Beamline setup

Layout of the TIMER beamline

The TIMER beamline is an independent branch of the FERMI photon transport [1]. The beam exiting from the Photon Analysis, Delivery and Reduction System (PADReS; not shown) is deflected towards the TIMER beamline by a switching plane mirror (SM). Then the beam impinges into a plane mirror (M1) working as a wavefront-division beam splitter, i.e. it intercepts just half of the beam diameter. One half of the beam is reflected upwards while the other half can propagate freely. The latter beam is further split, in the horizontal plane, by a second wavefront-division beamsplitter (M2). The beam reflected by M1 will be used as probe pulse, while the two FEL beams emerging from M2 are the pump pulses and are focused at the sample position by a pair of toroidal mirrors (FM_A and FM_B). Four pairs of such mirrors can be inserted and removed from the beam paths in order to change the crossing angle (2θ), and hence the TG period (L_TG) and wavevector (k_TG):

L_TG = λ_exc / 2 sin(2θ/2)     ;     k_TG = 2π / L_TG 

where λ_exc is the pump wavelength. In the current configuration the possible values of 2θ are: 18.4°, 27.6°, 79° and 105.4°. The polarization of the two pumps can be varied, but not independently, unless a very special FEL setup is used. The possible values of λ_exc are instead related to the chosen wavelength for the probe (λ_pr), which are listed in the table below. Typically: (i) when the FEL2 source is used λ_exc = λ_pr / N, with N in the range 2-5; (ii) when FEL1 is used λ_exc = λ_pr*(N1 / N2), where N1 is fixed and N2 is in the range 4-15. The shortest value of L_TG achieved do far is 17 nm, values down to ~7 nm are in principle possible, shorter values are very unlikely. 
Solid-state filters can be used to regulate the intensity of the pump pulses: a set of filters (Filter-AB; see picture above) is placed downstream M2 and it acts on both pumps, two sets of filters(Filter-A and Filter-B) are placed downstream M2, along the two separate paths of Pump-A and Pump-B. The transmission of the filters is shown in the page "specifications", along with the transmision of the other relevant beamline components. Other filters can be installed upon request. In addition, the filtering options available in the PADReS system can also be used, including the gas attenuator but excluding the solid-state filters located in experimental hall. Please note that any filtering in the PADReS system acts on both pump and probe pulses.
The time delay between the two crossed FEL pulses can be adjusted in the ~5 ps range. Please note that this is not yet a calibrated degree of freedom, we used this delay only to precisely set the time coincidence between the two excitation pulses. If you need to use it as a variable in your experiment, an ad hoc commissioning is needed.

The third FEL pulse, reflected upwards by M1 and made coplanar to the pump trajectories by two additional plane mirrors (M3 and M4, not shown), goes through a delay line equipped with 4 multilayer mirrors (ML) at 45° incidence. Currently, one can select one out of 4 sets of 4 narrow-band MLs (designed to reflect the FEL radiation at λ_pr = 20.6, 16.6, 13.4 and 8.34 nm) that act as bandpass filters centered around λ_pr; the overall transmission of the delay line is typically low (see the page "specifications" for more details). We have other 3 sets of ML mirrors (λ_pr = 22.8, 17.8 and 6.7 nm) that can be installed upon request. Other wavelengths might be possible if users can provide their own set of ML mirrors. We greatly thank our colleagues from MBI-Berlin (ref. C. von Korff-Schmising) and MIT (ref. Riccardo Comin) for having provided, respectively, the ML mirrors at 8.34 and 16.6 nm. Separate filters for the probe pulse are not available at present, but can be installed upon request.
The probe beam is focused onto the sample by 4 insertable toroidal mirrors (FM_pr) at an angle of incidence (with respect to the surface normal) of θ_pr = 3.05°, 4.56°, 12.24° and 15.4°. Not all the combinations of 2θ and θ_pr are possible.
The maximum range in the probe's time delay depends on the chosen values of 2θ and θ_pr and varies from ~3 ns (smaller angles) to ~1 ns (larger angles). The polarization of the probe beam is linear vertical.

