Beamline Description

Source Chamber The TeraFERMI source chamber intercepts the spent electron beam, before entering into the main beam dump. The chamber provides the possibility to operate different CTR screens, notably squared (5x5 cm^2) Si screens coated with 1000 nm of Al, or a free standing circular Al foil (1000 nm). It is also be possible to use two Si screens separated by an adjustable gap (1-5 mm), in order to use the chamber as a Coherent Diffraction Radiation (CDR) source. The screens are oriented with a fine-tunable 45° angle, with respect to the electron beam.  The TeraFERMI source chamber is also equipped with a YAG diagnostics screen, in order to precisely determine the electron beam position at source. A diamond window, with 20 mm clear aperture is located at 75 mm from the center of the screens. The diamond window separates the UHV of the source chamber, from the low-vacuum of the remaining part of the beamline.

The TeraFERMI source chamber is located between the multiscreen diagnostic (shown in yellow in the figure) and the beam dump (white). A sketch of the beginning of the TeraFERMI beamline is also shown in blue in the left figure.

Optical Layout

The THz light will then be propagated in the FERMI experimental room, which is located more than 20 meters far from the source, and separated by two shielding walls for radiation safety. Because of its large divergence properties, a collimated THz beam can not be propagated along distances of ten's of meters. For this reason, the TeraFERMI beamline employs 6 focusing and 4 mirrors which are thus allowing to keep the beam size within the optical pipes dimensions. With the present layout, the overall beamline length is of about 33 meters.





 

The propagation properties of the THz beam have been simulated with the help of the THzTransport code. THzTransport is a code based on scalar diffraction theory, developed at DESY in the group of B. Schmidt. The code calculates the complex electric field at any plane perpendicular to the optical axis, thus allowing to propagate the light emitted from a CTR source, throughout the whole set of 10 mirrors constituting the TeraFERMI beamline. An example of propagation at a frequency of 0.3 THz is provided in the figure below (simulation by C. Svetina).

The simulation shows that the efficiency of the beamline increases with frequency, up to 0.4-0.5 THz, and then saturates at very high transmission properties. This is a consequence of the decreased beam divergence and diffraction at the shorter wavelength. Above 0.5 THz (corresponding to 600 micrometer wavelength), diffraction effects become negligible with respect to the mirror size, and more than 90% of the radiation collected by the first mirror is transmitted throughout the beamline.

Experimental station

We presently offer to our users three different type of set-ups.

The fluence dependent set-up allows adressing the non-linear transmission or reflection properties of the materials of interest as a function of the THz fluence. The fluence can controlled over more than 3 orders of magnitude through a set of polarisers. The detection of the transmitted or reflected signal is performed through electro-optic sampling.

Our THz-pump/IR-probe set-up has been recently renewed. Here in particular non-equilibrium physics such as charge-carrier dynamics as well as nonnlinear effects like the Pockels and Kerr effects can be studied. Thereby NIR probe pulses have a 10 nm bandwidth centered at either 780 or 1560 nm. We designed a pump-probe setup whit which we can switch straightforward between two types of experiments: (i) a polarization state sensitive measurement of the NIR (THz-induced anisotropy) and (ii) a pure NIR intensity measurement probing the THz-induced change in reflection and transmission. The most interesting feature in this two-color pump-probe setup is a simultaneous data acquisition of reflection and transmission signals. Consequently, we gain the full information about the material's optical properties.

THz pump/THz probe experiments are also possible, by splitting the pulses from TeraFERMI and by recombining them on the sample after a suitable control of their intensity, polarisation, spectral content, and delay, thus allowing for many different class of experiments.