  Up to now, in the literature of optical manipulation, optical force due to
chirality usually coexists with the non-chiral force and the chiral force
usually takes a very small portion of the total force. In this work, we
investigate a case where the optical force exerted on an object is purely due
to the chirality while there is zero force on non-chiral object. We find that a
trapping force arises on chiral particles when it is placed in a field
consisted of two orthogonally polarized counter-propagating plane waves. We
have revealed the underlying physics of this force by modeling the particle as
a chiral diploe and analytically study the optical force. We find besides
chirality; the trapping force is also closely related to the dual
electric-magnetic symmetry of field and dual asymmetry of material. We also
demonstrate that the proposed idea is not restricted to dipolar chiral objects
only. Chiral Mie objects can also be trapped based on the technique proposed in
this article. Notably, such chiral trapping forces have been found robust by
varying several parameters throughout the investigation. This trapping force
may find applications in identifying object's chirality and the selective
trapping of chiral objects.
