Quantitative comparison of single-beam gradient-force optical traps and dual-beam optical traps

摘要

In this paper, we compare the performance of the single beam gradient-force trap (SBGFT) and the counter propagating dual-bam trap (CPDBT) quantitatively in terms of three performance parameters, namely, the transverse trapping efficiency, the width of the stable trapping zone, and the axial stiffness. Ray-Optics Model (for optical trapping efficiency, the width of the stable trapping zone, and the axial stiffness. Ray-Optics Model (for optical trapping of Mie particles) was used to obtain the numerical results. In the SBGFT, the particle is trapped in the vicinity of the focal spot of a strongly focused (N.A. approx 0.65 to 1.3) laser beam by gradient forces in both the transverse and the axial directions. In the CPDBT, with the two counter-propagating beams often mildly focused (N.A. <0.6), the particle is confined transversely by the tansverse gradient forces of the two beams, andstabilized axially by balancing the scattering forces from the two beams. Depending on the separation between the two beam waists, there can be more than one stable trapping zones in the CPDBT. Qualitatively, one obvious key advantage of SBGFT is that it is very simple to implement. In contrast, the CPDBT requires precised alignment of the two beams. The latter, however, allows longer working distance and offers more degrees of freedom. The theoretical values of the aforementioned performance parameters for the CPDBT vary over a wide range because they depend on the distance between two beam waists. This extra degree of freedom in the CPDBT allows us to trade off one performance parameter against the others. We have also measured these performance parameters experimentally to verify the general trend predicted by the theoretical model.