Under-filling trapping objectives optimizes the use of the available laser power in optical tweezers.

For optical tweezers, especially when used in biological studies, optimizing the trapping efficiency reduces photo damage or enables the generation of larger trapping forces. One important, yet not-well understood, tuning parameter is how much the laser beam needs to be expanded before coupling it into the trapping objective. Here, we measured the trap stiffness for 0.5-2 μm-diameter microspheres for various beam expansions. We show that the highest overall trapping efficiency is achieved by slightly under-filling a high-numerical aperture objective when using microspheres with a diameter corresponding to about the trapping-laser wavelength in the medium. The optimal filling ratio for the lateral direction depended on the microsphere size, whereas for the axial direction it was nearly independent. Our findings are in agreement with Mie theory calculations and suggest that apart from the choice of the optimal microsphere size, slightly under-filling the objective is key for the optimal performance of an optical trap.

[1]  E. Stelzer,et al.  Three‐dimensional high‐resolution particle tracking for optical tweezers by forward scattered light , 1999, Microscopy research and technique.

[2]  L. Oddershede,et al.  Optimizing immersion media refractive index improves optical trapping by compensating spherical aberrations. , 2007, Optics letters.

[3]  S. Hell,et al.  Aberrations in confocal fluorescence microscopy induced by mismatches in refractive index , 1993 .

[4]  D. R. Skinner,et al.  Measurement of the radius of a high-power laser beam near the focus of a lens , 1972 .

[5]  Francesco S. Pavone,et al.  Calibration of optical tweezers with positional detection in the back focal plane , 2006, physics/0603037.

[6]  Anita Jannasch,et al.  Measuring the complete force field of an optical trap. , 2011, Optics letters.

[7]  Alexander Rohrbach,et al.  Stiffness of optical traps: quantitative agreement between experiment and electromagnetic theory. , 2005, Physical review letters.

[8]  H. Rubinsztein-Dunlop,et al.  Multipole Expansion of Strongly Focussed Laser Beams , 2003 .

[9]  Jonathon Howard,et al.  Coated microspheres as enhanced probes for optical trapping , 2008, NanoScience + Engineering.

[10]  V. Mahajan.  Uniform versus Gaussian beams: a comparison of the effects of diffraction, obscuration, and aberrations , 1986 .

[11]  Jonathon Howard,et al.  Surface forces and drag coefficients of microspheres near a plane surface measured with optical tweezers. , 2007, Langmuir : the ACS journal of surfaces and colloids.

[12]  Norman R. Heckenberg,et al.  Optical tweezers computational toolbox , 2007 .

[13]  K. Svoboda,et al.  Biological applications of optical forces. , 1994, Annual review of biophysics and biomolecular structure.

[14]  J. Howard,et al.  LED illumination for video‐enhanced DIC imaging of single microtubules , 2007, Journal of microscopy.

[15]  E. Schäffer,et al.  Optical tweezers with millikelvin precision of temperature-controlled objectives and base-pair resolution. , 2009, Optics express.

[16]  A. Samadi,et al.  Optimal beam diameter for optical tweezers. , 2010, Optics letters.

[17]  Jonathon Howard,et al.  Optical trapping of coated microspheres. , 2008, Optics express.

[18]  Kishan Dholakia,et al.  Optical Tweezers With Increased Axial Trapping Efficiency , 1998 .

[19]  N. B. Viana,et al.  Characterization of objective transmittance for optical tweezers. , 2006, Applied optics.

[20]  A. Ashkin.  Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime. , 1992, Methods in cell biology.