Speckle light fields result from the interference of several optical waves with random phase. They can be generated by several techniques, e.g., (a) reflection from a rough surface, (b) transmission through a highly scattering medium (e.g., a biological tissue) and (c) mode mixing in a multimode optical fibre; the laser beam is sketched as a series of rays that interfere and generate the speckle. (d)–(f) The experimental trajectories (solid lines) of a silica bead (a = 1.03 ± 0.03 μm, np = 1.42) in water (nm = 1.33) under the action of the optical forces produced by a speckle show the progressive particle confinement as the average speckle intensity increases (⟨I⟩ = 0.12 μW/μm2 in (d), ⟨I⟩ = 1.43 μW/μm2 in (e) and ⟨I⟩ = 5.77 μW/μm2 in (f)). The backgrounds are the corresponding images of the speckle patterns generated by mode mixing in a multimode optical fibre. (g) Calculated optical force field (arrows) exerted on a silica bead in a simulated speckle pattern (background). (h)–(j) Corresponding simulated trajectories (solid lines) of silica particles moving in speckle fields of the same average intensity as in (d)–(f). The dashed lines delimit the area corresponding to the force field distribution in (g). The average modulus of the calculated force exerted by the speckle field is (h) ⟨F⟩ = 0.14 fN, (i) ⟨F⟩ = 1.82 fN, and (j) ⟨F⟩ = 7.3 fN. All trajectories are recorded or simulated for 420 s.
Figures (d)–(j) are reprinted from Volpe et al., Opt. Express 22, 18 159–67. Copyright (2014) The Optical Society.
Fig. 12.5 — Speckle optical tweezers