Newswise —
The angular momentum of electromagnetic waves in orbit, known as orbital angular momentum (OAM), corresponds to a twisted or helical wavefront. These helical patterns possess a specific topological charge, which distinguishes them. OAM beams that have different topological charges are mutually perpendicular, enabling them to transmit information and be multiplexed. The use of OAM multiplexing increases channel capacity and spectral efficiency, making it highly advantageous for communications in fiber-based and free-space systems. In addition, OAM beams possess characteristics that are beneficial for optical trapping, lattices, and other applications.
The exploration of OAM's potential has progressed considerably due to continuous research endeavors worldwide. According to Advanced Photonics, a team of researchers from The Hong Kong University of Science and Technology (HKUST) and City University of Hong Kong (CityU) have recently created time-varying OAM beams through a digital metasurface that uses space-time encoding. The team utilized a field-programmable gate array (FPGA) to govern the reflection phase of atoms on the metasurface in the microwave frequency range.
The researchers utilized the adaptable programmability of the metasurface to create various versions of time-varying OAM beams, each possessing a distinct time-dependent phase profile in every time layer. This feature allows for a time-varying topological charge, as well as a higher-order twist in the envelope wavefront structure of the OAM beam. The nonlinear time-dependence in phase creates an additional degree of freedom that enhances the beam's potential for a wider range of applications.
To demonstrate their experiment, the researchers developed a two-probe mapping approach to dynamically map the time-varying OAM field, including amplitude and phase patterns at various time instances. Furthermore, they performed a spectrum analysis of the measured field patterns, aiming for OAM mode decomposition. This analysis demonstrated the high degree of purity of the generated time-varying OAM, as well as the intended higher-order twist in the envelope wavefront structure.
Their innovative approach, combining the metasurface’s space-time digital encoding and the two-probe field mapping technique, results in a versatile platform for generating and observing time-varying OAM — as well as other spatiotemporal excitations.
The proposed time-varying OAM beams have application potential for dynamic particle trapping, time-division multiplexing, information encryption, and beyond.
Read the Gold Open Access article by J. Zhang et al., “Generation of time-varying orbital angular momentum beams with space-time-coding digital metasurface,” Adv. Photon. 5(3), 036001 (2023), doi 10.1117/1.AP.5.3.036001.
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