An attosecond electron microscope has been developed by physicists from Ludwig-Maximilians-Universitaet (LMU) in Munich, which allows them to visualize the dispersion of light in time and space, and witness the motions of electrons in atoms.
The most fundamental of all physical interactions in nature is that between matter and light. This interaction occurs in attosecond times (i.e. billionths of a billionth of a second). What precisely happens in such an amazingly short time has thus far remained essentially inaccessible. Currently a research team headed by Dr. Peter Baum and Dr. Yuya Morimoto at LMU Munich and the Max Planck Institute for Quantum Optics (MPQ) has created a new mode of electron microscopy, which enables one to see this central interaction in real space and real time.
To visualize marvels that occur on the attosecond scale, such as the interaction between light and atoms, one requires a technique that keeps pace with the ultrafast processes at a spatial resolution on the atomic scale. To match these requirements, Baum and Morimoto make use of the fact that electrons, as elementary particles, also have wave-like properties and can act as so-called wave packets. The researchers direct a beam of electrons onto a thin, dielectric foil, where the electron wave is moderated by irradiation using an orthogonally oriented laser. The interaction with the oscillating optical field alternately quickens and slows the electrons, which results in the formation of a train of attosecond pulses. These wave packets have approximately 100 separate pulses, each of which lasts for around 800 attoseconds.
Monitoring ultrafast processes
For the reasons of microscopy, these electron pulse trains have one significant benefit over sequences of attosecond optical pulses: They have a much shorter wavelength. They can thus be used to see particles with dimensions of less than 1 nm, such as atoms. These feature make ultrashort electron pulse trains an ideal tool with which to monitor, in real time, the ultrafast processes started by the impact of light oscillations onto matter.
In their initial two experimental tests of the new technique, the Munich researchers turned their attosecond pulse trains on a silicon crystal, and were able to view how the light cycles propagate and how the electron wave packets were refracted, diffracted, and dispersed in time and space. In the future, this concept will allow them to measure directly how the electrons in the crystal act in reaction to the cycles of light, the main effect of any light-matter interaction. Simply put, the procedure reaches sub-atomic and sub-light-cycle resolution, and the physicists can now observe these key interactions in real time.
Their subsequent goal is to make single attosecond electron wave packets, so as to follow what occurs during subatomic interactions with even higher precision. The new technique could find application in the creation of metamaterials. Metamaterials are artificial, i.e. engineered nanostructures, whose magnetic permeability and electrical permittivity diverge greatly from those of conventional materials. This in turn causes the unique optical phenomena, which open up unique perspectives in optoelectronics and optics. Indeed, metamaterials may well serve as regular components in light-driven computers of the future.