Public defence, Engineering Physics, M.Sc. Sagar Sehrawat

Title of the thesis: Scattering and near-field enhancement of light by plasmonic particles at higher-order multipole resonances

Thesis defender: Sagar Sehrawat 
Opponent: Professor Konstantin Bliokh, Donostia International Physics Center (DIPC), San Sebastián, Spain 
Custos: Professor Päivi Törmä, Aalto University School of Science

This study is focused on the nanoscale interaction of light with metal nanoparticles, such as silver and gold dimers and trimers, as well as periodic arrays of such nanostructures. While interaction of light with bulk matter is traditionally viewed via macroscopic reflection and refraction, engineering optical materials at the nanoscale unlocks unique optical properties. We explore scattering and near-field properties of light across varying particle sizes and shapes, utilizing both classical and scattering-current multipole expansions to map the complex current oscillations in the particle beyond the conventional electric-dipole approximation. This study is highly relevant to contemporary research in nanophotonics, metamaterials, and metasurfaces designed to control light at nanoscale. The research brings critical new information by addressing the limitations of the classical multipole expansion, which focuses primarily on far-field radiation patterns. By implementing the scattering-current multipole expansion technique, this work unravels the details of the charge and current excitations, providing deep quantitative and qualitative understanding of the intraparticle charge dynamics and local-field effects. The study demonstrates that, while smaller particles exhibit simple electric-dipole resonances, larger particles can show pronounced higher-order modes, such as quadrupoles and octupole. When particles are near each other, multipole hybridization effect takes place. Higher-order modes, traditionally neglected due to their weak far-field radiation, can contribute immensely to the near-field enhancement by the particles. One of the main results of this work shows that, in periodic dimer arrays coupled with surface lattice resonances, decreasing the array period causes octupole-assisted hotspots to significantly increase in local intensity while the conventional dipole excitation turns out to be self-quenched. This finding serves as a conceptually new approach to the creation of plasmonic sensors and other devices based on optical near-field enhancement. It can be used to engineer advanced metasurfaces, spectral filters, and molecular probes based on surface-enhanced Raman spectroscopy (SERS). Ultimately, it is concluded that higher-order multipoles provide an exceptional, underutilized platform for high-density plasmonic structures, for which the developed current-based multipole expansion can serve as a powerful design and characterization tool.

Keywords: multipole expansion, scattering current density, near-field enhancement, multipole hybridization, surface plasmon, surface lattice resonance, generalized scattering-current multipoles, optical metamaterials and metasurface.

Thesis available for public display 7 days prior to the defence at Aalto University's public display page. 

Contact information: 
sagar.sehrawat@aalto.fi 
https://www.aalto.fi/en/department-of-applied-physics/optics-and-photonics
https://www.linkedin.com/in/sagar-sehrawat-a7bb68352/