Attosecond Lasers Produce Electron Movies

Hoping to gain a clearer understanding of what happens during chemical reactions, a team of researchers is working to create a laser device that can release bursts of laser light to capture individual electrons as they orbit the nucleus — much like a movie. The researchers are from MIT’s Research Laboratory of Electronics, the University of Sydney in Australia, Polytechnic University of Milan in Italy and Hamburg University in Germany.

Achieving this phenomenon is difficult, considering that the electron in a hydrogen atom completes a full orbit around an atomic nucleus in just 151 attoseconds. An attosecond is a billionth of a billionth of a second, so capturing the act requires attosecond laser pulses.

Light waves of different frequencies (red and green) are combined to form a new wave (yellow), which in turn passes through a gas (blue). The light excites the atoms of the gas, which release their excess energy as light of an even higher frequency. (Image: Shu-Wei Huang)

Attosecond pulses have been demonstrated in the lab before, but they didn’t have the intensity required for so-called time-resolved spectroscopy, the technique typically used to measure electron dynamics. Not only should the new approach boost the pulses’ intensity, but it should require a simpler setup, making it more practical.

The key to producing ultrashort bursts of light is to combine light waves of various frequencies. When two waves intersect, they reinforce each other where their crests overlap, but the trough of one can cancel out the crest of another. The right combination of waves, however, can produce a new wave with a radically different shape.

Other researchers have tried to produce short bursts of light by combining laser beams, using a separate laser for each beam. This method makes it very difficult to synchronize the beams so that their troughs and crests coincide exactly where intended. The MIT group and its colleagues instead passed a single laser beam through a crystal that splits it into beams of different frequencies. Because the beams are derived from a single source, they remain perfectly synchronized.

Although this yields very short pulses of light, they are still not on the scale of attoseconds. So the next step in the process would be to send the pulses through a gas. When particles of laser light — photons — strike the atoms of the gas, they’re absorbed, but usually, their energy is immediately re-emitted as new photons. Those photons, however, have frequencies that can be many times that of the original photons. And higher frequencies mean even shorter bursts of light.

The researchers, however, have not yet performed this final step. Currently, they pass their laser beam through two amplifiers to increase its energy, but it needs more energy still to elicit enough higher-frequency photons from the gas. Adding another amplifier, they say, should do the trick, although it does pose some engineering challenges.

The work was published in Nature Photonics.

For more information, visit: www.mit.edu