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 a plethora of fascinating physical phenomena emerging from laser-matter interaction. The present work represents a significant step towards such an ideal control by manipulating the nonlinear optical properties of the gas medium at the quantum level. This is accomplished by engineering the intense laser pulse experiencing filamentation at the subcycle level with a relatively weak (about 1%) third-harmonic radiation. The control results from quantum interferences between a single and a two-color (mixing the fundamental frequency with its 3rd 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 that could be used for quasi-phase matched laser wakefield acceleration.