Coherent attosecond light sources based on high-order harmonic generation: influence of the propagation effects

[EN]During the past two decades, progress in the field of ultra-intense and ultra-short lasers has improved our understanding of the subject under intense laser fields. Unlike other nonperturbative fields of physics, the availability of intense laser technology in medium-sized laboratories worldwide has provided a fruitful interaction between theory and experiment. This thesis is an example of this interaction. Intense electromagnetic radiation induces a strong nonlinear response in the subject. The atomic electrons gain energy from the laser field, which can be subsequently released as high-frequency coherent radiation, a process known as generation of high order harmonics (HHG, from its acronym in English). Conventional lasers absence of these higher frequencies has fueled interest in developing technology as a tool for HHG coherent light sources of short wavelength. Until recently, ultra-intense laser technology was limited to wavelengths of near infrared (around 800 nm) and conversion into higher-order harmonics was limited to the far-ultraviolet region (XUV). Currently, with the improvement of the parametric inversion techniques, this limit is in soft X-rays. However, from the beginning recognized the potential applications of HHG process, beyond a natural high frequency coherent radiation. HHG spectrum consists of a comb of harmonics which extend to the so-called cutoff frequency. In the spectral region near this cut harmonics have similar intensities and, even more interestingly, the spectral phase distribution is smooth. With these two assumptions, after filtering the low frequency spectrum, the resulting radiation corresponds to a train of ultrashort pulses in the XUV region, lasting about a few hundred attoseconds (1 attosecond = 10-18 sec), spaced regularly every half cycle of the laser incident. Following early experimental confirmation of the century, they are considered coherent light pulses shorter ever created. This technology is starting to pay off, as already identified different applications for discriminating ultrafast processes (in the attosecond regime) in the dynamics of physical systems, chemical and biological. The aim of this thesis is to make a new and original contribution in this field. The core of this study is the development of theoretical methods to simulate the experiments. This strategy contains a double aspect. First, the theory used to understand the results obtained experimentally. We have developed our own experiments, we contrast directly with the theoretical results and also we have worked with two experimental groups at the international level in order to simulate their experiments. Second, we have applied our theory to predict new physical processes and thus serve as a guide for conducting new experiments. The starting point of this thesis is based on the theory SFA +, previously developed in the area of ​​optics at the University of Salamanca, for calculating the process of generating high order harmonics in a single atom, a microscopic level. This method, as well as an extensive introduction to the HHG process, is exposed along the chapter 1. Our first objective was to develop a propagation scheme of higher order harmonics which allows the simulation of the process on a macroscopic scale, so that the theory is comparable to the experiments. To do this, we implemented a new propagation technique based on the discrete dipole approximation. This technique, as well as the basics for understanding the propagation of high-order harmonics, are developed in chapter 2. In Chapter 3, studied the propagation of a high order harmonic fields generated by laser wavelength in the near infrared, targeted cells jets or low density gas. As a first test, we analyzed theoretically and experimentally the change in conditions of phase adjustment that results from positioning the gas stream at various positions along the axis of propagation. Once experimentally validated our theoretical method, we propose an alternative to shorten attosecond pulses, by detecting different angles from the axis of propagation. We then present a study of the transverse coherence length, comparing our theoretical and experimental results. Finally, we implemented the cell geometry of a semi-infinite gas in our propagation code, in order to understand the experimental results obtained by the group of M. Kovacev, University of Hannover (Germany). In Chapter 4 we have modified our method to study the harmonics generation ultra-high, produced using wavelength lasers in the mid-infrared (approximately 4 microns), in collaboration with the theoretical group A. Becker and A. Jaron-Becker, of JILA, University of Colorado (USA). The main result of this paper is the demonstration of the temporal coherence of X-rays with energy kiloelectronvoltio obtained in the experimental group M. Murnane and H. Kapteyn at JILA, University of Colorado (USA), who led an international collaboration which also included the Technical University of Vienna (Austria), Cornell University (USA) and our group. Further to this work, and thanks to the development of our theoretical methods, we have derived a way to obtain light pulses in the X-ray regime zeptosegundos (1 zeptosegundo = 10 to 21 seconds).

Author

• Hernández García, Carlos

• Tesis y disertaciones académicas
• Universidad de Salamanca (España)
• Tesis Doctoral
• Academic dissertations
• Física aplicada
• Láseres
• Óptica

Identifier

hdl:10366/121369

OAI identifier oai:identifier

oai:gredos.usal.es:10366/121369