The TG signal is emitted at the angle (θ_sig) that satisfies the diffraction condition:

[(sin(θ_sig) - sin(θ_pr)] = λ_pr / L_TG 

The TG signal intensity quadratically depends on the amplitude of the pump-induced modulations of the complex refractive index at λ_pr, which is the dynamical variable we can detect, and by several factors (absorption, phase matching, etc.) that can be calculated [2]. The beamline is designed to optimize the phase matching conditions (Bragg diffraction) for λ_exc = 3λ_pr. The TG signal can be measured both in transmission and in reflection geometry, with the additional option of signal polarization analysis, which is particularly useful for disentangling magnetic dynamics from thermoelastic and electronic ones [3]; further details are in the page "additional setups", along with some detials on the sample environment. Concerning the samples, please consider that the main parameters to be considered for EUV TG experiments in transmission and reflection geometry are the sample thickness and the reflectivity at λ_pr, respectively [2-3].

The optical layout is designed to provide (fixed) focal spots with a spatial profile that somehow optimizes the overlap conditions (see sketch below). The dimensions of the beams vary from about 250 x 250 um2 to 80 x 80 um2 (the larger the angles and the larger is the focal spot size). The wavefront tilt and finite spot size introduces a smearing of the time resolution, which is limited to 50-100 fs.

Main parameters of the probe's delay line: central wavelength (λpr), maximum transmission (Tmax), bandwidth (Δλ/λ) and the some elemental absorption edges that fall within this range; the most popular magnetic edges are underlined. Please note that an EUV probe resonant with a proper absorption absorption edge is needed to detect electronic and magnetic dynamics, while thermoelastic dynamics can be probed also by a non resonant probe.     λpr [nm] Tmax Δλ/λ Absorption edges (elements) 22.8 0.086 0.065 Li K ; Fe M2,3 ; Se M4,5 20.6 0.078 0.06 Ti M1 ; Co M2,3 17.8 0.093 0.034 Br N4,5 16.8 0.15 0.08 Al L2,3 ; Cr M1 ; Cu M2,3 ; Ru N1 ; Pt N6,7 13.4 0.19 0.045 Fe M1 ; Ba N4,5 8.34 0.014 0.011 Si L1 ; Gd N4,5 6.7 0.078 0.007 -

Important note

The full set of probe's wavelengths cannot be exploited in a single beamtime, since 22.8 and 20.6 nm is in the range of FEL1, while 13.4, 8.34 and 6.7 nm are in the range of FEL2. 17.8 and 16.8 nm are the only wavelengths that are, under given circumstances, accessible with both sources.

If the use of the time-delayed EUV probe is waived, then the aforementioned limitations in the values of λ_excdrop and the capability of generating EUV TGs can be exploited for other experimental approaches. Recently, two of them were succesfully implemented, namely: resonant self-diffraction and nanoscale structured illimination microscopy. The latter type of experiment requires the detection of optical fluorescence, which is now possible thanks to a dedicated setup; see the page "additional setups".

The TIMER instrument is designed for EUV TG, however, it is obviously possible to remove mirror M2 and perform non-collinear FEL-pump/FEL-probe experiments, in an extended time delay range (up to a few ns) and with ultrafast time resolution, with the constraint of the limited probe's wavelength, which are determined by the transmission of the delay line; see the page "specifications".

Important note

All the experimental setup is in high vacuum (10^-7 mbar). The samples, sample environment and any other equipment needed for the experiment needs to be vacuum compatible. Please consider that changing the samples or any other item in the setup requires several hours for venting/pumping.

