There have been many theoretical studies of the optomechanical behaviour of plasmonic nanoparticles. However, those that only take the nanoparticle polarizability into account are not able to explain the mechanical behaviour associated with the geometry and size of nanorods and nanoparticles. Now, Matthew Moocarme and co-workers from the City University of New York in the USA have found that significant mechanical forces are also produced by a surface plasmon that is widely neglected.
The team numerically investigated the Lorentz forces acting on a free electron gas that was bound to the surface of a gold nanowire under linearly-polarized electromagnetic plane waves in the wavelength range from 400 nm to 1,200 nm. The diameter and length of the nanowire was 75 nm and 1,025 nm, respectively. They found that plasmonically-induced Lorentz forces were significant and stronger than electric-dipole induced forces, particularly for the case of long-wavelength excitation between plasmon resonances. The existence of chiral plasmons coincided with spiral magnetic field patterns that produced net transverse forces and torques. Theoretical models that consider only electric-dipole induced torques cannot explain the experimental results — in particular, they fail to explain why the nanowires align perpendicular to the linear polarization of an optical trap. However, the model proposed by the researchers can convincingly explain it.
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