Under certain conditions, light incident on a metallic particle can excite a collective electron excitation known as a surface plasmon-polariton (SPP), a term that describes the nature of the interaction, which involves the metal’s free electrons (plasma) and light-induced polarization (polariton).

The surface plasmon mode is generally characterized by intense electric fields that decay exponentially away from the interface between the metal and the surrounding environment. Due to their unique properties, plasmonic systems have found a broad range of applications in various areas of science, including nanobiotechnology and biomedicine, where plasmons can be used for biosensing or for monitoring of biological cells and tissues. In optics, the large field strengths of surface plasmons can dramatically enhance a variety of phenomena, such as Raman scattering, photoluminescence and nonlinear optical effects. Also, from a more fundamental perspective, plasmonic systems constitute a very unique platform. Using two or more metallic elements in fact, it is possible to localize the electromagnetic radiation well below the limit of diffraction and generate electric field spots that are hundred times larger the incident radiation. These local hot-spots can be confined in deeply sub-wavelength volumes of the order of few cubic nanometers, making the light “sensitive” to the sub-atomic realm. In this regime, the classical description of light-matter interactions is not accurate enough and quantum aspects must be taken into account. The intrinsic multi-scale nature of such systems however makes their numerical investigation a real challenge.

Our work aims to develop novel numerical tools and theoretical methods to tackle multi-scale light-matter interaction problems. Plasmonic structures characterized by extremely small gaps are valuable candidates for achieving enhanced light-matter interactions, efficient nonlinear and quantum effects, thus leading to exotic new functionalities and applications.

Multiscale plasmonics
Our main goal is to develop simulation techniques to take into account quantum microscopic features at the scale of billions of atoms. A promising theory in this context is the quantum hydrodynamic theory, in which the quantum dynamics is solved via macroscopic hydrodynamic quantities such as the electron density, currents and quantum pressure. 

Nonlinear plasmonics
The richness of surface physics in plasmonic systems might be the key to greatly enhance nonlinear optical effects. In the quantum hydrodynamic theory the electron energy is expressed via energy functionals that are in general nonlinear functions of the electron density, and hence of the induced charge density, which is in turn proportional to the local electric field E. On top of the effects on the near-fields given by the interplay of the nonlocal properties, we expect a whole new class of nonlinear sources that are proportional to electron density gradients.

Light-matter strong coupling
Understanding and controlling emission properties of single quantum emitters is a decisive and timely research field for quantum information applications and biosensing. We are currently numerically investigating nonlocal effects in the limit of the hydrodynamic theory for a dipole emitting near metal nanostructures in both the weak and strong coupling regimes. 

Our group has access to the CBN HPC Laboratory hosting a cluster equipped with 224 cores and 1TB of memory