Abstract
Plasmonic antennas improve the stiffness and resolution of optical tweezers by producing a strong near-field. When the antenna traps metallic objects, the optically-resonant object affects the near-field trap, and this interaction should be examined to estimate the optical force accurately. We study this effect in detail by evaluating the force using both Maxwell's stress tensor and the dipole approximation. In spite of the strong optical interaction between the particle and the antenna, the results show that the dipole approximation remains accurate for calculating forces on Rayleigh particles. For particles whose sizes exceed the dipole limit, we observe different coupling regimes where the force becomes either attractive or repulsive. The distributions of field amplitudes and polarization charges explain such a behavior.
Highlights
Following a series of experiments by Ashkin [1,2], optical tweezers have established themselves as successful tools, in biological studies, where they allow viruses, bacteria, and even DNA molecules to be manipulated directly with light [3,4,5,6]
When we introduce the actual particle geometry for numerical calculations with Maxwell’s stress tensor (MST), the gaps formed between the particle and the corners of the antenna enhance the field, leading to significantly greater forces for larger particles
The discrepancy between the two approaches become noticeable for extreme cases, for example, when the antenna is on resonance, and the particle is in the location of high-intensity fields
Summary
Following a series of experiments by Ashkin [1,2], optical tweezers have established themselves as successful tools, in biological studies, where they allow viruses, bacteria, and even DNA molecules to be manipulated directly with light [3,4,5,6]. We can trap and manipulate small particles, typically in the order of microns, including dielectric and metallic ones [7,8]. Conventional optical trapping has a limit in its stiffness and resolution, especially when manipulating nanoscale particles [2,9,10]. The optical force scales down with the volume of the object [9]. Another issue is the diffraction limit of the trapping laser: an object much smaller than the focal spot or the beam waist experiences unstable trapping [2]. In order to enhance the trapping stiffness of such small objects, one can increase the power of the trapping laser for example [2], or implement an alternative trapping geometry such as counter-propagating beams [11]
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