Differentiation of right-handed and left-handed particles by exposure to light

Researchers studied the polarization dependence of the force exerted by circularly polarized light (CPL) by performing optical trapping of chiral nanoparticles. They found that left- and right-handed CPL exerted different strengths of the gradient optical force on the nanoparticles, and the D- and L-form particles were subjected to different gradient forces by CPL. The present results suggest that the separation of materials according to their handedness of chirality can be realized by the optical force.

Chirality is the property that the structure is not superimposable on its mirror image. Chiral materials have the property that they react differently to left-hand and right-hand circularly polarized light. When matter is irradiated with strong laser light, optical force is exerted on it. Theoretically, it was expected that the optical force exerted on chiral materials by left- and right-hand circularly polarized light would also be different.

The research group at the Institute for Molecular Science and three other universities used an experimental optical trapping technique to observe the circular polarization-dependent optical gradient force exerted on chiral gold nanoparticles. Chiral gold nanoparticles have either a D-shape (right-handed) or an L-shape (left-handed) structure, and the experiment was performed with both.

Although the gradient optical force acting on chiral nanoparticles has been theoretically predicted, no observation of the force has been reported so far. The group succeeded in observing the optical gradient force originating from chirality (ie the difference between the gradient force due to left- and right-circularly polarized light) by optically trapping the chiral gold nanoparticles.

The results showed that the gradient optical power was different for D-shape and L-shape particles. From the dependence of the force on the wavelength of the light used, the researchers also found that there is a previously unknown effect on the mechanism of the chirality-dependent optical forces.

The present study elucidated the properties of the circular polarization dependent optical gradient force on the mechanics of chiral gold nanoparticles. It demonstrates the possibility of optical force separation of chiral materials, which can be realized by using localized light generated on nanostructures to trap the materials and/or by using the optical force of other mechanisms.

The study was published in scientific advances.