The time-varying orbital angular momentum is generated by the meta surface
(Nanowerk News) Orbital angular momentum (OAM) of electromagnetic waves — a type of “structured light” — is associated with helical or bent wavefronts. The helical mode is characterized by topological loading. OAM beams with different topological loads are orthogonal to each other, which allows them to carry information and be multiplexed. OAM multiplexing provides increased channel capacity and spectral efficiency — particularly useful in fiber- and free-space-based communications. OAM beams also have qualities that are useful for optical trapping, grating and more.
Unlocking OAM’s potential has advanced thanks to persistent research efforts globally. As reported in Advanced Photonics (“Generation of time-varying orbital angular momentum beams with a space-time-encoded digital metasurface”), researchers from The Hong Kong University of Science and Technology (HKUST) and City University of Hong Kong (CityU) recently developed a time-varying OAM beam using a space-time-encoded digital metasurface. They used a field-programmable gate array (FPGA) to control the phase of the atomic reflections on the meta surface in the microwave regime.
By exploiting the flexible programmability of the metasurface, they construct distinct modes of time-varying OAM beams that have time-dependent phase profiles in each time layer. This allows not only time-varying topological loads but also high-order spins in the wavefront structure of the OAM beam sheath in terms of nonlinear time dependence in phase, which serves as an additional degree of freedom to allow greater capacity for applications.
For their experimental demonstration, the team developed a two-probe mapping method to dynamically map time-varying OAM fields including amplitude and phase patterns at various time instants. In addition, they performed a spectrum analysis targeting the decomposition of the OAM modes on the measured field pattern, demonstrating the high mode purity of the resulting time-varying OAM and the designed high-level spin within the envelope wavefront structure.
Their innovative approach, combining digital space-time coding of metasurfaces and two-probe field mapping techniques, results in a versatile platform for generating and observing time-varying OAMs – as well as other spatiotemporal excitations.
The proposed time-varying OAM beams have potential applications for dynamic particle trapping, time-division multiplexing, information encryption, and so on.