Ph.D Thesis

Ph.D StudentKfir Ofer
SubjectCircularly Polarized High-Order Harmonic Generation
of Bi-Circular Lasers
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


The irradiation of atoms by intense laser fields leads to emission of coherent high-order harmonics of the driving laser that spans the extreme-UV spectral range and reaches the X-ray regime. Until recently, the polarization of high-order harmonics, which is a fundamental property of electromagnetic waves, was considered to be limited to linear polarization, or close to linear. The conceptual constraint for polarization control of high-order harmonic generation changed when my colleagues and I at the Technion demonstrated high-order harmonics with circular polarization using a bi-circular driving field, comprising a circularly polarized fundamental field and its counter-rotating second harmonic. This work opened up a path towards the investigation of chiral phenomena using high harmonics, such as magnetism, chiral molecules, laser induced chirality, access the photon spin in high harmonic generation, and more.

This thesis is based on three published papers in the field of circularly polarized high harmonic generation. In the first paper, I present theoretically and show experimentally phase matched generation of circularly polarized high harmonics from the interaction of a bi-circular driving field with gases. These harmonics were bright enough to demonstrate X-ray magnetic circular dichroism (XMCD) with high harmonics for the first time. The magneto-optical interaction was used directly for circular polarimetry of the high harmonics, including their degree of circular polarization and the rotation helicity. In the second paper, I extend the theoretical framework for phase matching of circularly polarized high harmonics with their driving bi-circular field. I find that phase matching between circularly polarized high harmonics and their bi-circular driver occurs when the high harmonic field obeys the propagation symmetry induced by the driver. This symmetry based approach leads to two main results: first, the propagation symmetry constraint links the conservation of energy, linear momentum and angular momentum for the photons that participate in high harmonic generation - when two are given the third is imposed by the propagation symmetry. Second, the symmetry-based approach is extended to full- and quasi-phase matching when the driving bi-circular field is composed of a long and a short pulse. In addition, I proposed a path towards chiral attosecond pulses, which is supported by experimental results.

The third paper in this thesis employs an in-line scheme for generation of circularly polarized high-order harmonics that allows for a spin-mixing spectroscopy of the generation of high order harmonics. We identify channels in a high-order harmonic spectrogram according to the specific spin-mix of the optical field photons. That is, when the spin content of the driving field vary, some channels vanish and new ones appear in the spectrum, to match the mix of spins in the driving two-color field. 

The field of chiral high-order harmonic generation currently experiences a rapid expansion, spanning from strong field interaction with matter to the probing of materials with highly chiral extreme-UV and X-rays. This work marks important milestones in the field: phase matching of circularly polarized harmonics, interaction with chiral matter, and simplicity of operation. By the time this thesis is submitted, these papers have already made significant impact.