Optical tweezers in interaction with an apertureless probe
Résumé
We study the possibility of transfering a particle held in a far-field optical trap, namely the classical optical tweezers, to a near-field trap, in the form of a tapered metal tip (otherwise called an "apertureless probe"). The paper is theoretical: we compute the electromagnetic fields in both configurations, based on a vectorial nonparaxial representation of the Gaussian laser beam in the waist region. We afterwards use the coupled dipole method to compute the optical force acting on a spherical dielectric particle, in the 0.2−1 mum range in diameter. We find that either repulsion or attraction of the particle by the metal probe is possible, depending on the beam polarization state, particle size, and curvature radius of the metal tip. Equilibrium states of the particle in contact with the tip, under illumination by the laser beam, are predicted.