  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.
