Optical trapping and manipulation of particles in solutions has been first reported in 1970 by A. Ashkin. Traditional studies involving optical trapping are focused on manipulation of micrometer-sized particles, such as polystyrene beads. The ability to apply piconewton-level forces with simultaneous displacement measurements with nanometer precision of the micron-sized particles has paved the route for the optical tweezers for single-molecule studies, studies of the physics of colloids and mesoscopic systems, mechanical properties of the polymers and more. However, optical trapping becomes challenging, once the size of a manipulated object lies within intermediate size range e.g. nanoscale. The types of the structures, which fall within this range, are quite broad: quantum dots, nanowires, nanotubes, graphene flakes and metallic nanoparticles. However, only few studies explored nano-scale trapping as a tool for cross-disciplinary studies and applications
. Trapping of nanoparticles faces several major challenges, which could be briefly summarized in 3 categories – detection/imaging, trapping, and resolution. Traditional bright-field transmission microscopy cannot be employed for visualization and detection of deep subwavelength particles (below 100 nm) due to diffraction limit constrains. While several advanced optical techniques are capable to deliver imaging beyond the diffraction limit, dark field microscopy is the most preferable and already tested tool for detection of scattering from particles down to 5nm in size. Incorporation of dark-field imaging with optical tweezers will enable direct detection of colloidal nanoparticles (e.g. silver, gold and high-index dielectrics) and verification of trapping events.