The collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value. The complex self-focusing dynamics observed in the region below the critical power depends on the temporal evolution of the pulse and, also, on the interaction between the different spatial modes of the hollow-core fiber.

In this work we present a modification to conventional X-rays fluorescence using electrons as excitation source, and compare it with the traditional X-ray excitation for the study of pigments. For this purpose we have constructed a laser-based source capable to produce X-rays as well as electrons. Because of the large penetration depth of X-rays, the collected fluorescence signal is a combination of several material layers of the artwork under study. However electrons are stopped in the first layers allowing therefore a more superficial analysis. We show that the combination of both excitation sources can provide extremely valuable information about the structure of the artwork.

The advent of ultrafast laser science offers the unique opportunity to combine Floquet engineering with extreme time resolution, further pushing the optical control of matter into the petahertz domain. However, what is the shortest driving pulse for which Floquet states can be realised remains an unsolved matter, thus limiting the application of Floquet theory to pulses composed by many optical cycles. Here we ionized Ne atoms with few-femtosecond pulses of selected time duration and show that a Floquet state can be observed already with a driving field that lasts for only 10 cycles. For shorter pulses, down to 2 cycles, the finite lifetime of the driven state can still be explained using an analytical model based on Floquet theory. By demonstrating that the amplitude and number of Floquet-like sidebands in the photoelectron spectrum can be controlled not only with the driving laser pulse intensity and frequency, but also by its duration, our results add a new lever to the toolbox of Floquet engineering.

The development of new coherent and ultrashort light sources is of great relevance for exploring fundamental processes and different applications in science. The most successful technique for generating ultrashort laser pulses, in terms of energy and pulse duration, is using hollow capillary fibre compressors. The different strategies to further increase the pulse energy and to achieve shorter pulses at non-conventional wavelengths, lead to continuous improvement of this technique. In this work, we present the theoretical framework of the nonlinear propagation in hollow capillary fibres through the propagation equation and the spatio-temporal effects that appear in the nonlinear dynamics. To numerically study the nonlinear propagation of the pulse in the hollow capillary fibre, we present different numerical models considering only the spatial effects, (1+1)D model, the full spatio-temporal dynamics and ionization, (2+1)D model, or the case with lack of cylindrical symmetry, (3+1)D model. To show the performance of some of these models in a particular case, we study the generation of ultrashort and energetic dispersive waves inside the hollow capillary fibre. We show that the emission of a dispersive wave at a fixed wavelength for different pump wavelengths is possible by parameter scaling.

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