Quantum Optical Description of High-order Harmonic Generation

Sammanfattning: High-order Harmonic Generation (HHG) is a highly non-linear process in which an atom interacts with a strong laser field. The laser field lowers the atomic potential barrier allowing bound electrons to escape into the continuum through tunnel ionization, propagate, and, with some probability, recombine with the parent ion. As a result, coherent eXtreme UltraViolet (XUV) radiation is emitted in the form of ultrashort pulses with attosecond durations. This phenomenon has been successfully described with semi-classical models that consider the atoms as quantum systems and the radiation as a classical wave. However, there have been recent efforts to develop a fully quantum mechanical theory of HHG in which the radiation is also described as a quantum system. This quantum optical description has opened the doors to several applications of HHG radiation in quantum technology and fundamental physics research. In this thesis work, the newly developed Strong Field Quantum Electrodynamics (SFQED) theory applied to HHG is reviewed. The effects of considering a spatial distribution for the atoms and radiation's intensity are investigated using this framework. To do this, two Python simulations, where SFQED and the Strong Field Approximation (SFA) are used, were created. One of them considers single atom generation, and the other one considers few atoms generation using a Gaussian beam intensity profile. It is found that the spatial distribution affects the shape of the spectrum, but does not affect the final radiation state's statistics or purity. Several avenues to further explore the application of SFQED to HHG include applications in quantum technology, HHG driven by quantum light, and generation in more complex targets, such as diatomic molecules.