Tailoring the gradient and scattering forces for longitudinal sorting of generic-size chiral particles

Based on the concepts of conservative and non-conservative optical forces (COF and NCOF), we analyze the physical mechanism of longitudinal chirality sorting along the direction of light propagation in some simple optical fields. It is demonstrated, both numerically and analytically for particle of arbitrary size, that the sorting relies solely on the NCOF, which switches its direction when particle chirality is reversed. For particles larger than half of the optical wavelength λ, the NCOF far surpasses its counterpart COF, enabling the longitudinal chirality sorting. When the particle is much smaller than λ, however, the COF outweighs the NCOF, destroying the sorting mechanism. A scenario is thus proposed that totally eliminates the COF while leaving the sorting NCOF unchanged, extending the applicability of longitudinal chirality sorting to small particles.