image: a, A “Golden-ratio” phyllotaxis pattern producing “Fibonacci” OAM series inspires us to design phyllotaxis-alike vortex nanosieves. In the Fresnel region, the diffractive pattern of a Vogel spiral nanosieve with 936 florets is simulated and mode coefficients are calculated. The corresponding results reveal the link between phyllotaxis patterns and optical vortices, thus inspiring us to design phyllotaxis-alike vortex nanosieves. b, Free-space phyllotaxis-inspired vortex nanosieve generating four different on-axis optical vortices simultaneously in free-space. Left panel: The red, purple, green, and blue solid circles denote the four different motifs concealed in the free-space phyllotaxis-inspired vortex nanosieve structure. Right panel: The four different sets of concealed spirals and their relevant simulated Fresnel diffraction intensity profiles. c, A top-view scanning electron microscope (SEM) image of the sample. d, The measured free-space Fresnel diffraction intensity profiles upon 633nm, 532 nm, and 445 nm illumination, respectively. view more
Credit: by ZhongweiJin, David Janoschka, Junhong Deng, Lin Ge, Pascal Dreher, Bettina Frank, Guangwei Hu, Jincheng Ni, Yuanjie Yang, Jing Li, Changyuan Yu, Dangyuan Lei, Guixin Li, Shumin Xiao, Shengtao Mei, Harald Giessen, Frank Meyer zu Heringdorf, Cheng-Wei Qiu
The vortex is ubiquitous in nature, spanning from the galaxy, ocean flow, to phyllotaxis. In electromagnetic wave, the vortex can be found in spiral wavefront profiles, representing the orbital angular momentum (OAM) of light. Recently, multifunctional metasurfaces have been investigated to generate multiple optical vortices (OVs) within one nano-device. Such metasurfaces often resort to segmented or interleaved sub-array meta-atoms to multiplex OAMs in a single device. Unfortunately, such strategies degrade the device’s compactness and channel capacity since each inclusion seems only responsible for one specific topological charge and sufficient distance is required between meta-atoms to suppress cross coupling.
In a new paper published in eLight, a team of scientists, led by Professor Cheng-Wei Qiu from Department of Electrical and Computer Engineering, National University of Singapore, and his co-workers have developed phyllotaxis-alike vortex nanosieves which realize both free-space and near-field OAM multiplexing based on structure degeneracy in the space domain. The inspiration comes from the smart growing patterns of some phyllotaxis. The growing patterns of sunflower seeds and pine cones allow their seeds or leaves to enjoy the sun and rain to the greatest extend. When taking a deeper look at these patterns, one may find that such intelligent growing pattern is actually a special spiral- the Vogel spiral, which is also named the “golden-ratio” spiral. In a “golden-ratio” spiral nanosieve pattern, multiple sets of spiral structures can be encoded, and the numbers of spiral arms contained in different sets are in coincidence with the Fibonacci numbers. As is well known that the spiral structure is a common strategy for generating beams with orbital angular momentums (OAMs), we thus feel the curiosity to seek the relationship between the phyllotaxis pattern and OAM, which is also the starting point of this work.
To solve the puzzle, scientists simulated the diffraction pattern of a “golden-ratio” Vogel spiral nanosieve in its Fresnel region and found out that the diffracted pattern of such a mask contains a series of OAM modes (Figure 1a), in coincidence with the numbers of spiral arms which can be encoded from the pattern. Hence, the scientists infer that phyllotaxis-alike patterns concealing multiple spiral structures may enable the creation and multiplexing of OVs. Such beauty in nature inspires the scientists to design phyllotaxis-alike nanosieves which can generate beams containing multiple OAM modes for both free-space and on-chip optical systems.
The designed phyllotaxis-inspired vortex nanosieves are composed of judiciously arranged nanoholes on metal films (Figure 1c and 2b). As is shown in Figure 1b, the compact phyllotaxis-inspired vortex nanosieve conceals multiple sets of different spiral patterns, while each group of spiral pattern contribute for one specific OAM mode. The scientists summarize the journey of their findings about the phyllotaxis-inspired vortex nanosieve:
“First of all, through our theoretical derivation, we revealed why a “golden-ratio” spiral nanosieve can produce multiple OAM modes, which promoted us to design phyllotaxis-inspired vortex nanosieve to achieve multiple OAM modes within a single device. Meanwhile, we find that such principle applies both in free space and in the near field optical systems. In free space, the OAM orders are independent of incident spins; while in the on-chip optical system, we can get “vortex comb” containing spin-to-orbit conversion. Secondly, based on previous theoretical derivation, we designed and fabricated several phyllotaxis-inspired vortex nanosieves to realize multi-mode vortex manipulation. The phenomenon of multiplexed OAMs in our phyllotaxis-inspired vortex nanosieves comes from the embedded multiple spirals in a single device, each spiral set carrying a different vortex mode, which is confirmed by both steady-state and dynamic-state measured results.”
“Such strategy offers a different recipe with multimode OAM manipulation. Comparing with phase controlled metasurface OAM generators, our phyllotaxis-inspired vortex nanosieves is more robust, which means that even when some of the nanoholes in the nanosieve are destroyed, the vortex nanosieve can still keep its original function. This in turn lowers the normally stringent requirement of nanofabrication. The properties of our phyllotaxis-inspired vortex nanosieves open a new avenue for promising applications such as on-chip photonic devices, optical communication, and even quantum chiral optics.” the scientists added.
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Related references in past 5 years (contributed by Qiu’s group)
- Mehmood, M. et al. Visible‐frequency metasurface for structuring and spatially multiplexing optical vortices. Adv. Mater. 28, 2533-2539 (2016).
- Mei, S. et al. Flat helical nanosieves. Adv. Funct. Mater. 26, 5255-5262 (2016).
- Qiu, C.-W. & Yang, Y. Vortex generation reaches a new plateau. Science 357, 645-645 (2017).
- Huang, K. et al. Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum. Light Sci. Appl. 7, 17156-17156 (2018).
- Jin, L. et al. Dielectric multi-momentum meta-transformer in the visible. Nat. Commun. 10, 4789 (2019).
- Yang, Y. et al. Deuterogenic plasmonic vortices. Nano Lett. 20, 6774-6779 (2020).
- Bao, Y. et al. A minimalist single‐layer metasurface for arbitrary and full control of vector vortex beams. Adv. Mater. 32, 1905659 (2020).
- Sroor, H. et al. High-purity orbital angular momentum states from a visible metasurface laser. Nat. Photon. 14, 498 (2020).
- Ni, J. et al. Giant helical dichroism of single chiral nanostructures with photonic orbital angular momentum. ACS Nano 15, 2893-2900 (2021).
- Ni, J. et al. Gigantic vortical differential scattering as a monochromatic probe for multiscale chiral structures. Proc. Natl. Acad. Sci. U. S. A. 118, e2020055118 (2021).
- Ouyang, X. et al. Synthetic Helical Dichroism for Six-Dimensional Optical Orbital Angular Momentum Multiplexing, Nat. Photon., in press (2021).
- Ni, J. et al. Multidimensional phase singularities in nanophotonics. Science, in press (2021).
Journal
eLight