A Space-Time Sensor for Light-Matter Interactions

In this image, patterns captured at attosecond intervals have been superimposed, thus revealing, in real time, the kind of electron motions that underlie atomic and subatomic phenomena. (Source: Baum & Morimoto, LMU)

The most basic of all physical inter­actions in nature is that between light and matter. This inter­action takes place in atto­second times. What exactly happens in such an asto­nishingly short time has so far remained largely inacces­sible. Now a research team led by Peter Baum and Yuya Morimoto at LMU Munich and the Max Planck Insti­tute for Quantum Optics MPQ has developed a new mode of electron micro­scopy, which enables one to observe this funda­mental inter­action in real time and real space.

To visualize phenomena that occur on the atto­second scale, such as the inter­action between light and atoms, one needs a method that keeps pace with the ultrafast processes at a spatial reso­lution on the atomic scale. To meet these require­ments, Baum and Morimoto make use of the fact that electrons, as elemen­tary particles, also possess wave-like properties and can behave as wave packets. The researchers direct a beam of electrons onto a thin, dielec­tric foil, where the electron wave is modulated by irra­diation with an ortho­gonally oriented laser. The inter­action with the oscil­lating optical field alter­nately acce­lerates and decele­rates the electrons, which leads to the formation of a train of atto­second pulses. These wave packets consist of approxi­mately 100 individual pulses, each of which lasts for about 800 atto­seconds.

For the purposes of micro­scopy, these electron pulse trains have one great advantage over sequences of atto­second optical pulses: They have a far shorter wave­length. They can therefore be employed to observe particles with dimen­sions of less than 1 nanometer, such as atoms. These feature make ultra­short electron pulse trains an ideal tool with which to monitor, in real time, the ultrafast processes ini­tiated by the impact of light oscil­lations onto matter.

In their first two experi­mental tests of the new method, the Munich researchers turned their atto­second pulse trains on a silicon crystal, and were able to observe how the light cycles propagate and how the electron wave packets were refracted, dif­fracted and dispersed in space and time. In the future, this concept will allow them to measure directly how the electrons in the crystal behave in response to the cycles of light, the primary effect of any light-matter inter­action. In other words, the procedure attains sub-atomic and sub-light-cycle reso­lution, and the physicists can now monitor these funda­mental inter­actions in real time.

Their next goal is to generate single atto­second electron wave packets, in order to follow what happens during subatomic inter­actions with even higher precision. The new method could find application in the development of meta­materials. Met­amaterials are artificial, i.e. engi­neered nano­structures, whose electrical permit­tivity and magnetic permea­bility diverge signi­ficantly from those of conven­tional materials. This in turn gives rise to unique optical phenomena, which open up novel per­spectives in optics and opto­electronics. Indeed, meta­materials may well serve as basic compo­nents in future light-driven computers. (Source: LMU)

Reference: Y. Morimoto & P. Baum: Diffraction and microscopy with attosecond electron pulse trains, Nat. Phys., online 29 November 2017; DOI: 10.1038/s41567-017-0007-6

Link: Ultrafast Electron Imaging, Ludwig-Maximilians-University Munich, Garching, Germany

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