Photonic Bound States in the Continuum

Bound states in the continuum (BICs) are optical modes that, despite lying within the spectrum of freely propagating waves, remain perfectly confined without radiative loss. This paradoxical phenomenon arises from interference effects, topological constraints or symmetry protection, and permits exceptionally high quality factors in open photonic structures. In recent years, engineered BICs have been realised in photonic‐crystal slabs, metasurfaces and integrated waveguides, enabling unprecedented control over light–matter interaction. The capacity to tailor radiative coupling through geometrical design or near‐field hybridisation has led to quasi‐BICs with finite but extremely large quality factors, unlocking applications in sensing, nonlinear optics, lasing and quantum information. Emerging approaches exploit mode coupling, material nonlinearity and active tuning to achieve supercritical enhancement, dynamic resonance control and electro‐optic modulation. The global significance of photonic BICs is reflected in their potential to deliver low-threshold lasers, ultrasensitive detectors, narrowband filters and on-chip optical switches.

Research from Nature Portfolio

Recent studies have introduced the concept of supercritical coupling in photonic‐crystal nanoslab resonators, whereby near‐field interaction between dark and bright modes compensates for negligible direct radiation, yielding record near‐field enhancement even as radiative quality factors diverge. This approach enabled coherent upconversion luminescence enhanced by eight orders of magnitude, with controllable emission directivity and potential applications in light sources and energy harvesting. Foundational work on field enhancement mechanisms in subwavelength periodic dielectric films has derived analytical models linking partial resonance quality and material losses, predicting ultrahigh field enhancement factors near embedded eigenvalues. These findings provide design strategies for practical resonance elements based on optical BICs across a broad range of applications.

Photonic Bound States in the Continuum publication trend

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

Technical terms

Bound State in the Continuum (BIC): An optical eigenmode that is confined without radiation loss despite lying within the spectrum of free propagating waves.

Quality factor (Q): A dimensionless measure of a resonator’s energy retention, defined as the ratio of stored energy to energy lost per cycle.

Quasi-Bound State in the Continuum (quasi-BIC): A resonance with very high but finite quality factor produced by slight symmetry breaking or perturbation of an ideal BIC.

Photonic crystal slab: A dielectric structure with periodic refractive index modulation used to confine and manipulate light on wavelength scales.

Fano resonance: An asymmetric spectral line shape arising from interference between a discrete resonant state and a continuum of states.

Radiative losses: Energy dissipation from a resonator due to emission of waves into far‐field radiation modes.

References

1. Directive giant upconversion by supercritical bound states in the continuum. Nature (2024).
2. Experimental Observation of Dissipatively Coupled Bound States in the Continuum on an Integrated Photonic Platform. Laser & Photonics Review (2023).
3. Bound-state-in-continuum guided modes in a multilayer electro-optically active photonic integrated circuit platform.. Optica (2024).
4. Bound states in the continuum and Fano resonances in the strong mode coupling regime. Advanced Photonics (2019).
5. Critical field enhancement of asymptotic optical bound states in the continuum. Scientific Reports (2015).