Orbital Angular Momentum Locking via Bound States in the Continuum

Technical Summary: Orbital Angular Momentum Locking via Bound States in the Continuum

Problem Statement
Optical vortices, characterized by helical phase structures and quantized orbital angular momentum (OAM), are valuable for applications ranging from imaging to quantum information processing. While far-field vortices are well-established, their integration into compact photonic chips is hindered by the diffraction limit. Near-field polaritonic vortices (PVs) offer a solution by confining energy to deeply subwavelength scales. However, conventional methods for generating PVs—typically relying on the coupling of circularly polarized light to spiral nanostructures—suffer from poor mode selectivity. These approaches often produce multimode vortices with mixed OAM states due to imperfect polarization control, fabrication uncertainties, or finite laser linewidths. This OAM mixing degrades vortex purity, limiting their utility in high-fidelity optical information encoding and multi-dimensional imaging. Furthermore, existing strategies to improve selectivity often rely on strong nonlinear effects, which constrain material choices and spectral windows.

Methodology
The authors propose a platform that generates robust, deeply subwavelength PVs using quasi-bound states in the continuum (qBICs) within dielectric metasurfaces. The core design consists of periodic arrays of high-refractive-index silicon nanoresonators shaped as enclosed Archimedean spirals, placed beneath a thin layer of hexagonal boron nitride (hBN).

• qBIC Engineering: The qBICs are realized by breaking the in-plane symmetry of a square lattice of resonators. This is achieved by laterally offsetting resonator pairs in a checkerboard pattern, which converts a symmetry-protected bound state into a radiative qBIC with a finite quality factor (Q-factor). The resonance linewidth and spectral position are tunable via the lateral offset ($\Delta x$) and the unit cell geometry.
• OAM Generation: Unlike conventional methods that convert spin angular momentum (SAM) from circularly polarized light into OAM, this platform utilizes linearly polarized light. The OAM is intrinsically locked to the structural vorticity ($L$) of the spiral arms ($n$), such that the total topological charge is $\ell = L = n \cdot m$ (where $m=1$ is chosen to maximize resonator area). The photonic qBIC generates uniform out-of-plane electric fields ($E_z$) across the resonator, which launch hyperbolic phonon polaritons (HPhPs) in the hBN layer via in-plane momentum transfer.
• Characterization: The system was fabricated using electron-beam lithography on silicon resonators covered by exfoliated hBN flakes. Experimental validation was performed using transmission-mode scattering scanning near-field optical microscopy (s-SNOM) to map the amplitude and phase of the out-of-plane electric field. Far-field reflectance spectra were also measured to characterize resonance quality factors.

Key Contributions and Results

• Intrinsic OAM Locking: The study demonstrates that the qBIC mechanism intrinsically locks the PV to a single OAM value ($\ell$), independent of the incident polarization state. The system maintains a fixed topological charge for linear, elliptical, and circular polarization, and remains robust against polarization orientation variations of up to $\sim 70^\circ$.
• High Purity and Subwavelength Scale: Experimental results confirm the generation of PVs with high modal purity (overlap integrals $OI_\ell > 0.3$) for topological charges $\ell = \pm 1, \pm 2,$ and $\pm 4$. The resulting HPhPs exhibit wavelengths approximately 30–40 times smaller than the incident light ($\lambda_{HPhP}/\lambda_0 < 1/30$), enabling ultra-dense packing of multiple distinct PVs within a single subwavelength unit cell.
• Spectral and Geometric Tunability: The spectral window for OAM locking is governed by the Q-factor of the resonance, which can be adjusted through geometric modifications. Simulations indicate that the platform can support high-order PVs (up to $\ell > 50$) by increasing the number of spiral arms without significantly perturbing the underlying photonic mode.
• Multiplexing Capability: The authors successfully simulated and experimentally verified a multiplexed unit cell containing four resonators with different arm counts, generating four spatially separated PVs with distinct OAM values ($\ell = 1$ to $4$) simultaneously.

Significance
The paper claims that this platform addresses the critical limitation of OAM mixing in conventional PV generation, offering a route to high-fidelity, single-mode polaritonic vortices without the need for complex polarization control or nonlinear materials. By leveraging the unique properties of qBICs and HPhPs, the work brings PVs to the photonic chip scale. The authors suggest this technology could enable applications in structured optical information transfer, communications, and all-optical logic networks. Furthermore, the ability to generate robust, deeply subwavelength vortices with high purity opens possibilities for on-chip multiplexing, single-mode lasing, and the structured thermal emission of light. The robustness against polarization variations and the potential for high-density integration are highlighted as key advantages for practical technological implementation.