Abstract
Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time ( τ ≲ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature ( T e ≈ 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing.
Highlights
The interaction of an ultrafast laser pulse with a solid state specimen can induce phase transitions [1]
EIS-TIMEX was operated in a single-shot mode, moving the sample in a pristine region after exposure to a single pulse sequence, which consists of (i) three probing free electron laser (FEL) pulses arriving well before the pump; (ii) an optical-pump/FEL-probe sequence in which the two pulses are separated by a time delay (∆t) variable in the −1∼+10 ps range; and (iii) a FEL probing pulse arriving ∼100 ms after the pump
One can readily appreciate how Ir monotonically decreases in about 2 ps from the unexcited value to that recorded by the probe pulse arriving ∼100 ms after the main pump–probe sequence (Figure 1 top left), showing how the whole dynamics has been recorded in a time window of about 4 ps
Summary
The interaction of an ultrafast laser pulse with a solid state specimen can induce phase transitions [1]. Depending on the laser flux, the photo-excited sample can reach the liquid phase (ultrafast melting [2]) or more exotic states, such as, e.g., the warm dense matter [3] or the dense plasma phase [4], where the kinetic energy of the photo-excited free electrons compares or exceeds the potential energy of ion–electron interactions. Laser-driven excited states are characterized by short lifetimes, typically ranging from sub-ps to a few ps. After this narrow time window, the excited portion of the sample starts to interact with the surrounding (unexcited) regions. The pump–probe approach can tackle both issues, since it can ensure fs time resolution (in terms of both accuracy and reproducibility) and allows to perform sequential measurements on extended (homogeneous) samples; i.e., the repetition rate of the pump–probe pulse sequences can be synchronized with a sample translation, so that each sequence interacts with a fresh portion of the sample
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