Temporal dependence of optically-induced photophoretic force on absorbing airborne particles by a power-modulated laser

Photophoretic force due to the optically-induced thermal effect provides an effective way to manipulate the light-absorbing particles suspended in ambient gases. However, how this force temporally responds to the intensity modulation of the illumination light is unclear. Here, by vertically trapping a micron-sized absorbing particle with a negative photophoretic force generated by a focused Gaussian beam, we demonstrate that the temporal change in the photophoretic force in response to the intensity modulation is remarkably slow (with a time constant up to ~1 s) due to the slow change in the particle's temperature. When the trapping beam is turned off for a few tens or hundreds of milliseconds, the trapped particle is found to be pulled up towards the light source by the remained photophoretic force, whereas when the intensity of the trapping beam is increased for a short duration, the particle is pushed off by the radiation pressure. The instantaneous position of the trapped particles following the intensity modulation of the trapping laser is tracked and theoretically modeled. The understanding of the temporal behavior of the photophoretic force would be useful for the control of photophoretic-based optical pulling, transportation, and manipulation of atmospheric particles.