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Trapping force and optical lifting under focused evanescent wave illumination

Djenan Ganic, Xiaosong Gan, and Min Gu

Open Access Open Access

Abstract

A physical model is presented to understand and calculate trapping force exerted on a dielectric micro-particle under focused evanescent wave illumination. This model is based on our recent vectorial diffraction model by a high numerical aperture objective operating under the total internal condition. As a result, trapping force in a focused evanescent spot generated by both plane wave (TEM00) and doughnut beam (TEM* 01) illumination is calculated, showing an agreement with the measured results. It is also revealed by this model that unlike optical trapping in the far-field region, optical axial trapping force in an evanescent focal spot increases linearly with the size of a trapped particle. This prediction shows that it is possible to overcome the force of gravity to lift a polystyrene particle of up to 800 nm in radius with a laser beam of power 10 µW.

©2004 Optical Society of America

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Figures (4)
Fig. 1.
Fig. 1. Trapping efficiency mapping for a small and a large polystyrene particle of radius a, scanned in the X direction (light polarization direction) across the focused evanescent field. NA=1.65, λ=532 nm, ε=0.85, n 1=1.78 and n 2=1.33.

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Fig. 2.
Fig. 2. The calculated and measured maximal TTE of a polystyrene particle of 1 µm in radius as a function of the obstruction size ε for P and S scanning directions under plane wave and the doughnut beam illumination. The other conditions are the same as Fig. 1.

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Fig. 3.
Fig. 3. (a) The maximal ATE of a polystyrene particle of 1 µm in radius as a function of the obstruction size ε. (b) Dependence of the ATE on the virtual focus position for a small and large polystyrene particle (ε=0.85). The inset shows the phase of the Fresnel transmission coefficients as a function of the incident angle. The other conditions are the same as Fig. 1.

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Fig. 4.
Fig. 4. (a) The maximal ATE as a function of a polystyrene particle size (ε=0.85). The inset shows a schematic relation between the interaction cross-section area and the particle size. (b) The magnitudes of the axial force for a plane wave of power 10 µW and the gravity force for different particle sizes. The other conditions are the same as Fig. 1.

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