Got a light? Christian Hackenberger/Attoelectronics MPQ

The ultimate high-speed flashbulb just measured how quickly electrons inside atoms respond to light. The work could speed the development of light-based electronics.

At 380 attoseconds long – 380 x 10-18 seconds – the flashes are the shortest pulses of visible light ever created in the lab.


Eleftherios Goulielmakis at the Max Planck Institute of Quantum Optics in Garching, Germany, and his colleagues achieved a similar feat in 2008 when they generated pulses of extreme ultraviolet (EUV) light that were just 80 attoseconds long.

But making such short pulses of visible light is more challenging – and also more useful. EUV is energetic enough to strip electrons away from an atom altogether. Visible light makes a gentler probe: it energises electrons in an atom, encouraging them to emit light of their own, without actually removing them from the atom’s clutches.

This time, Goulielmakis’s key tool was a light field synthesiser, which carefully combines several light pulses of known wavelengths to generate the incredibly short flashes. Those pulses are brought together with their wavelengths slightly out of phase, so some parts of the combined light cancel each other out and leave a super-short pulse behind (see video, below). The same principle explains why two ocean waves that are perfectly out of sync will destroy each other on contact and leave an apparently calm surface.

Kicking out a photon

Theory suggested that electrons take a few hundred attoseconds to kick out a fresh photon after they’ve been hit by an incoming beam, but the precise figure was unknown. The 380-attosecond-long light pulses are ideal for testing this idea. Not only can the pulses energise the electrons, they can then act as a camera flash, illuminating the process just long enough for scientists to measure the time it takes the electrons to respond.

Goulielmakis and his colleagues aimed their short pulses at gaseous krypton atoms in a vacuum, and found that the electrons in the krypton kicked out UV photons 115 attoseconds later.

The atoms behaved a bit like an energy-saving light bulb, Goulielmakis says. “Turn on the switch and the lamp is a bit dim – it takes time to get bright,” he says. “An electron in an atom also needs time to respond and maximise its emission of radiation – it needs about 100 attoseconds.”

“This work indeed represents a major step forward in the control of electrons,” says Peter Hommelhoff at the University of Erlangen-Nuremberg in Germany.

Overtaking electrons

Goulielmakis and his colleagues plan to extend the work to examine the way electrons behave in other materials – particularly solids.

“[This] may lead to important new insights into the dynamics of electrons in a wide class of materials,” says Jon Marangos at Imperial College London. Those insights could help improve the design and efficiency of electronic devices.

Many people predict that computer circuits will eventually use photons rather than electrons to ferry information, but for that to work, photons have to interact with each other inside physical matter – things like the semiconductors used in today’s computers. So exploring how rapidly semiconductors and other solids respond to incoming light will help determine exactly how fast such light-based electronics will be able to operate. “This is the bridge between photonics and electronics,” says Goulielmakis. “We have to make sure we understand it.”

Journal reference: Nature, DOI: 10.1038/nature16528