Philip H. Jones, Onofrio M. Maragò & Giovanni Volpe
Fig. 2.8 — Counter-propagating optical tweezers
(a) A glass (radius a = 3 μm, np = 1.50) sphere immersed in water (nm = 1.33) and illuminated by a light ray is attracted towards the ray by the gradient force and pushed forward by the scattering force. (b) An air bubble (a = 3 μm, nm = 1.00) in water is pushed away from the ray. (c) The addition of a second counter-propagating ray manages to stabilise the glass sphere position. (d) Also two counter-propagating rays at an angle generate a stable trapping position towards which the glass sphere is attracted. The black arrows represent the forces. In all cases, only the deterministic optical forces are taken into account, while the Brownian motion of the particles is neglected. The times correspond to the overdamped motion of the particle in the viscous medium (see details in Chapter 7).