Fig. 2.11 — Optical trap stiffness

Trapping efficiencies for a glass (radius a = 3 μm, n_p = 1.50) sphere immersed in water (n_m = 1.33) as a function of the particle displacement (a) along the optical axis z (x = y = 0), where both the gradient and the scattering forces are aligned along the z-axis, and (b) along the transverse axis x (y = z = 0), where the gradient force is aligned along the x-axis and the scattering force is aligned along the z-axis. The focus is obtained overfilling a water-immersion objective (NA=1.20).