Longitudinal Radiation Force of Laser Pulses and Optics of Moving Particles

For a long time transverse and longitudinal optical forces are used for non-contact and noninvasive manipulation of small individual particles. The following question arises: What is the impact of these forces on the ensemble of thousand particles in continuous media? The aim of this work is to find analytical expression of the radiation force and potential densities creating from 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 obtained force is proportional to initial pulse energy and inversely proportional to its time duration. In the femtosecond region the force becomes strong enough to confine the neutral particles into the pulse envelope and translate them with group velocity. In 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. The dipole interaction with the electromagnetic field of the particles captured into the pulse generate at carrier-to-envelope frequency, instead of at the carrying ones. This oscillation is in sub-THz range in gases and in THz in solids. In nonlinear regime instead of third harmonics, the ensemble of moving particles generates at frequency proportional to three times the frequency of the envelope-carrier.