High-field THz cryo chamber (THC)

The THC is a chamber optimized for high-field THz excitation and (resonant) x-ray diffraction at sample temperatures between 5 and 500 K. Directly connected to the beamline vacuum and featuring Beryllium window and in-vacuum detector options, the chamber covers the full (2–12.7) keV energy range of the femtosecond x-ray pulses available at the Bernina endstation. For design details, please refer to the following publication:

The figure above shows the experimental geometry and the sample chamber as seen from top. The sample is mounted on the cold finger of a helium flow cryostat, which is motorized outside vacuum by a 6 degree of freedom hexapod (coordinate system x_h, y_h, z_h), mounted to a large rotation stage used for the main sample rotation η around the z_h-axis. All sample orientation and diffraction angles are defined according to a frequently used convention (You, H 1999 J. Appl. Crystallogr. 32 614–23 https://doi.org/10.1107/S0021889899001223 [71]), except for ν, which was replaced with γ to prevent mix-ups during the beamline control due to similarities between the detector and sample motions ν (nu) and μ (mu).

THz sample holder

Experiments using THz excitation are usually done at high incidence angles close to normal incidence. The laser pump pulses enter the chamber through an exchangeable window and are focused onto the sample with a 50mm diameter, 50mm focal distance parabolic mirror. The x-ray pulses are pass through a hole in the parabolic mirror and diffracted upwards from the sample. The 800 nm pulses used for characterization of the THz field by electro-optic sampling (EOS) take the same path as the x-rays and exit through the downstream window of the chamber.

The incidence angle can be varied from 0 to 90 degrees with the high precision μ rotation stage. To align the crystal axis with respect to the THz polarization vector and to access additional diffraction peaks, the sample can be rotated manually around its surface normal by 360° (ϕ) using a wobble stick.

Grazing incidence sample holder

Experiments using shorter pump wavelengths often require a non-collinear geometry to match the pump probe penetration depth. For these experiments, a different sample holder (shown above on the right hand side) is used to mount the sample. In this geometry, the sample surface is aligned to the large eta rotation stage. The diffraction conditions are reached using the eta rotation stage, keeping the x-ray incidence angle μ fixed to typically (0.2-1.5)°. The incidence angle can be varied using a big rotation stage, which moves the eta stage, hexapod and cryostat all together, keeping the chamber fixed. The pump laser incidence angle can be varied between (10-30)° using a prism, which replaces the parabola used for THz experiments.

Motorized range of motion

The motion ranges are summarized in the table below for the two different sample holders. Note that the motions of the hexapod  (x_h, y_h, rx_h, ry_h) are not independent of each other, limiting the individual ranges for large offsets.

For tender x-ray energies, two in-vacuum detector options are available for the THz chamber. The first detector is mounted into a flange 42mm above the sample. The second option is a motorized detector with 180mm distance to the sample. For higher x-ray photon energies, a 250um thick beryllium window offers highest flexbility. With the detector mounted on a robot arm, the detecor-sample distance can be varied. Both, the beryllium window and the fixed in-vacuum detector are mounted onto a CF250 flange wih a lateral offset from its center, increasing the range of acessible diffraction angles. Rotations require venting the chamber.

Please use the following online tool to calculate the accessible diffraction angles for the three detector options and different rotation angles of the Be window and fixed in-vacuum detector. A picture of the calculator is shown below.