The speed at which the electronic properties of a system can change is of great importance for the development of devices with ultra-fast response times. It is thus no surprise that time resolved probes are become an increasingly important tool in materials science. Time and angle resolved photoemission spectroscopy is one such tool, as it combines the capabilities of ARPES to map the electronic band structures of materials, with a femtosecond probe. Researchers based in Germany and the US have now managed to extend this technique into the extreme ultraviolet range, such that it is possible to study large areas of reciprocal space [Rohwer et al., Nature (2011) 471, 490].
The team has looked at 1T-TiSe2, a layered compound that possesses a charge-density wave below 200 K. In this phase, a charge modulation is present with a periodicity of double the unit cell in all three real-space directions. By combining the new probe with an infra-red laser pulse to heat the sample, the researchers attempted to answer the question, how fast does it take for the long-range charge order to melt in the sample?
As ARPES allows the electronic band structure to be mapped, previous studies have already observed significant differences between the charge ordered and non-charge ordered phases. At room temperature the distinct Se 4p and Ti 3d electronic bands are clear at different positions in reciprocal space; however, in the charge ordered phase the strong interaction between the two bands is clear, as the Se 4p band dominates the map in both positions.
The team was able to create a movie of the evolution of the electronic structure, after the heating pulse, by using stroboscopic imaging. They found that as they increased the intensity of the heating pulse, the charge order melted more quickly. Remarkably, for the most intense pulse, the charge order disappeared after just 20 femtoseconds.
Prof Michael Bauer explained how this study was able to answer this question, while previous experiments had not, “The most favorable techniques to monitor the melting of periodic long range order are time-resolved diffraction techniques such as time-resolved electron diffraction and time-resolved x-ray diffraction. As yet, these techniques are limited in their time-resolution and can monitor dynamics not faster than 100 femtoseconds or so. Time-resolved photoemission with femtosecond XUV pulses is in this context a more indirect probe, providing, however, a time-resolution of the order of 20 fs.”
The team is now hopeful that this technique can now be used to learn more about the ultrafast dynamics of a range of systems.


Stewart Bland