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 m2 relative to the ionic lattice m1. The plasma oscillations are represented with the elastic spring k2 = ω2pm2, where !p is the plasma frequency. Thus, the metallic particle vibrated with the external frequency ! is described by the effective mass m_eff = m₁ +m2ω2p/ω2p-ω2, which is negative when the frequency ω approaches ωp from above. The idea is exemplified with two conducting metals, namely Au and Li.