Polaritons in anisotropic van der Waals crystals

PhD Program

Speaker

Kirill Voronin

When

2025/09/12
11:30

Place

CFM Auditorium (Donostia / San Sebastían)

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PhD Thesis defense by Kirill Voronin

Supervisors: Alexey Nikitin (DIPC, Ikerbasque Research Professor) 

Nanophotonics of 2D materials - Photonics

Nano-optics investigates light–matter interactions at deep subwavelength scales, overcoming classical diffraction limits and enabling advances in biosensing, quantum technologies, and integrated photonics. Central to this field are polaritons, coupled states of electromagnetic waves and material excitations. This thesis presents a comprehensive study of polaritons in anisotropic van der Waals (vdW) crystals and their nanostructures, alongside theoretical developments in near-field optical microscopy. The work explores the unique properties of polariton modes in biaxial crystal layers, deriving analytical dispersion relations, revealing noncollinearity between real and imaginary components of the wavevector, and proving the equality of group and energy velocities in low-loss regimes. The thesis further investigates twisted multilayer structures, demonstrating how interlayer rotation angles modulate polariton behaviour and enable robust canalization, diffraction-free propagation of polaritons. A key theoretical contribution is the development of a Dyadic Green’s Function (DGF) framework for the calculation of polariton fields in highly anisotropic media. Additionally, the thesis addresses fundamental aspects of scattering-type scanning near-field optical microscopy (s-SNOM), presenting the first exact analytical solution for the commonly used spheroidal tip model. Numerical simulations validate this model and reveal how tip geometry influences resonant sample responses, enhancing data interpretation. Collectively, this work deepens the understanding of anisotropic polaritons and provides theoretical tools to guide next-generation nano-optical technologies.