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 (FMA and FMB). 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 (LTG) and wavevector (kTG):

LTG λexc / 2 sin(2θ/2)     ;     kTG = 2π / LTG 

where λexc 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 λexc are instead related to the chosen wavelength for the probe (λpr), which are listed in the table below. Typically: (i) when the FEL2 source is used λexc λpr / N, with N in the range 2-5; (ii) when FEL1 is used λexc λpr*(N1 / N2), where N1 is fixed and N2 is in the range 4-15. The shortest value of LTG 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 λpr 20.6, 16.6, 13.4 and 8.34 nm) that act as bandpass filters centered around λpr; 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 (λpr 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 (FMpr) at an angle of incidence (with respect to the surface normal) of θpr = 3.05°, 4.56°, 12.24° and 15.4°. Not all the combinations of 2θ and θpr are possible.
The maximum range in the probe's time delay depends on the chosen values of 2θ and θpr 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 (θsig) that satisfies the diffraction condition:

[(sin(θsig) - sin(θpr)] λpr / LTG 

The TG signal intensity quadratically depends on the amplitude of the pump-induced modulations of the complex refractive index at λpr, 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 λexc = 3λpr. 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 λpr, 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 λexcdrop 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.



[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)


Ultima modifica il Venerdì, 15 Marzo 2024 21:17