Recent developments in research on drivers for inertial confinement fusion identified the need to develop laser amplifiers with efficiencies of 80% or more. This article discusses a novel free electron laser amplifier that is based on vacuum interaction of laser beams with electrons, clusters of condensed matter, or neutral atoms. The scheme is based on the inversion of the ponderomotive expulsion of plasma from a focussed laser beam. The problem of Liouville conservation of phase space is overcome by employing the laser pulse transient processes. The optical energy for amplification is provided by the transfer of the translative kinetic energy of electrons, clusters, or neutral atoms that are injected nearly laterally into the laser pulse. The gains predicted to be achievable with this process are of interest for clusters, although the cluster densities presently available are too low to achieve the desired level of amplification. Neutral beams are shown to have the greatest potential for achieving large amplifications.