular orbitals.”

Shown is a view into the microcosm of carbon monoxide molecules. The detachment of electrons from carbon monoxide molecules by femtosecond laser pulses leads to a characteristic angular distribution of the molecular ions and their fragments. Courtesy of Matthias Kling/Max Planck Institute of Quantum Optics.

In the experiment, a neutral carbon monoxide molecule is ionized at the peak of the laser field so that an electron detaches. This released electron is driven by a strong, few-cycle laser field away and back to the molecule, and, upon recollision, the parent molecular ion gets excited. The resulting strongly coupled electron-nuclear motion can be steered by the remaining laser field.

“The speed of control is much faster compared with conventional electronic devices,” Kling said. “And the degree of control of electron dynamics increases with decreasing pulse duration.”

Although the commercial potential of the technique is promising, de Vivie-Riedle emphasized that, before electronic devices can be built, much more research is needed. “Our next step is to apply the newly developed techniques to more complex molecules and nanostructures.”

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