Broadband-operational orbital angular momentum generation in nonlocal metasurfaces with maximum efficiency approaching 80%

TL;DR

The paper tackles the challenge of broadband, high‑efficiency vortex beam generation with nonlocal metasurfaces, which are traditionally limited by narrow bandwidths and efficiency losses near BICs and DPs. It introduces a reflection‑type metasurface that hybrids a bound state in the continuum (BIC) with two degeneracy points (DPs) to sculpt dispersion, radiative Q factors, and polarization states, achieving quasi‑flat dispersion and robust cross‑polarized conversion. Simulations predict on‑resonance efficiencies near unity and overall efficiencies well above 90%, with OAM purity approaching 99.5% in idealized models and wavelength tunability from 1500 to 1600 nm; experiments demonstrate broadband operation from 1480 to 1600 nm with peak efficiency around 80% and OAM purity up to ~92%, including direct conversion of zero‑order Bessel beams to higher‑order OAM Bessel beams. This approach provides a practical, scalable route to broadband, high‑efficiency vortex generation for applications in high‑dimensional optical communications, advanced imaging, and quantum photonics.

Abstract

Nonlocal metasurfaces provide a compact route to generating momentum-space optical vortices but are limited by steep dispersion typically associated with high-quality (Q) factor resonances, resulting in narrowband and inefficient operation. Here, we introduce a reflection-type nonlocal metasurface that hybrid-couples a bound state in the continuum (BIC) with two degeneracy points (DPs). This engineered interaction enables on-demand control of dispersion, radiative Q-factors, and polarization states of guided resonances, yielding quasi-flat dispersion and enhanced scattering strength. Full-wave simulations predict near-unity on-resonance conversion and overall efficiencies above 90%, representing a three- to fourfold efficiency improvement and more than fifteenfold bandwidth expansion over conventional designs. Experiments confirm broadband operation from 1480 to 1600 nm, achieving peak efficiency approaching 80% and orbital angular momentum (OAM) purity up to 91.7% under flat-top illumination, while suppressing edge effects and mitigating positional sensitivity and numerical-aperture (NA) dependence. As a proof of concept, we demonstrate direct conversion of zero-order Bessel beams into OAM Bessel (perfect vortex) beams with enhanced wavelength tunability, underscoring the versatility of this approach over diverse illumination conditions. This record-high performance establishes a practical and scalable pathway toward broadband, high-efficiency vortex generation, opening new opportunities across high-dimensional optical communications, advanced imaging, and quantum photonics.

Figures (6)

• Figure 1: Overview of our theoretical principle. (a–c) For conventional nonlocal metasurfaces based on a single BIC approach, the presence of a narrow efficiency ring in $(k_x,k_y)$ space (a) and steep modal dispersion (b) severely limits overall vortex beam conversion efficiency (c). (d–f) In contrast, our reflection-type BZF nonlocal metasurface (d) employs tailored band engineering to construct quasi-flat dispersion harnessing both BIC and DP modes (e), leading to broadband high-efficiency operation (f).
• Figure 2: Overview of our implementation. (a) Top panel: BZF PCS structure converting an incident right-handed circularly polarized plane wave into a left-handed circularly polarized vortex beam. Middle panel: Periodic perturbation maintaining $C_4$ symmetry. Bottom panel Brillouin zone folding in momentum space. (b–d) Band-structure engineering diagrams; glowing regions represent mode bandwidth ($Q$ factor), red/blue colors denote AP/RP SOPs respectively. Broadband high-efficiency regimes I and II are formed in (d) through synergistic effects of BIC- and DP-related modes. (e,f) Simulated band structures for conventional and optimal designs, with line thickness and color indicating mode bandwidth.
• Figure 3: Numerical results of our design. (a) Detailed spectrum of conversion efficiency $\eta(\omega,\mathbf{k})$ for the comparson structure. Left: s-polarized reflection spectrum ($R_s$); right: cross-polarized conversion efficiency spectrum ($\eta$). (b) Colored panels: cross-sectional efficiency and phase at different wavelengths under NA=0.42. (c) Simulated overall efficiency of flat-top-beam illumination under different NA. (d–f) Same as (a–c) but for optimal structure.
• Figure 4: (a) Schematic of the momentum-space imaging optical setup employed in the experiment. (b) Top‐view SEM micrograph of the fabricated metasurface. (c-e) Three‐dimensional full $\omega$–$\mathbf{k}$ dispersion of the cross‐polarization conversion efficiency: (c) simulated results; (d-e) experimental measurements, with (e) highlighting broadband and high overall conversion efficiency across the 1500–1540 nm range. (f) Measured s-polarized reflection spectrum $R_s$. (g) Cross‐polarized conversion efficiency spectrum. Colored dashed lines in (g) mark the wavelengths—1500 nm (green), 1520 nm (blue) and 1540 nm (purple)—for which the right subpanels show the absolute intensities of the incident (black) and converted (red) beams.
• Figure 5: (a) Measured cross-polarization converted beam profile at different illumination positions. (b) Measured overall averaged efficiency under different wavelength and NA. (c) Measured generated beam profile. From top to bottom: intensity, interference pattern and extracted phase. (d) LG mode decomposition of the converted vortex beam under NA=0.21 at 1510, 1520, and 1530 nm. Inset: OAM ($l$) distribution, where the purity of $l=-2$ can reach as high as 91.7% at 1520 nm.