Researchers investigated the polarization-dependence of the force exerted by circularly polarized light (CPL) by performing optical trapping of chiral nanoparticles. They found that left-handed and right-handed CPL exert different forces of the optical gradient force on the nanoparticles, and that D-shaped and L-shaped particles are subjected to different gradient force by CPL. The present results suggest that the separation of materials according to their sense of chirality can be achieved by optical force.
Chirality is the property that the structure is not superimposable on its mirror image. Chiral materials have the particularity of reacting differently to left and right circularly polarized light. When matter is irradiated with strong laser light, an optical force is exerted on it. It was theoretically expected that the optical force exerted on chiral materials by left and right circularly polarized light would also be different.
The research group from 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 a D-shape (right-handed) or L-shape (left-handed) structure, and the experiment was performed using both.
Although the optical gradient force acting on chiral nanoparticles has been predicted theoretically, no observation of the force has been reported before. The research group succeeded in observing the optical gradient force arising from chirality (i.e. the difference between the gradient force by left and right circularly polarized light), by optical trapping of nanoparticles d chiral gold.
The results showed that the optical gradient force was different for D-shape and L-shape particles. The researchers also found, from the dependence of the force on the wavelength of the light used, that there is a hitherto unknown effect on the mechanism of chirality-dependent optical forces.
The present study clarified the characteristics of circular polarization-dependent optical gradient force on the mechanics of chiral gold nanoparticles. It shows the possibility of separation of chiral materials by optical force, which can be achieved by using locally confined light generated on the nanostructures to trap the materials and/or by using optical force for other mechanisms.
The research has been published in Scientists progress.
Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation
Junsuke Yamanishi et al, Optical Gradient Force on Chiral Particles, Scientists progress (2022). DOI: 10.1126/sciadv.abq2604. www.science.org/doi/10.1126/sciadv.abq2604
Provided by National Institutes of Natural Sciences
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