Enhancement of laser trapping force by spherical aberration correction using a deformable mirror

We have developed a method to enhance axial trapping force in optical tweezers by aberration correction of a laser beam with a membrane deformable mirror. The axial trapping force is strongly dependent on the quality of the laser beam spot, which is deteriorated by aberration due to the refractive index mismatch between a cover glass and water. The aberration correction, therefore, is crucial for stable trapping of a particle and for weak-force measurement with laser trapping. We have evaluated spring constants of the trapping force in the axial direction with and without aberration correction. The enhancement factor of the spring constants by the aberration correction has been achieved as 1.35 for the case of 5-μm sample thickness and as 1.83 for the case of 10-μm sample thickness. The numerical simulation is coincident with the experimental results.