The IR and THz user facility FELBE

The FELBE user facility located at the Helmhotz-Zentrum Dresden-Rossendorf operates two free-electron lasers (FELs). Here we discuss the basic parameters of the FELs and the experimental opportunities at the facility. The FELs are based on the superconducting electron linear accelerator ELBE, which provides short (picosecond) electron bunches with energies up to 35 MeV at a 13 MHz repetition rate. The two FELs of FELBE (FELBE stands for FEL@ELBE) are equipped with two undulators, one for the mid-infrared spectral range (wavelengths 4 – 21 µm) and one for the far-infrared or THz range (wavelengths 18 – 250 µm).
The key feature which distinguishes FELBE from other FEL user facilities is the possibility of “quasi cw” operation (meaning a continuous train of pulses, also called micropulses), made possible by the superconducting accelerator cavities. The FEL thus provides picosecond optical pulses at a repetition rate of 13 MHz. In this mode, the average power can reach up to 30 W (depending on the wavelength) corresponding to more than 1 µJ pulse energy. Additionally FELBE can be operated in a macrobunch mode and, via pulse-picking, a 1 kHz mode.
The two FELs can be synchronized to a number of tabletop femtosecond and picosecond lasers, enabling two-color experiments from the near-infrared to the THz frequency range. The main techniques at FELBE are pump-probe spectroscopy [1-3] and time-resolved photoluminescence [4]. Furthermore there is a lab devoted to near-field microscopy [5]. Spectroscopy with FELBE radiation is also possible in pulsed high magnetic fields up to 70 T (150 ms magnetic pulse duration) [6].
At the ELBE accelerator a new electron beamline, providing for femtosecond electron bunches with up to nC bunch charges and repetition rates in the 1-200 kHz regime has been constructed and is corrently tested. The electrons will be used for the generation of broad band and narrow bandwidth coherent THz pulses in the frequency range between 0.1 THz – 3 THz. The current status of this project is presented.
FELBE is operated as a user facility, i.e., scientists from other institutions are invited to submit short research proposals and apply for beamtime.

References

[1] D. Stehr et al., Appl. Phys. Lett. 92, 051104 (2008).
[2] E.A. Zibik, et al., Nature Mat. 8, 803 (2009).
[3] S. Winnerl et al., Phys. Rev. Lett. 107, 237401 (2011).
[4] J. Bhattacharyya et al., Rev. Sci. Instr. 82, 103107 (2011).
[5] S.C. Kehr et al., Nature Comm. 2, 249 (2011).
[6] O. Drachenko et al., Phys. Rev. B 79, 073301 (2009).