 

References:

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[1] Advances in instrumentation for FEL-based four-wave-mixing experiments
Ri. Mincigrucci, L. Foglia, D. Naumenko, E. Pedersoli, A. Simoncig, R. Cucini, A. Gessini, M. Kiskinova, G. Kurdi, N. Mahne, M. Manfredda, I. P. Nikolov, E. Principi, L. Raimondi, M. Zangrando, C. Masciovecchio, F. Capotondi, F. Bencivenga
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 907, pp. 132 - 148 (2018)
doi: 10.1016/j.nima.2018.03.051

[2] Extreme ultraviolet transient gratings: A tool for nanoscale photoacoustics
Foglia L., Mincigrucci R., Maznev A.A., Baldi G., Capotondi F., Caporaletti F., Comin R., De Angelis D., Duncan R.A., Fainozzi D., Kurdi G., Li J., Martinelli A., Masciovecchio C., Monaco G., Milloch A., Nelson K.A., Occhialini C.A., Pancaldi M., Pedersoli E., Pelli-Cresi J.S., Simoncig A., Travasso F., Wehinger B., Zanatta M., Bencivenga F.
Photoacoustics, Vol. 29, 100453 (2023)
doi: 10.1016/j.pacs.2023.100453 (Journal Article)

[3] Extreme ultraviolet transient gratings
F. Bencivenga, F. Capotondi, L. Foglia, R. Mincigrucci, C. Masciovecchio
Advances in Physics: X, Vol. 8 - 1, 2220363 (2023)
doi: 10.1080/23746149.2023.2220363 (Journal Article)
 

 

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.

 
EUV TG signal detection in reflection geometry.

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.

Polarization analysis of the EUV TG signal.

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.

Fluorescence microscope.

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.

Cryogenic setup.

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. 

 
Magnetic fields.

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°).

Beamline transmission:

Transmission of the pump branches of the beamline Plot of the beamline transmission for the pump branches, considering that pump A counts one reflection more than pump B due to the splitting mirror and assuming both splitting mirrors (M1 and M2) inerted.

Transmission of the probe branches of the beamline without the delayline Plot of the transmission of the probe beam without the delayline as function of wavelength, assuming that the splitting mirror of the probe (M1) is inerted.

Transmission of the delay line (current setup) Transmission of the delayline for the four currently mounted sets of multilayers tuned respectively at 8.34 nm, 13.4 nm, 16.8 nm and 20.8 nm. Note that the transmission at 8.34 nm is multiplied by a factor 10.

 

Intensity ratio between pump A and pump B Plot of the intesity ratio between pump A and pump B. Unbalanced intensities of the two pumps decreases the TG visibility and adds to it a spatially uniform distribution of light intensity; an intensity ratio of 0.5 reduces the visibility by about 10% and adds a 10% of uniform intensity, a ratio of 0.2 has an effect as large as 50%.

Possible TG periods in exemplificative cases Plot of the EUV TG period as a function of the excitation wavelength for given values of the crossing angle (2θ) and probe wavelength (λpr), this is our preferred choice for changing the TG period.

Transmission of the delay line (additional settings) Transmission of the delayline for the three currently dismounted sets of multilayers, tuned respectively at 6.7 nm, 17.8 nm and 22.8 nm. They can be installed upon request, but this cannot be done during a running experiment since it requires several days.

 

Filters along the pump A and pump B branchlines Transmission of the filters along the pump A and pump B branchlines, indicated as Filter-A and Filter-B in the beamline layout (see page "Beamline setup"). These filters are often used to compensate for the lower intensity in pump A, due to its additional reflection. Filters can be changed during the experiment. We are in the process of rearranging some filtering items, so please contact us for confirmation. If you need other filters please contact us.

Filters along upstream the pump A / pump B splitting   Transmission of the filters placed upstream M2, in the location indicated as Filter-AB in the beamline layout (see page "Beamline setup"). Since this filter is placed before thes pump A and pump B splitting, it equally affects the intensity of both pumps. Filters can be changed during the experiment. This filter should block the photons at the probe's wavelength that co-propagate in the pump branchlines, it is often very important. We are in the process of rearranging some filtering items, so please contact us for confirmation. If you need other filters please contact us.

Filters in front of the detector Transmission of the filters placed in front of the detector; "Pary" means Parylene-N; thicker filters of the same materials (except niobium) are available. The main scope of this filter is to suppress as much as possible the diffuse light from the pump and maximize the transmission of the probe. Sometimes this filter plays a critical role, in particular if the sample surface is rough. Changing these filters takes several hours, so we suggest to not plan changing them during the experiment. If you need other filters please contact us.

Beamline specifications:

SOFTWARE and DATA ANALYSIS:

Available detectors: