Intrinsic emergence of optical spin-orbit interaction at the nanoscale

The extension of spin-orbit interaction (SOI) to the framework of optics is attributed to the seminal work authored by Liberman and Zel’dovich [1]. Their approach is based on the conservation of the state of polarization... [ view full abstract ]

The extension of spin-orbit interaction (SOI) to the framework of optics is attributed to the seminal work authored by Liberman and Zel’dovich [1]. Their approach is based on the conservation of the state of polarization (SoP) when the light propagates in an optically inhomogeneous medium. Due to the vector character of the electromagnetic fields, light possess two types of rotations giving rise to the corresponding contributions termed as the spin angular momentum (SAM) and orbital angular momentum (OAM), respectively. Whereas the OAM is related with the spatial distribution and propagation of the optical field, SAM is generally determined by the SoP. Therefore, SOI is envisioned as the mutual influence between the SoP (spin) and the phase (orbit), i.e., provided the electromagnetic field can be expressed in a factorized form, that mutual influence is vanished, avoiding the occurence of SOI.

In the past few years, motivated by the rapid advance of nanotechnology, SOI of light has been subject of intense research activity [2]. Main efforts were devoted to investigate novel photonic applications [3], leaving elusive the fundamental theory underlying its origin. In this regard it has only been argued that SOI of light is inherent to Maxwell’s equations, arising from the transversality condition and described within the Berry phase formalism.

In this work we propose a new perspective to unveil the intrinsic emergence of optical SOI at the nanoscale. Taking into account the factorizability condition of the electromagnetic fields we show, by using an analytical full-vector description based on the spherical vector wave approach, that this condition is fulfilled only in the far-field limit [Fig.1a]. Indeed, since SOI come into play at the subwavelength scale, the usual treatment based on the scalar-like plane waves seems to be pretty naive. On the other side, in the near-field region, an additional relative phase hinders the factorization and reveals an intricate behavior capturing the main features of the spin-orbit coupling regime [Fig.1b].

[1]V.S. Liberman and B.Y. Zel’dovich, Phys. Rev. A 46, 5199 (1992).

[2]K.Y. Bliokh, et al., Nat. Photonics 9, 796 (2015).

[3]F. Cardano and L. Marrucci, Nat. Photonics 9, 776 (2015).