Optical sorting of small chiral particles by tightly focused vector beams

The identification and separation of substances by chirality has always been an important problem in biomedical research and industry. Light beams carry optical momentum, and can exert optical force on any object they impinge due to the transfer of momentum. Different chiral objects will experience different optical forces when illuminated by the same light beam. We demonstrate here, based on the dipolar approximation, that a tightly focused vector beam can selectively trap and rotate small chiral particles in the transverse plane via the chirality-tailored optical forces. The radial optical force can transversely trap the chiral particles off axis or push them away depending on the real part of the chirality parameter, while the lateral optical force manifesting as the azimuthal optical force can drive the trapped particles to orbitally rotate with opposite chiral absorption in opposite directions. The study reported here may find applications in discriminating and separating chiral objects with specified chirality.