Dynamically Reconfigurable Bipolar Optical Gradient Force Induced by Mid-Infrared Graphene Plasmonic Tweezers for Sorting Dispersive Nanoscale Objects

Existing techniques for optical trapping and manipulation of microscopic objects, such as optical tweezers and plasmonic tweezers, are mostly based on visible and near-infrared light sources. As it is in general more difficult to confine light to a specific length scale at a longer wavelength, these optical trapping and manipulation techniques have not been extended to the mid-infrared spectral region or beyond. Here, it is shown that by taking advantage of the fact that many materials have large permittivity dispersions in the mid-infrared region, optical trapping and manipulation using mid-infrared excitation can achieve additional functionalities and benefits compared to the existing techniques in the visible and near-infrared regions. In particular, it is demonstrated that by exploiting the exceedingly high field confinement and large frequency tunability of mid-infrared graphene plasmonics, high-performance and versatile mid-infrared plasmonic tweezers can be realized to selectively trap or repel nanoscale objects of different materials in a dynamically reconfigurable way. This new technique can be utilized for sorting, filtering, and fractionating nanoscale objects in a mixture.