Free-electron lasers for THz and IR radiation

Since 2016, the FLARE laser from the FELIX Laboratory is connected to the HFML. This combination of intense THz and IR radiation with high magnetic fields allows the set-up of a dedicated HFML-FELIX research team tasked with exploiting its world-unique status.

The HFML-FELIX research group uses THz spectroscopy to study the behavior of low-energy excitations in high magnetic fields, for instance paramagnetic, antiferromagnetic, and cyclotron resonance. Currently, magnetotransmission measurements in the Faraday and Voight configurations are offered to users, as well as (limited) reflectivity. The HFML-FELIX group is led by Hans Engelkamp

High power and pump-probe
All the beamlines between FELIX and HFML are functional. Two important lines of research are set out now: magnetic resonance and pump-probe experiments. The unique tunability of the FELIX lasers is a big advantage for the resonance experiments, as it allows to study resonance properties of solids over a unique range of frequencies (7-3600 cm-1, 0.25-120 THz, 2.7-1500 micron) and magnetic fields (up to 33 Tesla).

Another advantage of the FELIX lasers is the huge power of the radiation they provide. Such high-power irradiation of a sample may change intrinsic properties of the material. For some systems it is straightforward to compare their physical properties before, during, and after illumination. ‘The pulsed structure of the FELIX radiation creates a perfect playground for such measurements’, says postdoctoral researcher Dmytro Kamenskyi. ‘For example, we have successfully realized photoluminescence measurements synchronized with THz pumping, by studying the evolution of the photoluminescence signal after perturbation by the THz pulses.’ The next step in this direction is the design of transport pump-probe measurements synchronized with FELIX pulses.

Currently, one of the key projects of the group is the realization of a pump-probe experiment. Kamenskyi: ‘For this, we split the FELIX beam into two: one beam with a very high intensity, and one with a small fraction of that intensity. The intense pulse passes through the sample and its interrupting effect changes the samples properties. The weak pulse, also called the probe beam, is used to investigate the new properties of the sample after the reaction initiated by the intense pulse.’ A delay line for the probe beam allows for studying the relaxation of the system into its original state. However, the realization of such an experimental setup is challenging since restricted geometry is available in the Bitter magnet.

Apart from the high-power, single-frequency FELIX radiation in combination with a sweeping magnetic field, HFML also offers fixed field, low-power broad-band spectroscopy using a Bruker ifs113v FT spectrometer to quickly map out the modes of interest. ‘The Bruker spectrometer can be used for preliminary experiments, to check if the HFML-FELIX combination is really the way to go for the desired experiment’, Kamenskyi explains. ‘Since the combined set-up is challenging in use, we need to develop the most efficient way to achieve the aim of the desired experiment.’

Use the set-up 
Combining the radiation of the FELIX lasers with the continuous high magnetic fields of the HFML creates exiting new opportunities. However, not all desired experiments are feasible. When you want to use the set-up, your plans have to be complete, detailed, and it needs to be 100% obvious why their desired experiment should be performed here and not anywhere else in the world. The dedicated installation is quite expensive to use and we are still working on having it running perfectly.

Therefore, it is essential to contact Hans Engelkamp of the HFML-FELIX team before submitting an application for beam and magnet time. In this way, they can advise and assist you on your project plan and increase the chance of success. For more information on the application process for access to these facilities, see www.ru.nl/hfml/facility/access_to_the/.