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Orientation of flat particles in optical tweezers by linearly polarized light

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Abstract

Micrometer-sized objects with flat shapes have been oriented in optical tweezers formed by polarized light. The orienting torque originates from the anisotropic scattering of polarized light by the trapped particle. We investigated this effect experimentally on objects produced by photopolymerization. We determined and characterized the orienting torque acting on these particles, and the results were interpreted by model calculations. By manipulating particles with appropriately shaped optical tweezers, we can fully control the position of the particle in the trap. The torque exerted on the object can be measured and controlled. This angular trapping effect offers a useful extension of optical tweezer applications.

©2003 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. Cross-shaped test object to study angular trapping in optical tweezers formed by linearly polarized light: a and c, drawing of the object from two different views; b and d, photographs of the produced objects viewed from respective directions. The scale bar is 3 µm long.
Fig. 2.
Fig. 2. Cross-shaped test object shown in Fig. 1 held in the optical tweezers formed by linearly polarized light. The cross is positioned with the longer rod along the optical axis: the subfigures show different angular trapped positions. The scale bar is 3 µm long. A corresponding video clip can be downloaded form the webpage of our laboratory [17].
Fig. 3.
Fig. 3. Phase delay of the rotating particle to the rotating polarization as a function of the rotation rate.
Fig. 4.
Fig. 4. Torque exerted by the polarized light upon the trapped body as a function of the angle between the polarization plane and the long axis of the trapped flat object. ■, measured data; —, model calculation.

Equations (5)

Equations on this page are rendered with MathJax. Learn more.

F = 4 πvl { 0.5 C ln [ Rvρ ( 4 η ) ] } .
r = sin ( α β ) sin ( α + β ) , t = 2 sin β cos α sin ( α + β ) ,
r = tan ( α β ) tan ( α + β ) , t = 2 sin β cos α [ sin ( α + β ) cos ( α β ) ] ,
P = S c .
M = r × Δ P .
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