Optical properties of cluster plasma

It is shown that unlike a gas plasma or an electron plasma in a metal, an i onized clustered material ("cluster plasma") permits propagation below the plasma cut-off of electromagnetic (EM) waves whose phase velocity is close to but below the speed of light. Its unique properties allow a variety of a pplications, including direct acceleration of particles with its EM fields and the phase matching of waves of high harmonic generation (HHG). The part icle-in-cell simulation demonstrates the new optical mode, the cluster mode . (C) 1999 American Institute of Physics. [S1070-664X(99)01410-X].