Toroidal light pulses: generation, characterisation and light-matter interactions

Vignjevic, Luka (2026) Toroidal light pulses: generation, characterisation and light-matter interactions. University of Southampton, Doctoral Thesis, 189pp.

Record type: Thesis (Doctoral)

Abstract

Toroidal electrodynamics is a recently established research area handling the electromagnetic properties of objects of toroidal topology. Sparked by Zeldovich’s introduction of the concept of the static anapole in 1957, dynamic toroidal dipoles were discovered, and the field rapidly grew. Following predictions by Ziolkowksi, Hellwarth and Nouchi, propagating pulses of toroidal topology were experimentally generated leading to an expansion of toroidal electrodynamics to include free-space propagating electromagnetic pulses. Toroidal pulses were generalised to a broader family of supertoroidal pulses, which exhibit correlations in their spatial, temporal, and polarisation structures, leading to non-trivial propagation dynamics of such pulses exhibiting nondiffraction and isodiffraction. Their toroidal topology also results in features reminiscent of skyrmions in magnetic spin systems and leads to light-matter interactions that involve the excitation of toroidal and anapole modes. Toroidal pulses show promise in the fields of remote sensing and communications, owing to their complex topology and couplings between degrees of freedom.

As part of my project, I developed a tomographic method for characterising the space-polarisation and spatiotemporal coupling of toroidal pulses, then for the first time generated and characterised self-dual toroidal pulses. Further, I developed and experimentally demonstrated a device capable of detecting toroidal light pulses based on coherent perfect absorption in a cylindrically symmetric absorber. In addition, I derived analytical expressions for the absorption of cylindrical waves in a cylindrical absorber. These achievements are described in more detail in the paragraphs below.

In my experiments, I generated and characterised toroidal light pulses with parallel electric and magnetic field components formed by a superposition of azimuthally and radially polarised free space propagating Hellwarth and Nouchi pulses: The pulses were generated in the optical part of the spectrum with a central wavelength of 800nm and a 200nm bandwidth. The generation scheme involved sending circularly polarised pulses through a segmented waveplate to give them a topological charge of ±1. The pulse characterisation was performed using a state tomography technique that I developed (see next paragraph). The unique properties of these pulses make them candidates for the study of exotic light-matter interactions, such as magnetoelectric effects or even axions, speculative dark matter particles.

I developed a tomographic method for the characterisation of broadband ultrashort, topologically structured pulses. This method includes a space-polarisation tomography technique that characterises sequentially the individual narrow-band spectral components of the pulses, measuring their spatially varying polarisation and phase profiles. The pulses’ spatiotemporal (or equivalently spatiospectral) couplings were characterised by finding the Rayleigh range of each narrow-band spectral component. The tomography method was applied to the characterisation of 10fs toroidal light pulses covering the spectral range from 700nm to 900nm. Low dispersion composite wave plates were used for polarisation projection and a digital micromirror device for mapping the transverse space profile of the pulses. This method allows special propagation characteristics of the pulse, such as isodiffraction to be identified. Furthermore the method can discriminate between pulses with different topological charges and cylindrical vector polarisations.

A perfect topological absorber of electromagnetic waves with singularities that can be used for discriminating toroidal pulses from homogeneously polarised light was developed and experimentally demonstrated. It utilises the phenomenon of coherent perfect absorption in a cylindrical geometry with a one-dimensional co-axial absorber, where energy losses depend on the topology of the incident beam. The topological absorber was experimentally demonstrated in the microwave part of the spectrum (10 GHz – 27 GHz) with water in a capillary tube as a dissipating medium, which achieved more than 90% absorption of radially polarised waves. The absorption spectra were manipulated by varying salinity of the water.

To design topological light absorbers and investigate their behaviour, I developed an analytical Mie theory and derived the material property requirements for complete dissipation of light in the absorber. I designed an absorber of optical toroidal pulses based on Tungsten Ditelluride as an absorbing material, delivering over 95% absorption of energy in the spectral range between 700nm to 900nm.

To conclude, my work has contributed to the field of toroidal electrodynamics and will be of interest to a wide range of research areas within the fields of topologically structured light, its characterisation and applications. I expect the main applications of my work will be in communications, sensing and spectroscopy.