Optical forces and torques on eccentric nanoscale core-shell particles

The optical trapping and manipulation of small particles is an important tool for probing fluid properties at the microscale. In particular, microrheology exploits the manipulation and rotation of micron-scale particles to probe local viscosity, especially where these properties may be perturbed as a function of their local environment, for example in the vicinity of cells. To this end, birefringent particles are useful as they can be readily controlled using optically induced forces and torques, and thereby used to probe their local environment. However the magnitude of optical torques that can be induced in birefringent particles is small, and a function of the particle diameter, meaning that rotational flow cannot readily be probed on length scales much small than the micron level. Here we show modelling that demonstrates that eccentric spherical core-shell nanoparticles can be used to generate considerable optical torques. The eccentricity is a result of the displacement of the centre of the core from the shell. Our results show that, for particles ranging from 90~nm to 180~nm in diameter, we may achieve rotation rates exceeding 800~Hz. This fills a missing size gap in the rotation of microparticles with optical forces. The diameter of particle we may rotate is almost an order of magnitude smaller than the smallest birefringent particles that have been successfully rotated to date. The rotation of eccentric core-shell nanoparticles therefore makes an important contribution to biophotonics and creates new opportunities for rheology in nanoscale environments.