Davide Boschetto

Coherent optical phonons in different phases of Ge2Sb2Te5 upon strong laser excitation

The transient reflectivity response of phase-change Ge2Sb2Te5 films to intense femtosecond laser pulses is studied by ultrafast coherent phonon spectroscopy. The three different phases (amorphous, fcc-, and hcp-crystalline), as well as laser-crystallized films, are investigated, featuring different photoexcited carrier and coherent optical phonon dynamics. At least two main phonon frequencies are identified for each phase/material and their evolution for increasing pump fluences is investigated for the fcc-crystalline phase and the laser-crystallized material, revealing strong differences. We find evidence that a considerable fraction of amorphous phase remains in the laser-crystallized material, which features a different phonon frequency, not related to other phases. These results are important for emerging strategies aimed at driving ultrafast phase transitions via coherent phonon excitation for applications in data storage.

Spatial coherence properties of a compact and ultrafast laser-produced plasma keV x-ray source

The authors use Fresnel diffraction from knife-edges to demonstrate the spatial coherence of a tabletop ultrafast x-ray source produced by laser-plasma interaction. Spatial coherence is achieved in the far field by producing micrometer-scale x-ray spot dimensions. The results show an x-ray source size of 6μm that leads to a transversal coherence length of 20μm at a distance of 60cm from the source. Moreover, they show that the source size is limited by the spatial spread of the absorbed laser energy.

Transient Dielectric Function of Fs-Laser Excited Bismuth

We present time-resolved dual-angle single-wavelength pump-probe study of the reflectivity of femtosecond laser excited bismuth crystal. The recovered real and imaginary parts of the dielectric function show complex a transition to a new quasi-stable excited state at about 20 ps. This state lasts for about 4 ns after the excitation and is not an intermediate state between the solid and the liquid phase. The results suggest that the photo-excited state does not correspond to neither a warm nor a liquid phase, and open general questions on photoexcited phase transition.

Ultrafast dielectric function dynamics in bismuth

The time-dependent reflectivity at two angles from a fs-laser-excited single crystal of Bi were simultaneously measured up to 25 ps after a 50-fs pump pulse excitation. We demonstrate that bismuth did not have transition to liquid state even at the absorbed energy twice the equilibrium enthalpy of melting. We attribute this to the fast electrons transport that carries the excess energy away from the heated surface before the thermalisation of electrons and phonons has occurred.

Probing coherently excited optical phonons by extreme ultraviolet radiation with femtosecond time resolution

We present a pump-probe experimental approach to study time-resolve coherent optical phonons using light pulses in the extreme ultraviolet wavelength range with femtosecond time resolution. Using this technique, a Bi (111)-oriented single crystal was excited by near-infrared (1.55eV) pulses and probed by a high-order harmonic generation source, whose intrinsic flux instability was reduced by making use of a normalization procedure. This unconventional approach allowed us to perform measurements in a previously inaccessible range of phonon wavevectors within the Brillouin zone.

Large-angle convergent-beam setup for femtosecond X-ray crystallography

It is demonstrated how femtosecond plasma X-ray sources can be used to record X-ray diffraction images with many simultaneous reflections. A convergent-beam geometry was implemented and tested with a crystal of a small molecule [Fe-py3(tren)] as well as with a crystal of the protein lysozyme. The results pave the way for femtosecond X-ray crystallography.

Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott–Hubbard material V2O3 presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A1g coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron–lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems.

Mott transition in Cr-doped V2O3 studied by ultrafast reflectivity: Electron correlation effects on the transient response

The ultrafast response of the prototype Mott-Hubbard system (V1−xCrx)2O3 was systematically studied with fs pump-probe reflectivity, allowing us to clearly identify the effects of the metal-insulator transition on the transient response. The isostructural nature of the phase transition in this material made it possible to follow across the phase diagram the behaviour of the detected coherent acoustic wave, whose average value and lifetime depend on the thermodynamic phase and on the correlated electron density of states. It is also shown how coherent lattice oscillations can play an important role in some changes affecting the ultrafast electronic peak relaxation at the phase transition, changes which should not be mistakenly attributed to genuine electronic effects. These results clearly show that a thorough understanding of the ultrafast response of the material over several tenths of ps is necessary to correctly interpret its sub-ps excitation and relaxation regime, and appear to be of general interest also for other strongly correlated materials.

Demonstration of spatial coherence with a compact and ultrafast X-ray source

We used Fresnel diffraction from knife-edges to demonstrate spatial coherence of tabletop and ultrafast X-ray sources produced by laser-plasma interaction, and show that source size is limited by the spatial spread of absorbed laser energy.

Reflectivity oscillations of fs-laser excited Bismuth: excitation of coherent phonons

We present the experimental and theoretical studies of the optical response from the single-crystal of bismuth to the excitation by the femtosecond laser pulse. The experimental results revealed a complex, first - positive and a few picoseconds later - negative, change in time-dependent reflectivity, which could not be explained in the light of the existing theories. It is shown that reflectivity oscillations are related to the excitation of coherent phonons by the pulse with duration shorter of all relaxation times. We demonstrate that swiftly heated electrons are responsible for the phonon excitation due to the fast modification of the attractive (electronic) part of inter-atomic potential. The electronic perturbation of potential is also responsible for the red shift of phonon frequency and for the increase in the amplitude of phonons. The coherent phonon excitation process as well as the change in the reflectivity is related mainly to the modification of the electron-phonon momentum exchange frequency. The comparison between the theory and experiments shows an excellent agreement. Moreover, the reflectivity measurements allow direct recovery of the electron-phonon coupling rate in bismuth crystal, which has not been measured before.