Abstract
Structured plasmonic metals are widely employed for achieving nonlinear functionalities at the nanoscale due to their ability to confine and enhance electromagnetic fields and strong, inherent nonlinearity. Optical nonlinearities in centrosymmetric metals are dominated by conduction electron dynamics, which at the nanoscale can be significantly affected by the nonlocal effects. Here we show that nonlocal corrections, being usually small in the linear optical response, define nonlinear properties of plasmonic nanostructures. Using a full non-perturbative time-domain hydrodynamic description of electron plasma under femtosecond excitation, we numerically investigate harmonic generation in metallic Archimedean nanospirals, revealing the interplay between geometric and nonlocal effects. The quantum pressure term in the nonlinear hydrodynamic model results in the emergence of fractional nonlinear harmonics leading to broadband coherent white-light generation. The described effects present a novel class of nonlinear phenomena in metallic nanostructures determined by nonlocality of the electron response.
Highlights
Structured plasmonic metals are widely employed for achieving nonlinear functionalities at the nanoscale due to their ability to confine and enhance electromagnetic fields and strong, inherent nonlinearity
Being able to investigate the interplay between the topology of the nanostructure and various sources of nonlinearities of metal plasma, we demonstrate that electromagnetic nonlocality, manifesting itself via the quantum pressure term, has the prominent role in the nonlinear response of small and nanostructured geometries
The polarizability of metal structures can be introduced in this equation via natural hydrodynamic variables: the macroscopic position-dependent electron density n(r,t) and velocity t(r,t), which are subsequently related to the polarization current as
Summary
Structured plasmonic metals are widely employed for achieving nonlinear functionalities at the nanoscale due to their ability to confine and enhance electromagnetic fields and strong, inherent nonlinearity. Using a full non-perturbative time-domain hydrodynamic description of electron plasma under femtosecond excitation, we numerically investigate harmonic generation in metallic Archimedean nanospirals, revealing the interplay between geometric and nonlocal effects. Using a fully non-perturbative time-domain hydrodynamic model coupled with Maxwell’s equations, we demonstrate higher harmonics and supercontinuum generation from metal nanostructures of Archimedean spiral shapes. Being able to investigate the interplay between the topology of the nanostructure and various sources of nonlinearities of metal plasma, we demonstrate that electromagnetic nonlocality, manifesting itself via the quantum pressure term, has the prominent role in the nonlinear response of small and nanostructured (on B10 nm length scales) geometries. The properly tuned interplay between nonlocality and nonlinearity is responsible for very efficient harmonics mixing and the resulting broadband white light generation, as demonstrated below
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