Ponderomotive Self-focusing of Surface Plasma Wave

A large amplitude surface plasma wave (SPW) propagating over a conductor–vaccum interface with Gaussian intensity profile transverse to the direction of propagation ( $ widehat{z} $ ) and surface normal ( $ widehat{x} $ ) is shown to undergo periodic self-focusing due to ponderomotive nonlinearity. The ponderomotive force on electrons arises due to the rapid decline in surface wave amplitude with the depth inside the conductor. In case of plasma, this leads to ambipolar diffusion of plasma, whereas in metals, only electron displacement occurs until the space charge balances the ponderomotive force on electrons. For a surface plasma wave, having Gaussian amplitude profile in y, the maximum electron density depression occurs on the axis (y?=?0) and the effect weakens as |y| increases. The axial portion of SPW thus travels with slower phase velocity than the nonaxial portion leading to self-focusing.