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Bound states in the continuum

The ability to confine light is crucial for realizing high-Q resonators to enhance light-matter interactions and to guide light. The confinement is typically realized with a reflecting interface, such as using total internal reflection, metallic mirrors, or a photonic bandgap. In these common scenarios, the surrounding medium does not support outgoing waves, so the cavity is closed, and light has no choice but to remain trapped inside. The conventional wisdom used to be that once a cavity is opened (i.e., given access to a continuum of radiation states), light would inevitably leak out unless forbidden by symmetry.

Animation of a BIC in a photonic crystal slab.

We showed that light can in fact be perfectly confined even in an open photonic crystal slab with no symmetry separability, creating a “bound state in the continuum” (BIC) [1,2] whose mode profile is shown on the left. This can be understood as the destructive interference of radiating waves [3]. The disappearance of radiation corresponds to vortices of the far-field polarization vector in the Brillouin zone (experimental data on the right); the quantized topological charges ensure BICs’ robust existence and govern their generation, evolution, and annihilation [4,5].

Radiation vortices of BICs.

Coupling a BIC to a pair of leaky modes at the center of the Brillouin zone can spawn a ring of exceptional points out of a Dirac cone [6] (shown on the right). By breaking the mirror symmetry and suppressing radiation only to one side, one can realize perfect single-sided radiation or absorption [7]. We have predicted BICs in fiber Bragg gratings [8]. In addition to suppressing radiation into free space, it is also possible to suppress radiation into the modes of a photonic crystal environment [9,10]. Separability of the potential is another strategy to create BICs [11].

A ring of exceptional points in the momentum space.

Ref [12] provides a review on this topic, focusing on the common physical mechanisms underlying BICs across different wave systems.

Illustration of a BIC.

Related publications

  1. Bloch surface eigenstates within the radiation continuum, Chia Wei Hsu, Bo Zhen, Song-Liang Chua, Steven G. Johnson, John D. Joannopoulos, and Marin Soljačić. Light: Science & Applications 2, e84 (2013).
  2. Observation of trapped light within the radiation continuum, Chia Wei Hsu, Bo Zhen, Jeongwon Lee, Song-Liang Chua, Steven G. Johnson, John D. Joannopoulos, and Marin Soljačić. Nature 499, 188–191 (2013).
  3. Formation mechanism of guided resonances and bound states in the continuum in photonic crystal slabs, Xingwei Gao, Chia Wei Hsu, Bo Zhen, Xiao Lin, John D. Joannopoulos, Marin Soljačić, and Hongsheng Chen. Scientific Reports 6, 31908 (2016).
  4. Topological nature of optical bound states in the continuum, Bo Zhen, Chia Wei Hsu, Ling Lu, A. Douglas Stone, and Marin Soljačić. Physical Review Letters 113, 257401 (2014).
  5. Observation of polarization vortices in momentum space, Yiwen Zhang, Ang Chen, Wenzhe Liu, Chia Wei Hsu, Fang Guan, Xiaohan Liu, Lei Shi, Ling Lu, and Jian Zi. Physical Review Letters 120, 186103 (2018).
  6. Spawning rings of exceptional points out of Dirac cones, Bo Zhen, Chia Wei Hsu, Yuichi Igarashi, Ling Lu, Ido Kaminer, Adi Pick, Song-Liang Chua, John D. Joannopoulos, and Marin Soljačić. Nature 525, 354–358 (2015).
  7. Perfect single-sided radiation and absorption without mirrors, Hengyun Zhou, Bo Zhen, Chia Wei Hsu, Owen D. Miller, Steven G. Johnson, John D. Joannopoulos, and Marin Soljačić. Optica 3, 1079–1086 (2016).
  8. Bound states in the continuum in fiber Bragg gratings, Xingwei Gao, Bo Zhen, Marin Soljačić, Hongsheng Chen, and Chia Wei Hsu. ACS Photonics 6, 2996–3002 (2019).
  9. Bound states in the continuum through environmental design, Alexander Cerjan, Chia Wei Hsu, and Mikael C. Rechtsman. Physical Review Letters 123, 023902 (2019).
  10. Observation of bound states in the continuum embedded in symmetry bandgaps, Alexander Cerjan, Christina Jörg, Sachin Vaidya, Shyam Augustine, Wladimir A. Benalcazar, Chia Wei Hsu, Georg von Freymann, and Mikael C. Rechtsman. Science Advances 7, eabk1117 (2021).
  11. Controlling directionality and dimensionality of radiation by perturbing separable bound states in the continuum, Nicholas Rivera, Chia Wei Hsu, Bo Zhen, Hrvoje Buljan, John D. Joannopoulos, and Marin Soljačić. Scientific Reports 6, 33394 (2016).
  12. Bound states in the continuum, Chia Wei Hsu, Bo Zhen, A. Douglas Stone, John D. Joannopoulos, and Marin Soljačić. Nature Reviews Materials 1, 16048 (2016).
University of Southern California