Angular momentum of light has been applied to various applications such as mechanical motion manipulation, optical communication, and sensing. Furthermore, we have proposed that it is possible to form optical fields with angular momentum in nanometer scale area using plasmonic fields of multimer nanoantennas and have succeeded in experimentally manipulating orbital rotation of nanoparticles and controlling the chirality of crystals. However, the angular momentum of light is converted due to the interaction between light and matter in the process of transmission from incident lights to the plasmonic fields, and the laws of the conversion have not yet been clarified. To form plasmonic fields with the desired angular momentum on the nanometer scale, it is necessary to unveil thse conversion laws. Therefore, we analyzed the electromagnetic fields of the plasmonic fields of multimer nanoantennas and clarified the conversion law governing the spin and orbital angular momentum transfer. We also show that the interference of multiple fields with converted angular momentum enables the forming of nanometer scale fields with fractional angular momentum.
The spin angular momentum of light can induce the orbital rotation of matter via spin-orbit angular momentum conversion. In this work, we demonstrate the orbital rotation of nanoparticles using two different physical mechanisms. First, a nanoscale Poynting vector vortex is created above the nanogap of a plasmonic trimer nanoantenna upon circularly polarized laser irradiation. Using these trimer nanotweezers, single fluorescent nanodiamond trapping and rotation is experimentally achieved. Second, the orbital rotation of VO2 nanoparticles is achieved using a focused, circularly polarized Gaussian laser beam. We demonstrate that the non-linear optical response caused by the insulator-to-metal phase transition of VO2 leads to the formation of an annular trapping potential well around the center of the laser beam.
The ability to control nanoscale motions of nanomaterials is expected to play significant roles in various fields such as photophysics, photochemistry and biological applications. For the optical nanomanipulation, metal nanoantenna structures are widely used. These plasmonic structures can confine light into nano-sized volumes and enhance the nanoscale light-matter interactions. In this paper, we demonstrated that precise orbital rotational motion is driven by the angular momentum that is transferred from photon to plasmonic nanoantenna. We present the numerical simulation results and discussion on the mechanism of the angular momenta transfer. Then, we show the experimental results on the rotational manipulation of a nanodiamond using plasmonic trimer structure.
Optical tweezers are very effective for manipulating objects larger than the wavelength of light. But it is difficult at the
nanoscale because of the diffraction-limited focused spot size. In this context, there is a growing interest in the
manipulation nanoparticles using plasmonic nanostructures. Here, we investigated the optical rotational manipulation of
nanoparticle by using plasmonic multimer structures. Under some conditions such as using circularly polarized light or
optical vortex for incident light, spin angular momentum and orbital angular momentum are transferred to the localized
field. In this case, to clarify the force that the nanoparticles receive, we analyzed spatial distribution of Poynting vectors
closely related to scattering force.
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