Study of high-order harmonic generation effects under variations of focusing conditions of few cycle laser pulses

Resumen

Femtosecond laser pulses are nowadays routinely generated and, thanks to their high peak power and short duration, are used in a wide range of applications, either industrial, medical or scientific. They are used, for instance, to observe the molecular motion inside a chemical reaction, or to accelerate electrons or heavier particles. This PhD thesis is devoted to one of the many applications of femtosecond pulses: the high-order harmonic generation (HHG). HHG is a process where a high power pulse drives the generation of new frequencies, far beyond the current limit of wavelengths achievable with conventional lasers. Temporally, the radiation generated is emitted in bursts of pulses with a duration of hundreds of attosecond (even dozens in some cases), which adds interest to this application. In the experiments presented in this work we study the HHG dependence on key parameters in the generation, such as phase of the driving pulse, gas-jet position or focusing conditions. This allows us to control the characteristics of the emitted XUV radiation. In addition, macroscopic propagation of the new frequencies relevance in the emitted radiation is examined through comparison to theoretical simulations. Since HHG is driven by femtosecond pulses, it is essential to understand how to work with them. For this purpose, in the first chapter of the thesis, femtosecond pulses are described, including their generation, characterization and propagation. Novel techniques of characterization have been used, as STARFISH or developed, as single-shot d-scan (developed in Paper I), which are detailed in Section 1.3. Propagation of femtosecond pulses can lead to nonlinear effects that induce spectral and spatial changes in the pulse, as explained in Section 1.4. During this thesis work, we have studied two nonlinear propagation regimes: Paper II is devoted to study how the driving pulse filamentation affects on the HHG, and Paper III is dedicated to post-compression in hollow-core fiber. In the final section, laser systems used in this thesis work are described. In the second chapter, HHG is described and main results reported. In Section 2.2 the experimental setup employed is detailed, and the following sections are devoted to the experiments carried out. In Paper IV the influence of the phase-matching in the CEP dependence of the harmonics is studied, while in Paper V we analyze the generation of extreme ultraviolet (XUV) continuum. Finally, alternative focusing schemes are examined: in Paper VI, infrared generating pulses are focused by means of a chromatic singlet lens, while in Section 2.5.2, XUV vortices are generated by using a driving pulse with orbital angular momentum.