Temporal Dependence of Photophoretic Force Optically Induced on Absorbing Airborne Particles by a Power-Modulated Laser

Abstract

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.

Technique for rapid mass determination of airborne micro-particles based on release and recapture from an optical dipole force trap

“…Our drop-and-restore method may also be used to study highly absorbing particles confined in photophoretic traps provided the effects of amplitude modulation in such traps are carefully modeled [35,36]. Other extensions could involve the investigation of particulates trapped in liquids or media of higher viscosity.…”

“…This technique has been successful in characterizing masses on the femtogram scale with a precision of 0.25%. Other examples of mass determinations in the range of 10 −10 − 10 −15 kg involve photophoretic traps [35] and have achieved precision at the level of a few percent [36,37].…”

Controlled levitation of nanostructured thin films for sun-powered near-space flight

“…However, it is also possible to generate a photophoretic force in ultrathin structures that have a negligible temperature difference but instead have different surface properties on the top and bottom surfaces, as previously demonstrated for micron-sized particles (28)(29)(30). In the free-molecular regime, gas molecules colliding with a heated structure absorb energy from the surface and leave with a higher average speed.…”

Trapping multiple absorbing particles in air using an optical fiber by photophoretic forces

“…Note that here we neglect the radiation pressure force (F r ) as for absorbing particles, the value of (F r ) is much lesser than the gravitational force on the particle. A ball-park estimate can be found from the expression F r = (πa 2 )I(1 + R)/c, where I is the laser intensity, R is the reflectivity of the particle, and c is the velocity of light [11,15]. Thus, plugging in the values of I, and c-and assuming the value of R to be maximum, i.e.…”