Energy Flow and Group Velocity of Electromagnetic Surface Wave in Hydrodynamic Approximation

Abstract

Expressions for the energy density and the energy flow are derived for the electromagnetic surface wave (surface plasmon) on a semi-infinite metal surface by using the hydrodynamic approximation. A proof is given for the equality between the group velocity and the energy velocity. It is also shown that the direction of the energy flow in the metal is the same as that of propagation of the surface wave for sufficiently large wave numbers. This is contrary to the case in the local approximation.