Ultrafast microscope for the quantum world


The combination of ultra-short laser pulses (red) with a scanning tunneling microscope makes quantum processes visible like an electronic wave packet (colored) with atomic spatial resolution. (Image: Christian Hackenberger)

So far, scientists have been able to track processes at the atomic level either at extremely high resolution or at high speed – but not both. Now a team of researchers has succeeded in combining two common methods so that they receive a microscope with high spatial and temporal resolution – a kind of HD camera for the quantum world. Among other things, it can be used to track the movements of electrons down to an atom and in the attosecond range – this corresponds to the billionth part of a billionth of a second. The new microscope now offers the opportunity to make processes in molecules visible, but also to further accelerate computer processors, as the researchers report.

Whether microelectronics, nanotechnology or biomedicine: many processes that are important for today’s technologies take place in the realm of atoms and molecules – in size ranges that are not visible to our eyes. At the same time, these processes are usually extremely fast, so that the decay of a molecule in a chemical reaction takes only a fraction of a second. In order to understand and research such fundamental processes, scientists therefore need instruments that have both a high spatial and a high temporal resolution. So far, however, their technologies have mostly only achieved the required performance in one of the two areas.

Dilemma of time or place

The scanning tunneling microscope (RTM) is currently one of the most powerful instruments for looking down to the size of atoms. This scans a surface without contact with a fine tip, which usually consists of only a single atom. Due to the small distance between the surface and the tip of the microscope, there are interactions between the electrons of their atoms: They overcome the separating space – quantum physically speaking, they tunnel. From the distance between the tip and the tunnel current, the system can calculate where the individual atoms on the surface are and how big they are. This enables the microscope to image individual atoms and molecules on a surface. The problem, however, is that the scanning tunneling microscope is extremely high-resolution in spatial terms, but not in terms of time. He misses rapid processes at the level of the observed atoms.

Tools are also already available to monitor ultrafast processes such as the movement of electrons or atoms. Researchers use laser pulses that last only a few femtoseconds or attoseconds to virtually capture a snapshot of the current state. However, such laser techniques lack spatial resolution: they show a snapshot of an electron, so to speak, against a blurred background. “An atomic resolution on the time scale of femto- or attoseconds has so far been out of reach,” stated Manish Garg and Klaus Kern from the Max Planck Institute for Solid State Research in Stuttgart.

A microscope for quantum processes

However, Garg and Kern have now developed a technology that achieves exactly this goal. The two physicists use a combination of two proven methods – ultra-short laser flashes and a scanning tunneling microscope. For this purpose, specially tuned and focused infrared laser pulses of less than six femtoseconds in length are aimed at the tip of the microscope. This lowers the barrier to tunneling the electrons and at the same time enables the current position and state of the electrons to be read from subtle fluctuations in the tunnel current. This enables the scientists to measure down to a few hundred attoseconds when electrons are where – and this down to an atom. “By combining a scanning tunneling microscope with ultrafast pulses, we conveniently used the advantages of the two methods to compensate for their respective disadvantages,” says Garg.

There are some practical applications for the new technology, as the scientists explain. “Being able to film electrons live in molecules, in their natural spatial and temporal order, is crucial to understand, for example, the chemical reactivity and the conversion of light energy in charged particles such as electrons or ions,” explains Kern. This allows you to track, for example, what is happening in the tiny components of modern electronics and thus optimize the processes. “In today’s computers, electrons vibrate at a frequency of one billion Hertz,” says Kern. “With ultrashort flashes of light, their frequency can possibly be increased to a billion hertz.” This means that the new ultrafast microscope can not only film processes in the quantum world, but also intervene in these processes.

Source: Manish Garg and Klaus Kern (Max Planck Institute for Solid State Research, Stuttgart), Science, doi: 10.1126 / science.aaz1098

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