Highlights

Reflection setup

We implemented the new capability of measuring TG signals in reflection! 

During the last months the TIMER team, in collaboration with the K. Nelson group at the MIT (BT. 20174120), the Giordano group at the Université Claude Bernard Lyon 1 (BT. 20184032) and the Baldi group at the Trento University (BT. 20184081), implemented the reflection setup. This setup, shown in the above picture, allows to detect the phase-matched back-scattered transient grating signal. This capability opens up the possibility of investigating opaque samples, previously not available at TIMER, as well as measuring both the reflected and transmitted signal in thin, transparent membranes. 

 



First purely EUV, time-resolved TG signal

We measured all-EUV transient gratings with periods down to 28 nm!  

F. Bencivenga et al., Science Advances 5, eaaw5805 (2019)

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

 


First evidence of EUV FWM

We observed the first purely EUV four-wave-mixing signal! 

L. Foglia et al., Phys. Rev. Lett. 120, 263901 (2018) 

The extension of nonlinear optical techniques to the extreme-ultraviolet (EUV), soft and hard x-ray regime represents one of the open challenges of modern science since it would combine chemical specificity with background-free detection and ultrafast time resolution. We report on the first observation of a four-wave-mixing (FWM) response from solid-state samples stimulated exclusively by EUV pulses. The all-EUV FWM signal was generated by the diffraction of high-order harmonics of the FERMI freeelectron laser (FEL) from the standing wave resulting from the interference of two crossed FEL pulses at the fundamental wavelength. From the intensity of the FWM signal, we are able to extract the first-ever estimate of an effective value of ∼6 × 10−24 m2 V−2 for the third-order nonlinear susceptibility in the EUV regime. This proof of principle experiment represents a significant advance in the field of nonlinear optics and sets the starting point for a manifold of techniques, including frequency and phase-resolved FWM methods, that are unprecedented in this photon-energy regime.

 

First EUV-induced TG

Four wave mixing induced by two EUV FEL pulses and probed with a fs ultraviolet pulse was demonstrated at the DiProI beamline.

Bencivenga et al., Nature 520, 205 (2015).

A miniaturized delay line composed of three 70 mm long carbon-coated mirrors, with position and pointing remotely controlled with piezo-motor actuators, fits within the DiProI end station. Each beam can be synchronized with the ultraviolet pump monitoring the FEL-induced ultrafast reflectivity change of a Si₃N₄ target. The time dependence of the transient grating signal is featured by a clear peak, centered at zero time delay and with a FWHM ≈ 150 fs, and modulations likely due to vibrations (Raman and Brillouin modes). An appreciable signal (> 10⁴ photons/shot) was observed in the whole probed time delay range (0-100 ps).

Retrieve article

Four-wave mixing experiments with extreme ultraviolet transient gratings.

F. Bencivenga, R. Cucini, F. Capotondi, A. Battistoni, R. Mincigrucci, E. Giangrisostomi, A. Gessini, M. Manfredda, I.P. Nikolov, E. Pedersoli, E. Principi, C. Svetina, P. Parisse, F. Casolari, M.B. Danailov, M. Kiskinova, C. Masciovecchio. Nature 520, 205 (2015).

Testing the setup

First steps towards EUV transient grating: testing the setup in the laser lab! 

R. Cucini et al., Optics Letters 36, 1032-1034 (2018) 

The main experimental hurdle for FEL-based TG is to transpose the conventional optical layout into the EUV/soft X-ray range. The lack of reliably transmission optics in this wavelength range (1-100 nm) prevents the use of diffractive optical elements (phase masks) and achromatic doublets, that are at the basis of optical setups. Moreover, the lack of phase masks makes quite challenging the implementation of heterodyne detection, which greatly improve many aspects of the experiments. We tested various options at the EIS-TIMER laser laboratory and we evaluated that an "all-reflective" optical layout (top panel) is the best compromise for an FEL-based instrument; please note that the trasmission optics used for second harmonic (SH) generation are is not needed with the FEL.

 


The EUV TG project

Extending the transient grating approach to the EUV range: investigate dynamics at the nanoscale 

F. Bencivenga and C. Masciovecchio et al., Nuclear Instruments and Methods in Physics A 606, 785 (2009) 

The extension of the standard Transient Grating (TG) technique to the EUV spectral region will permit to investigate dynamics in the so-called mesoscopic regime, inaccessible with other experimental techniques. The unique characteristics of the FERMI source, moreover, are ideal for the development of the proposed experiment in Trieste.  The new instrument would allow a better understanding of the physics of disordered systems, and in particular of of the liquid-to-glass transition mechanisms, thermal anomalies at low temperatures, divergence of transport properties and relaxation phenomena. Further- more, the possibility to create and probe sub-picosecond transient gratings with spatial period in the nanometer range will be extremely useful also in other fields of research, since it will provide the unique capability to measure correlations, electronic excitation lifetimes, heat transport, intra-molecular dynamics and non-linear material responses. Finally, TG spectroscopy has proven to be able to reliably probe very weak excitations, such as heat waves in low-temperature fluids. TG is also an extremely sensitive probe for surfaces and interfaces, with potential applications in the study of thin films and nanostructured materials.