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Attosecond science with a metal nanotip

By Michael Förster (26.02.2013)

Nanotechnology has become crucial in science as well as daily life. In medicine, biology, and electronics tiny structures can routinely be fabricated and manipulated - arguably miniaturization has successfully been pushed to a limit. On the other hand, the advent of ultrafast laser technology has brought about the ability to observe and manipulate charged particles on a time scale typical for the movement of nuclei and even electrons in molecules and crystals. In my research group we aim at combining the best of both worlds to obtain high temporal and spatial resolution in one system. Taking advantage of recent developments, we study and control electron dynamics on the temporal scale of attoseconds (10-18 s) and the spatial scale of nanometers (10-9 m) at the surface of a sharp metallic tip.

We produce our samples by electrochemical etching of a metal wire, using materials such as tungsten or gold. After inserting them into a chamber at ultra-high vacuum (UHV), we apply a high voltage to the tip to clean the surface at the apex on an atomic level and ensure good surface quality in-situ by means of field ion microscopy. Tips produced in such a scheme are very sharp: They end in a hemisphere with a radius of curvature on the order of 10 nm.

On the laser side we generate pulses, in which the electric field only oscillates a few times in a well-controlled manner, so-called carrier-envelope-phase-stable few-cycle pulses. A high degree of control is essential at this point, as our experiment is extremely sensitive with respect to even small changes of the laser electric field. To be able to investigate the influence of parameters such as intensity or wavelength we use several different laser sources to observe qualitatively new behavior. To combine laser and nanostructure, we direct the beam into the vacuum chamber with the tip and focus it onto the apex.

Because the tip is so tiny - at the apex much smaller than the wavelength of the lasers - the different optical properties of vacuum and tip material lead to a strong electric field close to the tip. In fact, this field is much higher than the field achievable with just the laser, i.e. field enhancement takes place. This does not only come in handy as it reduces requirements with respect to our lasers, but it also helps to prevent damage from the structure by rendering the use of stronger lasers dispensable.

In our experiment, electric fields at the surface of the tip are so strong that electrons leave the metal. Once they are emitted, they are subject to the strong near-field in the vicinity of the tip and, e.g., for a suitable temporal form of the laser pulse can be steered back to the surface. There they can scatter elastically and subsequently acquire high kinetic energies in the laser field.

Figure 1: Upon being stimulated by a laser pulse (red), the metal nanotip (grey) emits an electron wave packet (blue).  (© Christian Hackenberger)[Bildunterschrift / Subline]: Figure 1: Upon being stimulated by a laser pulse (red), the metal nanotip (grey) emits an electron wave packet (blue). (© Christian Hackenberger)

The signal in this experiment is the spectrum of electrons, i.e. a graph that indicates how many electrons have which energy, under different experimental conditions. My group was able to show that this spectrum depends strongly on the exact temporal shape of the laser pulse. Just by varying the seemingly insignificant parameter of the carrier-envelope phase (CEP), structures appear or disappear in the spectrum and the current of electrons with high kinetic energies can even be switched on or off. Due to this high sensitivity, it is possible to reverse the experiment and characterize the laser pulse by observing the electron emission from the tip. This may lead to an application as a simple and compact sensor for the CEP.

We will explore different directions in the future. On the fundamental physics side, we aim at a deeper understanding of the underlying processes and dynamics, e.g. by studying field enhancement in detail by combining theory and experiment. However, we also want to explore possible applications of nanotips, such as a source for ultrafast electron diffraction experiments or even a switch on the attosecond time scale.

M. Schenk, M. Krüger, and P. Hommelhoff, Strong-field above-threshold photoemission from sharp metal tips, Phys. Rev. Lett. 105, 257601 (2010).

M. Krüger, M. Schenk, and P. Hommelhoff, Attosecond control of electrons emitted from a nanoscale metal tip, Nature 475, 78 (2011).

M. Krüger, M. Schenk, M. Förster, P. Hommelhoff, Attosecond physics in photoemission from a metal nanotip, J. Phys. B. 45 074006 (2012).

C. Homann, M. Bradler, M. Förster, P. Hommelhoff, and E. Riedle, Carrier-envelope phase stable sub-two-cycle pulses tunable around 1.8 μm at 100 kHz, Optics Letters 37, 1673 (2012).

Scientific Career
  • since 2011
  • Doctoral studies at the Max Planck Institute of Quantum Optics, Garching, Germany.
  • 2009
  • Semester abroad at the University of Adelaide, Adelaide, Australia.
  • 2006 - 2011
  • Bachelor of Science and Master studies within FOKUS Physik, Julius-Maximilians-Universität, Würzburg, Germany.

Awards and Scholarships
  • 2012
  • Participant of the 62nd Nobel laureate meeting in Lindau
  • Since 2011
  • Fellowship within IMPRS-APS
  • Since 2006:
  • Fellowship within Max Weber-Programm Bayern