Enhancing spin-orbit interaction of light by plasmonic nanostructures

Spin-Direction-Spin Coupling of Quasiguided Modes in Plasmonic Crystals

We report an unusual spin-direction-spin coupling phenomenon of light using the leaky quasiguided modes of a waveguided plasmonic crystal. This is demonstrated as simultaneous input spin-dependent directional guiding of waves (spin-direction coupling) and wave-vector-dependent spin acquisition (direction-spin coupling) of the scattered light. These effects, manifested as the forward and the inverse spin Hall effect of light in the far field, and other accompanying spin-orbit interaction effects are observed and analyzed using a momentum (k) domain polarization Mueller matrix. Resonance-enabled enhancement of these effects is also demonstrated by utilizing the spectral Fano resonance of the hybridized modes. The fundamental origin and the unconventional manifestation of the spin-direction-spin coupling phenomenon from a relatively simple system, ability to probe and interpret the resulting spin-orbit phenomena in the far field through momentum-domain polarization analysis, and their regulated control in plasmonic-photonic crystals open up exciting avenues in spin-orbit-photonic research.

Tunable spin Hall shift of light from graphene-wrapped spheres

Graphene has taken impressive roles in light manipulation and optical engineering. The most attractive advantage of graphene is its tunable conductivity that could be dynamically modulated by various means. In this paper, we show that the spin Hall shift of light is dynamically tunable via changing the Fermi level of the graphene-wrapped spheres. Such tunability is prominent when different modes interfere with each other, such as at the interference of electric and magnetic dipolar modes or at the interference of electric dipolar and electric quadrupole modes. The circular polarization degree in the near field clearly demonstrates the strength of spin-orbit interaction, which is associated with spin Hall shift of light in the far-field. In addition, the spin Hall effect is shown in far-field detection plane and should be observed in experiment. Our results provide insights into how the spin Hall effect could be tuned and add new perspective in designing optical super-resolution imaging techniques.

Topologically-tuned spin Hall shift around Fano resonance

The topological magnetoelectric effect is associated with the photonic spin-orbit interaction. However, due to the proportionate fine structure constant of the topological term, the topological magnetoelectric effect is usually weak. In this paper, we demonstrate that the axion term enables manipulation of the spin Hall shift of light around Fano resonance. And, the excited surface plasmon near the nanoparticle's interface could enhance the topological magnetoelectric effect for several orders. Numerical simulation of near field and far-field scattering confirms our theoretical results. Our work may pave the way to exploit the topological magnetoelectric effect in practical applications, such as optical sensing and nanoprobing.

Polarization-Tailored Fano Interference in Plasmonic Crystals: A Mueller Matrix Model of Anisotropic Fano Resonance

Fano resonance is observed in a wide range of micro- and nano-optical systems and has been a subject of intensive investigations due to its numerous potential applications. Methods that can control or modulate Fano resonance by tuning some experimentally accessible parameters are highly desirable for realistic applications. Here we present a simple yet elegant approach using the Mueller matrix formalism for controlling the Fano interference effect and engineering the resulting asymmetric spectral line shape in an anisotropic optical system. The approach is founded on a generalized model of anisotropic Fano resonance, which relates the spectral asymmetry to physically meaningful and experimentally accessible parameters of interference, namely, the Fano phase shift and the relative amplitudes of the interfering modes. The differences in these parameters between orthogonal linear polarizations in an anisotropic system are exploited to desirably tune the Fano spectral asymmetry using pre- and postselection of optimized polarization states. The concept is demonstrated on waveguided plasmonic crystals using Mueller matrix-based polarization analysis. The approach enabled tailoring of several exotic regimes of Fano resonance in a single device, including the complete reversal of the spectral asymmetry, and shows potential for applications involving control and manipulation of electromagnetic waves at the nanoscale.

Tunable Spin dependent beam shift by simultaneously tailoring geometric and dynamical phases of light in inhomogeneous anisotropic medium

Spin orbit interaction and the resulting Spin Hall effect of light are under recent intensive investigations because of their fundamental nature and potential applications. Here, we report an interesting manifestation of spin Hall effect of light and demonstrate its tunability in an inhomogeneous anisotropic medium exhibiting spatially varying retardance level. In our system, the beam shift occurs only for one circular polarization mode keeping the other orthogonal mode unaffected, which is shown to arise due to the combined spatial gradients of the geometric phase and the dynamical phase of light. The constituent two orthogonal circular polarization modes of an input linearly polarized light evolve in different trajectories, eventually manifesting as a large and tunable spin separation. The spin dependent beam shift and the demonstrated principle of simultaneously tailoring space-varying geometric and dynamical phase of light for achieving its tunability (of both magnitude and direction), may provide an attractive route towards development of spin-optical devices.

Complete polarization characterization of single plasmonic nanoparticle enabled by a novel Dark-field Mueller matrix spectroscopy system

Information on the polarization properties of scattered light from plasmonic systems are of paramount importance due to fundamental interest and potential applications. However, such studies are severely compromised due to the experimental difficulties in recording full polarization response of plasmonic nanostructures. Here, we report on a novel Mueller matrix spectroscopic system capable of acquiring complete polarization information from single isolated plasmonic nanoparticle/nanostructure. The outstanding issues pertaining to reliable measurements of full 4 × 4 spectroscopic scattering Mueller matrices from single nanoparticle/nanostructures are overcome by integrating an efficient Mueller matrix measurement scheme and a robust eigenvalue calibration method with a dark-field microscopic spectroscopy arrangement. Feasibility of quantitative Mueller matrix polarimetry and its potential utility is illustrated on a simple plasmonic system, that of gold nanorods. The demonstrated ability to record full polarization information over a broad wavelength range and to quantify the intrinsic plasmon polarimetry characteristics via Mueller matrix inverse analysis should lead to a novel route towards quantitative understanding, analysis/interpretation of a number of intricate plasmonic effects and may also prove useful towards development of polarization-controlled novel sensing schemes.

Linear mode conversion of terahertz radiation into terahertz surface magnetoplasmons on a rippled surface of magnetized n-InSb

A new mechanics of linear mode conversion of terahertz (THz) radiation into THz surface magnetoplasmons on a rippled surface of magnetized n-InSb is proposed. The normally incident THz radiation, polarized in the direction of a ripple wave vector, imparts oscillatory velocity to electrons in the ripple layer. This velocity beats with surface ripple to produce a nonlinear current that resonantly drives the THz surface magnetoplasmons. In the presence of an applied magnetic field, the surface plasmon (SP) mode splits into two modes-an upper mode and a lower mode. The amplitude of the SP for the upper branch mode is higher than that for the lower mode.

Giant Goos-Hänchen shift in scattering: the role of interfering localized plasmon modes

The longitudinal and transverse beam shifts, namely, the Goos-Hänchen (GH) and the Spin-Hall (SH) shifts are usually observed at planar interfaces. It has recently been shown that the transverse SH shift may also arise due to scattering of plane waves. Here, we show that analogous in-plane (longitudinal) shifts also exist in the scattering of plane waves from micro/nano systems. We study both the GH and the SH shifts in plasmonic metal nanoparticles/nanostructures and dielectric micro-particles employing a unified framework that utilizes the transverse components of the Poynting vector of the scattered wave. The results demonstrate that the interference of neighboring resonance modes in plasmonic nanostructures (e.g., electric dipolar and quadrupolar modes in metal spheres) leads to great enhancement of the GH shift in scattering from such systems. We also unravel interesting correlations between these shifts with the polarimetry parameters, diattenuation and retardance.