Generalized plasma waves in layered systems and their spectroscopic signatures.

In a layered system the hybrid modes provided by the propagation of electromagnetic waves in the matter identify two well-separated energy scales connected to a large intra-layer and to a small inter-layer plasma frequency. Despite the wide interest in their detection and manipulation by means of various experimental protocols the physical ingredients underlying a unified description of plasma waves in such systems, valid at arbitrary energy and momentum, have been for long time poorly understood. In this talk I will review our recent progresses in the description of the nature and the spectroscopic signatures of what we named generalized plasma waves in layered systems. I will first focus on their general properties, showing that the anisotropy of the current response leads to two intertwined hybrid light-matter modes with mixed longitudinal and transverse character, while a purely longitudinal plasmon is only recovered for wavevectors larger than the crossover scale set in by the plasma-frequency anisotropy. In the second part I will discuss how such modes emerge in EELS spectroscopy, that measures the density response at arbitrary energy and momentum.