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Data transmission rates in optical communication systems are approaching the limits of conventional multiplexing methods. Orbital angular momentum (OAM) in optical vortex beams offers a new degree of freedom and the potential to increase the capacity of free-space optical communication systems, with OAM beams acting as information carriers for OAM division multiplexing (OAM-DM). We demonstrate independent collinear OAM channel generation, transmission and simultaneous detection using Dammann optical vortex gratings (DOVGs). We achieve 80/160 Tbit s−1 capacity with uniform power distributions along all channels, with 1600 individually modulated quadrature phase-shift keying (QPSK)/16-QAM data channels multiplexed by 10 OAM states, 80 wavelengths and two polarizations. DOVG-enabled OAM multiplexing technology removes the bottleneck of massive OAM state parallel detection and offers an opportunity to raise optical communication systems capacity to Pbit s−1 level. Dammann gratings are used to realize multiplexing based on the generation, transmission and detection of optical angular momentum (OAM). The OAM of optical vortex beams offers a new degree of freedom for multiplexing and hence the promise of higher data communication rates, but massive parallel detection of OAM states has proved challenging. Now, researchers in China, Australia and Singapore have used Dammann optical vortex gratings (DOVGs) to realize multiplexing of massive OAM channels with individual modulation and simultaneous detection capabilities. They achieved a data capacity of 80 Tbit s−1 by multiplexing 1600 channels using ten OAM states, 80 wavelengths and two polarizations. This DOVG-enabled OAM multiplexing technology removes the bottleneck of massive parallel detection of OAM states and has the potential to increase optical communication capacities to the Pbit s−1 level.

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Massive individual orbital angular momentum channels for multiplexing enabled by Dammann gratings

A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied. We numerically analyze the effects of atmospheric turbulence on the system and find that turbulence induces attenuation and crosstalk among channels. Based on a model in which the constituent channels are binary symmetric and crosstalk is a Gaussian noise source, we find optimal sets of OAM states at each turbulence condition studied and determine the aggregate capacity of the multichannel system at those conditions. OAM-multiplexed FSO systems that operate in the weak turbulence regime are found to offer good performance. We verify that the aggregate capacity decreases as the turbulence increases. A per-channel bit-error rate evaluation is presented to show the uneven effects of crosstalk on the constituent channels.

Turbulence-induced channel crosstalk in an orbital angular momentum-multiplexed free-space optical link

Scattering forces in focused light beams push away metallic particles. Thus, trapping metallic particles with conventional optical tweezers, especially those of Mie particle size, is difficult. Here we investigate a mechanism by which metallic particles are attracted and trapped by plasmonic tweezers when surface plasmons are excited and focused by a radially polarized beam in a high-numerical-aperture microscopic configuration. This contrasts the repulsion exerted in optical tweezers with the same configuration. We believe that different types of forces exerted on particles are responsible for this contrary trapping behaviour. Further, trapping with plasmonic tweezers is found not to be due to a gradient force balancing an opposing scattering force but results from the sum of both gradient and scattering forces acting in the same direction established by the strong coupling between the metallic particle and the highly focused plasmonic field. Theoretical analysis and simulations yield good agreement with experimental results.

Focused plasmonic trapping of metallic particles

We experimentally demonstrate optical rotation and manipulation of microscopic particles by use of optical vortex beams with fractional topological charges, namely fractional optical vortex beams, which are coupled in an optical tweezers system. Like the vortex beams with integer topological charges, the fractional optical vortex beams are also capable of rotating particles induced by the transfer of orbital angular momentum. However, the unique radial opening (low-intensity gap) in the intensity ring encompassing the dark core, due to the fractional nature of the beam, hinders the rotation significantly. The fractional vortex beam’s orbital angular momentum and radial opening are exploited to guide and transport microscopic particles.

Fractional optical vortex beam induced rotation of particles

In magnetic materials, skyrmions are nanoscale regions where the orientation of the electron spin changes in a vortex-type manner1–4. Electromagnetic waves carry both spin and orbital angular momenta5,6. Here we show that spin–orbit coupling7–12 in a focused vector beam results in a skyrmion-like structure of local photonic spin. While diffraction limits the spatial size of intensity variations, the direction of the electromagnetic field, which defines the polarization and local photonic spin state, is not subject to this limitation. We demonstrate that the local spin direction in the skyrmion-like structure varies on the deep-subwavelength scale down to 1/60 of the light wavelength, which corresponds to a length scale of about 10 nm. The application of photonic skyrmions may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the local spin state of the field, not its intensity, can be applied to achieve deep-subwavelength optical patterns. Magnetic textures known as skyrmions have gathered much attention in recent years. It is now shown that focused vector beams can also give rise to photonic skyrmion-like structures.

Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum

We report an approach to the increase of signal channels in free-space optical communication based on composed optical vortices (OVs). In the encoding process, conventional algorithm employed for the generation of collinearly superimposed OVs is combined with a genetic algorithm to achieve high-volume OV multiplexing. At the receiver end, a novel Dammann vortex grating is used to analyze the multihelix beams with a large number of OVs. We experimentally demonstrate a digitized system which is capable of transmitting and receiving 16 OV channels simultaneously. This system is expected to be compatible with a high-speed OV multiplexing technique, with potentials to extremely high-volume information density in OV communication.

High-volume optical vortex multiplexing and de-multiplexing for free-space optical communication

In magnetic materials, skyrmions are nanoscale regions where the orientation of electron spin changes in a vortex-type manner. Here we show that spin-orbit coupling in a focused vector beam results in a skyrmion-like photonic spin distribution of the excited waveguided fields. While diffraction limits the spatial size of intensity distributions, the direction of the field, defining photonic spin, is not subject to this limitation. We demonstrate that the skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of light wavelength, which corresponds to about 10 nanometre lengthscale. The application of photonic skyrmions may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the spin structure of the field, not its intensity, can be applied to achieve deep-subwavelength optical patterns.

We studied and demonstrated optical trapping capabilities of an Airy beam generated with a cubic phase plate incorporated into a conventional optical tweezer system. The power efficiency and damage threshold of the cubic phase plate were found to be much higher when spatial light modulators were employed in beam generation.

Microfabricated continuous cubic phase plate induced Airy beams for optical manipulation with high power efficiency

Propagating surface plasmon (PSP) excitation, based on the total internal reflection configuration, was introduced into the nanoparticle (NP)-plane junction Raman spectroscopy. Experimental results demonstrated that silver nanospheres within the propagation region of PSP are effectively activated and detected by CCD camera due to their impressive Raman enhancement, which presents around 20 times improvement compared with the conventional NP-induced PSP/LSP co-enhanced Raman spectroscopy. This impressive Raman enhancement along with its high reproducibility of NP-plane junctions makes our configuration an attractive candidature for the PSP-assisted gap-mode surface-enhancement Raman spectroscopy and tip-enhanced Raman spectroscopy.

Xiaocong Yuan

Papers

Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum

High-volume optical vortex multiplexing and de-multiplexing for free-space optical communication

Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum

Microfabricated continuous cubic phase plate induced Airy beams for optical manipulation with high power efficiency

Tightly Focused Radially Polarized Beam for Propagating Surface Plasmon-Assisted Gap-Mode Raman Spectroscopy