Manipulation of resonant metallic nanoparticle using 4Pi focusing system.

Metallic nanoparticles have fascinated scientists for over a century and are now heavily utilized in biomedical sciences and engineering. Due to its noncontact and holding nature, optical trapping is suitable to be combined with various applications to manipulate metallic nanoparticles. However, stable trapping of resonant metallic nanoparticles remains challenging due to the strong axial scattering force and severe optical heating effect. In this work, we propose novel optical tweezers constructed around a 4Pi focusing system that is capable of manipulating metallic nanoparticles even under the resonant condition. By properly modulating the spatial distribution of the illumination and adjusting the focusing condition, specific numbers of spherical spots with controllable locations can be generated in the focal region, providing multiple probes to interrogate the sample properties. Besides, stable three-dimensional optical trapping can be formed since the axial scattering force is canceled by the counter-propagating light. The greatly enhanced optical force arising from the extremely high focusing efficiency of the 4Pi focusing system enables to avoid the overheating effect by reducing the input power without destroying the mechanical stability. Moreover, complex motion trajectory of the metallic nanoparticles can be realized via introducing specific phase modulation to the illumination sequentially. The technique demonstrated in this work may open up new avenues for optical manipulation and their applications in various scientific fields.