Radiation forces and confinement of neutral particles into the pulse envelope. New regime of collision ionization

Abstract

For a long time, transverse optical forces with continuous-wave laser beams have been used for non-contact manipulation of micro-scale objects. By replacing the continuous-wave lasers with pulse-mode lasers many novel physical effects related with the influence of Poynting vector on the longitudinal part of the optical force can be observed. The following question arises: What is the impact of this longitudinal addition to the optical force, connected with the flow of energy, on the ensemble of 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 a dielectric media. This allows us to find an effective averaged longitudinal real force at the level of the laser pulse’s spot. The obtained force 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. Additionally, if the trapped particles into the pulse’s spot are with high density, the probability of collision with the free atoms and molecules in air become significant. The collision energies are in the range of 12–24 GeV and high enough to ionize the neutral atoms. Thus, a new type of collision ionization can be observed, when powerful femtosecond pulses propagate in gaseous media.