High harmonics radiation is a brilliant, coherent XUV delivering femtosecond/attosecond pulses in the XUV spectral range. The radiation presents a good beam quality, which allows focusing the harmonic radiation to very tight spots a high brightness and a high repetition rate ~up to 1 kHz.nThe harmonic radiation is also naturally synchronized with the fundamental driving IR laser. These last two properties are very important for experiments of the pump probe type.
XUV femtosecond Pump-probe experiments:
Applications of this ultra-short duration, coherence source have been performed in collaboration with diffrent laboratories: ultrafast evolution of electronic densities in denses plasmas using XUV interferometry (reference below), ultra-fast hot electron dynamics in dielectircs has been characterized using ultrafast UPS spectroscopy (references below), C2H4 fragmentation has been studied in a pump(UV)-probe(XUV) experiment in the femtosecond regime (references will appear soon).
“Frequency-domain interferometry in the X-UV with high order harmonics”
P. Salières, L. le Déroff, T. Auguste, P. Monot, P. d’Oliveira, D. Campo, J.-F. Hergott, H. Merdji, B. Carré
Phys. Rev. Lett. 83(26) (1999) 5483.
Abstract: We demonstrate that frequency-domain interferometry can be performed in the extreme ultraviolet range using
high-order harmonics. We first show that two phase-locked harmonic sources delayed in time can be generated in the same
medium despite ionization. This gives insight into the dynamics of the generation and ionization processes.
We then apply the technique to the study of the temporal evolution of an ultrashort laser-produced plasma
at the femtosecond time scale.
“Extreme ultraviolet interferometry measurements with high-order harmonics”
D. Descamps, C. Lyngå, J. Norin, A. L'Huillier, C.-G. Wahlström, J.-F. Hergott, H. Merdji, P. Salières, M. Bellini, and T. W. Hänsch, Optics Letter 25, (2), 135-137 (2000).
Abstract: We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2–1020 electrons/cm3 are measured.
“First observation of hot electron relaxation in quartz using high order harmonics”
F. Quéré, S. Guizard, G. Petite, Ph. Martin, H. Merdji, B. Carré, J-F Hergott, L. LeDéroff
Physical Review B 61(15) (2000) 9883.
Abstract: Using ultrafast time-resolved ultraviolet photoelectron spectroscopy, we have followed the energy relaxation kinetics of conduction electrons in quartz, for energies of the order of 30 eV above the conduction-band minimum. We measure energy-loss rates three orders of magnitude lower than those estimated for energies close to this minimum. Likewise, the impact ionization rate obtained (1/40 ps-1) is much lower than the values generally assumed in optical breakdown models.
Non-linear regimes in the XUV:
The development of high-flux (~µJ), ultrashort harmonic sources with controllable spatial, temporal and spectral properties is of primary importance for the realization of a number of applications such the study of non-linear processes in the XUV spectral range in atoms, molecules or cluster. We have already performed two-photons ionization of rare gazes experiments. The results will be soons published. In particular high intensities with interaction with a solid leads to femtosecond XUV ablation of matter which is a completly new regime of interaction. At higher XUV intensities (above 1016W/cm2) matter will rapidly turn into a strongly coupled plasma allowing to explore warm dense matter properties. Apart the high intensity regime, a high number of photons is required even in a linear regime, for example in the realization of coherent control of atomic or molecular processes in the XUV or in interferometry.