Attosecond Camera for Nanostructures

When laser light interacts with a nanoneedle (yellow), electromagnetic near fields are formed at its surface. A second laser pulse (purple) ejects an electron (green) from the needle, which can be used to characterize the near fields. (Source: C. Hackenberger)

When laser light interacts with a nanoneedle (yellow), electromagnetic near fields are formed at its surface. A second laser pulse (purple) ejects an electron (green) from the needle, which can be used to characterize the near fields. (Source: C. Hackenberger)

When light strikes a metal, its electro­magnetic field excites vibrations of the electrons in the metal. This interaction results in the formation of so-called near fields – electromagnetic fields that are localized close to the surface of the metal. Precisely how such near fields behave under the influence of light has now been inves­tigated by an inter­national team of physicists at LMU Munich and the Max Planck Institute for Quantum Optics MPQ, in close colla­boration with researchers at the Chair of Laser Physics at the Friedrich-Alexander-Univer­sität Erlangen-Nürnberg.

The researchers focused intense infrared laser pulses onto a gold nano­needle. These pulses are so short that they consist of only a few oscil­lations of the light field. When the light impinges on the nanowire it excites collective vibrations of the electrons associated with the gold atoms near the surface of the wire. These electron motions are responsible for the generation of near fields at the surface of the wire.

To study the timing of the near field’s response to the light field, the physi­cists directed a second light pulse with an extremely short duration of just a couple of hundred atto­seconds onto the nano­structure very shortly after the first light pulse. This second flash actually detaches some electrons from the nanowire. When they reach the surface, they are accelerated by the near fields and can be detected, allowing the dynamics of the near fields to be charac­terized. Analysis of these electrons showed that the near fields were oscillating with a time shift of about 250 attoseconds with respect to the incident light, and that they were leading in their vibrations. In other words, the near-field vibra­tions reached their maximum amplitude 250 atto­seconds earlier than the vibrations of the light field.

“Fields and surface waves generated in nano­structures are of central importance for the development of opto-elec­tronics. With the imaging technique we have demonstrated here, they can now be sharply resolved,” explains Matthias Kling, the leader of the Ultrafast Nano­photonics group in the Department of Physics at LMU. The expe­riments pave the way for more complex studies of light-matter inter­actions in metals that are of interest for nano-optics and the light-driven elec­tronics of the future. Such electronics would work at the fre­quencies of light. Optical fields oscillate at rates of a million billion times per second, i.e. with petahertz fre­quencies – about 100,000 times faster than the clock frequencies attainable in conven­tional elec­tronic devices. (Source: LMU / MPQ)

Reference: B. Förg et al.: Attosecond nanoscale near-field sampling, Nat. Comm. 7, 11717 (2016), DOI: 10.1038/ncomms11717

Links: Ultrafast Nanophotonics Group (M. Kling, F. Krausz), Department of Physics, Ludwig-Maximilians-University, Munich, Germany • Max Planck Institute of Quantum Optics, Garching, Germany

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