Ph.D Thesis

Ph.D StudentLerner Gavriel
SubjectNew Symmetries of Light and New Selection Rules in High
Harmonic Generation
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


High harmonic generation (HHG) is an extreme up-conversion process where the frequencies of the electromagnetic (EM) field interacting with the nonlinear medium are multiplied many times to generate a broad coherent frequency comb in the extreme ultraviolet (EUV) spectral region. This process is sensitive to symmetries in the light-medium system, which dictate the selection rules, (i.e., restrictions of the allowed properties of generated field, such as frequency and polarization). Many types of symmetries and selection rules have been studied in the past, such as static symmetries of media in the sixties, dynamical symmetries (DSs) in the nineties (including time operations), and a recent comprehensive microscopic DS theory. However, these theories did not consider the macroscopic degrees of freedom (DOF) of the light-matter system, which are responsible for the momentum, orbital angular momentum (OAM), and other important features of light.

This thesis presents analytical study of multi-scale light-matter interactions and numerical and experimental investigations of HHG. Chapter 3 presents a method for the selective suppression of different harmonics by varying the phase-velocity of the driver field along the propagation. In chapter 4, we develop a new concept in EM theory - symmetries in EM fields - that mix the temporal part, microscopic field polarization components, and macroscopic spatial configurations of light. We formulate a theory for these multi-scale symmetries and derive the selection rules for nonlinear wave-mixing. We also implement this new concept in HHG experiments and theoretical calculations. In chapter 5, we show how the photonic conservation laws arise from the general symmetries of EM fields. We discover two new photonic conservation laws and investigate them numerically. These photonic conservation laws describe selection rules with a simple photonic picture. In the last section presented here, we develop an analytic method that (i) identifies symmetries of light-matter systems, and (ii) easily retrieves its selection rules.