In an electron microscope, electrons are excited by sharp metal tips, so they can achieve high precision manipulation and control. Recently, such metal tips have also been used as a high-precision electron source for X-ray generation. A team of researchers from the Vienna Technical University (Vienna), in collaboration with researchers from the University of Erlangen-Nürnberg (Germany), has developed an electron-emitting method with higher precision control than in the past. With the help of two high power laser pointer pulses, it is now possible to achieve switching of the electron flow on a very short time scale.
“The basic idea is like a lightning rod,” says Christoph Lemell (from the Technical University of Vienna). “The strongest point of the electric field around the needle happens to be at the tip of the needle, which is why lightning always hits the tip of a rod, and for the same reason, the electrons stay on top of a needle” The needle may be prepared using modern nanotechnology methods. Their tip is only a few nanometers wide, so launching electrons at this point can achieve very high accuracy. In addition, it is important to control at which time point electrons are emitted.
Using a new approach, this time control becomes possible: “Two different burning laser pen pulses are emitted at the metal tip,” explained Florian Libisch of the Technical University of Vienna. The color of the two lasers is chosen so that the photons of such a laser have exactly twice the energy of the other laser photons. At the same time, it is also important to ensure perfect synchronism of the optical oscillations. With the help of computer simulations, the University of Vienna’s research team was able to predict that a tiny time lag between the two laser pulses could be used as an “electronic switch.” This prediction has been experimentally confirmed by the research team of Professor Peter Hommelhoff of the University of Erlangen-Nuremberg. On the basis of these experiments, it is now possible to learn more about the process.
When a 50mw laser pointer pulse is emitted at a metal tip, its electric field allows electrons to crack out of the metal, a well-known phenomenon. The new idea is that a combination of two different lasers can be used to control the emission of electrons on a femtosecond time scale.
There are different ways to make an electron get enough energy to leave the metal tip: it can absorb two photons from a high-energy laser or four electrons from a low-energy laser. Both mechanisms have the same result. “Like particles in a double-slit experiment, two different paths at the same time, electrons can participate in two different processes at the same time,” says Prof. Joachim Burgdörfer of the Technical University of Vienna. “Essentially, you do not have to choose one of two possibilities, both of which are equally authentic and interfere with each other.”
By carefully adjusting the two 100mw laser pointer, it is possible to control whether the two quantum physics processes are mutually magnified, which leads to an increase in electron emission or whether they cancel each other, which means that there is almost no electron emission. This is a simple and effective method of controlling electron emission. This is not only a new way to achieve high-energy electronic experiments, this new technology also opens the door to achieve the era of X-ray control. “The new X-ray sources have been implemented using narrow metal tip arrays as electron sources,” Lemell said. “Using our new approach, these nanotubes can be triggered in exactly the right way and produce coherent X-ray radiation.”
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