Fig. 1. Concept and working principle of our rGO broadband multifocal vortex generator (BMVG). (a) Schematic of working principle of our BMVG. (b) Simulated intensity and phase profiles of continuous OAM patterns captured at corresponding focal planes. Scale bar, 4 µm.

Fig. 2. Simulated light fields of our BMVG in the x–z plane. (a) Simulated intensity profile in the x–z plane of the light field generated by our BMVG. (b) Zoom-in simulated intensity profiles (x–z plane) of the non-diffracting regions of generated OAM modes from l=1 to l=10.

Fig. 3. Fabricated sample and measured results of the BMVG. (a) The scanning electronic microscopy (SEM) image of the entire fabricated sample. (b) The topographic profile (top) of the rGO sample measured by an optical profiler and the cross-sectional profile (bottom) marked by the white dashed line in the top image of (b). (c) The interference patterns of the generated OAMs captured before and after the designed focal planes upon 633 nm illumination. Scale bar, 6 µm.

Fig. 4. Demonstration of wavelength tunability and focal distance tunability of our BMVG. (a) Simulated and measured interference patterns of generated OAMs captured before (left panel) and after (right panel) z = 450 µm under the incident light of 886 nm, 739 nm, and 633 nm. Scale bar, 6 µm. (b) The measured interference patterns of the generated OAMs under 633 nm (near z = 720 µm), 760 nm (near z = 600 µm), 800 nm (near z = 570 µm), 850 nm (near z = 536 µm), 886 nm (near z = 514 µm), 900 nm (near z = 506 µm), and 1000 nm (near z = 456 µm). Scale bar, 6 µm. (c) The measured interference patterns of the generated OAMs captured before and after the designed focal planes upon 900 nm illumination. Scale bar, 6 µm.