Midinfrared Optical Force for Sorting Materials Depending on their Chemical Structures

Mechanical control of micro- and nanometer-scale materials using lasers forms an indispensable class of techniques for advancing nanoscience and nanotechnology. Optical force (radiation pressure) is exerted on such materials when the momentum of a laser is transferred to them through light scattering and absorption. The magnitude and the sign of optical force depends on the light-matter interactions. For instance, fluorescent nano-diamonds can be sorted by exciting the electronic transition energies of their nitrogen-vacancy centers [1]; the optical force on a dye-doped polystyrene particle is resonantly enhanced by using a 1064 nm trapping laser and 488 nm wide-field excitation laser [2]. In addition, based on circular dichroism, liquid crystal chiral droplets can be sorted using counter-propagating laser beams which are right and left circularly polarized [3]. Importantly, the microscopic intrinsic quantum features of materials are reflected in their macroscopic movement due to an optical force through light-matter interactions.