Subcycle engineering of laser filamentation in gas by harmonic seeding

Manipulating at will the propagation dynamics of high power laser pulses is a long-standing dream whose accomplishment would lead to the control of fascinating physical phenomena emerging from laser-matter interaction. The present work represents a significant step towards such a control by manipulating the nonlinear optical response of the gas medium. This is accomplished by shaping an intense laser pulse experiencing filamentation at the subcycle level with a relatively weak (similar or equal to 1%) third-harmonic radiation. The control results from quantum interference between a single- and a two-color (mixing the fundamental frequency with its third-harmonic) ionization channel. This mechanism, which depends on the relative phase between the two electric fields, is responsible for wide refractive index modifications in relation with significant enhancement or suppression of the ionization rate. As a first application, we demonstrate the production and control of an axially modulated plasma channel.