M.Sc Thesis

M.Sc StudentKfir Ofer
SubjectAttosecond Pulses with Sophisticate Waveforms:
Spatio-Spectral Airy and Auto-Focusing Beams
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


Optical attosecond pulses are the shortest events that are systematically produced by mankind. These pulses have been used for time-domain investigations of ultrafast electronic processes in atoms, molecules, and solids. Attosecond pulses are produced through high harmonic generation (HHG) in which high-intensity visible/IR laser light is up-converted into extreme ultraviolet and x-ray radiation. The semi-classical model of HHG consists of three steps. First, an electron is released from the binding potential through strong-field tunneling. Second, the oscillating laser field accelerates the released electron, increasing its kinetic energy. And third, upon re-collision with its parent ion, the electron and the ion recombine and a high-energy photon is emitted. In the first work of this thesis, we propose a scheme for producing isolated attosecond pulses with sophisticated spatio-spectral waveforms. Spatial and spectral waveforms of a seed attosecond pulse are shaped and its central frequency is up-converted through interaction with an infrared pump laser pulse in a gas of atoms or ions with large binding potential. The pump field, which is too weak to release electrons to the continuum, amplifies the energy of electrons that were ionized by the seed pulse through single photon absorption. An x-ray attosecond pulse is emitted when the energetic electrons recombine with their parent ions. At maximum amplification, the x-ray emission spectrum obtains the shape of an Airy function with huge bandwidth which corresponds to an attosecond pulse with a flat-top pulse-shape. Generation of attosecond pulses with fast leading and trailing edges are also shown. In addition, we present settings for generation of attosecond pulse beams with sophisticate spatial waveforms, and discuss two examples: (i) An attosecond "spatio-spectral Airy beam" that exhibit Airy profile in both spectrum and transverse coordinate, and displays prismatic effect where each spectral component propagates along a different curved path. (ii) A pulsed beam that auto-focuses to a nano-scale hot spot at a controlled propagation distance. In the second part of this thesis we generalize the semi-classical model of HHG. In the common model, the electron is excited to the continuum with initial zero velocity. We extend the semi-classical 3-step model to a regime in which the ionized electronic wave-packet exhibits broad momentum distribution, as occurs, for example, when the ionization step is driven by attosecond extreme ultraviolet pulses. A comparison between quantum calculations and the generalized semi-classical model show that the momentum distribution is significant for ~100  attosecond ionizing pulses.