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

Ebrahim Karimi,Robert W Boyd,Hammam Qassim,Israel De Leon,Sebastian A Schulz,Jeremy Upham

doi:10.1038/lsa.2014.48

Ebrahim Karimi, Robert W Boyd + Show 4 more

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https://doi.org/10.1038/lsa.2014.48

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Abstract

AbstractLight beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation. Until recently, it was thought that these two ‘rotational’ motions of light were largely independent and could not be coupled during light–matter interactions. However, it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling, where a change of the spin angular momentum will modify the orbital angular momentum and vice versa. In this work, we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum. A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas. Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of ±2ħ (with conversion efficiency of at least 1%). This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy, nanoscale sensing and classical or quantum communications using integrated photonic devices.

Highlights

Spin angular momentum (SAM) and orbital angular momentum (OAM) are associated with the polarisation and phase of the optical field, respectively.[1]
This relationship can be described by the Pancharatnam–Berry phase,[4] and is what allows a beam to experience different optical paths associated with the trajectory of the polarisation evolution on the Poincaresphere.[5]. This phenomenon has enabled Pancharatnam– Berry phase optical elements:[6,7] devices that control the output beam’s wavefront according to the polarisation of the input beam. These devices could 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]
Interference patterns between the converted light and either planar or spherical waves are imaged onto a charge coupled device (CCD) camera to determine the OAM value of the converted beam (Figure 3b) showing a double pitch-fork and double helix, respectively

Summary

Introduction

Spin angular momentum (SAM) and orbital angular momentum (OAM) are associated with the polarisation and phase of the optical field, respectively.[1]. 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 Poincaresphere.[5] Recently, this phenomenon has enabled Pancharatnam– Berry phase optical elements:[6,7] devices that control the output beam’s wavefront according to the polarisation of the input beam. These devices could 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]

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Conclusion