Figure 3
From: Enhancement of the ‘tractor-beam’ pulling force on an optically bound structure
(a) Comparison of the measured trajectories with the calculated lobes and particle velocities. The background is proportional to the calculated optical intensity (shown in log scale) of the total electric field averaged in y over one fringe length λSW/2. The white thin curves have the same meaning as in Figure 2a. Calculated velocities of the particle pair along the z axis are encoded in the length of the triangles (proportional to 
). For comparison, the velocity of an isolated particle visol is shown at the center of particle 1 placed at the origin of the system of coordinates. The solid zigzag curves represent experimental results, and their colors encode the direction of the particle pair motion (see Supplementary Information for more experimental results). (b) Comparison of the particle pair velocity, vpair, and the relative pair velocity, vpair/visol, for the selected configuration groups that are distinguished here by colors (the same as in Figure 2b) and denoted by Roman numerals as in Figure 3a. Different markers correspond to different experimental data sets performed under the same experimental conditions (incident angle of the S-polarized tractor beam α=2.15°). The curves show the calculated velocities of the particle pair if the hydrodynamic interaction between the particles is (dashed) and is not (full) added to the theoretical model (see Supplementary Information for more details). (c) Comparisons of the calculated velocities, vpair, of the particle pair along the z axis for different angles of incidence α and polarizations of the incident beam. Each horizontal gray segment corresponds to one steady-state configuration of the pair for considered α and polarization calculated in the same region of the y and z axis as in Figure 3a. The yellow curve in the top plots denotes the velocity, visol, of a single isolated particle in the tractor beam, and the bottom plots reveal the relative enhancement of the pulling (but also pushing) force (proportional to vpair/visol) in particular configurations of the incident beam and the particle pair. The velocity extremes are highlighted with the thick curves, pulling and pushing is encoded in the curve color as above. We considered the incident power density 1.8 mW μm−2 in all theoretical results presented in this figure.
