The volume plasmon is traditionally defined as a one-dimensional collective oscillation of the free-charge carriers in a metallic volume. Here, we use an alternative approach with the geometry of a lossy Fabry–Perot cavity in a metallic slab. The field equations now show singularities at the plasma resonance, but these can be worked away. We find that the volume plasmon is not purely longitudinal, as in the classical picture; that it does not show evanescence, that its magnetic field is zero, and that at resonance the Fabry–Perot reflectance of the resonant slab equals one. These attributes differentiate the volume plasmon more fundamentally from the surface plasmon than was thought up to now.
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