Can the electron move faster than light
"This is the fastest electric current ever measured"
On the way to faster electronics, the flow of electrons within a circuit plays a decisive role. With conventional methods such as batteries, electron oscillations up to the gigahertz range can be generated. With the help of ultra-short laser pulses, researchers have now been able to drive electrons in a solid to up to eight quadrillion oscillations per second - around a million times faster than was previously possible. The team reports on their approach in the journal "Nature". Welt der Physik spoke to Eleftherios Goulielmakis from the Max Planck Institute for Quantum Optics in Garching, who was involved in the study.
World of Physics: How do electrons move in a conventional circuit?
Eleftherios Goulielmakis: You can imagine that the electrons are driven by electrical fields that are generated by batteries, for example. If you apply a static field, the electrons are only driven in one direction, they flow. In electronic devices, however, alternating current is used, which is why the electrons vibrate at a certain frequency. In conventional electronic devices, these frequencies can be up to around one hundred gigahertz, with one gigahertz corresponding to one billion oscillations per second.
How is the speed of the electrons limited?
The electrons collide with the atoms and molecules in the solid through which they move. This limits their speed, because if you try to use ever faster electric fields, the electrons can practically no longer keep up - and become slower. That is why researchers have tried in the past to cool solids to very low temperatures. Then the atoms are more or less frozen and the electrons can move more easily. But this effect is also limited.
Why would higher speeds of the electrons be desirable?
The fundamental question here is how electronic devices can be made even faster. For decades, scientists have been making circuits that are getting smaller and smaller. As a result, the distances that the electrons have to travel are getting shorter and shorter and the electronics become faster. This approach worked for decades, but it is now reaching its limits because the electronics are now so thin that they only measure a few atoms through. Therefore, we would have to get the electrons to move even faster than we want them to. Because that is ultimately electronics: the controlled movement of electrons within a solid.
What is your approach to driving the electrons faster?
We used light and sent extremely short laser pulses onto a solid body made of silicon dioxide. Of course, light is also an electric field that can drive the electrons and make them swing back and forth. The electric fields of these laser pulses oscillate at the speed of light, and they can be very strong. Although such extremely strong fields can also destroy a solid, it works in our case because the fields are only so strong for an extremely short time. Therefore, a laser can exert forces on electrons that are about ten times stronger than what would be possible with standard methods.
Illustration of the ultrafast electron movements
Which frequencies did you reach?
We drive the electrons very quickly, so quickly that the much heavier atoms cannot move at all and thereby disrupt the movement of the electrons. The electrons not only vibrate as fast as the electric field of the laser pulse, but even faster, so their reaction is not linear. We were able to achieve frequencies of eight petahertz, which corresponds to eight quadrillion oscillations per second. This is the fastest electric current ever measured, and about a million times faster than conventional electronics.
How did you measure this current?
That is exactly the point. Just as no electronic device can generate these high frequencies, neither can we build a device that measures them. But in our research area of attosecond physics, we have developed techniques in recent years with which we can measure the extremely fast oscillations of electromagnetic radiation - mind you, radiation, not electrons. Because we drive the electrons in the solid body so quickly and many electrons carry out these oscillating movements, the electrons themselves emit radiation in the extreme ultraviolet range. This radiation contains all the secrets of the actual vibratory motion, and by measuring it we can infer the motions of the electrons. For us, this is the real breakthrough.
Is it purely basic research or can you already imagine applications?
We look at electronics from a physicist's point of view. We explore the principles and want to find out what is actually physically possible. It was known that the interaction between light and matter causes electrons to move. But for many years it wasn't clear that lasers could really do this and that this movement could also be observed. Of course, we can't put a transistor in this circuit at the moment, but that is the basis that could one day also lead to applications.
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