Figure 2.12 — Dependence of optical forces on numerical aperture

Fig. 2.12 — Dependence of optical forces on numerical aperture.
Figure 2.12 — Dependence of optical forces on numerical aperture. Optical forces in the longitudinal (zx) and transverse (xy) planes as a function of the NA produced by a focused circularly polarised light beam on a glass (radius a = 3 μm, np = 1.50) sphere immersed in water (nm = 1.33). As the NA decreases, the trapping in the longitudinal plane is lost due to the overwhelming presence of scattering forces. In the transverse plane, trapping is possible also at low NA because it is mainly due to gradient forces.