An approach to cylindrical vector beam (CVB) multiplexing that is based on off-axis polarization control, developed by a research team from Shenzhen University, could help realize the potential of CVB multiplexing as a pathway to high-capacity optical communications. Coupling and separating CVB modes are critical procedures in CVB multiplexing and demultiplexing. Although scientists have previously investigated off-axis control solutions for coupling and separating light beams, most of these technologies, because of their phase-only grating structures, have been useful only for light beams with homogeneous polarization. For CVBs with nonhomogeneous polarization, a gradient phase device is needed to create a gradient phase difference between the left- and right-handed circularly polarized (LHCP and RHCP) components. This allows CVBs with off-axis incident angles to coaxially propagate and at the same time carry different phase structures. After CVBs are decomposed into LHCP and RHCP components, they can be off-axis controlled by independently modulating the phase of the two components. The mode coupling and separating can be achieved by introducing gradient phase changes. (a1) Polarization and intensity distributions in each diffraction order with Gaussian beam incident, respectively. (a2) Polarization and intensity distributions in each diffraction order with cylindrical vector beam (CVB) (m = −1) incident. (b1) Measured coaxial CVB with four modes. (b2-b5) Measured Gaussian points after demultiplexing of the different CVB mode channels. Courtesy of S. Chen, Z. Xie, H. Ye, X. Wang, Z. Guo, Y. He, Y. Li, X. Yuan, and D. Fan. The Shenzhen research team developed an approach to off-axis polarization control for CVB multiplexing and demultiplexing that is based on a plasmonic metasurface. To allow for off-axis phase modulation of the LHCP and RHCP components to occur independently and support the off-axis, spin-independent control of CVBs that have nonuniform polarization distributions, the geometric phase and the propagation phase are combined on the metasurface. The metal-dielectric-metal metasurface consists of a subwavelength gold nanoantenna on a silica dioxide-gold-silicon substrate. The optical metasurface is capable of phase, amplitude, and polarization control. Further, the metasurface enables circular polarization-dependent phase modulation via the spin-orbit interaction, thus providing a route for independent phase control of two circularly polarized components (LHCP and RHCP). The light path is reversible and the vector mode is conserved, enabling the LHCP and RHCP components to be simultaneously multiplexed and demultiplexed. The multiplexers and demultiplexers used in the approach are broadband (from 1310 to 1625 nm) and are compatible with wavelength-division-multiplexing. As proof of concept, the researchers demonstrated a four-channel CVB multiplexing communication that combined wavelength-division-multiplexing and polarization-division-multiplexing with a transmission rate of 1.56 Tbit/s and a bit-error-rate of 10-6 at the receive power of −21.6 dBm. The CVB multiplexer/demultiplexer based on off-axis polarization control shows a broadband response characteristic and an orthogonal polarization response, which makes it compatible with wavelength-division-multiplexing and polarization-division-multiplexing. CVB multiplexing could enable robust transmission in atmospheric turbulence due to its spatially nonhomogeneous polarization distribution and its ability to transmit over ultralong distances. Because it is independent of wavelength and polarization states, CVB multiplexing could offer a way to support optical communication that is compatible with both conventional wavelength-division-multiplexing and polarization-division-multiplexing. The off-axis polarization control approach could lead to new applications for CVB applications, such as CVB multiplexing and integrated photonics. It could also be applied to CVB holography and particle capture technology, and it could be combined with active metasurfaces to achieve dynamic off-axis control. Owing to the stability of the gold metal in the metasurface, the CVB multiplexers/demultiplexers can be used in high-temperature, high-pressure, and other complex environments. The metasurface is flat and compact, making it suitable for system integration and miniaturization. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00667-7).
