In a groundbreaking advancement in wave physics, researchers led by scientists from Nanyang Technological University (NTU) in Singapore have unveiled a novel framework that harnesses bound states in the continuum (BICs) to create highly degenerate flatbands. This innovation, detailed in a paper published today in Nature Communications, enables the compact localization of orbital angular momentum (OAM) carrying waves. The achievement addresses a longstanding challenge in flatband materials, where zero-dispersion energy bands typically allow for tight localization of simple wavefunctions but struggle with more complex structures like those bearing OAM—a helical phase front crucial for high-capacity communications and quantum technologies.

The international team, including Zheyu Cheng and corresponding author Baile Zhang from NTU's School of Physical and Mathematical Sciences and Centre for Disruptive Photonic Technologies, collaborated with experts from Jiangsu University in China and The Chinese University of Hong Kong. Their work demonstrates flatbands with unprecedented degeneracy—four-fold in two-dimensional (2D) acoustic crystals and twelve-fold in three-dimensional (3D)—paving the way for selective OAM filtering and topologically structured waves in acoustics and beyond.

Understanding Flatbands: The Foundation of Localization

Flatbands represent energy bands in a material's band structure where the energy remains constant across momentum space, resulting in zero group velocity. This peculiar property confines wavefunctions to the scale of a single unit cell, fostering strong electron-electron interactions in solids or enhancing light-matter coupling in photonics. Think of it step-by-step: in conventional dispersive bands, waves propagate freely; in flatbands, they are pinned in place, much like electrons trapped in a lattice site without hopping to neighbors.

Historically, flatbands have driven phenomena like superconductivity in twisted bilayer graphene and lasing in photonic lattices. However, achieving flatbands for waves with internal degrees of freedom, such as OAM—characterized by a phase singularity and helical wavefronts—requires not just flatness but high degeneracy to 'host' the wave's topological charge (denoted as ℓ, an integer representing twists per wavelength).

Pioneering works on kagome or Lieb lattices provided nondegenerate flatbands, but the NTU framework elevates this by engineering degeneracy through BICs, making OAM localization feasible.

Bound States in the Continuum: Trapped Waves in Open Space

Bound states in the continuum (BICs), first theorized in quantum mechanics, are localized states embedded within a radiating continuum that do not decay due to destructive interference or symmetry protection. In photonics and acoustics, BICs yield infinite quality factors (Q-factors), ideal for cavities and lasers. The process unfolds as: (1) design a unit cell hosting multiple BICs at the same frequency; (2) lattice them periodically; (3) the BICs evolve into flatbands whose degeneracy matches the BIC count per cell.

This BIC-to-flatband transition is generalizable across waves—acoustic, photonic, even electronic—offering a blueprint beyond moiré superlattices. In the study, quasi-BICs (with finite but high Q) ensure practical fabrication.

The NTU Framework: Engineering Degenerate Flatbands

The core innovation is a systematic method: start with a primitive cell embedding N BICs at target frequency ω; form a superlattice where each supercell contains M such cells, yielding N×M degenerate flatbands. Mathematically, the Bloch Hamiltonian's eigenvalue problem reveals flat dispersion when BICs couple symmetrically.

Step-by-step: (1) Simulate unit cell for BICs via symmetry-protected or accidental mechanisms; (2) Verify infinite Q in isolated cell; (3) Periodize into lattice, observing flatbands; (4) Probe degeneracy via Hilbert space dimensionality. This universality stems from BIC's insensitivity to k-space folding.

NTU's Baile Zhang, an expert in metamaterials, emphasizes the framework's versatility: "Our approach unlocks flatbands for structured waves, revolutionizing wave manipulation."

Experimental Breakthrough in Acoustic Metamaterials

The team fabricated 2D acoustic crystals from brass plates with slotted resonators, hosting four BICs per unit cell for four-fold flatbands at 6.5 kHz. Near-field scanning confirmed compact OAM modes (ℓ=±1, ±2) localized to ~1 unit cell diameter.

In 3D, cubic lattices of Helmholtz resonators achieved twelve-fold degeneracy, visualizing vortex-like OAM fields via pressure mapping. These experiments validate the theory, with measured Q-factors exceeding 1000.

Supporting data and code are openly available via NTU's research repository, promoting reproducibility.

Orbital Angular Momentum Localization: A New Frontier

OAM waves carry helical phase fronts, Σ exp(iℓφ), enabling multiplexing in fiber optics (terabit speeds) and quantum entanglement. Localizing them compactly requires matching degeneracy to |ℓ|max +1 per polarization/dimension.

Here, four-fold flatbands support ℓ up to ±1 (2D); twelve-fold up to higher ℓ (3D). Selective excitation via phased sources pins OAM vortices precisely, ideal for on-chip processing. Compared to dispersive OAM, flatband versions resist diffraction, enhancing stability.

• Compact size: ~λ/10 footprint
• High purity: >90% OAM fidelity
• Directional control: arbitrary lattice sites

Applications in Acoustic Signal Processing

Immediate payoff: OAM-compatible flatband filters separate channels in noisy environments, boosting underwater comms or medical ultrasound. Imagine sonar distinguishing OAM-encoded signals amid reverberations.

A link to higher education jobs in acoustics at institutions like NTU underscores career opportunities in this niche.Read the full paper for simulations.

Extending to Photonics and Quantum Technologies

Though demonstrated acoustically, the framework translates to photonics via photonic crystals or metasurfaces. NTU's photonics expertise (e.g., prior BIC lasers) positions Singapore for OAM flatband chips in 6G or quantum networks.

Topological implications: BIC flatbands host edge states for robust transport, potentially fractional quantum Hall analogs in waves. Future: hybrid opto-acousto devices.

Explore Singapore university jobs for photonics roles amid RIE2030 investments.

NTU Singapore's Leadership in Disruptive Photonics

NTU ranks globally top-10 in physics (QS 2026), with the Centre for Disruptive Photonic Technologies pioneering metamaterials. Baile Zhang's group advances BIC platforms, aligning with Singapore's quantum hub ambitions.

This publication elevates NTU's profile, attracting talent. Check university jobs or academic CV tips for entry.

Challenges, Future Outlook, and Global Impact

Challenges: scaling to optics (fabrication losses), higher ℓ (more BICs). Outlook: integrate with AI for adaptive OAM muxing; explore electronic analogs for 2D materials.

Singapore's ecosystem—NRF funding, A*STAR—accelerates translation. Globally, this democratizes structured flatbands, impacting telecoms (OAM fibers) and sensing.

Photo by Mari Ganesh Kumar on Unsplash

This NTU breakthrough exemplifies how Singapore universities drive wave physics frontiers. For researchers eyeing photonics careers, platforms like Rate My Professor offer insights into NTU faculty. Explore higher ed jobs, career advice, and university jobs to join the revolution. Share your thoughts in comments below.