Bound States in the Continuum in Photonic Systems

Bound states in the continuum (BICs) are non-radiative resonances that, despite existing within the spectral range of radiating modes, remain perfectly confined in open photonic structures. They arise either from symmetry protection, destructive interference between resonances or the interplay of guided and Fabry–Pérot modes, and are categorised broadly as symmetry-protected, accidental or Friedrich–Wintgen BICs. Theoretically infinite quality factors of BICs have driven their implementation in photonic crystal slabs, metasurfaces and heterostructures, enabling ultrahigh field localisation and enhanced light–matter interactions. Their topological nature, marked by integer charges in momentum space, underpins robustness to perturbations and guides device design. Applications span low-threshold nanolasers, ultrasensitive sensors, nonlinear frequency conversion and platforms for structured light and quantum photonics. Recent advances in Brillouin zone engineering, symmetry breaking and mode merging have extended ultrahigh quality factors across wide momentum ranges and achieved dynamic polarisation control over the Poincaré sphere. A growing classification framework and phase diagrams now inform universal design principles, accelerating the translation of BICs into integrated photonic circuits and quantum technologies.

Research from Nature Portfolio

Recent studies have exploited periodic perturbations to fold guided modes into the light cone, yielding Brillouin-zone-folding BICs that maintain ultrahigh quality factors over large and tunable momentum spaces while remaining robust against structural disorder, paving the way for silicon metasurface cavities in terahertz, nonlinear and quantum applications. Work on coherent excitation by multiple beams has demonstrated real-time, full-sphere polarisation control in a photonic crystal slab supporting a symmetry-protected BIC, utilising topological features around the singularity to achieve highly efficient, dynamic polarisation conversion. Seminal research on supercavity modes has merged symmetry-protected and accidental BICs in finite-size resonators to create an ultralow-threshold nanolaser with dramatically enhanced quality factors and reduced footprint, illustrating the potential of BIC merging strategies for active photonic devices.

Bound States in the Continuum in Photonic Systems publication trend

The graph below shows the total number of articles in bound states in the continuum in photonic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Bound state in the continuum (BIC): A non-radiative resonance embedded within the radiation continuum that confines electromagnetic energy despite open boundaries.

Quality factor (Q factor): A measure of energy confinement in a resonator, defined as the ratio of resonant frequency to energy loss rate.

Photonic crystal slab: A two-dimensional periodic dielectric structure supporting guided resonances and BICs through bandgap and symmetry engineering.

Brillouin zone: The fundamental region in reciprocal space of a periodic structure, determining the dispersion and symmetry of photonic modes.

Topological charge: An integer index characterising the winding of phase or polarisation singularities in momentum space, indicating BIC robustness.

Poincaré sphere: A geometric representation of all possible polarisation states of light, with BIC singularities corresponding to poles and equatorial states.

References

1. Arbitrarily polarized bound states in the continuum with twisted photonic crystal slabs. Light: Science & Applications (2023).
2. Brillouin zone folding driven bound states in the continuum. Nature Communications (2023).
3. Ultralow-threshold laser using super-bound states in the continuum. Nature Communications (2021).
4. Merging bound states in the continuum by harnessing higher-order topological charges. Light: Science & Applications (2022).
5. Global phase diagram of bound states in the continuum. Optica (2022).
6. Coherent full polarization control based on bound states in the continuum. Nature Communications (2022).