No abstract

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

The present study, focused on the sol-gel synthesis of rare-earth-doped zinc oxide, highlights the crucial points involved in the design and development of ZnO/Eu nanosystems with peculiar and easily tunable photophysical properties. ZnO nanopowders containing different europium amounts were prepared starting from zinc and europium acetate salts as the sol precursors. The densification process and the evolution of the structural-optical properties were studied as a function of annealing performed in air between 100 and 1000°C. The microstructure and composition of the samples and their dependence on the synthesis procedure were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, whereas the emission properties were studied by photoluminescence spectroscopy in the energy and time domains as a function of the structural evolution. Crystalline ZnO powders in the wurtzite structure were formed after heat treatment at 400°C, with an average nanocrystal size of ca. 20 nm. Stronger annealing conditions allowed a more extended densification of the oxide-based network and the removal of most -OH groups but also promoted the crystallization of Eu 2 O 3 at 800°C. The most intense emission bands around 600 nm due to Eu 3+ transitions were clearly observed in the annealed samples (T g 600°C). Interestingly, the observed overlap of the typical Eu 3+ red emission with the characteristic green luminescence of the nanostructured ZnO matrix can be synergistically exploited for single or multicolor emission through the proper choice of the excitation wavelength.

For the first time single-shot harmonic spectra generated by few-optical-cycle pulses have been measured. Clear carrier-envelope phase effects have been observed in the cutoff harmonic spectral structure. Results have been interpreted in terms of the nonadiabatic single-atom response of the nonlinear medium excited by few-optical-cycle pulses.