Orbital-to-spin angular momentum conversion employing local helicity

Abstract

Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane-wave by a nanoparticle, converting spin to orbital angular momentum of light. Here, we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian beams of first order ( = ±1) exhibit an -dependent spatial distribution of helicity density in the focal volume. By using a dipolar scatterer the helicity density can be manipulated locally, while influencing globally the spin and orbital angular momentum of the beam. Specifically, the scattered light can be purely circularly polarized with the handedness depending on the orbital angular momentum of the incident beam. We corroborate our findings with theoretical calculations and an experimental demonstration. Our work sheds new light on the global and local properties of helicity conservation laws in electromagnetism.

Unveiling dipolar spectral regimes of large dielectric Mie spheres from helicity conservation

“…In striking contrast, high-refractive index (HRI) dielectric nanospheres present strong magnetic and electric dipolar resonances in the visible [5] region, as well as in telecom and near-infrared frequencies [6]. Notably, when the electric and magnetic polarizabilities of the scatterer are identical, at the so-called first Kerker condition [7], the EM helicity is preserved [8][9][10] and the intensity in the backscattering direction is exactly zero [11][12][13]. These anomalous light scattering effects have been exploited in multiple branches of physics ranging from boosting light collection efficiency in plants [14], to dynamic control of cryptographic nanoprints [15], to maximizing the sensitivity of circular dichroism spectroscopy of chiral molecules [16][17][18][19].…”

Kerker Conditions upon Lossless, Absorption, and Optical Gain Regimes

“…On the other hand, for cylindrically symmetric particles, the absence of backscattered light follows from the preservation of EM helicity [14][15][16][17][18]. Conservation of helicity has proven crucial in many applications such as enhanced chiral light-matter interactions [19][20][21][22][23][24][25], or in the spin-orbit interactions of light [26][27][28][29][30][31][32]. In this vein, it has been reported that from a relatively simple far-field measurement of the EM helicity at a right angle, the radiation pattern of the dipolar particle is inferable [33].…”

Mimicking chiral light-matter interaction

“…2(e) have a helical phase distribution as a direct consequence of the phase-shifted superposition of azimuthal polarization of the excitation beam and radial polarization emitted by the nanoparticle [23]. Finally, the presented effect is determined by the longitudinal field components, accentuating their importance in the description of light-matter interaction [19,[42][43][44][45][46][47][48][49].…”