Testing the response time of electrons

The first visible attosecond pulses allow researchers to determine the delay with which electrons in atoms respond to the electromagnetic forces of light

Light could be the driving force which makes electronics even faster in the future. This is how physicists pursue their goal of using short pulses of light to control electric currents in circuits at the same rate as the frequency of light. The attophysics discovery made by an international team working with Eleftherios Goulielmakis, Leader of the Attoelectronics Research Group at the Max Planck Institute of Quantum Optics, may make it possible in future to use light to control electrons much more precisely than ever before. This is because electrons apparently follow the electromagnetic forces of light with a slight delay. The researchers determined the time it takes the electrons to react to light by exciting electrons in krypton atoms with attosecond pulses of visible light. They observed that it takes around 100 attoseconds (one attosecond is a billionth of a billionth of a second) until the particles’ reaction to the light pulses becomes noticeable. Physicists previously had to assume that the force of light has an immediate effect because they were unable to measure the delay.

An electron weighs almost nothing at all. If you want to express its mass in grammes, you have to write 27 zeros after the decimal point before you can write the first number. But even this lightweight is sluggish, a little bit at least. Quantum mechanics predicts that an electron also needs a certain, albeit very short, period of time to react to the forces of light. Since this takes only several tens to hundreds of attoseconds, this process was considered to be unmeasurably fast – until now. Researchers from the Max Planck Institute of Quantum Optics working with colleagues at Texas A&M University (USA) and Lomonosov Moscow State University (Russia) are now the first to have stopped this reaction time, as it were.

“Our research thereby puts an end to the decade-long debate about the fundamental dynamics of the light-matter interaction,” says Eleftherios Goulielmakis. In recent decades, researchers were already in a position to track both the rotations as well as the nuclear motions in molecules. “This is the first time that we are able to also track the reaction of the electrons bound in the atoms in real time,” stresses Goulielmakis. “But at the same time we are now standing on the threshold of a new era in which we will investigate and manipulate matter by influencing electrons.” In the current publication, the researchers namely present not only the first measurements of how long an electron takes to respond to a light pulse. They also present the means that made this measurement possible in the first place, and which will enable completely new experiments with electrons to be carried out in the future: a way of tailoring pulses of visible light.

Visible attosecond pulses are formed from light of different wavelengths

“One prerequisite for capturing such a brief event is a pulse of light that causes the electrons to start moving extremely quickly – it polarizes them, to use the scientific term – and thus tests their reaction time,” explains Mohammed Hassan from Eleftherios Goulielmakis’ Research Group. The researchers use a so-called light-field synthesizer to produce such light pulses. They manipulate the properties of visible, near-infrared and ultraviolet light in order to be able to compose a light pulse in the visible range with a duration of only 380 attoseconds. The pulses are so short that they entail barely more than a half oscillation of the light field. They are thus the shortest pulses ever generated in the visible range. “We can not only manipulate visible light with attosecond precision, we can also limit its waves to attosecond time intervals,” explains Tran Trung Luu, one of the scientists in Goulielmakis’ team.

Physicists have already been controlling flashes of UV and X-ray light, which have a much shorter wavelength, for a number of years with similar precision. But these wavelengths do not incite electrons to execute small movements, but instead directly eject the particles out of an atom, molecule or solid body.

Which path will lead to novel electronics and photonics?

The scientists used this new tool of attosecond pulses of visible light to excite krypton atoms. They varied the two properties of the pulses which characterize them precisely: the intensity and the phase. The latter gives the point on the light wave which the electromagnetic oscillation passes through at a specific point in time. The small changes to the pulses meant that slightly different forces acted on the electrons in the atoms in different experiments. After being excited, the electrons emitted ultraviolet light. It was this radiation which ultimately told the researchers that it takes roughly 100 attoseconds until the electrons respond to the force of the light.

One of the next steps planned by Goulielmakis and his team is to extend the investigations to the electron dynamics in solid bodies. “This will tell us the best way to realize novel, ultrafast electronics and photonics which operate on time scales of a few femtoseconds – a femtosecond is one millionth of a billionth of a second – and with petahertz clock rates,” explains Goulielmakis.

OMS/PH