Optical-trapping forces exerted on polystyrene microspheres are predicted and measured as a function of sphere size, laser spot size, and laser beam polarization. Axial and transverse forces are in good and excellent agreement, respectively, with a ray-optics model when the sphere diameter is ≥ 10 μm. Results are compared with results from an electromagnetic model when the sphere size is ≤ 1 μm. Axial trapping performance is found to be optimum when the numerical aperture of the objective lens is as large as possible, and when the trapped sphere is located just below the chamber cover slip. Forces in the transverse direction are not as sensitive to parametric variations as are the axial forces. These results are important as a first-order approximation to the forces that can be applied either directly to biological objects or by means of microsphere handles attached to the biological specimen.
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In micrometers, ±0.03 μm.
The effect of the phase ring was not accounted for in the model calculations. Previous calculations12 indicate that the difference in Q is ∼2%.
Phase-contrast objective (ph 3).
This objective had an internal aperture that determined the effective numerical aperture. The aperture was continuously variable from 0.8 to 1.25.
Table 2
Maximum Backward Axial Trapping Q in Water as a Function of Sphere Diametera
Sphere Type
Diameter (μm)
Q
Amorphous silica
1
−0.006 ± 0.001
Polystyrene
5
−0.070 ± 0.015
Polystyrene
10
−0.077 ± 0.014
Polystyrene
15
−0.091 ± 0.015
Polystyrene
20
−0.10 ± 0.015
Trapping wavelength of 1.06 μm. Microscope objective was Zeiss Neofluor 100× /1.3 without phase ring.
Table 3
Maximum Backward Axial Trapping Q as a Function of the Numerical Aperture and the Distance Below the Cover Glassa
Polystyrene microspheres in water; trapping wavelength of 1.06 μm and a spot size of ≈ 0.4 μm, based on the data in Fig. 3.
The symbol ║ denotes parallel and ┴ denotes perpendicular to the polarization direction of the laser beam.
Table 5
Maximum Transverse Trapping Q as a Function of Maximum Convergence Anglea
20 μm polystyrene microspheres in water; trapping wavelength of 1.06 μm and sphere motion perpendicular to the laser beam polarization.
This objective had an internal aperture that determined the effective numerical aperture. The aperture was continuously variable from 0.8 to 1.25.
Tables (5)
Table 1
Measured Spot Size for Various High Numerical Aperture Microscope Objectives and the Corresponding Maximum Backward Axial Trapping Q
In micrometers, ±0.03 μm.
The effect of the phase ring was not accounted for in the model calculations. Previous calculations12 indicate that the difference in Q is ∼2%.
Phase-contrast objective (ph 3).
This objective had an internal aperture that determined the effective numerical aperture. The aperture was continuously variable from 0.8 to 1.25.
Table 2
Maximum Backward Axial Trapping Q in Water as a Function of Sphere Diametera
Sphere Type
Diameter (μm)
Q
Amorphous silica
1
−0.006 ± 0.001
Polystyrene
5
−0.070 ± 0.015
Polystyrene
10
−0.077 ± 0.014
Polystyrene
15
−0.091 ± 0.015
Polystyrene
20
−0.10 ± 0.015
Trapping wavelength of 1.06 μm. Microscope objective was Zeiss Neofluor 100× /1.3 without phase ring.
Table 3
Maximum Backward Axial Trapping Q as a Function of the Numerical Aperture and the Distance Below the Cover Glassa
Polystyrene microspheres in water; trapping wavelength of 1.06 μm and a spot size of ≈ 0.4 μm, based on the data in Fig. 3.
The symbol ║ denotes parallel and ┴ denotes perpendicular to the polarization direction of the laser beam.
Table 5
Maximum Transverse Trapping Q as a Function of Maximum Convergence Anglea
20 μm polystyrene microspheres in water; trapping wavelength of 1.06 μm and sphere motion perpendicular to the laser beam polarization.
This objective had an internal aperture that determined the effective numerical aperture. The aperture was continuously variable from 0.8 to 1.25.