Beamline setup

Additional Setups: 

EUV TG experiments on solid samples, sufficiently thin for allowing transmission of the probe beam and with no need to determine the polarization state of the signal beam (e.g. for studying the thermoelastic response in isotropic media), can be carried out without no additional setups. In this case you have only to attach your samples in the sample holder, which provides a 30 x 200 mm² area. Please remember that the main requirements for samples are: thickness comparable with the absorption length of the EUV probe (for transmission measurements) and high reflectivity at the probe's wavelength (for reflection measurements); in both cases a high surface quality (say roughness << λ_pr) is important. A number of additional setups are also available, both for what concerns signal detection and sample environment, namely:

i) EUV TG signal detection in reflection geometry, which is needed to probe the surface response and enables using bulk samples.
ii) Polarization analysis of the EUV TG signal, which can be used for isolating the dichroic response and may be essential, e.g., for magnetic samples if the thermoelastic and electronic response is comparable with the magnetic one.
iii) A fluorescence microscope with a spatial resolution of about 0.8 μm (at a fluorescence wavelength of 550 nm), which can be used for structured illumination microscopy.
iv) A cryogenic setup for low temperature measurements (down to about 50 K); but here we warn the reader that the interaction with the EUV beam induces a substantial heating of the sample, which is normally larger at low temperatures.
v) An electromagnet able to provide an out of plane field up to ±70 mT and a setup based on permanent magnets able to provide a fixed field (up to 200 mT) with a variable angle in the scattering plane; other magnetic setups based on permanent magnets can be easily realized.

Many of these setups can be combined and are compatible with EUV TG experiments in transmission, which can thus be carried out during the same experimental run, but the available space for the samples can substantially reduce.

In the near future we are going to implement a setup for detecting the self-diffraction signal in reflection geometry, in order to perform surface spectroscopy in bulk samples. We are also happy to help users to develop their own setup, which may become part of our list of additional setups, so please contact us if you have special requirements or interesting idea.

The figure below displays a scheme of the setup (Panel a)) and a picture (Panel b)). The backward diffracted signal from the surface of the sample (green dashed line) is collected by a multilayer mirror (ML), which is designed to efficiently reflect the light at a given value of λ_pr and impinging at near normal incidence, typically 10° from the surface normal. The signal is thus folded into the detector (CCD). The sample should be slightly tilted (normally 10°) in order to allow the EUV TG signal to be emitted out of the scattering plane. The mirror ML is mounted on motorized stages in order to cover a large angular range, normally θ_sig = 0-30°; other ranges are possible. The system permits having two mirrors (ML1 and ML2) that can be swapped, therefore two values of λ_pr are possible in the same experiment. Replacing mirrors needs several hours. In certain circumstances we may add a second detector to collect in parallel both the transmitted and reflected signals.

The figure below displays a scheme of the setup (Panel a)) and a picture (Panel b)). The EUV polarizer simply consists in a multilayer mirror (ML) placed close to the Brewster angle, that directs the signal into a detector (labeled as CCD B). The probe pulse is linearly polarized orthogonally to the scattering plane, which is parallel to the floor. Therefore, a reflection in the vertical plane (as shown in the figure) can be used to isolate the component of the EUV TG signal beam having a linear polarization parallel to the scattering plane. Such a depolarized diffraction might arise from a modulation of the dichroic component of the complex refractive index, as expected, e.g., from a magnetization grating.
When we use the EUV polarizer we normally use an additional detector without polarizer (labeled as CCD A) to collect the total signal. These two detectors cannot work in parallel. In principle we may add a second polarizer and detect the linear vertical and linear horizontal components of the signal, instead of the linear horizontal component and the total signal.

The image below shows a sketch of the fluorescence microscope set-up. It consists in a high numerical aperture objective placed in the experimetal chamber to image the sample of interest. It is equipped with two rotations (θ,φ), to ensure parallelism of the objective plane and the sample of interest, and three translations for alignment (X, Y) and focusing (Z). The objective steers the image of the sample outside the experimental chamber where it is collected by a lens tube and focused on a camera. The set-up may accommodate optical filters to select the wavelength of interest; the Abbe resolution is about 0.8 μm at a fluorescence wavelength of 550 nm. The set-up has been employed to demonstrate structured illumination microscopy with extreme ultraviolet pulses, an approach that permitted of extending the resolution, i.e. the finest details that can be resolved, down to 0.27 μm. Modifications of the set-up or the realization of new set-ups for implementing other super-resolution techniques can be discussed.

A cryogenic set-up is available at the beamline. It consists in a continuous flow ST-400 cryostat mounted on a CF100 flange and equipped with a custom-made oxygen-free copper braid of 35 cm length and Lake Shore 336 temperature controller. The terminal side of braid, designed to be flexible, is conceived to be attached to the manipulator and hold samples than can be clamped on an area of about 30 x 40 mm². The braid natively supports experiment in reflection geometry, but special adapters can be designed to accommodate thin films and perform transmission experiments. The braid is screened with an Aluminum foil, connected to the cryostat shield that permits to decrease the losses. The cryostat can be operated with liquid Helium or Nitrogen depending on the required temperature at the sample position. Liquid Nitrogen permits to reach 120 K, while with Helium the minimum temperature can be decreased to about 50 K. The value of the temperature can be feedbacked both at the cold finger level or at the sample holder level; both temperatures are continuously monitored. The set-up is completed by a custom designed transfer line with 2.5 m flexible section capable to operate with liquid Helium Dewars of 500 L and a 100 L nNtrogen Dewar that can be refilled during the experiments. 

The picture below (Panel a)) displays a scheme of a conical electromagnet, provided by the Stockholm University (ref. Stefano Bonetti), specially designed for having a large clear aperture in both space (17 mm diameter) and angle (±60°). Please note that this is a bulky piece of equipment! This device can provide a variable magnetic field (H) up to ±70 mT, oriented along the bisector of the excitation beams, which is normally orthogonal to the sample plane.
The picture below (Panel b)) shows a picture of an assembly based of permanent magnets, able to provide a magnetic field (H) of about 200 mT, that lies in the scattering plane and has a variable angle (φ) with respect to the sample surface. The system can provide an angular range of 0-360°, however, depending on the specific configuration the pump and probe beams, the dimensions of the magnets can interfere with the beams, thus reducing the exploitable angular range. The setup is conceived for fields parallel to the sample surface or with a moderate tilt (φ from 0° to ≈ ±30°).