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Static optical sorting in a laser interference field
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View: Figures


Image of FIG. 1.
FIG. 1.

Experimental configuration. Linearly polarized laser beams from laser Verdi V5 (Coherent, , ) are focused with lenses into the optical glass cuvette (23/G/5, Starna Cells). The white arrows and dots mark beam polarizations that are controlled by polarizing beamsplitters with half-wave plates. All three beams overlap on the top surface of the cuvette and form a sorting region of size elongated along the axis. The distance between the interference fringes is adjusted by changing the distance between the two beams coming from the left and is given by , where is the laser wavelength in the medium and angles and are defined in the figure. The Roman numbers I–IV in the inset indicate four different types of object behavior, which is further described in Fig. 2.

Image of FIG. 2.
FIG. 2.

Calculated optical forces as a function of particle diameter in the arrangement of three incident plane waves with and (top) or (bottom). The intensities of both copropagating beams correspond to the experimental values and the intensity of the third beam is higher. (thick curves) corresponds to the extremal optical force across the fringes (positive for equilibrium position located at the fringe intensity maximum). (thin curves) corresponds to the optical force along the fringes for the object placed at fringe intensity maximum (regions I and II) or minimum (regions III and IV). The ripples come from Mie scattering resonances. The refractive indices of polystyrene and water were and .

Image of FIG. 3.
FIG. 3.

Sorting of polystyrene microspheres of diameters (Duke Scientific 5100A, ) and (Duke Scientific 240, ) placed into interference fringes of width and vertically oriented in the figure. Smaller particles settle in the fringe intensity maximum (type I) and the bigger ones in intensity minimum (type IV). The time scale in seconds is denoted above the figure.

Image of FIG. 4.
FIG. 4.

Sorting of polystyrene microspheres of types I and II of diameters (Duke Scientific 4K-02, ) and (Duke Scientific 240, ) placed into intensity maxima of interference fringes of width . For the sake of clarity, one selected particle is marked with a cross and the fast motion of two smaller ones is followed with the arrows.

Image of FIG. 5.
FIG. 5.

Sorting of living yeast cells and spores. The smaller objects are yeast spores; they settle in the fringe maxima and are pushed along the positive direction of the axis while the larger objects (cells) are drawn to the intensity minima and subsequently pushed in negative axis direction. The width of the fringes was set to .


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Scitation: Static optical sorting in a laser interference field