TIMER
The understanding of dynamics in disordered systems still represents a challenge for modern scientists and is motivating several experimental and theoretical studies. On the experimental side, the main limitation is represented by the lack of an experimental method able to probe collective dynamics over “mesoscopic” length-scales (i.e., ~10-100 nm). These scales are of the highest relevance for the study of disordered systems, since they can be related to the topological disorder inherent to the amorphous phases. Our aim is to develop a Free Electron Laser (FEL)-based time-resolved technique able to extend the conventional Transient Grating (TG) method in the VUV spectral range [1]. The unique properties in terms of coherence, stability, time and frequency band-width provided by the FERMI@Elettra source will make possible the realization of this FEL-based instrument (TIMER) at Elettra. TIMER will open up the possibility to study collective dynamics at the nanoscale, thus allowing probing dynamics in the mesocopic range that, nowadays, cannot be investigated by existing laser or synchrotron based instruments. Moreover, the proposed experimental method will also be a sensitive probe for dynamics, heat transport and electron correlations in nanostructured materials.
Figure 1: (q,ω)- and (λ,τ)-ranges accessible by the transient grating technique with conventional laser source (TG) and by the available inelastic scattering techniques: inelastic light (ILS), ultraviolet (IUVS), neutron (INS) and x-ray (IXS) scattering. The shaded area is the range accessible by TIMER instrument.
In a naïve picture disordered systems are those characterized by the lack of translational invariance. This greatly complicates the understanding of collective dynamics, which is even more complicated when other degrees of freedoms come into play. These introduce different time- (τ) and length-scales (λ) affecting the dynamical behaviour differently, depending on their values as compared to the average period of molecular vibrations (tv) and the average interparticle distance (δ). Inelastic scattering methods can experimentally probe dynamics by measuring the spectrum of density fluctuations, naturally occurring over a broad range of frequencies (ω=2π/τ) and wave vectors (q=2π/λ). However, available techniques cannot explore all the (λ,τ)-range of interest (see Fig.1). Particularly, any technique can access the mesoscopic λ-range (~10-100 nm), mainly because of severe constraints concerning the frequency resolution. This hurdle can be circumvented by using time resolved spectroscopies, such as the TG [2].
Figure 2: Rationale of the transient grating method.
The rationale of TG experiments is sketched in Fig.2: two photon pulses (pump) of wavelength λ1 interfere into the sample creating a standing electromagnetic wave (i.e., the transient grating) with period: Λ=(λ1/2)·sin(θ), where 2θ is the angle between the pulses. This standing wave interacts with the sample generating a modulation of sample properties such as, e.g., density. This density modulation can be probed by diffraction of a third pulse (probe) of wavelength λ2 impinging into the sample at the Bragg angle: θB=asin(λ1sin(θ)/λ2). Finally, the time evolution of diffracted intensity can be monitored by properly delaying the probe pulse with respect to the pump ones, and it is related to the back time-Fourier transform of density fluctuations [3]. The TG signal therefore embeds the same information as an inelastic scattering experiment. However, the λ-range exploitable by TG method is presently limited by the rather large wavelength (>400 nm) of available sources of coherent and short time-width radiation. FERMI@Elettra will make available a coherent photon pulses of short wavelength radiation (1-100 nm), thus allowing the extension of TG method in the VUV spectral range with sub-ps time resolution. This FEL-based TG technique will be developed in a dedicated experimental end-station (TIMER) of FERMI@Elettra. This instrument will be able to probe the “mesoscopic” (λ,τ)-range never exploited before by a single instrument (see Fig.2). Furthermore, TG experiments at the nanoscale would potentially be of great impact in other fields of research, since they can measure correlations, electronic excitation lifetimes, heat transport, intra-molecular dynamics and non-linear material responses and, moreover, TG technique has recently proven to be a reliable probe for surfaces, interfaces and nanostructured materials.
References:
[1] F. Bencivenga and C. Masciovecchio, Nucl. Instr. and Meth. in Phys. Res. A 606, 785 (2009).
[2] H.J. Eichler, P. Gunter, and D.W. Pohl, Laser-Induced Dynamic Gratings, Berlin: Springer (1986).
[3] P. Bartolini, A. Taschin, R. Eramo, and R. Torre, Time resolved spectroscopy in complex liquids: an experimental perspective, edited by R. Torre, Springer (2008).
