Experimental observation of particle cones formed by optical trapping

We report on observation of particle cones formed by optical trapping of absorbing particles in air using two sets of simple geometric optical schemes. Further, the trapped particles on a cone in both schemes are size-sorted with large particles or particle ensembles close to the cone vertex. This new experimental observation shows an excellent example of 3D particle trapping between the two extreme cases, photon radiation trapping of nonabsorbing particles and photophoretic trapping of strongly absorbing particles; and the observation may challenge theoretical calculations of the trapping forces applied in this case.