Physicists can control light waves with absolute virtuosity nowadays. They not only generate light pulses which last only femtoseconds, i.e. a time span in which light travels a distance roughly equivalent to the diameter of a chromosome. They can even form the femtosecond pulse as desired. A light wave oscillates so fast that even during the short period of a femtosecond pulse several wave crests and wave troughs occur. With complex laser technology based not least on the work of the Nobel Laureate Theodor W. Hänsch from the Max Planck Institute in Garching, laser physicists very precisely adjust where within the laser pulse the wave crests and troughs of the light waves occur. They thus shift the phase of the light wave within the pulse as desired, or in technical terms: they control the phase difference between wave and pulse.
The team working with physicist Peter Hommelhoff at the Max Planck Institute of Quantum Optics has bombarded a metallic tungsten tip measuring only a few nanometres (one millionth of a millimetre) with laser pulses of six femtoseconds duration. The researchers varied the phase difference between shots in this process. They thus controlled when the wave crests and troughs of a pulse impinged on the nanotip with a precision of around 80 attoseconds.
The physicists were very surprised by what they observed. The phase difference affects the emission process of electrons with certain energy values, i.e. very fast electrons. The number of fast electrons increased or decreased when the physicists shifted the arrival of the wave crests and troughs of the pulse in steps of 160 attoseconds.
“We can therefore use the phase difference to control the process whereby fast electrons are emitted, and which takes a mere 450 attoseconds,” summarises Hommelhoff. This was an important step towards a field-effect transistor with which an electric current could be switched on and off within attoseconds. This would correspond to a frequency of several hundred terahertz, i.e. around 10,000 times faster than the field-effect transistors which process data in modern computer chips.A ping-pong effect accelerates the electron
Hommelhoff can, however, also envisage that the experiment will have more direct application possibilities. Since the fine metal tip amplifies the electric field of the light enormously, the laser pulse need not have a particularly high intensity. “A simple laser oscillator would do,” says Hommelhoff. The setup was therefore relatively simple and thus provided a low-cost alternative for measuring the phase difference between pulse and wave in laser laboratories, which is currently done using interferometers which can cost up to 25,000 euros.
The extremely short red laser pulse – shown here as a red ball – releases electrons (blue waves) from the tungsten tip when the light wave impinges on the tip with the right phase difference. From a quantum physics point of view, electrons are matter waves that can superimpose like water waves. The researchers use this superposition to deduce which ultrafast processes are taking place when electrons are released from the metal. © Christian Hackenberger / MPI of Quantum Optics
What surprised the scientists in Garching almost more than the research result itself was the fact that they can explain it with a relatively simple theoretical model. According to this, the electric field of the laser pulse draws an electron out of the tungsten tip when it is at maximum and its polarity is pointing into the tip. The electron released is pulled away from the tip by the electric field and accelerated in the process. After a few hundred attoseconds the electric field of the light wave switches into the opposite direction, just like a water wave first increases and then decreases again. The reversed field hurls the electron back to the tip. The elementary particle recoils elastically from the tip like a table tennis ball from a bat. Meanwhile, the electric field of the light has reversed its polarity again and accelerated the electron further away from the tip. It collects so much energy in the process that it escapes from the vicinity of the pin.
At certain phase differences two electrons are released from the tip one after the other within the femtosecond pulse and both participate in the to-ing and fro-ing. From a quantum physics point of view these two electrons are matter waves. They can superimpose and amplify each other like normal waves, thus increasing the electric current measured at the detector. Quite amazingly, this also works with a single electron whose matter wave is emitted at two points in time. In this process, the wave packet of the electron virtually splits into two parts; these dissolve slowly and superimpose as they fly away from the tip. “We were able to qualitatively describe the experimental results very well with the help of this model,” says Hommelhoff.The next step to a transistor with attosecond speed is being planned
The mechanism is familiar from similar experiments with gases of atoms and molecules. “We observed it on a solid body for the first time,” says Hommelhoff. Two particular obstacles had to be overcome with the Garching experiment, says the physicist. The measurement of the energy of the released electrons, on the one hand, and how to keep the tungsten tip clean during the measurements, on the other. “The build-up of a single layer of foreign atoms on the tip would have crucially falsified the results,” says Hommelhoff. The tip must therefore be kept in a sensitive ultra-high vacuum.
After the current breakthrough, some questions remain unanswered which the Garching team wants to clarify with further work. “We have no precise knowledge about the mechanism that releases the electrons from the tip,” says Hommelhoff. It could be a so-called multi-photon effect where the energy of several light particles from the laser pulse is transferred to an electron so that it can escape from the metal. It is also possible that the electric field of the light can influence the electric potential of the metal in such a way that the electrons are released by the so-called quantum mechanical tunnel effect.
In order to get a step closer to the attosecond transistor, Hommelhoff and his team want to set up an experiment where the electrons are transferred from one metal tip to another, controlled by laser light. This would be an electric switch that was controlled by the phase of a light wave.
Source: Max Planck Institute