Optical Spin-to-Orbital Angular Momentum Conversion in Inhomogeneous Anisotropic Media

Abstract

We demonstrate experimentally an optical process in which the spin angular momentum carried by a circularly polarized light beam is converted into orbital angular momentum, leading to the generation of helical modes with a wavefront helicity controlled by the input polarization. This phenomenon requires the interaction of light with matter that is both optically inhomogeneous and anisotropic. The underlying physics is also associated with the so-called Pancharatnam-Berry geometrical phases involved in any inhomogeneous transformation of the optical polarization.

Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface

“…These devices could easily be inserted into the beam path of existing spectroscopic, nano-imaging or communication systems, as they do not rely on diffraction, adding OAM-based functionality that has the potential to distinguish between molecules of different chirality, enhance optical circular dichroism 8 and encode multiple bits of information onto a single photon. 9 Existing Pancharatnam-Berry phase optical elements include qplates, 3 made of liquid crystals, and computer-generated subwavelength gratings, 6,10 made of micron-size dielectric features. These devices transform polarisation superposition states into complex structures rich in polarisation and phase singularities.…”

“…In an anisotropic and inhomogeneous medium, these otherwise independent momenta can be made to interact, changing both the polarisation and phase of the beam. 3 This change depends on the incident beam's polarisation and the medium's topology stemming from its inhomogeneity. This relationship can be described by the Pancharatnam-Berry (geometrical) phase, 4 and is what allows a beam to experience different optical paths associated with the trajectory of the polarisation evolution on the Poincaré sphere.…”

Interface between path and orbital angular momentum entanglement for high-dimensional photonic quantum information

“…Recently, first experiments of an integrated OAM beam emitter demonstrated an interface between both fields, although not yet in the quantum regime 15 . In another approach, q-plates have been demonstrated to interface the OAM with the polarization of photons 16,17 .…”

Giant photonic spin Hall effect in momentum space in a structured metamaterial with spatially varying birefringence

“…40 It is expected that the light beam will acquire a space-variant PB phase when propagating through an inhomogeneous anisotropic medium with homogeneous phase retardation, such as an inhomogeneous subwavelength grating, a liquid crystal q-plate, or a plasmonic metasurface. [26][27][28] The PB phase can be written as W G (x,y) 5-2s 6 a(x,y), where a(x,y) is the local optical axis direction of the anisotropic medium. The factor 22 arises because the anisotropic medium reverses the handedness of the circular polarization and applies an additional geometric phase factor 2s 6 a(x,y) to the output light.…”

“…In certain inhomogeneous anisotropic media, a spatially varying PB phase has been produced to generate and manipulate vortex beams, vector beams, vector vortex beams, etc. [26][27][28][29][30][31] In this work, we theorize a unified description of the photonic SHE caused by the two types of geometric phases. It is predicted that the spin-dependent shift induced by the PB phase gradient can be very large because it occurs in k (momentum) space.…”