The most basic of all physical interactions in nature is that between light and matter. This interaction takes place in attosecond times (billionths of a billionth of a second). What exactly happens in such an astonishingly short time has so far remained largely inaccessible. Now a research team led by Dr Peter Baum and Dr Yuya Morimoto at LMU Munich and the Max Planck Institute for Quantum Optics has developed a mode of electron microscopy, which has enabled them to observe this fundamental interaction in real time and real space.
To visualise phenomena that occur on the attosecond scale, a method is needed that keeps pace with the ultrafast processes at a spatial resolution on the atomic scale. To meet these requirements, Baum and Morimoto made use of the fact that electrons, as elementary particles, also possess wave-like properties and can behave as so-called wave packets.
The researchers directed 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 lasting around 800 attoseconds.
For the purposes of microscopy, these electron pulse trains have one big advantage over sequences of attosecond optical pulses: They have a far shorter wavelength. Therefore, they can be employed to observe particles with dimensions of less than 1 nanometre, such as atoms. These features 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, the researchers turned their attosecond pulse trains on a silicon crystal, and observed how the light cycles propagate and how the electron wave packets were refracted, diffracted and dispersed in space and time.
In future, this concept will allow the researchers 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 to follow what happens during subatomic interactions with even higher precision. This method could find application in the development of metamaterials – artificial, or 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. These metamaterials may well serve as basic components in future light-driven computers.