Anomalous wave phenomena in 2D materials

PhD Program

Speaker

Kateryna Domina

When

2025/12/12
10:00

Place

Assembly Hall of the Faculty of Psychology (Salón de Grados), EHU, Donostia/San Sebastián

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PhD Thesis defense by Kateryna Domina

Supervisor: Alexey Nikitin (DIPC, Ikerbasque)

Nanophotonics of 2D materials

Recent advances in two-dimensional (2D) van der Waals (vdW) materials (layered systems in which individual atomic planes are bonded by weak vdW attraction) have established them as a versatile platform supporting a rich family of polaritonic excitations, including plasmon, phonon, and exciton polaritons among other. These materials exhibit remarkable optical tunability, anisotropy, and field confinement, making them ideal for exploring light-matter interactions at the nanoscale. This thesis investigates several fundamental and applied aspects of polaritonics in 2D vdW materials, combining theoretical analysis, numerical modeling, and experimental near-field characterization using scattering-type scanning near-field optical microscopy (s-SNOM), providing new strategies for nanoscale light control, enhanced molecular sensing, and quantum photonic applications.

The thesis is organized into five chapters. The first chapter introduces the fundamental principles of light confinement and the diffraction limit, discussing how evanescent fields and surface polaritons enable subwavelength optical control. Different types of polaritons supported by 2D vdW materials are described, with emphasis on hyperbolic phonon polaritons in anisotropic vdW crystals, as well as quantum plasmon polaritons in magnetized charge-neutral graphene (CNG). The chapter concludes with a discussion of the conceptual foundations of wave vortices, which represent phase singularities in scalar wave fields.

The second chapter presents the experimental and computational methods used in this thesis. The s-SNOM technique is described in detail as a means of probing optical near-fields with nanometer-scale resolution, enabling direct imaging of polaritons in vdW materials. The chapter also introduces the finite element method, implemented in COMSOL Multiphysics, for full-wave electromagnetic simulations of complex structures studied in the thesis, and the Fourier-Floquet plane-wave expansion method, used in this thesis for investigation of isofrequency curve (IFC) topology of periodic structures such as metasurfaces. Together, these tools establish a comprehensive methodological foundation for the subsequent chapters.

The third chapter is focused in studying of wave vortices in 2D scalar wave field of very different nature. This chapter introduces a new class of wave vortices, referred to as type-II vortices, that form around subwavelength "holes" or exclusions in homogeneous 2D wave fields. In contrast to conventional type-I wave vortices, characterized by zero intensity at the core and orbital angular momentum localized on the wavelength scale, type-II vortices exhibit high-intensity and subwavelength localization of the angular momentum. We demonstrate, both theoretically and experimentally, that such vortices arise in diverse systems - from electromagnetic waves near metallic nanodiscs and nanoholes in hBN, to oceanic tidal waves around large islands. These results establish a new framework for subwavelength orbital angular momentum confinement, with potential applications in vortex microlasers, nonlinear optics, and acoustic or microfluidic systems.

Chapter four studies quantum polaritons in magnetized CNG, arisen from inter-Landau-levels transitions. It is demonstrated, for the first time, the existence of quantum hyperbolic magnetoexciton polaritons in metasurfaces formed by periodically arranged CNG nanoribbons under external magnetic fields, whose anisotropic optical conductivity is derived from tight-binding calculations. Our study reveals the formation of quantum polaritons with magnetic-field-tunable hyperbolic dispersion, featuring topological transitions in their IFCs from elliptical to hyperbolic geometries. Moreover, we identify a canalization regime, where polaritons propagate in a diffractionless, unidirectional manner along the ribbons, originating from the effective decoupling of high-index modes in individual nanoribbons. This phenomenon constitutes the first theoretical observation of tunable quantum hyperbolic polaritons.

The fifth chapter explore in-plane hyperbolic phonon polaritons in molybdenum trioxide for creating broadband anisotropic polaritonic nanoresonator. The proposed nanoresonator is based on an molybdenum trioxide slab placed over a circular nanohole in a gold membrane. The in-plane hyperbolicity of molybdenum trioxide enables formation of a continuum of Fabry–Perot resonances along distinct crystallographic directions, giving rise to a broadband optical response within a single nanocavity. Unlike conventional resonators, whose resonance frequency is fixed by geometry, these anisotropic cavities exhibit multi-resonant broadband behavior without any structural modification. This property makes them a good candidate, for example, for mid-infrared molecular fingerprint sensing, as demonstrated by detecting fingerprints of pentacene molecules placed inside the cavity. The ability to achieve broadband spectral coverage in a single resonator structure opens new avenues for compact, high-sensitivity molecular sensing and spectroscopy.