Abstract
We propose an optical tweezers setup based on an annular-shaped laser beam that is efficient to trap 2.8 m-diameter superparamagnetic particles. The optical trapping of such particles was fully characterized, and a direct absolute comparison with a geometrical optics model was performed. With this comparison, we were able to show that light absorption by the superparamagnetic particles is negligible for our annular beam tweezers, differing from the case of conventional Gaussian beam tweezers, in which laser absorption by the beads makes stable trapping difficult. In addition, the trap stiffness of the annular beam tweezers increases with the laser power and with the bead distance from the coverslip surface. While this first result is expected and similar to that achieved for conventional Gaussian tweezers, which use ordinary dielectric beads, the second result is quite surprising and different from the ordinary case, suggesting that spherical aberration is much less important in our annular beam geometry. The results obtained here provide new insights into the development of hybrid optomagnetic tweezers, which can apply simultaneously optical and magnetic forces on the same particles.
Highlights
The optical and magnetic manipulation of microparticles has become in the past few years indispensable tools in biophysics and soft matter research [1,2,3,4]
Before performing the force measurements and calculations, the relevant optical parameters of our system were carefully measured. The knowledge of such parameters is fundamental to characterize the trap that was used to perform the experiments. They are essential to perform the numerical calculations of the optical forces and to obtain the theoretical prediction of the trap stiffness κ that we compare with our experiments
Iyengar et al have previously demonstrated that the trap stiffness decreases with the laser power when there is a considerable absorption of light by the superparamagnetic particles, due to heating effects [14]
Summary
The optical and magnetic manipulation of microparticles has become in the past few years indispensable tools in biophysics and soft matter research [1,2,3,4] Fields such as single-molecule biophysics [5,6], cell biology [7,8] and colloidal science [9] have experienced considerable advances with the advent of the tweezers techniques, namely the optical and magnetic tweezers. These techniques allow one to apply forces and torques on the particles of interest, which are usually attached to the systems that one intends to manipulate, stretch and/or rotate. An apparatus that allows one to explore the best features of the two tweezers techniques would be of great interest
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