Figure 3.13 — Optical binding Figure 3.13 — Optical binding. Longitudinal binding between two dipoles. (a) The first particle radiates towards the second particle in the same direction as the incident field and with the same wavevector size; therefore, the phase difference between the incident and scattered fields is independent of the inter-particle distance and the optical binding force on the second particle is always directed towards the first particle [Eq. (3.72)]. (b) In contrast, the second particle radiates backwards against the incident wave propagation; thus the phase difference between the incident and scattered waves depends on the inter-particle distance as 2k0Δz and results in an oscillatory behaviour of the binding force [Eq. (3.74)].