HELIOS - High Energy Laser Induced Overtone Source
In the HELIOS laboratory we are studying the temporal evolution of electronic processes on an atomic and/or molecular scale, these experiments are typically called time-resolved or pump-probe experiments.
In a pump-probe experiment, first a pump-pulse is used to somehow perturb a sample. Thereafter, a second photon-pulse is used after a variable time to probe how the initial perturbation has developed over time. It probes how the pump-pulse has interacted with the sample, hence it is called probe-pulse. Much of the high-time-resolution pump-probe spectroscopy in the world today is (and has been) using two laser pulses which in turn gives extremely high temporal resolution and also molecular information.
In the HELIOS laboratory we have chosen a slightly different path. We use a commercial laser (pulsed with < 35 fs pulse duration) to generate ultrashort pulses of Extreme Ultraviolet radiation (XUV) as a probe pulse. This whole technique is called High Harmonic Generation (HHG) and leads to a XUV pulse of about < 20 fs. To get a feeling for how short the femtosecond time-scale is, one can think about the following comparison. 1 femtosecond relates to 1 second in the same way that 1 second relates to the 32 million years. These timescales are almost impossible to imagine.
The XUV photons produced at HELIOS have a tremendous advantage over "traditional laser light", namely their energy, which is in the region of 20 eV to 72 eV. This allows doing reasonable photoelectron spectroscopy which is very difficult with traditional laser-laser pump-probe. Additionally, one can sometimes use the different properties of the electron shell of different elements to tell them apart from other elements in a spectrum. This allows following a process started by the pump pulse with element specificity.
Today HELIOS is a working setup. We generate XUV photons on a daily basis and we have shown that the pump-probe aspect of HELIOS is working as expected.
We aim to cover a vast array of experimental fields, including
- Electron dynamics in liquid environment
- Ultrafast (de)magnetization
- Electron dynamics in energy-relevant materials, e.g. solar-cells and batteries
- Transient ARPES (Angular Resolved Photoelectron Spectroscopy)
- Artificial photosynthesis
- Fundamental physics (gas-phase dynamics)
HELIOS is built to accommodate several types of experimental chambers with different spectrometers as well as samples and vacuum requirements. Since we said that HELIOS is routinely producing XUV photons we would love to share a figure of this, in the figure below you can see a typical HHG spectrum
Below a schematic drawing of the entire experimental setup is shown. In the two foci it is possible to mount any experimental station that physically fits in.
Presently, two experimental stations are available at HELIOS. One being an in-house developed reflectometer primarily used for studies of magnetic materials (time resolved T-MOKE measurements). The other will soon be equipped with a semi-permanently mounted ARTOF electron spectrometer used for angular and time-resolved photoelectron spectroscopy of solid materials in ultra-high vacuum (UHV) as well as for measurements on gases.
HELIOS is a powerful probing instrument used by many researchers in the division; however there is a core team of members which are more dedicated to the operation of HELIOS as an experimental facility. Over the years members have joined us and later finished. Below is a list of some of the key individuals involved:
Main responsible for the HELIOS lab.
Joachim Terschlüsen (PhD student)
Robert Stefanuik (post doc)
Somnath Jana (post doc)
Stefan Plogmaker (former PhD student and later on post doc)
Nils Krebs (former post doc)
Mathias Svanquist (former post doc)
Undergraduate students at HELIOS (master projects etc.)
Are you interested in an undergraduate research project at HELIOS? Do not hesitate to contact Johan Söderström. Below is a list of ongoing and completed projects at Helios:
Extending the pump energy range for a pump-probe system using High Harmonic Generation
Work in progress
If you are interested in working with HELIOS, either as a researcher or student do not hesitate to contact Johan Söderström. If you are in the vicinity of Uppsala you can also visit the HELIOS lab at Ångström - house 6 floor 0.
Constructing and Commissioning HELIOS – A High Harmonic Generation Source for Pump-Probe Measurements with sub 50 fs Temporal Resolution: The Development of Experimental Equipment for Extreme Ultraviolet Spectroscopy
Doctoral thesis - Joachim. A. Terschlüsen
Electronic structure dynamics in a low bandgap polymer studied by time-resolved photoelectron spectroscopy
Phys. Chem. Chem. Phys., 2016,18, 21921-21929
Ute B. Cappel, Stefan Plogmaker, Joachim A. Terschlüsen, Torsten Leitner, Erik M. J. Johansson, Tomas Edvinsson, Anders Sandell, Olof Karis, Hans Siegbahn, Svante Svensson, Nils Mårtensson, Håkan Rensmo and Johan Söderström
HELIOS—A laboratory based on high-order harmonic generation of extreme ultraviolet photons for time-resolved spectroscopy
Rev. Sci. Instrum. 86, 123107 (2015); http://dx.doi.org/10.1063/1.4937463
S. Plogmaker, J. A. Terschlüsen, N. Krebs, M. Svanqvist, J. Forsberg, U. B. Cappel, J.-E. Rubensson, H. Siegbahn and J. Söderström
Measuring the temporal coherence of a high harmonic generation setup employing a Fourier transform spectrometer for the VUV/XUV
Nuclear Instruments and Methods in Physics Research Section A, Volume 768, 21 December 2014, Pages 84–88
J.A. Terschlüsen, , M. Agåker, M. Svanqvist, S. Plogmaker, J. Nordgren, J.-E. Rubensson, H. Siegbahn, J. Söderström
Techniques and Application of Electron Spectroscopy Based on Novel X-ray Sources
Doctoral Thesis - Stefan Plogmaker