Negative Effective Mass in Plasmonic Systems.

We report the negative effective mass (density) metamaterials based on the electro-mechanical coupling exploiting plasma oscillations of a free electron gas. The negative mass appears as a result of the vibration of a metallic particle with a frequency of ω, which is close the frequency of the plasma oscillations of the electron gas m 2 relative to the ionic lattice m 1 . The plasma oscillations are represented with the elastic spring k 2 = ω p 2 m 2 , where ω p is the plasma frequency. Thus, the metallic particle vibrated with the external frequency ω is described by the effective mass m e f f = m 1 + m 2 ω p 2 ω p 2 - ω 2 , which is negative when the frequency ω approaches ω p from above. The idea is exemplified with two conducting metals, namely Au and Li.