Ph.D Thesis

Ph.D StudentNeufeld Ofer
SubjectSymmetry and Chirality in High Harmonic Generation and
Ultrafast Spectroscopy
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


In the late 1980’s, it was experimentally observed that irradiating atoms with intense laser fields can lead to the emission of a coherent comb of high energy photons. The efficiency of this emission followed a non-perturbative scaling with the incident light’s intensity, and a non-exponential scaling with the emitted wavelength, unlike previously familiar nonlinear optical phenomena. The process behind this unique radiation source is now known as high harmonic generation (HHG), and it holds great promise for several technological frontiers. For instance, HHG has already found applications in high resolution imaging, ultrafast spectroscopy of materials, table-top generation of broad-range X-ray radiation, and has practically opened-up the field of attosecond science. Not surprisingly, it has been extensively studied in the past decades. Still, many aspects of HHG remain not fully understood. Among these, it was only recently shown that the polarization of the HHG emission can be robustly controlled by manipulating the symmetry properties of the laser-matter system. However, the connection between symmetries and HHG was made ad-hoc, and a general theory is missing. Moreover, it is not clear how this polarization control can be extended to the entire harmonic spectrum, and to the attosecond pulses that are emitted in the time-domain. Another open question in the field regards the possible utilization of HHG for spectroscopic purposes - HHG spectra are known to contain a great deal of information about the medium and the ultrafast dynamics that occurred in it, but it is not clear what part of this information can be extracted out, and if so, how.

This thesis presents theoretical research that explores these open questions. It focuses on three main frontiers: (i) understanding the role of symmetry in nonlinear optical processes and HHG. In this regard, the thesis derives a general group theory for symmetries of light-matter interactions and the selection rules that they impose on the emission spectra. This understanding is utilized to derive control schemes for the polarization of X-ray light and attosecond pulses. (ii) Exploring the role of chirality in electromagnetic (EM) theory and chiral light-matter interactions. Here, we have derived new measures for the chirality density of EM fields, which can be used to design laser beams that interact extremely efficiently with chiral matter. We demonstrate the effectiveness of these new chiral-light beams in HHG from chiral molecular media. (iii) Ultrafast spectroscopy based on HHG. In this avenue we have developed several new techniques that can probe a variety of properties of matter and light with ultrafast temporal resolution, including: laser carrier envelope phase, atomic orbital structure, molecular chirality, atomic and molecular ring-currents, and electronic-correlation.

Overall, this work introduces many new fundamental concepts in nonlinear optics and EM theory. Most notably, the newly derived symmetry and chirality properties of light, and the presented novel techniques for probing unique properties of matter. These underlying advancements can lead to many future applications, ranging from ultrafast spectroscopy, to material design and control.