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Longitudinal radiation force of laser pulses and nonlinear optics of moving neutral particles

For a long time, transverse and longitudinal optical forces have been used for non-contact manipulation of small individual particles. The following question arises: What is the impact of these forces on an ensemble of a thousand particles in continuous media? The aim of this work is to find analytical expressions of the radiation force and potential densities arising from a laser pulse propagating in dielectric media. This allows us to find an effective averaged longitudinal real force at the level of the laser pulse spot. The force obtained is proportional to the initial pulse energy and inversely proportional to its time duration. In the femtosecond region, the force becomes strong enough to confine neutral particles into the pulse envelope and translate them with the group velocity in gases. In solids, as silica for example, the longitudinal force of a femtosecond pulse is significantly greater than the molecular forces. Thus, the fine ablation in silica with short pulses may be due to this longitudinal force, which breaks down the molecular bonds. Additionally, after confinement into the pulse envelope, the moving particles produce new linear and nonlinear effects.