The ability to confine light is crucial in science and technology. Light can be confined with a wide range of methods ranging from metallic reflectors, total internal reflection, photonic bandgap, to Anderson localization and symmetry separability. In all of these known methods, the "resonator" is surrounded by a medium that does not support outgoing waves, so light has no choice but to remain trapped inside. Unexpectedly, we found that light can sometimes be perfectly confined even though there do exist outgoing waves that the trapped light can couple to.
The trapped light is an optical bound state in continuum (BIC), namely a localized state whose frequency (or energy) lies within the continuous band where normally only spatially extended states can be found.
BIC on a surface
Our theoretical paper describes an optical BIC on the surface of a 2D photonic crystal:
"Bloch surface eigenstates within the radiation continuum," Chia Wei Hsu, Bo Zhen, Song-Liang Chua, Steven G. Johnson, J. D. Joannopoulos, and Marin Soljacic, Light: Science & Applications 2, e84 (2013); doi:10.1038/lsa.2013.40.
BIC in a slab
Our experimental paper describes the observation of such trapped light at optical wavelength, on a photonic crystal slab:
"Observation of trapped light within the radiation continuum," Chia Wei Hsu, Bo Zhen, Jeongwon Lee, Song-Liang Chua, Steven G. Johnson, J. D. Joannopoulos, and Marin Soljacic, Nature 499, 188-191 (2013); doi:10.1038/nature12289.
This work was featured on MIT home page on July 11, 2013.
Press coverages also include Physics Today, Scientific American, Science News, Science Daily, Optics & Photonics News, Science World Report, Materials Today, Phys.org, Photonics.com, Photonics Spectra, How It Works magazine, Nanowerk, EurekAlert!, Laboratory Equipment, TG Daily, New Electronics, Kurzweil, RenewEconomy, and Francis (th)E mule Science's News.
Topological charge of a BIC
BICs may carry topological charges that ensure their robust existence and govern their generation, evolution, and annihilation:
"Topological nature of optical bound states in the continuum," Bo Zhen, Chia Wei Hsu, Ling Lu, A. Douglas Stone, and Marin Soljacic, Phys. Rev. Lett. 113, 257401 (2014); doi:10.1103/PhysRevLett.113.257401.
This work was featured on MIT home page on December 22, 2014.
We wrote a review paper on BICs, focusing on the common physical mechanisms underlying BICs across different wave systems:
"Bound states in the continuum," Chia Wei Hsu, Bo Zhen, A. Douglas Stone, John D. Joannopoulos, and Marin Soljacic, Nature Reviews Materials 1, 16048 (2016); doi:10.1038/natrevmats.2016.48.