Interaction of angular momentum carrying optical beams with a single nanowire

Abstract

Angular momentum (AM) is an inherent property of electromagnetic fields. A light beam can possess both spin angular momentum (SAM) and orbital angular momentum (OAM). SAM and OAM are results of transverse spin and orbital energy/momentum flows of the light beam respectively. Transverse spin momentum flow originates from the rotating electric field vector of a circularly polarized light beam. SAM can have two orthogonal states corresponding to left and right-handedness of circular polarization. On the other hand, transverse orbital momentum flow is a result of the azimuthal phase gradient present in optical vortex beams. Systematic control of the spatial phase of a vortex beam can result in an infinite number of orthogonal states of OAM. AM carrying beams can be easily generated in the laboratory by engineering polarization and spatial phase of the Gaussian output of a LASER source. One of the commonly used and easy to prepare OAM carrying beams is Laguerre-Gaussian (LG) beam. AM of the light beam has been utilized as an extra degree of freedom to enhance optical information transfer. Interaction of AM carrying light beams with matter not only provides a fundamental understanding of light-matter interaction but has been harnessed in applications such as quantum memory gadgets, optical trapping, directional coupling and more. The effects of optical AM interaction with matter are small at large scale and can be neglected. But it is important to consider them at scales comparable to the wavelength of the light beam and has been utilized in nanophotonics based applications. Herein, we have studied the interaction of AM carrying beams with a single plasmonic nanowire (NW). Our study aims at how transverse spin and orbital momentum flows control the scattering of AM carrying beams from an individual NW and generation of plasmons in the NW. We show the optical Spin-Hall effect in the scattering of a light beam from NW caused by spin flow, the detection of OAM states using a single NW at the subwavelength scale and OAM controlled surface plasmon polaritons generation in a plasmonic NW. In order to experimentally probe the above concepts, we have utilized a home-built dual-channel Fourier plane microscopy setup and corroborated the measurement with finite element based numerical simulations. Structured light beams such as Hermite-Gaussian and Laguerr-Gaussian beams are generated using spatial light modulator working in off-axis hologram configuration for experimental observations. Using the developed experimental methods, we have also explored nonlinear optical effects such as second-harmonic generation and two-photon excited fluorescence from organic mesowire using conventional optical Gaussian beams.