Flying Doughnuts: Space-Time non-Separable Electromagnetic Pulses.

Abstract

Flying doughnuts (FD) are solutions of Maxwell’s equations that exist only in the form of short bursts of electromagnetic (EM) energy propagating in free space at the speed of light. They are distinguished from transverse waves by a toroidal configuration of EM fields and strong field components along the propagation direction. They possess a unique space-time coupling (STC) while their broadband spectrum and toroidal structure makes them ideal candidates for the study of toroidal excitations and dynamic anapoles. Their generation is expected to significantly boost the research in the field of toroidal electrodynamics while the developed technology for the control of STCs could open a new field of ultrafast optics. In this thesis I report for the first time the generation of few cycle doughnut pulses in the near-IR part of the spectrum through a spatio-temporal transformation scheme of conventional laser pulses. The transformer is a properly engineered spatially gradient metasurface that has been designed and applied for the realization of the desired space-time coupling. I have developed a new experimental technique for the complete spatio temporal characterization of cylindrical vector pulses that was not previously possible and I confirmed the generation of FDs. Furthermore, the method characterizes the aberrations of axially symmetric pulses that attract great attention for applications. I have derived closed form expressions for the Fourier and Hankel transforms of FDs that led to better understanding of their properties and provide an important tool for their further study. In particular, the pulses have been proven to be isodiffracting, a feature rendering them resistant to changes of their shape other than scaling upon propagation. Finally, I report the first topological study of complex toroidal pulses that provides the opportunity to expand the topological description of EM fields from the well-studied area of monochromatic beams to broadband pulsed fields. A first theoretical analysis of FDs towards this direction revealed the existence of a fine topological structure with the formation of time dependent vortices and areas of instantaneous energy back-propagation, indicating the presence of rich topological effects.

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Identifiers

ORCID for Apostolos Zdagkas: orcid.org/0000-0002-1734-9722

ORCID for Nikolai Zheludev: orcid.org/0000-0002-1013-6636

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