In an electron microscope, electrons are emitted by pointy metal tips, that way the can be steered and controlled with high precision. Recently, such metal tips have also been used as high precision electron sources for generating x-rays. A team of researchers at TU Wien (Vienna), together with colleagues from the FAU Erlangen-Nürnberg (Germany), have developed a method of controlling electron emission with higher precision than ever before. With the help of two different laser pulses it is now possible to switch the flow of electrons on and off on extremely short time scales.
"The basic idea resembles a lightning rod," says Christoph Lemell (TU Wien). "The electrical field around a needle is always strongest right at the tip. That's why the lightning always strikes the tip of a rod, and for the same reason, electrons leave a needle right at the tip." Extremely pointy needles can be fabricated with the methods of modern nanotechnology. Their tip is just a few nanometres wide, so the point at which the electrons are emitted is known with very high accuracy. In addition to that, it is also important to control at which point in time the electrons are emitted.
This kind of temporal control has now become possible, using a new approach: "Two different laser pulses are fired at the metal tip," explains Florian Libisch (TU Wien). The colours of these two lasers are chosen such that the photons of one laser have exactly twice the energy of the other laser's photons. Also, it is important to ensure that both light waves oscillate in perfect synchronicity.
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